Aluminum-triggered structural modifications and aggregation of β-amyloids

We investigated the structural effects induced by Al3+ on different beta-amyloid (Abeta) fragments at pH 7.4 and T=25 degrees C, with particular attention given to the sequences 1-40 and 1-42. Al3+ caused peptide enrichment in beta sheet structure and formation of solvent-exposed hydrophobic clusters. These intermediates evolved to polymeric aggregates which organized in fibrillar forms in the case of the Al3+-Abeta(1-42) complex. Comparative studies showed that Zn2+ and Cu2+ were much less efficient than Al3+ in stimulating the spontaneous aggregation/fibrillogenesis of Abetas. Studies with liposomes as membrane models showed dramatic changes in the structural properties of the lipid bilayer in the presence of Al3+-Abeta complexes, suggesting a major role of Al3+ in Abeta-induced cell dysfunction. Al3+ effects were abolished by desferrioxamine mesylate (DFO) only in solution. We concluded that, in vivo, DFO may act as a protective agent by preventing or reverting Abeta aggregation in the extracellular spaces.

The effects of singlet oxygen produced by photodynamic action on the mitochondrial permeability transition differ in accordance with the localization of the sensitizer

We have examined whether the effects of singlet oxygen (1O2) produced by photodynamic action on the mitochondrial permeability transition (PT) can be modulated by the localization of photosensitizers in irradiated mitochondria. We have previously shown that oxidation due to 1O2 photogenerated in hematoporphyrin (HP)-loaded mitochondria can prevent opening of the PT pores, likely after degradation of some critical histidines (Salet et al, 1997, J. Biol. Chem. 272, 21938-21943). Equally, in the present study we have irradiated mitochondria in the presence of a structurally different photosensitizer producing 1O2, namely 4,5',8-trimethylpsoralen (TMP). Fluorescence studies show that TMP binds to protein sites which differ from those of HP. In sharp contrast with HP, TMP-driven photodynamic action triggers per se pore opening. Interestingly, this inducing effect is inhibited when TMP-treated mitochondria are irradiated after addition of mersalyl, a specific reagent protecting thiol groups of the inner mitochondrial membrane that are oriented toward the external hydrophilic phase. This fact suggests that 1O2-mediated thiol oxidation is responsible for TMP-photoinduced pore opening. Taken together, these findings suggest that 1O2 can activate or inactivate a cellular function such as mitochondrial PT depending on the site where it is produced in the mitochondrial membrane.

Disaccharide modulation of the mitochondrial membrane fluidity changes induced by the membrane potential

The influence of the medium composition on the dynamic properties of mitochondrial membranes on depolarization was studied by following the fluorescence anisotropy changes of mitochondria-bound 1,6-diphenyl-1,3,5-hexatriene (DPH) and hematoporphyrin (HP) as reporters, respectively, of lipid and protein regions. On collapse of the potential, the membrane fluidity increased in NaCl-, KCl-, and monosaccharide-based media and decreased in disaccharides. Infrared spectroscopy experiments suggested that disaccharides likely change water's structure and association on the membrane surface. These results indicate that disaccharides induce membrane perturbation, which may interfere in the study of structure-function correlation in biological membranes.

Implications of the generation of reactive oxygen species by photoactivated calcein for mitochondrial studies

Calcein is a fluorescent probe that is widely used in studies of cell viability and mitochondrial function by microscopy fluorescence imaging. It was found to have a strong photosensitizing action that prevalently involves the generation of reactive oxygen species (ROS). The photooxidation properties of calcein in solution were studied in the presence of histidine and tryptophan as oxidizable substrates. The photodegradation of histidine was mainly mediated by singlet oxygen (1O2), as shown by the inhibitory effect of sodium azide, a specific 1O2 scavenger. On the other hand, mixed photosensitization mechanisms were present when tryptophan was used as the target of the calcein-stimulated photoprocess. In addition to 1O2, hydroxyl radicals and hydrogen peroxide were involved as reactive species, as shown by using mannitol and catalase as scavengers. The calcein-photosensitized alterations of mitochondria as a potential source of artifacts in confocal microscopy studies of cells were considered. Irradiation of isolated mitochondria with visible light (500-600 nm) in the presence of calcein induced opening of the permeability transition (PT) pore. The extent of the mitochondrial membrane photodamage, however, was modulated by the nature of the calcein environment. Thus, pore opening was triggered at short irradiation times and low dye concentrations when calcein was dissolved in the bulk medium. On the contrary, calcein concentrated in the matrix space was rather inefficient as photosensitizer even at concentrations 10 times higher than those present in the external medium.

Changes of the fluidity of mitochondrial membranes induced by the permeability transition

The dynamic properties of protein and lipid regions of mitochondrial membranes during the permeability transition (PT) process were studied by following the anisotropy changes of hematoporphyrin (HP) and 1,6-diphenyl-1,3,5-hexatriene (DPH), respectively. We show that opening of the PT pore is accompanied by a remarkable increase of mitochondrial membrane fluidity which is specifically localized to protein sites, while lipid domains are unaffected. The increased membrane fluidity is not related to the collapse of transmembrane potential that follows the PT, as demonstrated by a comparison between the anisotropy properties of permeabilized mitochondria and impermeable, depolarized organelles. Parameters such as osmotic swelling and temperature, which are shown to affect the mitochondrial membrane dynamics in the absence of permeability transition, cannot alone account for the pore dynamical properties. We suggest that the observed increase in fluidity is mainly due to a conformational change of pore-forming protein(s) during the "assembly" of the PT pore.

Effect of delivery system on the pharmacokinetic and phototherapeutic properties of bis(methyloxyethyleneoxy) silicon-phthalocyanine in tumor-bearing mice

A Si(IV)-phthalocyanine bearing two methoxyethyleneglycol axial ligands bound to the central metal ion (SiPc) has been prepared by chemical synthesis and analyzed for its phototherapeutic activity after administration in a Cremophor or liposome formulation to C57B1/6 mice bearing a subcutaneously transplanted Lewis lung carcinoma (LLC). The maximum drug accumulation in the tumor is found at 24 h after intraperitoneal injection, independent of the delivery system. However, the tumor concentration of SiPc in the Cremophor formulation is about two-fold higher, while the drug concentration in liver and skin shows similar trends with the two delivery systems. The drug accumulation and retention in the brain is much larger when using Cremophor emulsion. Photodynamic therapy (672 nm, 370 mW m-2, 360 J cm-2) at 24 h after the injection of Cremophor emulsion- or DPPC liposome-formulated SiPc causes a very efficient and similar response for the LLC (approximately 8 versus 22 mm mean tumor diameter for the control groups at 21 days after phototreatment). These very promising effects, obtained both at higher and lower tumor drug concentrations, clearly demonstrate the potential phototherapeutical activity of the newly synthesized SiPc.

Photodynamic action of porphyrin on Ca2+ influx in endoplasmic reticulum: a comparison with mitochondria

We have studied the distribution properties of haematoporphyrin (HP) and protoporphyrin (PP) in mitochondria and endoplasmic reticulum after isolation from rat liver. The photosensitizing efficiency of porphyrin on the Ca2+ influx function of microsomes has been compared with that obtained on Ca2+ uptake in mitochondria. HP and PP are accumulated in microsomes to a greater extent than in mitochondria, both porphyrins binding to membrane protein sites. The Ca2+ influx functions of mitochondria and microsomes, before and after irradiation in the presence of HP or PP, were studied by following the changes in the free Ca2+ concentration in the medium as revealed by the variations in fluorescence intensity of the Ca2+ indicator Calcium Green-1. For the same amount of incorporated porphyrin, the Ca2+ influx function of microsomes is degraded by irradiation more rapidly than that of mitochondria. The protective effect of dithiothreitol suggests that thiol groups in the Ca2+-transporting enzyme are the preferential targets of the photodynamic effect. These results suggest that intracellular Ca2+ movements are altered primarily by the endoplasmic reticulum rather than by mitochondrial damage, in good agreement with other observations made in porphyrin-loaded irradiated cells.

Effects of photodynamic action on respiration in nonphosphorylating mitochondria

We have studied the effects of singlet oxygen produced by photodynamic action on respiration in nonphosphorylating mitochondria (state 4). Isolated rat liver mitochondria were incubated with 3 microM hematoporphyrin and irradiated at 365 nm with a fluence rate of 25 W/m2. After short durations of irradiation, state 4 respiration with beta-hydroxybutyrate as substrate increases while respiration with succinate is negligibly affected. When mitochondria have been uncoupled with carbonylcyanide-p-trifluoromethoxyphenyl hydrazone before irradiation, no change occurs in beta-hydroxybutyrate-driven respiration, while succinate-driven respiration strongly decreases. Stimulation of state 4 NADH respiration cannot be explained by slippage of the NADH ubiquinone oxidoreductase because the stoichiometry of the redox pump was found insensitive to photodynamic action. In the light of the metabolite theory for linear enzymatic chains applied to state 4 respiration (Brand et al., Biochem. J. 255, 535-539, 1988), these results suggest that stimulation of NADH respiration is simply due to an increase of membrane leaks which occurs after irradiation. In the case of succinate-driven respiration, a strong inhibition of succinate dehydrogenase activity has been demonstrated after irradiation. It can be suggested that this inhibition introduces a negative control coefficient over state 4 respiration, counterbalancing the effects due to leakage.

Photophysical properties of porphyrin planar aggregates in liposomes

This paper describes studies of some photophysical properties of non-covalent planar aggregates of hematoporphyrin and protoporphyrin. This porphyrin species has been recently discovered and can be generated in lipid bilayers such as liposomes and inner mitochondrial membranes. The relative weight of this species in different media, as compared to porphyrin monomers and stacked aggregates, has been deduced by fluorescence decay studies. In contrast with what is observed for stacked aggregates, promotion of planar suprastructures can occur both in aqueous and lipid environments. The spectroscopic properties are very similar to those of the corresponding monomers, in particular as regards the shape of the absorption and emission spectra. The fluorescence decay times are generally higher than those of the monomers, and depend on the medium in which the planar aggregates are formed. The photooxidation properties of porphyrin planar aggregates, as revealed by oxygen consumption and histidine photodegradation upon irradiation at 365 nm, were compared to those of the monomers. The extent of the photooxidation process is nearly 20-30% higher in planar aggregates than in the monomers. In contrast, it is well known that cofacial aggregates are photochemically inert and only monomeric species of porphyrin are efficient photosensitizers. The biological relevance of these findings is discussed.

Si(IV)-methoxyethylene-glycol-naphthalocyanine: synthesis and pharmacokinetic and photosensitizing properties in different tumour models

A Si(IV)-naphthalocyanine bearing two methoxyethylenglycol axial ligands to the centrally coordinated metal ion (SiNc) was prepared by chemical synthesis and assayed for the phototherapeutic activity after administration in a Cremophor formulation to C57BI/6 mice bearing a subcutaneously transplanted Lewis lung carcinoma or B16 pigmented melanoma. Pharmacokinetic studies indicate that the maximal accumulation in the tumour occurs at 24 h after intraperitoneal injection of 0.5 mg kg-1 of SiNc, although the naphthalocyanine concentration in the Lewis lung carcinoma (0.70 microgram g-1) is significantly larger than that in the B16 pigmented melanoma (0.15 microgram g-1). This results in a higher selectivity of tumour targeting in the case of the lung carcinoma. Photodynamic therapy (782 nm, 370 mW cm-2, 360 J cm-2) at 24 h after SiNc injection causes an efficient tumour response for Lewis lung carcinoma (50% lower tumour diameter on day 19 post-treatment as compared to untreated controls) while the pigmented melanoma shows only a minor response regarding the rate of tumour growth.

Singlet oxygen produced by photodynamic action causes inactivation of the mitochondrial permeability transition pore

We have studied the effects of singlet oxygen produced by photodynamic action on the cyclosporin A-sensitive permeability transition (PT) in isolated rat liver mitochondria. Mitochondria were incubated with 3 microM hematoporphyrin and irradiated at 365 nm with a fluence rate of 25 watts/m2. For short durations of irradiation (60 s) the adenine nucleotide translocase was inactivated, but mitochondria retained their ability to form a proton electrochemical gradient and accumulated Ca2+ and Pi at the same rate as non-irradiated controls. Strikingly, however, the oxidative effects of photodynamic action prevented opening of the PT pore which is normally induced by Ca2+ plus Pi or by treatment with diethyl pyrocarbonate (a histidine reagent) or diamide (a thiol oxidant). We show that the most likely targets for photodynamic action are critical histidines that undergo degradation. Irradiated, hematoporphyrin-loaded mitochondria treated with diethyl pyrocarbonate or diamide still undergo the PT when treated with phenylarsine oxide, which reacts with a critical dithiol involved in pore modulation (Petronilli, V., Costantini, P., Scorrano, L., Colonna, R., Passamonti, S., and Bernardi, P. (1994) J. Biol. Chem. 269, 16638-16642). These data suggest (i) that the dithiol cysteines are not oxidized by photodynamic action, but rather became inaccessible to oxidants; and (ii) that irradiation of hematoporphyrin-loaded mitochondria does not lead to pore denaturation, but rather to site-selective inactivation of discrete pore functional domains.

Effects of Photofrin photodynamic action on mitochondrial respiration and superoxide radical generation

Cyanide-resistant respiration increases after irradiation of isolated mitochondria in the presence of Photofrin. This suggests an enhancement of electron leakage which has been evaluated by measuring superoxide radical formation in submitochondrial particles incubated with 6 micrograms/ml Photofrin in the medium and irradiated with increasing doses of light at 365 nm. After a dose of 4.5 kJ/m2 has been delivered, superoxide generation increases by a factor of approximately 2.5 at the level of NADH dehydrogenase and by a factor approximately 1.5 in the cyt bc1 region. These effects have been compared with changes observed in NADH-, succinate- and ascorbate-driven respiration and their implications discussed.

Fluence rate effects on photodynamic therapy of B16 pigmented melanoma

In vivo experiments were performed to evaluate the effect of fluence rate on the efficiency of Zn(II)-2,3 naphthalocyanine (ZnNc) photosensitization of B16 pigmented melanoma subcutaneously transplanted in C57B1/6 mice. The tumour was irradiated with 774 nm light at 24 h after an injection of liposome--which incorporated ZnNc (0.5 mg kg-1 b.w.). A total light dose of 360 J cm-2 was delivered at fluence rates of 260, 320, 380, 440 and 500 mW cm-2. Separate groups of mice utilized to monitor tumour temperature changes during irradiation without or after anaesthesia. Tumour response was determined by measuring the mean tumour diameter of the treated towards the untreated animals for a period of 21 days following PDT, as well as the percentage of cured animals. The most promising result (40% complete tumour response) was obtained with anaesthetized mice following 380 mW cm-2. Higher dose rates (440 and 500 mW cm-2) led to less promising results for both anaesthetized and non anaesthetized mice. These results outline the potential of PDT with longer wavelengths for the treatment of highly pigmented tumour tissues. The optimal fluence rate for tumour treatment should be chosen in order to avoid inflammatory effects (tissue swelling) and oxygen suppression with sublethal injury to the tumour cells.

Temperature-induced changes in fluorescence properties as a probe of porphyrin microenvironment in lipid membranes. 1. The partition of hematoporphyrin and protoporphyrin in liposomes

Temperature-induced fluorescence changes were studied for hematoporphyrin and protoporphyrin, incorporated into liposomes of dipalmitoylphosphoglycerocholine (Pam2GroPCho) or dimiristoylphosphoglycerocholine (Myr2GroPCho). In some cases, cholesterol or cardiolipin were added to the vesicles for better mimicking the lipid composition of biological membranes. The experimental conditions were appropriately chosen in order to reproduce different possible configurations of the porphyrin molecule in lipid membranes: namely, at the polar water/headgroups, headgroups/lipid and lipid/lipid interfaces. A peculiar feature observed in some of the above liposomal systems was the appearance of discontinuities in the Arrhenius plots of the fluorescence quantum yields, with relevant changes of the values of activation energies. These discontinuities were due to an increase of the fluorescence signal in a temperature range corresponding to the transition of the different lipids from the gel-to-liquid crystal state. The observed phenomena are consistent with the formation of non-covalent linear dimers or linear higher aggregates of the porphyrin molecules. The intermolecular contacts required for the formation of these species are favoured by at least three situations: disruption of the ordered lipid structure during the gel-to-liquid crystal phase transition; competition of other polar groups (e.g., the -OH group of cholesterol) with the porphyrin carboxylate groups for the polar phospholipid headgroups; and steric constraints due to overcrowding of porphyrin molecules in a restricted space.

Temperature-induced changes in fluorescence properties as a probe of porphyrin microenvironment in lipid membranes. 2. The partition of hematoporphyrin and protoporphyrin in mitochondria

The temperature dependence of hematoporphyrin and protoporphyrin fluorescence quantum yields (phi F) was studied after delivery to whole mitochondria or isolated inner (IMM) and outer (OMM) mitochondrial membranes, obtained from liver of Wistar rats. These studies are very sensitive to variations of the porphyrin lipid environment. Before incorporation, the porphyrins were dissolved in 0.01 M sodium phosphate, 0.15 M NaCl, pH 7.4) NaCl/Pi (only hematoporphyrin) or dispersed into liposomes of dipalmitoylphosphoglycerocholine (Pam2GroPCho), sometimes enriched with cholesterol or cardiolipin. Whole mitochondria show higher incorporation capacity of hematoporphyrin and protoporphyrin than isolated IMM and OMM, probably because additional, energy-sensitive transport mechanisms for the porphyrin uptake occur in intact organelles. A small decrease in protoporphyrin uptake is observed in OMM in comparison with IMM; in contrast, the decrease in hematoporphyrin uptake by OMM is rather significant. A comparison between the results obtained with IMM, OMM and whole mitochondria show that both porphyrins, when released to the intact organelles, preferentially localize in the IMM, irrespective of the lipid carrier used. NaCl/Pi-dissolved hematoporphyrin probably interacts with some membrane proteins, due to the similarity of the Arrhenius plots with those obtained for liposome-entrapped human serum albumin/hematoporphyrin complexes which were used as models to mimic hematoporphyrin-membrane protein binding sites. Liposomal hematoporphyrin and protoporphyrin bind to lipid domains. Hematoporphyrin accumulates in specific, localized lipid regions, perhaps in the boundary lipids area surrounding some inner-mitochondrial carriers; protoporphyrin accomodates in more rigid, lipid areas. On these bases, the higher photoactivity of hematoporphyrin, previously observed in mitochondria, in comparison with protoporphyrin, can be easily explained. Formation of linear dimers/aggregates, endowed with higher phi F than that of the monomers, are postulated to occur for both porphyrins only in the inner mitochondrial membrane.

Photophysical properties of porphyrins in biological membranes

This review illustrates the photophysical properties of some porphyrins, especially those used for biomedical applications, in relation to their photosensitizing efficiency in biological membranes. Porphyrin absorption and luminescence properties are mainly examined. The factors influencing the affinity of porphyrins for biological membranes, including the dye hydrophobicity, the charge and aggregation state, the pH of the medium and the physicochemical properties of the dye environment, are discussed. These factors determine the differences in the photophysical properties of porphyrins in biological membranes. Particular attention is paid to the porphyrin aggregation state: only monomeric species and possibly planar end-to-end aggregates are endowed with significant photosensitizing ability. Many conclusions presented are based on data obtained on membrane model systems such as micelles or liposomes which can mimic specific situations occurring in cells.

Interaction of phthalocyanines with lipid membranes: a spectroscopic and functional study on isolated rat liver mitochondria

Absorption and emission spectroscopic studies on Zn(II)-phthalocyanine (ZnPc) incorporated into unilamellar liposomes of dipalmitoylphosphatidylcholine, sometimes added with cholesterol or cardiolipin, and released to rat liver mitochondria via the three types of liposomal vesicles indicated that: (a) ZnPc predominantly dissolves in all lipid domains of biological membranes with the exception of cardiolipin-containing regions; a partial localization of ZnPc in protein binding sites is also postulated; (b) the spectroscopic properties of ZnPc, although mainly determined by the aggregation state of the dye, are somewhat influenced by the physico-chemical characteristics of the lipid environment; (c) ZnPc-binding lipid domains in mitochondria are mainly localized in the outer membrane; this conclusion is clearly deduced from the trends of Arrhenius plots of the ZnPc fluorescence quantum yield in whole mitochondria and isolated inner or outer membrane in the temperature range -10 degrees C-(+)45 degrees C; (d) the nature of the ZnPc-binding site in mitochondria is not dependent on the chemical composition of the liposome carrier, contrary to what observed for other hydrophobic dyes, such as porphyrins. This has been also confirmed by photosensitization experiments. Actually, illumination of ZnPc-loaded mitochondria by 600-700 nm light causes a decline of the respiratory control ratio, which is essentially dependent on the amount of incorporated photosensitizer, irrespective of the composition of the carrier.

Hematoporphyrin derivative (Photofrin) photodynamic action on Ca2+ transport in monkey kidney cells (CV-1)

After 24 h incubation with Photofrin (PF), photodynamic action has been studied on Ca2+ transport in CV-1 cells. A moderate increase of the cytosolic free Ca2+ concentration [Ca2+]i is observed immediately after a dose of irradiation which yields a survival rate of less than 5% at 48 h. In parallel, studies on digitonin-permeabilized cells indicate that such a treatment inhibits endoplasmic reticulum Ca2+ uptake with few alterations of this process in mitochondria. In contrast, ADP-stimulated respiration is impeded and intracellular ATP level decreases. It is suggested that direct damage to endoplasmic reticulum as well as mitochondrial disturbance are the primary mechanisms responsible for a nontransient elevation of [Ca2+]i preceding cell death.

Photosensitization of mitochondria by liposome-bound porphyrins

We have compared the photodynamic activities of hematoporphyrin (HP) and protoporphyrin (PP) on isolated rat liver mitochondria by measuring the decline of the respiratory control ratio (RCR) after irradiation at 365 nm. Before addition to the respiratory medium, the dyes were dissolved in phosphate-buffered saline (PBS) or incorporated into unilamellar liposomes of dipalmitoyl-phosphatidylcholine (DPPC), sometimes enriched with cholesterol (Chol) or cardiolipin (Card), which are naturally present in mitochondrial membranes. Chol and especially Card strongly increase the porphyrin uptake by mitochondria. In all experimental conditions, PP is taken up by mitochondria to a higher extent than HP. Nevertheless, under conditions giving the same amount of mitochondria-bound dye, HP is a more efficient photosensitizer than PP. As the efficiency of singlet oxygen production has been shown to be equivalent for the two porphyrins in monomeric state, the resulting photobiological effects are explained in terms of different localization of HP and PP in the mitochondrial membranes. In particular, HP preferentially localizes in the protein-rich polar domains of the inner mitochondrial membrane, whereas PP dissolves in the lipid regions of the membranes.

Comparative pharmacokinetic and photodynamic studies with zinc(II) phthalocyanine in hamsters bearing an induced or transplanted rhabdomyosarcoma

Comparative pharmacokinetic studies in hamsters bearing an induced or first-generation transplanted rhabdomyosarcoma that were injected with liposome-incorporated zinc(II) phthalocyanine (ZnPc) show a higher drug concentration in the induced tumour. The selectivity of tumour targeting is underlined by the fact that, 24 h after injection, larger amounts of ZnPc are found in the tumour than in the liver. Photodynamic therapy investigations were carried out using 673 nm light from an argon-dye laser. On the basis of different assessment criteria (changes in mean tumour diameter with time, tumour mass regression, survival time of the treated groups of animals, and histological determination of the necrotic tissue) the photosensitizing effect of ZnPc appears to be comparable for both kinds of tumour in spite of the higher uptake of photosensitizer by the induced tumour.

Liposomes as models to study the distribution of porphyrins in cell membranes

Unilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) have been chosen as suitable models of cell membranes in studies aimed at defining the influence of specific parameters on the distribution properties of selected hydrophobic photosensitizers, namely hematoporphyrin (HP) and protoporphyrin (PP), in normal and tumour tissues. To better mimic in vivo situations, DPPC liposomes were sometimes mixed with cardiolipin (Card) or cholesterol (Chol). Two techniques were mainly used: the quenching of porphyrin fluorescence by methyl viologen, which can discriminate different dye populations inside the vesicles as well as their degree of accessibility to the external medium, and the polarization of porphyrin fluorescence, which gives information on the dye microenvironment through its degree of rotational freedom. The nature of the porphyrin binding sites in each phospholipid monolayer is found to be a function of the degree of hydrophobicity and the concentration of the dye as well as the chemical composition of the liposomes. In DPPC and DPPC-Chol liposomes, all PP molecules are deeply embedded into very rigid, hydrophobic domains of the inner lipid monolayer. Only in the presence of cardiolipin, for [PP] greater than 2.5 microM, a partial shift of the dye molecules towards the outer lipid monolayer is observed. HP mostly localizes at the inner lipid/water interface in all liposomes: at very low concentrations ([HP] approximately equal to 0.5 microM) the dye is bound to the polar heads of the lipids through its carboxylate groups, leaving the rest of the molecule dissolved in the inner aqueous pool. At higher concentrations, HP molecules change their orientation: the ionized propionic chains still interact with the polar heads while the hydrophobic core lies in the lipid phase in DPPC and DPPC-Card vesicles. HP incorporated into DPPC-Chol mixed liposomes projects from the inner lipid phase into the aqueous compartment in all the concentration range studied by us. A very small fraction of HP population (corresponding to 5-10% of the overall fluorescence) is localized at the water/lipid external interface in DPPC and DPPC-Chol liposomes. This fraction increases in the presence of cardiolipin (up to 30% of the overall fluorescence). The possible implications of these findings for the nature of the targets of photosensitization in cell membranes are discussed.

Factors influencing the distribution pattern of porphyrins in cell membranes

The mechanism of the sensitizer-membrane interactions has been studied by following the distribution properties of selected porphyrins, including haematoporphyrin (HP) and protoporphyrin (PP), into unilamellar liposomes of dipalmitoyl phosphatidylcholine (DPPC). The endomembrane distribution of HP and PP has been checked as a function of the membrane fluidity and composition by fluorescence polarization and quenching techniques. At porphyrin concentrations below 0.5 microM, HP and PP exclusively localize in the inner phospholipid monolayer; at higher concentrations, the outer monolayer also becomes populated. The porphyrin binding sites in liposomes, however, are different for HP and PP: HP preferentially distributes into water-accessible lipid regions, while PP localizes in the most hydrophobic loci of the lipid matrix. A porphyrin redistribution occurs when the fluidity properties of the liposomes are changed by addition of cholesterol or cardiolipin. In DPPC-cholesterol vesicles, all HP molecules dissolve in DPPC-rich regions while all PP molecules partition in cholesterol-rich environments. In DPPC-cardiolipin vesicles both porphyrins preferentially localize in regions accessible to the external medium. The effect of the nature of the carrier on porphyrin distribution in membranes has been studied by following the uptake and photosensitization properties of free and DPPC-incorporated PP and HP with rat liver mitochondria. The porphyrin photosensitizing efficiency has been checked by following the impairment of the respiratory function of mitochondria upon irradiation. Liposome-bound HP is less active than aqueous HP in determining membrane photodamage in mitochondria. On the contrary, aqueous PP is a very poor sensitizer as compared to a DPPC liposome-entrapped drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Cobalt substitution studies on bovine erythrocyte superoxide dismutase: evidence for a novel cobalt-superoxide dismutase derivative

Three cobalt derivatives of bovine erythrocyte superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) have been prepared under different pH conditions using a cobalt-thiocyanate complex which has already proved to yield specific substitutions on other copper proteins. The cobalt-protein derivatives have been characterized by optical, circular dichroism and fluorescence spectroscopies. One derivative, referred to as Co2Co2-protein, contains Co(II) ions specifically bound at both Zn(II) and Cu(II) sites. On the basis of their spectroscopic properties, the other two derivatives can be referred as E2Co2- and Co2E2-superoxide dismutase, with cobalt substituting, respectively, at the zinc and the copper sites leaving the contiguous site empty (E). The Co2E2-protein complex represents a novel derivative, since it has never been described in literature. The optical spectrum in the visible region of Co2-Co2-protein well corresponds to the sum of the spectra of the other two derivatives. The circular dichroism spectrum of Co2Co2-derivative, however, is not the sum of individual E2Co2- and Co2E2-proteins, suggesting that the presence of Co(II) in one site strongly affects the geometry of the neighbouring site. Some discrepancies between our spectroscopic data and those reported in literature are discussed. The results obtained from fluorescence experiments indicate that Co(II) ions exert a different quenching effect on the tyrosine emission, depending on whether they are located in the Zn(II) or in the Cu(II) site. The fluorescence quenching can be attributed to a 'heavy atom' and 'paramagnetic ion' effect by Co(II) ions.

Influence of the gel-liquid phase transition on hematoporphyrin triplet deactivation in liposomes

The deactivation of the triplet state of hematoporphyrin and its dimethyl ester in unilamellar liposomes of dipalmitoyl phosphatidylcholine was studied by nanosecond laser flash photolysis. It was found that the rate of deactivation increases abruptly on raising the temperature in the region of the gel-liquid phase transition of the lipid bilayer (41 degrees C). The rate of change has its maximum at 38.4 +/- 0.5 degree C for both porphyrins. This variation is due to the high lateral mobility of the porphyrins in the liquid-crystal bilayer, which enhances the rates of concentration triplet quenching and triplet-triplet annihilation.

Steady-state and time-resolved spectroscopic studies on the hematoporphyrin-lipoprotein complex

The interaction of hematoporphyrin (Hp) with the isolated rabbit lipoprotein fractions very low density lipoproteins, low-density lipoproteins, and high-density lipoproteins has been studied by steady-state and time-resolved spectroscopy. The porphyrin appears to be bound to both the apoprotein and the lipid phase. The two populations of lipoprotein-bound Hp molecules can be distinguished on the basis of the fluorescence excitation spectrum, decay constants of the lowest excited singlet and triplet states, and accessibility to oxygen. Upon Hp binding, the intrinsic fluorescence emission of apolipoproteins is quenched at least in part via singlet-singlet energy transfer from tryptophyl residues to the porphyrin moiety. The binding of Hp with the protein matrix can be adequately described on the basis of Scatchard analysis, whereas the interaction of Hp with the lipid core can be described as the partitioning of the dye between a hydrophobic and an aqueous phase. The Hp binding capacity of lipoproteins is maximal for very low density lipoproteins.

The role of copper and quaternary structure on the conformational properties of Octopus vulgaris hemocyanin

Some structural properties of Octopus vulgaris hemocyanin have been investigated by fluorescence spectroscopy. The three-dimensional structure of Octopus hemocyanin is remarkably tight, resulting in a deep burial of almost all the tryptophyl residues of the protein. The hemocyanin conformation has been studied in the two main aggregation states (11 S, 50 S) of the protein, and with respect to the presence or absence of copper in the active site. Upon changing the pH of the solution, Octopus hemocyanin in the 50 S aggregation state can assume at least three different conformations. During the transition between each conformation the fluorescence quantum yield changes, but the environment of tryptophans does not change. Dissociation of the protein from 50 S to 11 S strongly enhances its susceptibility toward denaturating agents such as pH or temperature, and modifies the effects of fluorescence quenchers such as acrylamide. Moreover, these effects are more pronounced when copper is removed from the active site. A comparative analysis of the results shows that the subunit-subunit interactions exerted within the 50 S species are more important in the maintenance of the conformational stability than the copper ions present in the active sites. This behavior can be accounted for by the large amount of Ca(II) ions linked to 50 S hemocyanin.

Removal of copper from Octopus vulgaris haemocyanin. Preparation of the half-apo and apo derivatives

The two copper ions bound in the active site of Octopus vulgaris haemocyanin can be removed by cyanide. The two metal ions react with the ligand sequentially. In this paper the preparation of Octopus half-apo-haemocyanin, containing at the active site a single copper ion, is described. Moreover, the conditions to obtain Octopus apo-haemocyanin, containing less than 3% of copper still bound, are given.

Intramolecular distances between tryptophan residues and the active-site serine residue in alkaline bacterial proteinases as measured by fluorescence energy-transfer studies

Singlet-singlet energy transfer from the tryptophan residues to an active-site-serine-bound 5-dimethylaminonaphthalene-1-sulphonyl group was investigated in four subtilisins. The transfer distances for subtilisin Novo and mesentericopeptidase are 1.93 +/- 0.20 nm (19.3 +/- 2.0 A) and 1.81 +/- 0.20 nm (18.1 +/- 2.0 A) respectively. The positions of the indole groups in the three-dimensional structures of the two pairs of proteinases, namely subtilisin Novo and mesentericopeptidase on the one hand and subtilisins Carlsberg and DY on the other, are essentially identical.

Effects of pH and urea on the conformational properties of subtilisin DY

Subtilisin DY is very resistant to the denaturing action of urea: the conformational properties are not affected up to 4.5 M-urea, and even in the presence of 8 M-urea there is only a slow loss of ordered structure and caseinolytic activity. C.d. and fluorescence-emission studies also show that this proteinase is stable in the 5.5-10.0 pH range, whereas below pH 5.5 a sharp denaturation occurs that is complete at pH 4.5. Protein denaturation leads to a change of the emission quantum yield; in particular, in the native protein, indole fluorescence is quenched by some amino groups. Moreover, subtilisin DY possesses two classes of tyrosine residues: one class of exposed residues titrates normally, with pKapp. = 10.24, whereas one class of partially buried or hydrogen-bonded residues ionizes with pKapp. = 11.58. In general, such conformational properties resemble those of other subtilisins. However, some differences occur: e.g., subtilisin DY is less stable at acidic pH values and its tyrosine residues are more accessible to the solvent. Such differences are probably due to small variations of the three-dimensional structure; e.g., subtilisin DY has a slightly lower alpha-helix content.

Chemical, photochemical and spectroscopic characterization of an alkaline proteinase from Bacillus subtilis variant DY

Circular-dichroism and fluorescence studies indicate that the 5-dimethylaminonaphthalene-1-sulphonyl and phenylmethanesulphonyl derivatives of subtilisin DY have three-dimensional structure closely similar to that of native enzyme. The single tryptophan residue is largely accessible to the aqueous solvent, and is not directly involved in the enzyme-substrate interactions, since its photochemical modification causes only a partial inhibition of the enzyme activity. It appears very likely that the location of the single tryptophan residue in the three-dimensional structure of subtilisin DY is similar to that of the single tryptophan residue in subtilisin Carlsberg. Fluorescence-quenching experiments further indicate that the 14 tyrosine residues are also largely accessible to the aqueous solvent, and probably interact with hydrated peptide carbonyl groups. The charge environment for tryptophan and tyrosine residues in subtilisin DY, as deduced by quenching experiments with ionic species, is also discussed. In general, subtilisin DY displays strong similarities to subtilisin Carlsberg, as suggested by a comparative analysis of the amino acid composition and fluorescence properties.

Interaction of human serum albumin with hematoporphyrin and its Zn(2)+-and Fe(3)+-derivatives

Human serum albumin at pH values above 6.8 has one strong binding site for hematoporphyrin; the stability constant of the 1:1 complex is about 10(6) M-1 as determined by Scatchard plot after estimation of the bound hematoporphyrin-induced quenching of the fluorescence emitted by the single tryptophanyl residue of the protein. Determination of the tryptophan-to-hematoporphyrin energy transfer efficiency yields a Förster parameter R0 of 6.2 - 6.9 nm, depending on the value chosen to represent the donor-acceptor mutual orientation, and a tryptophan-to-hematoporphyrin distance of about 1.7 nm. Zn2+- and Fe3+-hematoporphyrin also give a 1:1 complex with albumin, probably binding at the same site as hematoporphyrin, as shown by the identity of the energy transfer parameters; however, the metal ions do not appear to be involved in the formation of the albumin-porphyrin complex. The albumin-hematoporphyrin interaction is drastically affected by the pH of the medium; below pH 6.5 we find a large number of binding sites with weak affinity for hematoporphyrin, which disappear upon increasing the pH. The main site, below pH 6.5, has an affinity comparable with that of the secondary sites. Circular dichroism studies show that the pH effect is due to a change in the protein conformation leading to different interactions between bound porphyrin and specific amino acid side chains.

Fluorescence properties of native and chemically modified mesentericopeptidase

Studies on the fluorescence properties of native mesentericopeptidase as a function of the temperature and/or in the presence of either neutral or ionic fluorescence quenchers demonstrate that the intrinsic emission f this protein is dominated by a partially exposed tryptophyl residue, which is probably located in a site of high dielectric constant containing positively charged amino acid side chains. One largely exposed tryptophan contributes about 14% of the total emission, whereas one deeply buried tryptophan is virtually non-fluorescent. The conversion of the active site serine to cysteine and the insertion of either one phenylmethanesulfonyl or one dansyl substituent into the active site induce only subtle differences in the conformational properties with respect to the native protein; in particular, the mutual distances and orientation between the 13 tyrosyl and 3 tryptophyl residues are unaffected, as shown by singlet-singlet energy transfer experiments.

Synthetic enkephalins. Addicting properties and conformational studies in solution

The addicting properties of [Leu5]enkephalin in mice are conserved in the LSer3 analogue and lost both in the L-Ser2 analogue and in all the L-Cha4 derivatives of the above peptides. Fluorescence measurements in water show the presence of hydrogen-bonded tyrosul OH groups in [Leu5]-enkephalin and in its LSer2 analogue. The Phe4/Cha replacements do not influence these equilibria, but they affect the near u.v. dichroism of the hydrogen bonded tyrosyl residues. In the peptide absorption region in water solution, only [Leu5]-enkephalin and its cyclohexylalnyl derivative show a positive dichroism towards high frequencies, which is maintained in 8 M ura. No clear relation is found between conformation(s) in solution and biological activity. A II' beta-turn, with residues in positions 2 and 3 at the corners, is suggested for the conformation of enkephalin bound to the receptors involved in the bioassay here used.

The binding of 1-anilino-8-naphthalene sulfonate to the hemocyanin of Octopus vulgaris

The binding of 1-anilino-8-naphthalene sulfonate (ansyl) to native and copper-free hemocyanin of Octopus vulgaris has been studied in different conditions by measuring the fluorescence properties of the probe in the presence of hemocyanin. Native hemocyanin, either in the oxygenated or in the deoxygenated state, does not bind ansyl. The binding of ansyl with apohemocyanin induces a strong increase (from 0.004 to 0.6 -- 0.7) of the quantum yield and a blue shift from 520 nm to 460 nm of the emission maximum indicating the presence of ansyl binding sites in the protein. Experimental evidence is reported that the binding occurs at the copper-binding site of the protein. The dissociation constants of the ansyl-hemocyanin complexes are equal to about 10(-4) M, i.e. they are of the same order of those obtained with other proteins. The number of binding sites (n) of apohemocyanin for ansyl depends on the conformational state of the protein and ranges from 0.15 -- 0.80 mol/mol protein (Mr 50,000), depending on pH, ionic strength, and urea concentration. A negative interaction between the ansyl binding sites has been suggested.