Association between the proportion of Plasmodium falciparum and Plasmodium vivax infections detected by passive surveillance and the magnitude of the asymptomatic reservoir in the community: a pooled analysis of paired health facility and community data

BACKGROUND

Passively collected malaria case data are the foundation for public health decision making. However, because of population-level immunity, infections might not always be sufficiently symptomatic to prompt individuals to seek care. Understanding the proportion of all Plasmodium spp infections expected to be detected by the health system becomes particularly paramount in elimination settings. The aim of this study was to determine the association between the proportion of infections detected and transmission intensity for Plasmodium falciparum and Plasmodium vivax in several global endemic settings.

METHODS

The proportion of infections detected in routine malaria data, P(Detect), was derived from paired household cross-sectional survey and routinely collected malaria data within health facilities. P(Detect) was estimated using a Bayesian model in 431 clusters spanning the Americas, Africa, and Asia. The association between P(Detect) and malaria prevalence was assessed using log-linear regression models. Changes in P(Detect) over time were evaluated using data from 13 timepoints over 2 years from The Gambia.

FINDINGS

The median estimated P(Detect) across all clusters was 12·5% (IQR 5·3-25·0) for P falciparum and 10·1% (5·0-18·3) for P vivax and decreased as the estimated log-PCR community prevalence increased (adjusted odds ratio [OR] for P falciparum 0·63, 95% CI 0·57-0·69; adjusted OR for P vivax 0·52, 0·47-0·57). Factors associated with increasing P(Detect) included smaller catchment population size, high transmission season, improved care-seeking behaviour by infected individuals, and recent increases (within the previous year) in transmission intensity.

INTERPRETATION

The proportion of all infections detected within health systems increases once transmission intensity is sufficiently low. The likely explanation for P falciparum is that reduced exposure to infection leads to lower levels of protective immunity in the population, increasing the likelihood that infected individuals will become symptomatic and seek care. These factors might also be true for P vivax but a better understanding of the transmission biology is needed to attribute likely reasons for the observed trend. In low transmission and pre-elimination settings, enhancing access to care and improvements in care-seeking behaviour of infected individuals will lead to an increased proportion of infections detected in the community and might contribute to accelerating the interruption of transmission.

FUNDING

Wellcome Trust.

Predictors of residual antimalarial drugs in the blood in community surveys in Tanzania

Background

Understanding pattern of antimalarials use at large scale helps ensuring appropriate use of treatments and preventing the spread of resistant parasites. We estimated the proportion of individuals in community surveys with residual antimalarials in their blood and identified the factors associated with the presence of the most commonly detected drugs, lumefantrine and/or desbutyl-lumefantrine (LF/DLF) or sulfadoxine-pyrimethamine (SP).

Methods

A cross-sectional survey was conducted in 2015 in three regions of Tanzania with different levels of malaria endemicity. Interviews were conducted and blood samples collected through household surveys for further antimalarial measurements using liquid chromatography coupled to tandem mass spectrometry. In addition, diagnosis and treatment availability was investigated through outlet surveys. Multilevel mixed effects logistic regression models were used to estimate odds ratios for having LF/DLF or SP in the blood.

Results

Amongst 6391 participants, 12.4% (792/6391) had LF/DLF and 8.0% (510/6391) SP in the blood. Factors associated with higher odds of detecting LF/DLF in the blood included fever in the previous two weeks (OR = 2.6, p<0.001), living in districts of higher malaria prevalence (OR = 1.5, p<0.001) and living in a ward in which all visited drug stores had artemisinin-based combination therapies in stocks (OR = 2.7, p = 0.020). Participants in older age groups were less likely to have LF/DLF in the blood (OR = 0.9, p<0.001). Factors associated with higher odds of having SP in the blood included being pregnant (OR = 4.6, p<0.001), living in Mwanza (OR = 3.9, p<0.001 compared to Mbeya), fever in the previous two weeks (OR = 1.7, p<0.001) and belonging to older age groups (OR = 1.2, p<0.001).

Conclusion

The most significant predictors identified were expected. History of fever in the past two weeks and young age were significant predictors of LF/DLF in the blood, which is encouraging. Antimalarial drug pressure was high and hence the use of recommended first-line drugs in combination with malaria Rapid Diagnostics Tests should be promoted to ensure appropriate treatment.

Appropriateness of malaria diagnosis and treatment for fever episodes according to patient history and anti-malarial blood measurement: a cross-sectional survey from Tanzania

Background

Monitoring the impact of case management strategies at large scale is essential to evaluate the public health benefit they confer. The use of methodologies relying on objective and standardized endpoints, such as drug levels in the blood, should be encouraged. Population drug use, diagnosis and treatment appropriateness in case of fever according to patient history and anti-malarials blood concentration was evaluated.

Methods

A cross-sectional survey took place between May and August 2015 in three regions of Tanzania with different levels of malaria endemicity. Interviews were conducted and blood samples were collected by dried blood spots through household surveys for further anti-malarial measurements. Appropriate testing when individuals attended care was defined as a patient with history of fever being tested for malaria and appropriate treatment as (i) having anti-malarial in the blood if the test result was positive (ii) having anti-malarial in the blood if the person was not tested, and (iii) no anti-malarial in the blood when the test result was negative.

Results

Amongst 6391 participants included in the anti-malarial analysis, 20.8% (1330/6391) had anti-malarial drug detected in the blood. Only 28.0% (372/1330) of the individuals with anti-malarials in their blood reported the use of anti-malarials within the previous month. Amongst all participants, 16.0% (1021/6391) reported having had a fever in the previous 2 weeks and 37.5% of them (383/1021) had detectable levels of anti-malarials in the blood. Of the individuals who sought care in health facilities, 69.4% (172/248) were tested and 52.0% (129/248) appropriately treated. When other providers were sought, 6% (23/382) of the persons were appropriately tested and 44.2% (169/382) appropriately treated. Overall, the proportion of individuals treated was larger than that being tested [47.3% (298/630) treated, 31.0% (195/630) tested].

Conclusion

This study showed high prevalence of circulating anti-malarial drug in the sampled population. Efforts should be made to increase rapid diagnostic tests use at all levels of health care and improve compliance to test result in order to target febrile patients that are sick with malaria and reduce drug pressure. Objective drug measurements collected at community level represent a reliable tool to evaluate overall impact of case management strategies on population drug pressure.

LC-MS/MS method for the simultaneous analysis of seven antimalarials and two active metabolites in dried blood spots for applications in field trials: Analytical and clinical validation

In epidemiological studies, antimalarials measurements in blood represent the best available marker of drugs exposure at population level, an important driver for the emergence of drug resistance. We have developed a liquid chromatography-tandem mass spectrometry method (LC-MS/MS) for the simultaneous quantification of 7 frequently used antimalarials (amodiaquine, chloroquine, quinine, sulfadoxine, pyrimethamine, mefloquine, lumefantrine) and 2 active metabolites (N-desethyl-amodiaquine, desbutyl-lumefantrine) in 10-μl dried blood spots (DBS). This sampling approach is suitable for field studies wherein blood samples processing, transportation and storage are problematic. Sample preparation included extraction from a 3 mm-disk punched out of the DBS with 100-μl of methanol + 1% formic acid containing deuterated internal standards for all drugs. Good performances were achieved in terms of trueness (-12.1 to +11.1%), precision (1.4-15.0%) and sensitivity, with lower limits of quantification comprised between 2 ng/ml (sulfadoxine) and 20 ng/ml (chloroquine, quinine, pyrimethamine, mefloquine, lumefantrine and desbutyl-lumefantrine). All analytes were stable in DBS kept for 24 h at room temperature and at 37 °C. The developed assay was applied within the frame of a pharmacokinetic study including 16 healthy volunteers who received a single dose of artemether-lumefantrine. Lumefantrine concentrations in plasma and in DBS were highly correlated (R = 0.97) at all time points, confirming the assumption that lumefantrine concentrations determined in DBS confidently reflect blood concentrations. The blood/plasma ratio of 0.56 obtained using the Bland-Altman approach (and corresponding to the slope of the linear regression) is in line with very low penetration of lumefantrine into red blood cells. This sensitive multiplex LC-MS/MS assay enabling the simultaneous analysis of antimalarials in DBS is suitable for epidemiological studies in field conditions.

Dynamics and orientation of amphipathic peptides in solution and bound to membranes: a steady-state and time-resolved fluorescence study of staphylococcal delta-toxin and its synthetic analogues

The environment of both the hydrophilic and hydrophobic sides of alpha-helical delta-toxin are probed by tryptophanyl (Trp) fluorescence, when self-association occurs in solution and on binding to membranes. The fluorescence parameters of staphylococcal delta-toxin (Trp15 on the polar side of the amphipathic helix) and synthetic analogues with single Trp at position 5 or 16 (on the apolar side) were studied. The time-resolved fluorescence decays of the peptides in solution show that the local environment of their single Trp is always heterogeneous. Although the self-association degree increases with concentration, as shown by fluorescence anisotropy decays, the lifetimes (and their statistical weight) of Trp16 do not change, contrary to what is observed for Trp15. The first step of self-association is then driven by hydrophobic interactions between apolar sides of alpha-helices, whilst further oligomerization involves their polar side (Trp15) via electrostatic interactions. This is supported by dissociation induced by salt. For all self-associated peptides, the polarity of the Trp microenvironment was not significantly modified upon binding to phospholipid vesicles, as indicated by the small shifts of the fluorescence emission spectra and lifetime values. However, the relative populations of the lifetime classes vary with bound-peptide density similar to the rates of their global motions in bilayers or smaller particles. Quenching experiments by water or lipid-soluble compounds show changes of the orientation of membrane-inserted peptides, from probably dimers lying flat at the interface at low peptide density, to oligomers spanning the membrane and inducing membrane fragmentation at high peptide density.

Solution structure and backbone dynamics of the defunct domain of calcium vector protein

CaVP (calcium vector protein) is a Ca(2+) sensor of the EF-hand protein family which is highly abundant in the muscle of Amphioxus. Its three-dimensional structure is not known, but according to the sequence analysis, the protein is composed of two domains, each containing a pair of EF-hand motifs. We determined recently the solution structure of the C-terminal domain (Trp81-Ser161) and characterized the large conformational and dynamic changes induced by Ca(2+) binding. In contrast, the N-terminal domain (Ala1-Asp86) has lost the capacity to bind the metal ion due to critical mutations and insertions in the two calcium loops. In this paper, we report the solution structure of the N-terminal domain and its backbone dynamics based on NMR spectroscopy, nuclear relaxation, and molecular modeling. The well-resolved three-dimensional structure is typical of a pair of EF-hand motifs, joined together by a short antiparallel beta-sheet. The tertiary arrangement of the two EF-hands results in a closed-type conformation, with near-antiparallel alpha-helices, similar to other EF-hand pairs in the absence of calcium ions. To characterize the internal dynamics of the protein, we measured the (15)N nuclear relaxation rates and the heteronuclear NOE effect in (15)N-labeled N-CaVP at a magnetic field of 11.74 T and 298 K. The domain is mainly monomeric in solution and undergoes an isotropic Brownian rotational diffusion with a correlation time of 7.1 ns, in good agreement with the fluorescence anisotropy decay measurements. Data analysis using a model-free procedure showed that the amide backbone groups in the alpha-helices and beta-strands undergo highly restricted movements on a picosecond to nanosecond time scale. The amide groups in Ca(2+) binding loops and in the linker fragment also display rapid fluctuations with slightly increased amplitudes.

The polar headgroup of the detergent governs the accessibility to water of tryptophan octyl ester in host micelles

Many attempts have been made to rationalize the use of detergents for membrane protein studies [J. Biol. Chem. 264 (1989) 4907]. The barrier properties of the detergent headgroup may be one parameter critically involved in protein protection. In this paper, we analyzed these properties using a model system, by comparing the accessibility of tryptophan octyl ester (TOE) to water-soluble collisional quenchers (iodide and acrylamide) in three detergent micelles. The detergents used differed only in the chemical nature of their polar headgroups, zwitterionic for dodecylphosphocholine (DPC) and nonionic for octa(ethylene glycol) dodecyl monoether (C(12)E(8)) and dodecylmaltoside (DM). In all cases, in phosphate buffer at pH 7.5, the binding of 5 microM TOE was complete in the presence of a slight excess of detergent micelles over TOE molecules, resulting in a significant blue shift and greater intensity of TOE fluorescence emission. The resulting quantum yield of bound TOE was between 0.08 (in DPC) and 0.12 (in DM) with an emission maximum (lambda(max)) of approximately 335 nm whatever the detergent micelle. Time-resolved fluorescence intensity decays of TOE at lambda(max) were heterogeneous in all micelles (3-4 lifetime populations), with mean lifetimes of 1.7 ns in DPC, and 2 ns in both C(12)E(8) and DM. TOE fluorescence quenching by iodide, in detergent micelles, yielded linear Stern-Volmer plots characteristic of a dynamic quenching process. The accessibility of TOE to this ion was the greatest with C(12)E(8), followed by DPC and finally DM (Stern-Volmer quenching constants K(sv) of 2 to 5.5 M(-1)). In contrast, the accessibility of TOE to acrylamide was greatest with DPC, followed by C(12)E(8) and finally DM (K(sv)=2.7-7.1 M(-1)). TOE also presents less rotational mobility in DM than in the other two detergents, as shown from anisotropy decay measurements. These results, together with previous TOE quenching measurements with brominated detergents [Biophys. J. 77 (1999) 3071] provide reference data for analyzing Trp characteristics in peptide (and more indirectly protein)-detergent complexes. The main finding of this study was that TOE was less accessible (to soluble quenchers) in DM than in DPC and C(12)E(8), the cohesion of DM headgroup region being suggested to play a role in the ability of this detergent to protect function and stability of solubilized membrane proteins.

Annexin A5 D226K structure and dynamics: identification of a molecular switch for the large-scale conformational change of domain III

The domain III of annexin 5 undergoes a Ca(2+)- and a pH-dependent conformational transition of large amplitude. Modeling of the transition pathway by computer simulations suggested that the interactions between D226 and T229 in the IIID-IIIE loop on the one hand and the H-bond interactions between W187 and T224 on the other hand, are important in this process [Sopkova et al. (2000) Biochemistry 39, 14065-14074]. In agreement with the modeling, we demonstrate in this work that the D226K mutation behaves as a molecular switch of the pH- and Ca(2+)-mediated conformational transition. In contrast, the hydrogen bonds between W187 and T224 seem marginal.

Origin of laurdan sensitivity to the vesicle-to-micelle transition of phospholipid-octylglucoside system: a time-resolved fluorescence study

The fluorescent probe laurdan has been shown to be sensitive to the vesicle-to-micelle transition of phosphatidylcholine/octylglucoside (M. Paternostre, O. Meyer, C. Grabielle-Madelmont, S. Lesieur, and, Biophys. J. 69:2476-2488). On the other hand, a study on the photophysics of laurdan in organic solvents has shown that the complex de-excitation pathway of the probe can be described by two successive processes, i.e., an intramolecular charge transfer followed by dielectric relaxation of the solvent if polar. These two excited-state reactions lead to three emitting states, i.e., a locally excited state, a charge transfer state, and a solvent relaxed state (M. Viard, J. Gallay, M. Vincent, B. Robert and, Biophys. J. 73:2221-2234). Experiments have been performed using time-resolved fluorescence on the probe inserted in amphiphile aggregates (mixed liposomes, mixed micelles) different in detergent-to-lipid ratios. The results have been compared with those obtained for laurdan inserted in dipalmitoyl phosphatidylcholine liposomes in the gel and in the fluid lamellar phase. Except for laurdan in dipalmitoyl phosphatidylcholine liposomes in the gel lamellar phase, the red part of the emission spectra originates from the de-excitation of the relaxed excited state of laurdan, indicating that indeed the dielectric relaxation process is an important phenomena in the ground-state return pathway of this probe. On the other hand, the maximization entropy method (MEM) analysis of the fluorescence decay recorded in the blue part of the emission spectra indicates that the dielectric relaxation is not the only reaction occurring to the excited state of laurdan. Moreover, the analysis of the fluorescence decays of laurdan inserted in gel lamellar dipalmitoylphosphatidylcholine (DPPC) liposomes indicates excited-state reactions, although dielectric relaxation is impossible. These results are in agreement with the de-excitation pathway determined from laurdan behavior in organic solvent even if, in most of the aggregates studied in this work, the major phenomenon is the dielectric relaxation of the solvent. All along the vesicle-to-micelle transition, we have observed that the lifetime of the relaxed excited state of laurdan continuously decreases probably due to a dynamic quenching process by water molecules. On the other hand, the time constant of the dielectric relaxation process remains almost unchanged in the lamellar part of the transition but abruptly decreases as soon as the first mixed micelle is formed. This decrease is continuous all over the rest of the transition even if it is more pronounced in the mixed liposomes' and mixed micelles' coexistence. The increase of the octylglucoside-to-lipid ratio of the mixed micelles via the change of the size and the shape of the aggregates may facilitate the penetration and the mobility of water molecules. Therefore, during the vesicle-to-micelle transition, laurdan probes the evolution of both the amphiphile packing in the aggregates and the increase of the interface polarity. This study finally shows that the detergent-to-lipid ratio of the mixed micelles is an important parameter to control to limit the penetration and the mobility of water within the amphiphile aggregates and that laurdan is a nice tool to monitor this phenomenon.

Substrate-induced fit of the ATP binding site of cytidine monophosphate kinase from Escherichia coli: time-resolved fluorescence of 3'-anthraniloyl-2'-deoxy-ADP and molecular modeling

The conformation and dynamics of the ATP binding site of cytidine monophosphate kinase from Escherichia coli (CMPK(coli)), which catalyzes specifically the phosphate exchange between ATP and CMP, was studied using the fluorescence properties of 3'-anthraniloyl-2'-deoxy-ADP, a specific ligand of the enzyme. The spectroscopic properties of the bound fluorescent nucleotide change strongly with respect to those in aqueous solution. These changes (red shift of the absorption and excitation spectra, large increase of the excited state lifetime) are compared to those observed in different solvents. These data, as well as acrylamide quenching experiments, suggest that the anthraniloyl moiety is protected from the aqueous solvent upon binding to the ATP binding site, irrespective of the presence of CMP or CDP. The protein-bound ADP analogue exhibits a restricted fast subnanosecond rotational motion, completely blocked by CMP binding. The energy-minimized models of CMPK(coli) complexed with 3'-anthraniloyl-2'-deoxy-ADP using the crystal structures of the ligand-free protein and of its complex with CDP (PDB codes and, respectively) were compared to the crystal structure of UMP/CMP kinase from Dictyostelium discoideum complexed with substrates (PDB code ). The key residues for ATP/ADP binding to CMPK(coli) were identified as R157 and I209, their side chains sandwiching the adenine ring. Moreover, the residues involved in the fixation of the phosphate groups are conserved in both proteins. In the model, the accessibility of the fluorescent ring to the solvent should be substantial if the LID conformation remained unchanged, by contrast to the fluorescence data. These results provide the first experimental arguments about an ATP-mediated induced-fit of the LID in CMPK(coli) modulated by CMP, leading to a closed conformation of the active site, protected from water.

Modulation by Ca(2+) and by membrane binding of the dynamics of domain III of annexin 2 (p36) and the annexin 2-p11 complex (p90): implications for their biochemical properties

The modulation of the local structure and dynamics of domain III of annexin 2 (Anx2), in both the monomeric (p36) and heterotetrameric forms (p90), by calcium and by membrane binding was studied by time-resolved fluorescence intensity and anisotropy measurements of the single tryptophan residue (W212). The results yield the same dominant excited-state lifetime (1.4 ns) in both p36 and p90, suggesting that the conformation and environment of W212 are very similar. The fluorescence anisotropy decay data were analyzed by associative (two-dimensional) as well as nonassociative (one-dimensional) models. Although no statistical criterion is decisive for one model versus the other, only the associative model allows recovery of a physically relevant value of the Brownian rotational correlation of the protein. Using the associative model, a nanosecond flexibility is detectable in p90 but not in p36. When Ca(2+) binds in the millimolar concentration range to both forms of Anx2, a conformational change takes place leading to an increase of the major excited-state lifetime (2.6 ns) and to a suppression of the W212 local flexibility of p90. Binding to membranes of either p36 or p90 in the presence of Ca(2+) does not induce any conformational change other than that provoked by Ca(2+) binding alone. The W212 local flexibility in both proteins increases significantly, however, in their membrane-bound forms. In the presence of membranes, the conformation change of domain III in p90 displays a sensitivity to Ca(2+) 2 orders of magnitude higher than that of p36, reaching intracellular sub-micromolar concentration ranges. This higher Ca(2+) sensitivity correlates with the Ca(2+)-dependent membrane aggregation but not with their Ca(2+)-dependent binding to membranes. The significance of these structural and dynamical changes for the function of the protein is discussed.

Wavelength-resolved fluorescence emission of proteins using the synchrotron radiation as pulsed-light source: cross-correlations between lifetimes, rotational correlation times and tryptophan heterogeneity in FKBP59 immunophilin

The time-resolved fluorescence intensity and anisotropy decays of the immunophilin domain of FKBP59 (FKBP59-I)--a protein containing two tryptophan residues (the W89, buried in a hydrophobic pocket and the W59, water exposed)--were studied using the time-correlated single photon counting (TCSPC) technique. The synchrotron radiation machine Super-ACO (Orsay, France) was used as a pulsed light source (approximately 8MHz). A mainly dual and discrete excited state lifetime distribution was previously evidenced (Rouvière et al., 1997). The lifetime heterogeneity has been suggested to be relevant to the topological tryptophan heterogeneity. Indeed, taking into account the spectroscopic properties of the single tryptophan residue of the immunophilin FKBP12, a highly homologous protein containing a single tryptophan residue, the short- and the long-lived lifetime species were assumed to be related to the solvent-buried and to the solvent-exposed fluorescent residues, respectively. We definitely demonstrate this point by describing the dynamical properties of each tryptophan residue of the FKBP59-I as a function of the emission wavelength. The data of the polarized components of the fluorescence emission were analyzed by the Maximum Entropy Method using a one-dimensional model (each excited-state lifetime tau being associated with each rotational correlation time theta) and a two-dimensional model (without any a priori association constraint between the tau's and the theta's). The two dimensional analysis of the polarized fluorescence intensity decays by MEM show the existence of a correlation between fast picosecond dynamics of the indole ring with the shortest-lived and blue emitting species. Conversely, the long-lived and red emitting population is mainly associated to the Brownian motion of the protein. A protein flexibility of the region located around the W59 residue, but slightly contributing to the light depolarization process, is also evidenced and can be specifically attributed to the red emitting population.

Tryptophan octyl ester in detergent micelles of dodecylmaltoside: fluorescence properties and quenching by brominated detergent analogs

The fluorescence properties of tryptophan octyl ester (TOE), a hydrophobic model of Trp in proteins, were investigated in various mixed micelles of dodecylmaltoside (DM) and 7,8-dibromododecyl beta-maltoside (BrDM) or 10,11-dibromoundecanoyl beta-maltoside (BrUM). This study focuses on the mechanism via which these brominated detergents quench the fluorescence of TOE in a micellar system. The experiments were performed at a pH at which TOE is uncharged and almost completely bound to detergent micelles. TOE binding was monitored by its enhanced fluorescence in pure DM micelles or its quenched fluorescence in pure BrUM or BrDM micelles. In DM/BrUM and DM/BrDM mixed micelles, the fluorescence intensity of TOE decreased, as a nonlinear function of the molar fraction of brominated detergent, to almost zero in pure brominated detergent. The indole moiety of TOE is therefore highly accessible to the bromine atoms located on the detergent alkyl chain because quenching by bromines occurs by direct contact with the fluorophore. TOE is simultaneously poorly accessible to iodide (I(-)), a water-soluble collisional quencher. TOE time-resolved fluorescence intensity decay is heterogeneous in pure DM micelles, with four lifetimes (from 0.2 to 4.4 ns) at the maximum emission wavelength. Such heterogeneity may arise from dipolar relaxation processes in a motionally restricted medium, as suggested by the time-dependent (nanoseconds) red shift (11 nm) of the TOE emission spectrum, and from the existence of various TOE conformations. Time-resolved quenching experiments for TOE in mixed micelles showed that the excited-state lifetime values decreased only slightly with increases in the proportion of BrDM or BrUM. In contrast, the relative amplitude of the component with the longest lifetime decreased significantly relative to that of the short-lived species. This is consistent with a mainly static mechanism for the quenching of TOE by brominated detergents. Molecular modeling of TOE (in vacuum and in water) suggested that the indole ring was stabilized by folding back upon the octyl chain, forming a hairpin conformation. Within micelles, the presence of such folded conformations, making it possible for the entire molecule to be located in the hydrophobic part of the micelle, is consistent with the results of fluorescence quenching experiments. TOE rotational correlation time values, in the nanosecond range, were consistent with a hindered rotation of the indole moiety and a rotation of the complete TOE molecule in the pure DM or mixed detergent micelles. These results, obtained with a simple micellar model system, provide a basis for the interpretation of fluorescence quenching by brominated detergents in more complex systems such as protein- or peptide-detergent complexes.

Conformational flexibility of domain III of annexin V at membrane/water interfaces

The conformational dynamics of domain III in annexin V bound to negatively charged phospholipid vesicles of 1-palmitoyl-2-oleoyl-sn-glycerophosphocholine and 1-palmitoyl-2-oleoyl-sn-glycerophosphoserine or incorporated into reverse micelles of water/sodium bis(2-ethylhexyl) sulfosuccinate in isooctane, used to mimic the phospholipid/water interface, was studied by steady-state and time-resolved fluorescence of its single tryptophan residue (W187). Upon interaction with sonicated phospholipid vesicles in the presence of calcium, or upon incorporation into reverse micelles without calcium, a progressive 12-14 nm red shift of the fluorescence emission spectrum of W187 is observed. The indole environment becomes therefore more polar than in the unbound protein. Three major lifetime populations describe the fluorescence intensity decays of W187 in both systems. A long-lived excited-state population characterizes the membrane-bound state of the protein. The existence of local conformers with different subnanosecond mobility is suggested by specific association between lifetimes and correlation times both for the protein in buffer and in interaction with the membrane surface. The interaction of the protein with the membrane surface preserves the existence of a rapid unhindered rotational motion, which is coupled with all three lifetimes. The longest lifetime is coupled to restricted motions in subnanosecond and nanosecond time scales. The overall amplitude of rotation of the indole ring is increased in the membrane-bound conformation of the protein. In reverse micelles, the local dynamics reported by W187 is also considerably increased whereas the overall folding of the protein remains unaffected. The same conformational change of domain III can therefore be provoked by different conditions: calcium binding at high concentration, mild acidic pH [Sopkova, J., Vincent, M., Takahashi, M., Lewit-Bentley, A. , and Gallay, J. (1998) Biochemistry 37, 11962-11970] and the interaction of the protein with the membrane surface. The high flexibility of domain III in the membrane-bound protein suggests that this domain may not be crucial for the interaction of the protein with the membrane, in contrast with previous models. Our data are compatible with atomic force microscopy results which suggest that domain III of annexin V does not interact strongly with the membrane surface [Reviakine, I., Bergma-Schutter, W., and Brisson, A. (1998) J. Struct. Biol. 121, 356-361].

Conformational flexibility of domain III of annexin V studied by fluorescence of tryptophan 187 and circular dichroism: the effect of pH

The conformation and dynamics of domain III of annexin V was studied by steady-state and time-resolved fluorescence of its single tryptophan residue (Trp187) as a function of pH in the absence of calcium. At neutral pH, the maximum of emission occurs at 326 nm, in agreement with the hydrophobic location of the tryptophan residue seen in the three-dimensional structure. Upon decreasing the pH, a progressive red-shift by about 12 nm of the fluorescence emission spectrum is observed. The effect is complete between pH 6 and 4.5, and most likely involves at least one and maybe two carboxylic group(s). Circular dichroism mesurements give evidence for a preservation of the native folding of the protein in these mild acidic conditions. A fluorescence red-shift of smaller amplitude is also observed at high pH (approximately 11). The aggregation state of the protein is affected by pH: while at neutral pH, the protein is monomeric (rotational correlation time = 14 ns); it forms aggregates larger than a dimer (rotational correlation time > 40 ns) in acidic pH conditions. These results suggest that electrostatic interactions are probably important for the stabilization of the folding of domain III without calcium. The conformational change may be related to the aggregation state of the molecule. Examination of the protein crystal structures with and without calcium ion in domain III shows an interplay of salt bridges implying charged amino acid side chains at the molecule surface of domain III. These observations may provide a further clue to the mechanism of the conformational change of domain III of annexin V induced by high calcium concentrations and interaction at the membrane/water interface.

Time resolved fluorescence properties of phenylalanine in different environments. Comparison with molecular dynamics simulation

Time resolved fluorescence of the phenylalanine residue (Phe) alone and included in the transmembrane domain (TMD) sequences of the epidermal growth factor receptor (EGFR) and ErbB-2 was studied using the synchrotron radiation source of light, and compared to molecular dynamics (MD) simulations. The fluorescence intensity decay is strongly sensitive to the environment. A mono-exponential decay was obtained for Phe amino acid alone in two different solvents and for Phe included in EGFR transmembrane sequence, with fluorescence lifetime values varying from 1.7 ns (EGFR) to 7.4 ns (Phe dissolved in water). In ErbB-2 transmembrane sequence three lifetimes were detected. The relative amplitude of the shortest one (0.14 ns) is smaller than 10%, whereas the others (0.6 and 2.2 ns) are almost equally represented. They have been attributed to different rotamers exchanging slowly. This interpretation is supported by MD simulations which evidence transitions in time series of the chi 1 dihedral angle of Phe observed in the case of ErbB-2. The anisotropy decays are similar for both peptides and indicate the presence of a correlation time in the nanosecond range (1-4 ns) and the probable existence of a very fast one (< 0.05 ns). Autocorrelation functions computed from MD simulations corroborate these results.

Fluorescence heterogeneity of tryptophans in Na,K-ATPase: evidences for temperature-dependent energy transfer

The intrinsic fluorescence emission kinetics of Na,K-ATPase, a large membrane protein containing 16 tryptophan residues, was studied by time-resolved techniques. The lifetime distributions recovered by the Maximum Entropy Method exhibit a strong dependence on the emission wavelength at temperatures between 37 degrees C and -70 degrees C. From the 'blue' edge of the fluorescence emission spectrum up to the maximum of emission, the lifetime distribution at room temperature is the result of four broad peaks which cover the time range 0.3-7 ns. With increasing emission wavelength, these peaks move to longer lifetimes and the peak at shorter times are suppressed at the red edge, while the longest component (6-7 ns) becomes dominant. With decreasing temperature, the number of lifetime components is reduced for the benefit of the long one. At cryogenic temperatures, the emission decay in the red-edge of the fluorescence spectrum consists of one major slow component (6-7 ns) and a fast one (0.5 ns) associated with a negative pre-exponential term. This is a characteristic feature of an excited-state reaction. The temperature dependence of this fast component and the fluorescence anisotropy decay at low temperature in the red-edge, indicate that this excited state reaction may be accounted for a unidirectional inter-tryptophan fluorescence energy transfer from 'blue' populations of donors to 'red' populations of acceptors. This is also illustrated by the time-resolved emission spectra. In the blue edge of the fluorescence emission spectrum, moreover, the time course of the anisotropy decay suggests the existence of homo-transfer of excitation energy involving 'blue' tryptophan residues. The steady-state anisotropy excitation spectrum in vitrified solvent agrees with this suggestion. These different energy transfer mechanisms may be used as structural probes to detect more accurately conformational changes of the protein elicited by effectors and ion binding or release.

Laurdan solvatochromism: solvent dielectric relaxation and intramolecular excited-state reaction

Absorption, steady-state, and time-resolved fluorescence measurements have been performed on laurdan dissolved either in white viscous apolar solvents or in ethanol as a function of temperature. The heterogeneity of the absorption spectra in white oils or in ethanol is consistent with semiempirical calculations performed previously on Prodan. From steady-state and time-resolved fluorescence measurements in apolar media, an excited state reaction is evidenced. The bimodal lifetime distribution determined from the maximum entropy method (MEM) analysis is attributed to the radiative deexcitation of a "locally excited" (LE) state and of a "charge transfer" (CT) state, whereas a very short component (20 ps), the sign and the amplitude of which depend on the emission wavelength, is attributed to the kinetics of the interconvertion reaction. The observation of an isoemissive point in the temperature range from -50 degrees C to -110 degrees C in ethanol suggests an interconvertion between two average excited-state populations: unrelaxed and solvent-relaxed CT states. A further decrease in temperature (-190 degrees C), leading to frozen ethanol, induces an additional and important blue shift. This low temperature spectrum is partly attributed to the radiative deexcitation of the LE state. Time-resolved emission spectra (TRES) measurements at -80 degrees C in the ethanol liquid phase show a large spectral shift of approximately 2500 cm(-1) (stabilization energy of the excited state: 7.1 kcal x M(-1)). The time-dependent fluorescence shift (TDFS) is described for its major part by a nanosecond time constant. The initial part of the spectral shift reveals, however, a subnanosecond process that can be due to fast internal solvent reorientation and/or to intramolecular excited-state reactions. These two relaxation times are also detected in the analysis of the fluorescence decays in the middle range of emission energy. The activation energy of the longest process is approximately 3 kcal x M(-1). At -190 degrees C, one subnanosecond and one nanosecond excited-state reactions are also evidenced. They are likely due to intramolecular rearrangements after the excitation, leading to the CT state and not to solvent relaxation, which is severely hindered in these temperature conditions. Therefore, both intramolecular and solvent relaxations are responsible for the large Stokes shift displayed by this probe as a function of solvent polarity. A possible scheme is proposed for the deexcitation pathway, taking into account the kinetics observed in these different solvents.

Immunosuppressor binding to the immunophilin FKBP59 affects the local structural dynamics of a surface beta-strand: time-resolved fluorescence study

The interaction of the immunophilin domain of FKBP59 (FKBP59-I) with immunosuppressant drugs was investigated by steady-state and time-resolved fluorescence of tryptophan. One of the two Trp residues present in this protein (W89), conserved in almost all immunophilins, is buried in the hydrophobic core and participates in the immunosuppressant binding. By comparison with the highly homologous protein FKBP12, containing only the buried Trp, it has been concluded that its weak fluorescence is due to an atypical H-bond interaction involving the indole nitrogen and the Phe129 benzene ring. The second Trp residue (W59) in FKBP59-I is located on the external hydrophilic side of the 50-60 beta-sheet [Craescu, C. T., Rouvière, N., Popescu, A., Cerpolini, E., Lebeau, M.-C., Baulieu, E.-E., & Mispelter, J. (1996) Biochemistry 35, 11045-11052] and is responsible for >95% of the fluorescence emission. The long lifetime of the major excited state, the large activation energy of thermal quenching, and the rotational correlation time distribution pattern suggest that its environment is not highly mobile. Binding of the immunosuppressant drugs FK506 and rapamycin leads to a approximately 60% decrease of the fluorescence intensity without any change in the fluorescence emission maximum. Time-resolved measurements show that this "quenching" is due to a conformational change which depletes the long excited-state lifetime population to the profit of a more quenched minor excited state, which becomes prominent in the complexes. This is accompanied by a strong slowing of the indole ring dynamics in the case of FK506 and by a complete immobilization in the case of rapamycin, as shown by two-dimensional (tau, theta) maximum entropy analysis of the polarized fluorescence decays. Binding of the immunosuppressant drugs therefore modifies the structure and the dynamics of the external side of the 50-60 beta-sheet in FKBP59-I, which could be relevant for the formation of ternary complexes with other protein targets.

Conformational transitions within the calmodulin-binding site of Bordetella pertussis adenylate cyclase studied by time-resolved fluorescence of Trp242 and circular dichroism

The sequence situated around Trp242 in Bordetella pertussis adenylate cyclase, a bifunctional protein of 1706 amino acid residues, forms the core of the calmodulin-binding site. Peptides varying in size and in affinity for calmodulin, and preserving the same sequence around Trp242 were analyzed by time-resolved fluorescence spectroscopy. Their dynamic properties were compared to those of the catalytic domain of B. pertussis adenylate cyclase corresponding to the first 400 amino acid residues of the protein and in which the Trp69 residue was replaced by Phe. The heterogeneity of the fluorescence intensity decays of Trp242 is likely due to the existence of conformers in equilibrium as is suggested by the effect of trifluoroethanol both on the secondary structure content and the lifetime distributions. Binding to calmodulin leads to striking effects on the lifetime distribution profiles by selecting a major excited state population and therefore one major conformer. Trp242 still presents some degree of rotational freedom in the complexes. The reduction of rotational freedom is more important for the shorter peptides than for the longest one. A similar selection of one major conformer with the same lifetime was also observed for the Trp242 in the mutant protein when bound to calmodulin, as in the complexes with the peptides. We conclude that the site of interaction of B. pertussis adenylate cyclase with calmodulin has similar conformational flexibility as that evidenced in the isolated peptides. This property of the molecule allows a better adjustment of the enzyme upon interaction with calmodulin.

Photophysics of the fluorescent K+ indicator PBFI

The fluorescent indicator PBFI is widely used for the determination of intracellular concentrations of K+. To investigate the binding reaction of K+ to PBFI in the ground and excited states, steady-state and time-resolved measurements were performed. The fluorescence decay surface was analyzed with global compartmental analysis yielding the following values for the rate constants at room temperature in aqueous solution at pH 7.2: k01 = 1.1 x 10(9) s-1, k21 = 2.7 x 10(8) M-1s-1, k02 = 1.8 x 10(9) s-1, and k12 = 1.4 x 10(9) s-1. k01 and k02 denote the respective deactivation rate constants of the K+ free and bound forms of PBFI in the excited state. k21 represents the second-order rate constant of binding of K+ to the indicator in the excited state whereas k12 is the first-order rate constant of dissociation of the excited K(+)-PBFI complex. From the estimated values of k12 and k21, the dissociation constant Kd* in the excited state was calculated. It was found that pKd* (-0.7) is smaller than pKd (2.2). The effect of the excited-state reaction can be neglected in the determination of Kd and/or the K+ concentration. Therefore, intracellular K+ concentrations can be accurately determined from fluorimetric measurements by using PBFI as K+ indicator.

Photophysics of the fluorescent Ca2+ indicator Fura-2

The photophysics of the complex forming reaction of Ca2+ and Fura-2 are investigated using steady-state and time-resolved fluorescence measurements. The fluorescence decay traces were analyzed with global compartmental analysis yielding the following values for the rate constants at room temperature in aqueous solution with BAPTA as Ca2+ buffer: k01 = 1.2 x 10(9)s-1, k21 = 1.0 x 10(11) M-1 s-1, k02 = 5.5 x 10(8) s-1, k12 = 2.2 x 10(7) s-1, and with EGTA as Ca2+ buffer: k01 = 1.4 x 10(9) s-1, k21 = 5.0 x 10(10) M-1 s-1, k02 = 5.5 x 10(8) s-1, k12 = 3.2 x 10(7) s-1. k01 and k02 denote the respective deactivation rate constants of the Ca2+ free and bound forms of Fura-2 in the excited state. k21 represents the second-order rate constant of binding of Ca2+ and Fura-2 in the excited state, whereas k12 is the first-order rate constant of dissociation of the excited Ca2+:Fura-2 complex. The ionic strength of the solution was shown not to influence the recovered values of the rate constants. From the estimated values of k12 and k21, the dissociation constant K*d in the excited state was calculated. It was found that in EGTA Ca2+ buffer pK*d (3.2) is smaller than pKd (6.9) and that there is negligible interference of the excited-state reaction with the determination of Kd and [Ca2+] from fluorimetric titration curves. Hence, Fura-2 can be safely used as an Ca2+ indicator. From the obtained fluorescence decay parameters and the steady-state excitation spectra, the species-associated excitation spectra of the Ca2+ free and bound forms of Fura-2 were calculated at intermediate Ca2+ concentrations.

The dynamic behavior of annexin V as a function of calcium ion binding: a circular dichroism, UV absorption, and steady-state and time-resolved fluorescence study

The binding of calcium ions to annexin V in the absence of phospholipids has been studied by UV-difference spectroscopy, circular dichroism, and steady-state and time-resolved fluorescence. In the absence of calcium, the unique tryptophan 187, located in domain III of annexin V, is surrounded by a strongly hydrophobic environment, as indicated by its "blue" fluorescence emission maximum (325 nm). This corresponds well with the description of the structure determined by X-ray crystallography of several crystal forms. The Trp187 time-resolved fluorescence decay shows the existence of a fast (picosecond) excited-state reaction which can involve the formation of an H-bond between the indole NH group and the proximate epsilon-OH and/or alpha-carbonyl groups of Thr224. Titration with calcium tends to stabilize the overall structure, as shown by circular dichroism, while leading to large modifications of the local structure around Trp187 making it accessible to the solvent as shown by UV-difference spectra, circular dichroism spectra, and the displacement of its fluorescence emission maximum at saturating concentrations of calcium (350 nm). A rapid (picosecond) formation of an excited-state complex, probably involving one or a few water molecules of the solvation shell, is observed. These observations correlate well with the conformational change observed in crystal structures obtained in high calcium concentrations, involving the removal of Trp187 from the buried position to the surface of the molecule [Sopkova, J., Renouard, M., & Lewit-Bentley, A. (1993) J. Mol. Biol. 234, 816-825; Concha, N. O., Head, J. F., Kaetzel, M. A., Dedman, J. R., & Seaton, B. A. (1993) Science 261, 1321-1324]. In the solvent-exposed conformation, the indole ring becomes mobile in the subnanosecond and nanosecond time range. This conformational change and the increase in local flexibility can be important for the accommodation of the protein on the surface of phospholipid membranes.

Stereospecificity of the interaction of porcine pancreatic phospholipase A2 with micellar and monomeric inhibitors. A time-resolved fluorescence study of the tryptophan residue

The binding effect of enantiomeric substrate analogs under micellar form on the local conformation and dynamics of the N-terminal region of porcine pancreas phospholipase A2 was examined by time-resolved fluorescence measurements of its single tryptophan residue (Trp3). The complexity of the fluorescence intensity decay of the unliganded protein (four excited-state lifetime populations) suggests a conformational heterogeneity of the N-terminal region of the protein. A considerable simplification of the excited-state lifetime profile was specifically observed in the complex with one of the stereoisomers [(R)-2-tetradecanoylamino)-hexanol-phosphocholine] at low inhibitor/protein molar ratio of approximately 9. This indicates the existence of a definite conformation of the N-terminal region of the protein in the complex. No effect was detected for the S-enantiomer. In parallel, the rotational mobility of the Trp residue in the complex with the R-enantiomer was reduced. At a higher inhibitor/protein molar ratio of approximately 130, the stereospecificity of the interaction was lost and complexes were formed with both stereoisomers. These complexes were, however, not similar to the specific one either in terms of the local Trp3 environment or of the volume of the rotating unit. The local effects of low amounts of monomeric inhibitors added to a preformed protein/micelle complex of a phospholipase A2 double mutant in which a Trp residue was genetically inserted near the active site at position 31 while the natural Trp3 was replaced by Phe [Kuipers, O., Vincent, M., Brochon, J. C., Verheij, H. M., de Haas, G. H. & Gallay, J. (1991) Biochemistry 30, 8771-8785], were also monitored by time-resolved fluorescence of this single Trp residue. A stereospecific dependence of the local perturbations was again observed. These results support the idea that the active conformation of the protein is reached in solution only after formation of a ternary complex: protein-interface-inhibitor.

Protein flexibility and aggregation state of human epidermal growth factor. A time-resolved fluorescence study of the native protein and engineered single-tryptophan mutants

A time-resolved fluorescence spectroscopic study of the recombinant human epidermal growth factor (hEGF), a bis(tryptophan)-containing protein (Trp49-Trp50), and of the two single-tryptophan-containing engineered mutants with Trp49 or Trp50 replaced by Phe ([W49F]hEGF, [W50F]hEGF), was undertaken in order to gain insight into the conformational dynamics of the C-terminal region. Quite different position-dependent microenvironments for the two Trp residues are shown by comparing the fluorescence intensity decay of both mutants. Trp50 in the single-tryptophan mutant [W49F]EGF probably undergoes a dominant interaction with the solvent. A more heterogeneous environment of Trp49 in the [W50F]hEGF mutant is found. Moreover, the fluorescence decay of the native hEGF is not simply the additive result of the decays of both mutants: the Trp2 sequence confers a conformation of the C-terminal sequence which is more in contact with the rest of the protein molecule. By contrast, the fluorescence anisotropy decay of the native protein is quite similar to that of the single-tryptophan mutants. A high degree of rotational freedom in the C-terminal region of the protein is demonstrated. The resonance energy transfer, which could contribute to the anisotropy decay, appears therefore not to be highly efficient with respect to the depolarization motions. In addition to these local conformational and dynamic aspects of the hEGF C-terminal sequence, the fluorescence anisotropy decay data demonstrate the existence of a dimerization process of the native protein which is dependent on pH and protein concentration. This phenomenon influences the excited-state lifetime profiles and, therefore, the local conformational equilibrium of the C-terminal region.

Time-resolved fluorescence study of human recombinant interferon alpha 2. Association state of the protein, spatial proximity of the two tryptophan residues

Human recombinant interferon alpha 2 belongs a to family of proteins active against a wide range of viruses. It contains two tryptophan residues located at positions 77 and 141 in the peptide sequence. The fluorescence emission spectrum of these tryptophan residues displays a maximum at 335 nm. The fluorescence intensity decay is described by one broad excited-state-lifetime population centered around a value of 1.7 ns (full width at half maximum, 1.5 ns). These observations suggest that in the native protein, both tryptophan residues emit from similar environments, not directly exposed to the surrounding solvent. The anisotropy decay is essentially biexponential. The correlation-time value characterizing the Brownian rotation of the protein varies linearly with the viscosity/temperature ratio. The calculated hydrodynamic volumes are compatible with the existence of a dimer and a tetramer, at pH 5.5 and 9.4, respectively. Addition of urea at pH 5.5 disrupts the dimer and modifies to some extent the excited-state-lifetime distribution which becomes more heterogeneous. Disulfide-bond reduction also dissociates the dimer and leads to a highly heterogeneous fluorescence-intensity decay with four excited-state-lifetime populations. An opening of the local structure in the Trp region of the protein is likely to occur in these conditions. The fast-anisotropy-decay components can be due to either fast rotation or energy transfer between the indoles. Close proximity of the two Trp residues (less than 1 nm) is suggested from steady-state and time-resolved fluorescence-anisotropy measurements in vitrified medium [95% (by mass) glycerol at -38 degrees C]. This suggestion is in agreement with the recently published three-dimensional structure of the homologous protein murine interferon beta [Senda, T., Shimazu, T., Matsuda, S. Kawano, G., Shimizu, H., Nakamura, K. T. & Mitsui, Y. (1992) EMBO J. 11, 3193-3201].

Interaction of recombinant human epidermal growth factor with phospholipid vesicles. A steady-state and time-resolved fluorescence study of the bis-tryptophan sequence (Trp49-Trp50)

The interaction of recombinant human epidermal growth factor with small unilamellar phospholipid vesicles was studied by steady-state and time-resolved fluorescence of the bis-tryptophan sequence (Trp49-Trp50). Steady-state anisotropy measurements demonstrate that strong binding occurred with small unilamellar vesicles made up of acidic phospholipids at acidic pH only (pH < or = 4.7). An apparent stoichiometry for 1,2-dimyristoyl-sn-phosphoglycerol of about 12 phospholipid molecules per molecule of human epidermal growth factor was estimated. The binding appears to be more efficient at temperatures above the gel to liquid-crystalline phase transition. The conformation and the environment of the Trp-Trp sequence are not greatly modified after binding, as judged from the invariance of the excited state lifetime distribution and from that of the fast processes affecting the anisotropy decay. This suggests that the Trp-Trp sequence is not embedded within the bilayer, in contrast to the situation in surfactant micelles (Mayo et al. 1987; Kohda and Inigaki 1992).

Competitive inhibition of lipolytic enzymes. VIII: Inhibitor-induced aggregation of porcine pancreatic phospholipase A2

Several 2-acylaminophospholipid analogues have been demonstrated to behave as potent competitive inhibitors of porcine pancreatic phospholipase A2 (De Haas, G.H., Dijkman, R., Ransac, S. and Verger, R. (1990) Biochim. Biophys. Acta 1046, 249-257). Their inhibitory power appeared to be strictly controlled by the stereoconfiguration around the chiral C-2 atom and effective inhibition of the enzyme was observed only when incorporated into a micellar substrate-water interface. In the present study various direct binding techniques were applied to investigate the interaction of the enzyme with pure micelles of the stereoisomeric forms of 2-tetradecyl-amino-hexanol-1-phosphocholine (R-C14-PN and S-C14-PN). Upon equilibrium gel filtration of the enzyme (monomeric molecular mass = 14 kDa) on calibrated Superdex columns running in micellar solutions of R-C14-PN, the phospholipase eluted as a lipid-protein complex of 74 kDa. Under identical conditions, micellar solutions of S-C14-PN did not give rise to high-molecular mass aggregates and the enzyme eluted at its normal 14 kDa position. Light scattering experiments, ultrasedimentation and time-resolved fluorescence spectroscopy studies confirmed the formation of a high-molecular mass aggregate between enzyme and R-C14-PN micelles. The ultimate complex was shown to consist of four protein and about ten inhibitor molecules. Using time-resolved fluorescence spectroscopy the interaction was studied between the active site of phospholipase A2 and R-C14-PN molecules, both incorporated in an inert lipid matrix.

Insight into the conformational dynamics of specific regions of porcine pancreatic phospholipase A2 from a time-resolved fluorescence study of a genetically inserted single tryptophan residue

The effects of Ca2+ and substrate analogue binding on the conformational dynamics of porcine pancreas phospholipase A2 (PLA2) in different regions was explored by combining site-directed mutagenesis and time-resolved fluorescence measurements. The single tryptophan residue (Trp-3) of the wild-type protein (W3), in the alpha-helix A, was replaced by a phenylalanine residue (W3F), whereafter Trp was substituted either for leucine-31 (W31), located in the calcium binding loop, or for phenylalanine-94 (W94), located at the "back side" of the enzyme. Furthermore, mutants lacking the 62-66 sequence were constructed with the Trp at position 3 (delta W3) or 31 (delta W31). The total fluorescence intensity decays of Trp in each protein, in the protein-calcium and the protein-calcium-substrate analogue complexes, analyzed by the maximum entropy method (MEM) can be interpreted as distributions of separated lifetime classes. In the case of the W94 mutant, a major short-lived excited-state population (tau approximately 50 ps) is observed, probably deactivated by the interaction with two proximate disulfide bridges via a radiationless process. For the four other mutants, the respective barycenters of the four lifetime classes display comparable values, but the amplitude distributions are different for Trp-3 and Trp-31. The rotational mobility of the Trp residue varies along the peptide chain. Trp-3 experiences only a fast hindered motion. Trp-31 is sensitive to an additional local flexibility that is absent in the N-terminal part of the protein. The largest wobbling angle is observed at position 94. No effect of calcium binding occurs on the lifetime distribution of the Trp-3 and Trp-94 residues. Their mobilities are not affected. In contrast, calcium binding displays a strong influence on the excited-state population distribution of Trp-31. A major population decaying with the longest lifetime is selected in the W31 protein and contributes to approximately 50% of the decay. The local flexibility and the amplitude of motion of Trp-31 is wider in the protein-calcium complex than in the unliganded protein. Binding of the monomeric substrate analogue n-dodecylphosphocholine (C12PN) in the presence of calcium slightly affects the Trp-3 excited-state population distribution and its mobility. Trp-31 is more sensitive to this binding. In particular, a more restricted rotation of the Trp-31 residue and a decrease of the peptide local flexibility as protein-calcium complexes are observed in both the W31 and delta W31 mutants.(ABSTRACT TRUNCATED AT 400 WORDS)

The interactions of horse heart apocytochrome c with phospholipid vesicles and surfactant micelles: time-resolved fluorescence study of the single tryptophan residue (Trp-59)

The interactions of horse heart apocytochrome c with membrane interfaces were studied on membrane models including micelles of the anionic surfactant sodium dodecyl sulfate (SDS), the micelle forming lipid analogs dodecylphosphoglycol (C12PG), tetradecylphosphoglycol (C14PG), and dodecylphosphocholine (C12PN), and the negatively charged phospholipid 1-palmitoyl-2-oleoylsn-glycero phosphocholine (POPS) forming small unilamellar vesicles (SUV). The time-resolved fluorescence of the single tryptophan residue (Trp-59) emission was monitored to characterize the modifications of the conformational equilibrium and of the internal dynamics of the protein, which can be brought about by its binding to these model membranes. In most of the cases, as for the protein in solution, the excited state lifetime distribution of the Trp emission was described by four discrete classes, whose relative proportions and barycenters vary significantly in the different complexes formed. In the complex with POPS, however, the decay analysis showed only 3 lifetime classes: the long lifetime class displayed a barycenter value smaller than that observed for the protein in aqueous solution but with a much higher proportion, indicating a stabilization of this conformer in the membrane-bound form of the protein. A similar sensitivity of the Trp-59 excited state to deactivation by thermal collisions in water and in the protein/POPS complex was observed, indicating a probable location of Trp-59 at the membrane/water interface. The effects of protein binding to C12PN, C12PG and C14PG micelles on the long lifetime class proportion were similar to that of POPS but, in addition, there was a large contribution of a short lifetime component which was absent in POPS vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Time-resolved fluorescence of the single tryptophan residue in rat alpha-fetoprotein and rat serum albumin: analysis by the maximum-entropy method

The time-resolved fluorescence emissions of the lone tryptophan residues in rat alpha-fetoprotein (RFP) and rat serum albumin (RSA) were studied. The total fluorescence intensity decays in both proteins were multiexponential. Analysis of the data by nonlinear least squares as a sum of discrete exponentials showed that four exponentials were needed for a satisfactory fit for both proteins. Analysis by the maximum entropy method using 150 logarithmically equally spaced exponentials yielded four well-resolved excited-state lifetime classes with barycenters and relative amplitudes values (ci) that corresponded to those obtained from the nonlinear least-squares method. Changing the temperature affected the relative amplitudes of the lifetime classes but had little effect on the lifetime values themselves. This suggests that the four classes reflect local conformational substates that exchange slowly with respect to the time window of observation defined by the longest lifetime. The internal rotational dynamics of the tryptophan in each protein was monitored by fluorescence anisotropy decay measurements. The mobility of the tryptophan appeared to be larger and faster in RFP than in RSA. The nonlinear least-squares analysis suggests the existence of three rotational correlation times of 0.1, 3, and 55 ns for this protein. As a function of temperature, the long correlation time did not follow the Perrin's law expected for a rigid rotating body. This suggests that this correlation time may reflect not only the Brownian rotation of the whole protein but also the flexibilities of domains in the protein. For RSA a two-component model with correlation times of 0.4 and 31 ns was sufficient to describe the data.(ABSTRACT TRUNCATED AT 250 WORDS)

Relation between membrane phospholipid composition, fluidity and function in mitochondria of rat brown adipose tissue. Effect of thermal adaptation and essential fatty acid deficiency

Male weanling rats were maintained either at 28 degrees C (thermoneutrality) or at 5 degrees C (cold adaptation). During 9 weeks they were fed either a 2% hydrogenated coconut oil diet deficient in essential fatty acids or a diet containing 2% sunflower oil. The respective incidences of cold adaptation and of EFA deficiency on lipid composition of mitochondrial membranes from brown adipose tissue (BAT) were investigated. Using 1,6 diphenylhexatriene (DPH) as a probe, the parameters of membrane fluidity were estimated by steady-state fluorescence polarization measurements (rs) and by time-resolved fluorescence anisotropy decay (order parameter S). Cold acclimation induced a decrease of phosphatidylcholine to phosphatidylethanolamine (PC/PE ratio), an increase of the total fatty acid unsaturation index (T.U.). EFA deficiency had the same effect as cold on the PC/PE ratio, but decreased T.U. Cold adaptation induced a larger decrease of S than of rs, whereas EFA deficiency only increased rs and did not modify S. In liposomes prepared from mitochondrial lipids, rs values were smaller than in whole mitochondria. Both in cold-adapted and in EFA-deficient rats the variations of rs were correlated with lipid unsaturation. Comparison between BAT thermogenic activity, assessed by GDP binding and proportions of PE and PC showed a high correlation suggesting a change in the membrane occurring with the increase of mitochondrial activity that could be related to phospholipid composition rather than to membrane fluidity.

The physical state of the erythrocyte membrane in myotonic dystrophy

The molecular pathology of myotonic dystrophy is believed to be expressed at the plasma membrane level. Previous assessments of membrane fluidity, a marker of the biochemical state of the membrane, have yielded conflicting results. In this study, erythrocyte membrane fluidity was reevaluated using highly sensitive fluorescence probe techniques. Steady-state anisotropy was measured with diphenylhexatriene (DPH), trimethylaminophenyl-hexatriene (TMA-DPH) and phenylhexatrienylphenylpropionic acid, probing different regions of the membrane. In the patients, significantly increased steady-state anisotropy was obtained with DPH, probing the hydrophobic core of the membrane, while slightly reduced anisotropy was found with TMA-DPH. The dynamic properties of the membrane lipids were further examined by means of time-resolved measurements with DPH. The excited state decay kinetics could best be described by a bi-exponential decay model. A large redistribution of the probe populations and a reduction of the average order parameter were found in the patients indicating a less ordered or more fluid lipid matrix. These perturbations might be induced by a protein abnormality and altered protein-lipid interaction within the erythrocyte membrane.

Nanosecond dynamics of horse heart apocytochrome c in aqueous solution as studied by time-resolved fluorescence of the single tryptophan residue (Trp-59)

The time-resolved fluorescence emission characteristics of the single tryptophan residue (Trp-59) of horse heart apocytochrome c--the precursor of the intramitochondrial cytochrome c--were studied in aqueous solution. The total fluorescence intensity decay measured over the whole emission spectrum was analyzed as a sum of three or four exponentials by the nonlinear least-squares method, the last model always providing a slight but significant decrease in the chi 2 values. Maximum entropy analysis, recently developed for time-resolved fluorometry (Livesey et al., 1987; Livesey & Brochon, 1987), strongly suggests the existence of a distribution including at least four separate classes of lifetimes. The center values were around 0.1-0.2, 1, 3, and 5 ns, in agreement with the lifetime values obtained by nonlinear least-squares regression analysis. As a function of the emission wavelength, these values remained constant within the experimental error, whereas a redistribution of the fractional amplitudes was observed: the contributions of the short components increased in the blue edge region of the emission spectrum. Temperature increase led essentially to a redistribution of the fractional amplitudes, affecting mostly that of the 5-ns component, which almost totally disappeared at high temperature (35-40 degrees C). The lifetime values were not significantly affected except for the 3-ns component, which decreased by about 15% in the temperature range studied. Such observations strongly suggest that the protein exists under different conformational substates in thermal equilibrium. Time-resolved fluorescence anisotropy measurements evidenced the existence of fast internal rotation of the Trp residue. An average maximum restricted angle of rotation of around 55 degrees was calculated. A second internal motion, slower by 1 order of magnitude, corresponding likely to a local motion of the peptide chain involving the Trp-59 residue, was detected on the anisotropy decay curve. Finally, the longest correlation time (5 ns) should correspond to the average rotation of the overall protein. Its value doubled as a function of the protein concentration, revealing an association process leading most likely to a dimer in the concentration range studied (2-139 microM). The flexibility of the peptide chain was more restrained in the associated than in the monomeric form, but the fast internal rotation of the Trp residue was not.

Alcohol abuse increases the lipid structural order in human erythrocyte membranes. A steady-state and time-resolved anisotropy study

The effect of ethanol abuse on the lipid ordering of the human erythrocyte membranes was studied by steady-state and time-resolved fluorescence anisotropy measurements of DPH and its polar analogue TMA-DPH, which probe different membrane regions. Steady-state anisotropy values with DPH as a probe were slightly but significantly increased (+3%) in erythrocyte membranes from alcoholic patients. A resistance to the ethanol fluidizing effect was evidenced in these membranes with DPH and TMA-DPH. No difference in the probe lifetimes was detected between the control and the alcoholic subjects. In the alcoholic patients as compared to the healthy controls, the residual anisotropy for DPH was significantly increased (+7%) corresponding to an increase in the orientational order parameter of 4%; a decrease of the apparent correlation time value was also observed. Nevertheless, no differences between the two erythrocyte populations were observed with TMA-DPH.

Effects of temperature and benzyl alcohol on the structure and adenylate cyclase activity of plasma membranes from bovine adrenal cortex

Adenylate cyclase activation by corticotropin (ACTH), fluoride and forskolin was studied as a function of membrane structure in plasma membranes from bovine adrenal cortex. The composition of these membranes was characterized by a very low cholesterol and sphingomyelin content and a high protein content. The fluorescent probes 1,6-diphenylhexa-1,3,5-triene (DPH) and a cationic analogue 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) were, respectively, used to probe the hydrophobic and polar head regions of the bilayer. When both probes were embedded either in the plasma membranes or in liposomes obtained from their lipid extracts, they exhibited lifetime heterogeneity, and in terms of the order parameter S, hindered motion. Under all the experimental conditions tested, S was higher for TMA-DPH than for DPH but both S values decreased linearly with temperature within the range of 10 to 40 degrees C, in the plasma membranes and the liposomes. This indicated the absence of lipid phase transition and phase separation. Addition to the membranes of up to 100 mM benzyl alcohol at 20 degrees C also resulted in a linear decrease in S values. Membrane perturbations by temperature changes or benzyl alcohol treatment made it possible to distinguish between the characteristics of adenylate cyclase activation with each of the three effectors used. Linear Arrhenius plots showed that when adenylate cyclase activity was stimulated by forskolin or NaF, the activation energy was similar (70 kJ.mol-1). Fluidification of the membrane with benzyl alcohol concentrations of up to 100 mM at 12 or 24 degrees C produced a linear decrease in the forskolin-stimulated activity, that led to its inhibition by 50%. By contrast, NaF stabilized adenylate cyclase activity against the perturbations induced by benzyl alcohol at both temperatures. In the presence of ACTH, biphasic Arrhenius plots were characterized by a well-defined break at 18 degrees C, which shifted at 12.5 degrees C in the presence of 40 mM benzyl alcohol. These plots suggested that ACTH-sensitive adenylate cyclase exists in two different states. This hypothesis was supported by the striking difference in the effects of benzyl alcohol perturbation when experiments were performed below and above the break temperature. The present results are consistent with the possibility that clusters of ACTH receptors form in the membrane as a function of temperature and/or lipid phase fluidity.(ABSTRACT TRUNCATED AT 400 WORDS)

Conformational aspects and rotational dynamics of synthetic adrenocorticotropin-(1-24) and glucagon in reverse micelles

The tryptophan (Trp) rotational dynamics and the secondary structure of the peptide hormones adrenocorticotropin-(1-24) [ACTH(1-24)]--the fully active N-terminal fragment of adrenocorticotropin-(1-39)--and glucagon were studied in aqueous solutions and in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/water/isooctane, a system selected to mimic the membrane-water interface. In aqueous solutions, the total fluorescence intensity decays of their single Trp residue [Trp-9 and Trp-25 for ACTH(1-24) and glucagon, respectively] are multiexponential. This is also the case for ACTH(5-10), a fragment of the adrenocorticotropin "message" region. Time-resolved fluorescence anisotropy data evidence a high degree of rotational freedom of the single Trp residue. Transfer of these peptides from water to the aqueous core of reverse micelles induces severe restrictions of the Trp internal motion and of its local environment. The results indicate that the Trp-9 residue in ACTH(1-24 is maintained in the close neighborhood of the water-AOT molecular interface where the water molecules are strongly immobilized. By contrast, the Trp residues in ACTH(5-10) and glucagon are likely to be located closer to the center of the micellar aqueous core where the water molecules are in a more mobile state. Furthermore, the above location of Trp can be extended to the peptide chains themselves as evidenced by the overall correlation time values of the peptide-containing micelles. Nevertheless, in all peptides, the indole ring remains susceptible to oxidation by N-bromosuccinimide. Circular dichroism measurements evidence the induction in glucagon of alpha-helices remaining unaffected by the micellar water content. Conversely, beta-sheet structures are favored in ACTH(1-24) at low water-to-surfactant molar ratios (w0) but are disrupted by subsequent additions of water. These results are discussed in terms of the possible role of the micellar interfaces in selecting the preferred peptide dynamical conformation(s)

Adenylate cyclase of bovine adrenal cortex plasma membranes. Divergence between corticotropin and fluoride combined effects with forskolin

The diterpene forskolin maximally stimulated bovine adrenal cortex adenylate cyclase activity 9-fold with a concentration producing half-maximum effect (ED50) of about 4 microM. The effects of forskolin and the fully active corticotropin fragment ACTH (I 24) were additive over nearly the whole range of concentration of both effectors, indicating separate and independent mechanisms of action. By contrast, 10 mM NaF blocked forskolin action in the nanomolar range of the diterpene concentration, while it allowed a partial stimulation by forskolin in the micromolar range. NaF thus reveals a heterogeneity of forskolin action in the adrenal cortex plasma membranes. Moreover, our data suggest that ACTH and NaF activation effects, both mediated by the stimulatory regulatory protein Gs, proceed through different mechanisms.

Cardiolipin-cholesterol interactions in the liquid-crystalline phase: a steady-state and time-resolved fluorescence anisotropy study with cis- and trans-parinaric acids as probes

The potency of cholesterol to affect the acyl chain order and dynamics of cardiolipin membranes in the liquid-crystalline state was monitored by steady-state and time-resolved fluorescence anisotropy as well as excited-state lifetime measurements with cis- and trans-parinaric acids as probes. Up to a cholesterol mole fraction (mean chl) of congruent to 0.20, no measurable effect on any of the fluorescence parameters of either probe in cardiolipin bilayers was evidenced. This was in striking contrast to the situation in dioleoylphosphatidylcholine (DOPC), for which a cholesterol mole fraction of 0.20 corresponded to the half-maximal effect on the fluorescence parameters, reflecting the classical ordering effect of cholesterol observed in lecithin systems in the liquid-crystalline phase. Whereas in DOPC bilayers this order effect plateaued at mean chl = 0.50, in cardiolipins the increase in acyl chain order was observable up to a mole fraction as high as 0.80. This indicated that cardiolipins were able to incorporate about 4 mol of cholesterol/mol of cardiolipin (i.e., 1 mol of cholesterol per fatty acyl chain). Besides, 31P NMR spectra of multilamellar liposomes obtained from pure cardiolipins and cardiolipin--cholesterol mixtures evidenced a line shape characteristic of lamellar structures. These results clearly indicate that the presence of high levels of cardiolipins in inner mitochondrial membrane does not impede cholesterol uptake by these membranes. However, the absence of an effect of cholesterol in the physiological range of the cholesterol mole fraction (congruent to 0.20) would signify weaker sterol-cardiolipin interactions than with lecithins and in turn would explain the relative dearth of cholesterol in these membranes.

Membrane proteins in reverse micelles: myelin basic protein in a membrane-mimetic environment

The solubility, reactivity, and conformational dynamics of myelin basic protein (MBP) from bovine brain were studied in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-isooctane and water. Such a membrane-mimetic system resembles the aqueous spaces of native myelin sheath in terms of physicochemical properties as reflected in the high affinity of MBP for interfacial bound water. This is marked by the unusual profile of the solubility curve of the protein in reverse micelles, which shows optimal solubility at a much lower molar ratio of water to surfactant ([ H2O]/[AOT] = w0) than that reported for other water-soluble proteins. The role of counterions and/or charged polar head groups in the solubilization process is revealed by comparison of the solubility of MBP in nonionic surfactant micellar solutions. Whereas MBP is unfolded in aqueous solutions, insertion into reverse micelles generates a more folded structure, characterized by the presence of 20% alpha-helix. This conformation is unaffected by variations in the water content of the system (in the 2.0-22.4 w0 range). The reactivity of epsilon-amino groups of lysine residues with aqueous solutions of o-phthalaldehyde demonstrates that segments of the peptide chain are accessible to water. Similar results were obtained with the sequence involved in heme binding. In contrast, the sole tryptophan residue, Trp-117, is shielded from the aqueous solvent, as indicated by lack of reaction with N-bromosuccinimide. The invariance of the wavelength maximum emission in the fluorescence spectra as a function of w0 is consistent with this result.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiolipin vesicles can accommodate cholesterol up to 0.80 mole fraction, i.e. one molecule per cardiolipin fatty acid chain

Cardiolipin-cholesterol interactions were studied by time-resolved fluorescence spectroscopy with 1,6-diphenyl-1,3,5-hexatriene as probe. The residual anisotropy parameter, r infinity (reflecting the fatty acid chain packing), was measured in the liquid crystalline phase as a function of cholesterol addition. Two main results are reported: (i) a slight increase of the order parameter, S, computed from the r infinity value as S = (r infinity/r0)1/2, in the physiological concentration range of cholesterol; (ii) a sharp enhancement of S from a cholesterol mole fraction (X chl) of 0.20 and up to X chl of 0.80. This is in contrast to unsaturated lecithin systems for which a continuous increase of the order parameter was monitored, culminating at X chl = 0.50, the well-known maximum level of incorporation of cholesterol into lecithin model membranes.

Steady-state and time-resolved fluorescence anisotropy study of phospholipid molecular motion in the gel phase using 1-palmitoyl-2-[9-(2-anthryl)-nonanoyl] -sn-glycero-3-phosphocholine as probe

1-Palmitoyl-2-[9-(2-anthryl)-nonanoyl]-sn-glycero-3-phosphocholine (Anthr-PC), a non-perturbing phospholipid probe [de Bony, J. and Tocane, J. F. (1983) Chem. Phys. Lipids 32, 105-121], has been designed in order to obtain insight into the membrane lipid organization at a 'microscopic' level, in terms of lateral distribution both in model and in natural membranes [de Bony, J. et al. (1984) Eur. J. Biochem. 143, 373-379; FEBS Lett. 174, 1-6]. In the present study, the molecular motions of this new fluorescent probe embedded in a lipid matrix have been investigated by fluorescence anisotropy techniques in steady-state and time-resolved modes. The results indicate that long axis rotation, monitored by the out-of-plane mode of rotation of the fluorophore, is fast even in the phospholipid gel state. It is moderately sensitive to the phase transition. The data suggest that this rotation is anisotropic. Cholesterol exhibits little effect on this rotation. The rotation of the long axis itself is sensitive to the transition. It is hindered as inferred from measurements at wavelength where both the in-plane and out-of-plane motions contribute to the depolarization of the emitted fluorescence light. Cholesterol restricts this motion. The behaviour of the free 9-(2-anthryl)-nonanoic acid is not significantly different from that of Anthr-PC. These results are discussed with respect to the influence of orientational constraints on the photodimerization process when this lipid probe is used to monitor phospholipid lateral distribution.

Corticosteroids as effectors of lipid polymorphism of dielaidoylglycerophosphoethanolamine. A study using 31P NMR and differential scanning calorimetry

The influence of corticosteroids on the lipid polymorphism of dielaidoylglycerophosphoethanolamine was studied by 31P NMR spectroscopy and differential scanning calorimetry. Both techniques evidenced two transitions in the pure lipid samples. The first one corresponded to the gel----liquid crystalline phase transition. It occurred at a temperature of 38.9 degrees C, as measured by differential scanning calorimetry and at 35-40 degrees C as detected by 31P NMR. The second transition corresponded to the bilayer----hexagonal HII phase transition. It occurred at 64.2 degrees C as measured by differential scanning calorimetry and at 60 degrees C as detected by NMR. Addition of corticosteroids led to different specific effects on the bilayer----hexagonal HII phase transition, according to their chemical structure. These effects appear to be the result of low amounts of incorporated steroids, according to binding studies (partition coefficient values range between 5 and 54). The presence of a conjugated 3-keto group in the steroid molecule (progesterone) promoted a downward shift in the bilayer----hexagonal HII phase transition temperature by about 6 -7 degrees C as compared to the 3 beta-OH-bearing compound (pregnenolone), which did not exhibit any appreciable effect. No change in the delta H of transition could be measured. The presence of the 21-OH group (like in deoxycorticosterone) induced the formation of a structure, characterized by an isotropic lineshape of the 31P NMR spectrum at temperatures where the 'hexagonal' type of lineshape is present, without steroid. The transition from the bilayer to this other structure occurred at a slightly higher temperature than the bilayer----hexagonal HII phase transition. It corresponded to a peak in differential scanning calorimetry scans with a delta H of 2.1 kJ X mol-1. The presence of the 17 beta-OH group as present in 17 beta-OH-progesterone and 11-deoxycortisol suppressed the two former effects. These compounds had no influence on the bilayer----hexagonal HII phase HII phase transition. The additional presence of the 11 beta-OH group like in corticosterone and cortisol, evoked a stabilization of the bilayer organization as the bilayer----hexagonal HII phase transition temperature is shifted upward by about 10 degrees C. This was accompanied by a decrease of the delta H to 0.8 kJ X mol-1. Besides this, the corticosteroids did not affect to a large extent the gel----liquid crystalline phase transition: a general slight downward shift of the transition temperature and a small broadening of the transition were observed without significant change in the delta H.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of cholesterol and protein content on membrane fluidity and 3 beta-hydroxysteroid dehydrogenase activity in mitochondrial inner membranes of bovine adrenal cortex

The steroid biosynthetic enzymes in the adrenal cortex are localised in endoplasmic reticulum and mitochondrial membranes. For some of the enzymes in endoplasmic reticulum the activity appears to be modulated by lipid fluidity, (21-hydroxysteroid hydroxylase and 3 beta-hydroxysteroid dehydrogenase). A mechanism for the regulation of corticosteroid biosynthesis mediated by the membrane fluidity has been suggested. Therefore a study of the mitochondrial inner membrane of the bovine adrenal cortex has been undertaken in comparison with a previous study of the endoplasmic reticulum. The kinetic parameters of the 3 beta-hydroxysteroid dehydrogenase were studied as a function of pH and temperature. No thermal transition can be observed in the Arrhenius plot for this enzyme in contrast with the results obtained for the microsomal enzyme. Membrane fluidity using, as fluorescent probes, diphenylhexatriene and a set of n-(9-anthroyloxy)fatty acids has been also studied as a function of temperature with or without addition of cholesterol. No thermal transition in the lipid phase can be observed. The addition of cholesterol to total mitochondrial membrane as to a lipid extract of the membrane decreases fluidity to the same extent as it does with microsomes. The presence of a large amount of protein in mitochondria has an effect which is additive to that of the cholesterol.

Sterol-phospholipid interactions in model membranes. Effect of polar group substitutions in the cholesterol side-chain at C20 and C22

The interactions of phospholipids with four different cholesterol derivatives substituted with one OH or one keto group at position C20 or C22 of the side-chain were studied. The derivatives were the 22,R-hydroxy; 22,S-hydroxy; 22-keto- and 20,S-hydroxycholesterol. Two aspects of the interactions were investigated: (1) the effect of the cholesterol derivatives on the gel leads to liquid crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) and of dielaidoylphosphatidylethanolamine (DEPE) monitored by differential scanning calorimetry and (2) The effect on the lamellar leads to hexagonal HII phase transition of DEPE monitored by DSC and by 31P-NMR to determine structural changes. The gel leads to liquid crystalline phase transition was affected by the cholesterol derivatives to a much larger extent in the case of DPPC than of DEPE. In both cases, there was a differential effect of the four derivatives, the 22,R-hydroxycholesterol being the less effective. In DPPC-sterol 1:1 systems, 22,R-hydroxycholesterol does not suppress the melting transition, the delta H values becomes 7.1 kcal X mol-1 as compared to 8.2 kcal X mol-1 for the pure lipid. 22,S-OH cholesterol has a much stronger effect (delta H = 3.1 kcal X mol-1) and 22-ketocholesterol suppresses the transition completely. In DEPE mixtures of all these compounds, the melting transition of the phospholipid is still observable. The transition temperature was shifted to lower values (-13.5 degrees C in the presence of 20,S-OH cholesterol). The delta H of the transition was lowered by these compounds except in DEPE-22,R-OH cholesterol mixtures and the cooperativity of the transition (reflected by the width at half peak height) was reduced. The lamellar leads to hexagonal HII phase transition was also affected by the presence of these cholesterol derivatives. The transition temperature value was depressed with all these compounds. 20,S-OH cholesterol was the most effective followed by 22,R-OH cholesterol. The delta H of the transition was not strongly affected. The molecular interfacial properties of these derivatives were studied by the monomolecular film technique. It is most likely that 22,R-OH cholesterol due to the hydroxyl groups at the 3 beta- and 22,R-positions orients with the sterol nucleus lying flat at the air/water interface, since the compression isotherm of either the pure sterol or the DOPC-sterol mixture (molar ratio, 1:1) monomolecular film exhibits a transition at approx. 103 A2.(ABSTRACT TRUNCATED AT 400 WORDS)

Steroid-lipid interactions in sonicated dipalmitoyl phosphatidyl choline vesicles: a steady-state and time-resolved fluorescence anisotropy study with all trans-1,6-diphenyl-1,3,5-hexatriene as probe

The effect of steroid chemical structure on steroid-lipid interaction is studied in sonicated dipalmitoyl phosphatidylcholine vesicles by steady-state and time-resolved fluorescence anisotropy techniques using diphenylhexatriene as probe. A large number of cholesterol derivatives and steroids, some of them being precursors involved in corticosteroids biosynthesis, has been studied. Addition of a polar group on the side chain in carbon-20 or 22 position evokes considerable difference for the interaction with the bilayer. The carbon-22 position appears to be of critical importance. This observation may be relevant for the functioning of mitochondria from steroid producing tissues, where a stereospecific hydroxylation of cholesterol at the carbon-22R position is required for pregnenolone synthesis.