Kinetics of the interactions of a dicarboxylic porphyrin with unilamellar lipidic vesicles: interplay between bilayer thickness and pH in rate control

Diffusion of chlorin-p6 across phosphatidyl choline liposome bilayer probed by second harmonic generation

We have investigated the diffusion of the photosensitizer Chlorin-p(6) (Cp(6)) across a egg lecithin lipid bilayer at different pH by the Second Harmonic Generation (SHG) method. Cp(6) has three ionizable carboxylic acid groups, and consequently, neutral and several ionic forms of Cp(6) are expected to be present in the pH range 3-8. The absorption spectra of Cp(6) get considerably modified in the presence of liposomes as the pH is decreased indicating that the drug liposome binding is pH dependent. The first pK(a) of interconversion (D-C) has been identified at pH ~7.0 by fluorescence measurement in an earlier work. In this work, the second pK(a) of interconversion (C-B) has been identified at pH ~4.8 by the hyper-Rayleigh scattering method. At acidic pH (3, 4, and 5), where species A, B, and C are dominant, the addition of liposomes to a Cp(6) solution generates an instantaneous rise (less than 1 s) in the second harmonic (SH) signal followed by decays whose time constants ranged from ten to hundreds of seconds. The instantaneous rise is attributed to the adsorption of Cp(6) to the outer lipid bilayer, and the decay is attributed to the diffusion of the neutral and charged (A and B) species of the drug. The observed fast and slow time constants for diffusion in the pH range 3-5 are attributed to the neutral (A) and ionic form (B) of Cp(6), respectively. At pH 6, the intensity of the generated SH signals on the addition of liposome reduced, and at physiological pH, it was too weak to be detected. These results are consistent with previous studies that show that the interaction between Cp(6) and egg-PC liposomes is pH dependent. At lower pH due to the presence of the hydrophobic species (A and B) of Cp(6), its interaction with liposomes is strong, and at higher pH, the abundance of the negatively charged hydrophilic species (C and D) decreases the interaction with the like charged liposomes. We have also studied the effect of increasing the bilayer rigidity by decreasing the temperature of the medium or by incorporating 50 mol % cholesterol in the lipid bilayer and observed that lowering of temperature has more profound effect on the diffusion rates. The characteristics of the SH signal changed significantly when liposomes incorporating 50 mol % cholesterol were used at a low (3 °C) temperature. Under these conditions, the SH signal consisted of an instantaneous (<1s) followed by a slower rise (10-90s), and then, it decayed on a much longer time scale. This slow rise of the SH signal at pH 3 and 4 may be attributed to the temperature dependent adsorption of the anionic species (B) of Cp(6) with the liposomes. Further investigations are required in order to understand clearly the pH dependent diffusion of this drug across lipid bilayers.

Binding and acid-base properties of novel photosensitizing drugs in micellar and liposome solutions

The binding and acid-base equilibria of the two novel mesoporphyrin derivatives, PB07 and PB109 (quino[4,4a,5,6-efg]- annulated 7-demethyl-8-deethylmesoporphyrin and 2'-cyano-8'-formyl-N'-methyl-1',1a',5a',6'-tetrahydroacrido [4,5,5a,6-bcd]- annulated 2,3-dihydromesoporphyrin, resp.), which are promising agents for photodynamic therapy (PDT), were studied in aqueous solutions of different surfactants (Triton X-100 (TX-100), dodecyl maltoside (DDM), cetyltrimethylammonium bromide (CTAB), lithium dodecyl sulfate (LDS)) and phosphatidyl choline (PC) liposomes. In all cases, the porphyrins are solubilized at neutral/alkaline pH in monomeric form and remain micelle/liposome-bound independently of their ionization state. The dissociation constants of the solubilized porphyrins are found to be influenced by the charge of the surface groups of the carrier. The protonation of pyrrole/quinoline nitrogens of the studied porphyrins is facilitated in the following order: LDS ≫TX-100 (DDM, PC liposomes) > CTAB. The dissociation constants of PB 07 carboxylic groups are similar in neutral/cationic micellar and liposome solutions and are significantly decreased for LDS-bound pigment. The results provide necessary information for the optimization of delivery systems for PB 07 and PB 109 when applied as sensitizers in PDT.

The effect of lipid composition, bilayer phase and temperature on the uptake of hematoporphyrin by liposomal membranes

In this study we investigated, spectroscopically, the binding of hematoporphyrin (HP) to non-charged lipid vesicles as a function of temperature and the molecular structure of the phospholipid. The temperature dependence of partitioning was employed to evaluate the thermodynamic parameters of the process. We studied the binding of HP to liposomes composed of different phospholipids: natural lecithin and three chemically defined phosphatidylcholines: dimiristoyl-phosphatidylcholine (DMPC), 1-palmitoyl-2-myristoyl-phosphatidylcholine (PMPC) and 1-stearoyl-2-myristoyl-phosphatidylcholine (SMPC), at different temperatures. The last three lipids differ only in the length of the fatty acid on 1 position of the glycerol backbone. Consequently, they have different phase transition temperatures and different order parameters. For SMPC, PMPC and DMPC, we checked the effect of temperatures above and below the phase transition while for lecithin, whose phase transition temperature is well below 0 °C, only temperatures above the phase transition could be tested. A very distinct effect of the phase transition on the binding constant was observed. Below this temperature a dramatic decrease in the binding was observed as the temperature was increased. Above the phase transition, the effect of temperature declined and the changes were minor compared to the changes observed when the bilayers undergo the solid-gel phase transition. Differences in HP binding to the various bilayers were attributed to the differences in the order parameters of DMPC, PMPC, SMPC and lecithin bilayers.

Transfer mechanism of temoporfin between liposomal membranes

The transfer kinetics of temoporfin, a classic photosensitizer, was analyzed by investigating the influence of total lipid content, temperature, as well as charge, acyl chain length, and saturation of the lipids in donor vesicles using a mini ion exchange column technique. The obtained results are consistent with an apparent first order kinetics in which the transfer proceeds through both liposome collisions and through the aqueous phase. We present a corresponding theoretical model that accounts for the detailed distribution of drug molecules in donor and acceptor liposomes and predicts the transfer rates as a function of drug concentration and number of donor and acceptor liposomes. The experimentally observed transfer rates depended strongly on the temperature and comply with the Arrhenius equation. Thermodynamic calculations indicate the transfer process to be entropically controlled. In terms of the charge of donor liposomes, positively charged liposomes showed transfer rates faster than negatively charged liposomes whereas the maximum amount transferred was almost the same. A more rigid structure of the donor liposomes increases the transfer rate of temoporfin, which is caused by expelling the drug from the membrane interior, as proposed in former work. In summary, our combined theoretical/experimental approach offers a systematic way to study the mechanism of drug release from liposome-based delivery systems.

pH-dependent distribution of chlorin e6 derivatives across phospholipid bilayers probed by NMR spectroscopy

The pH-dependent membrane adsorption and distribution of three chlorin derivatives, chlorin e6 (CE), rhodin G7 (RG), and monoaspartyl-chlorin e6 (MACE), in the physiological pH range (pH 6-8) were probed by NMR spectroscopy. Unilamellar vesicles consisting of dioleoyl-phosphatidyl-choline (DOPC) were used as membrane models. The chlorin derivatives were characterized with respect to their aggregation behavior, the pK(a) values of individual carboxylate groups, the extent of membrane adsorption, and their flip-flop rates across the bilayer membrane for pH 6-8. External membrane adsorption was found to be lower for RG than for CE and MACE. Both electrostatic interactions and the extent of aggregation seemed to be the main determinants of membrane adsorption. Rate constants for chlorin transfer across the membrane were found to correlate strongly with the pH of the surrounding medium, in particular, for CE and RG. In acidic solution, CE and RG transfer across the membrane was strongly accelerated, and in basic solution, all compounds were retained, mostly in the outer monolayer. In contrast, MACE flip-flop across the membrane remained very low even at pH 6. The protonation of ionizable groups is suggested to be a major determinant of chlorin transfer rates across the bilayer. pK(a) values of CE and RG were found to be between 6 and 8, and two of the carboxylate groups in MACE had pK(a) values below 6. For CE and RG, the kinetic profiles at acidic pH indicated that the initial fast membrane distribution was followed by secondary steps that are discussed in this article.

Effect of cholesterol and sugar on the penetration of glycodendrimeric phenylporphyrins into biomimetic models of retinoblastoma cells membranes

Photodynamic therapy (PDT) is considered one efficient treatment against retinoblastoma. The specificity of a photosensitizer and its penetration into cancerous cells are crucial for achieving tumor necrosis. The selection of photosensitizers such as porphyrin derivatives by tumor cells thus depends to a large extent on their ability to interact with the biological membrane. In this work, we have studied by surface pressure measurements and fluorescence spectroscopy the interaction between three newly synthesized dendrimeric phenylporphyrins and monolayers or liposomes with increasing cholesterol content mimicking the retinoblastoma cell membrane. The morphology of phospholipid-cholesterol-porphyrin mixed monolayers was also analyzed by Brewster angle microscopy. The results showed that the increase in cholesterol content in the model membranes had almost no effect on the effective penetration of the drugs into the lipid layers. Conversely, the chemical structure of the glycodendrimeric phenylporphyrins and the presence of sugar moieties especially appeared to play a crucial role. Although the non-glycoconjugated phenylporphyrin penetrated to a greater extent than glycodendrimeric ones into the liposome membrane, this could be achieved at a high lipid/porphyrin ratio only. Glycodendrimeric porphyrins exhibited improved surface properties compared to the non-glycoconjugated derivative and could penetrate into lipid layers even at low lipid/porphyrin ratios and high surface pressures. Our work highlights the role in the passive diffusion of porphyrins into biomimetic cancer cell membranes, of complex interactions among the lipid molecules, the sugar moieties, and the hydrophobic macrocycle of the porphyrins.

Inhibition of aggregation of [Pd]-bacteriochlorophyllides in mesoporous silica

Aggregation is a major factor affecting the photophysical properties of chlorophylls. For two [Pd]-bacteriochlorophyll derivatives that are currently under clinical testing as sensitizers for photodynamic therapy, aggregation control in aqueous solution has been studied with folded-sheet mesoporous silica (FSM) of different pore sizes (20, 45, 83 A) and with detergent (Triton X-100). With both the moderately polar WST09 and the highly polar WST11, no pigment oligomers were formed in FSM, and the monomer-dimer equilibrium was shifted toward the monomer with decreasing pore diameter.

Structural analysis of fish versus mammalian hemoglobins: effect of the heme pocket environment on autooxidation and hemin loss

The underlying stereochemical mechanisms for the dramatic differences in autooxidation and hemin loss rates of fish versus mammalian hemoglobins (Hb) have been examined by determining the crystal structures of perch, trout IV, and bovine Hb at high and low pH. The fish Hbs autooxidize and release hemin approximately 50- to 100-fold more rapidly than bovine Hb. Five specific amino acid replacements in the CD corner and along the E helix appear to cause the increased susceptibility of fish Hbs to oxidative degradation compared with mammalian Hbs. Ile is present at the E11 helical position in most fish Hb chains whereas a smaller Val residue is present in all mammalian alpha and beta chains. The larger IleE11 side chain sterically hinders bound O(2) and facilitates dissociation of the neutral superoxide radical, enhancing autooxidation. Lys(E10) is found in most mammalian Hb and forms favorable electrostatic and hydrogen bonding interactions with the heme-7-propionate. In contrast, Thr(E10) is present in most fish Hbs and is too short to stabilize bound heme, and causes increased rates of hemin dissociation. Especially high rates of hemin loss in perch Hb are also due to a lack of electrostatic interaction between His(CE3) and the heme-6 propionate in alpha subunits whereas this interaction does occur in trout IV and bovine Hb. There is also a larger gap for solvent entry into the heme crevice near beta CD3 in the perch Hb (approximately 8 A) compared with trout IV Hb (approximately 6 A) which in turn is significantly higher than that in bovine Hb (approximately 4 A) at low pH. The amino acids at CD4 and E14 differ between bovine and the fish Hbs and have the potential to modulate oxidative degradation by altering the orientation of the distal histidine and the stability of the E-helix. Generally rapid rates of lipid oxidation in fish muscle can be partly attributed to the fact that fish Hbs are highly susceptible to oxidative degradation.

Photosensitizing properties of chlorins in solution and in membrane-mimicking systems

The photosensitizing properties of three chlorins, meso-tetra(3-hydroxyphenyl)chlorin (m-THPC), chlorin e6 (Ce6) and meso-tetraphenylchlorin substituted by two adjacent sulfonated groups (TPCS(2a)) are compared in solution and when incorporated in dioleoyl-sn-phosphatidylcholine (DOPC) liposomes. In solution, the three chlorins possess a similar efficacy to generate singlet oxygen (quantum yield approximately 0.65). The formation of conjugated dienes was used to determine their ability to induce the peroxidation of methyl linoleate as a target of singlet oxygen. In ethanol solution, the apparent quantum yield for this process is the same for the three chlorins and its value agrees with that expected from the known rates for the decay of singlet oxygen and its reaction with methyl linoleate. When incorporated in liposomes, the order of efficacy is m-THPC > TPCS(2a) > Ce6. This order is tentatively assigned to the relative embedment of the photosensitizer within the lipidic bilayer, TPCS(2a) and Ce6 being anchored by their negative chains nearer to the water-lipid interface. The photoinduced permeation of the lipidic bilayer by these chlorins was investigated by measuring the release of carboxyfluorescein entrapped into DOPC liposomes. The charged chlorins, in particular TPCS(2a), are the most efficient, a result discussed in relation with the technology of photochemical internalization, PCI.

Time-dependent interactions of the two porphyrinic compounds chlorin e6 and mono-L-aspartyl-chlorin e6 with phospholipid vesicles probed by NMR spectroscopy

The distribution processes of chlorin e6 (CE) and monoaspartyl-chlorin e6 (MACE) between the outer and inner phospholipid monolayers of 1,2-dioleoyl-phosphatidylcholine (DOPC) vesicles were monitored by 1H NMR spectroscopy through analysis of chemical shifts and line widths of the DOPC vesicle resonances. Chlorin adsorption to the outer vesicle monolayer induced changes in the DOPC 1H NMR spectrum. Most pronounced was a split of the N-methyl choline resonance, allowing for separate analysis of inner and outer vesicle layers. Transbilayer distribution of the chlorin compounds was indicated by time-dependent characteristic spectral changes of the DOPC resonances. Kinetic parameters for the flip-flop processes, that is, half-lives and rate constants, were obtained from the experimental data points. In comparison to CE, MACE transbilayer movement was significantly reduced, with MACE remaining more or less attached to the outer membrane layer. The distribution coefficients for CE and MACE between the vesicular and aqueous phase were determined. Both CE and MACE exhibited a high affinity for the vesicular phase. For CE, a positive correlation was found between transfer rate and increasing molar ratio CE/DOPC. Enhanced membrane rigidity induced by increasing amounts of cholesterol into the model membrane was accompanied by a decrease of CE flip-flop rates across the membrane. The present study shows that the movement of porphyrins across membranes can efficiently be investigated by 1H NMR spectroscopy and that small changes in porphyrin structure can have large effects on membrane kinetics.

Cellular uptake and subcellular distribution of chlorin e6 as functions of pH and interactions with membranes and lipoproteins

The uptake and more importantly the subcellular distribution of photosensitizers are major determinants of their efficacy. In this paper, the cellular internalization of chlorin e6 (Ce6), a photosensitizer bearing three carboxylic chains, is considered with emphasize on pH effects. Small unilamellar vesicles are used as models to investigate the dynamics of interactions of Ce6 with membranes. The entrance and exit steps from the outer lipid hemileaflet are very fast (~ms). A slow transfer of Ce6 through the membrane was observed only for thin bilayers made of dimyristoleoyl-phosphatidylcholine. Ce6 did not permeate through bilayers consisting of longer phospholipids more representative of biological membranes. These results along with previous data on the interactions of Ce6 with low-density lipoproteins (LDL) are correlated with cellular studies. After 15 min incubation of HS68 human fibroblasts with Ce6, fluorescence microscopy revealed labeling of the plasma membrane and cytosolic vesicles different from lysosomes. When vectorized by LDL, Ce6 was mainly localized in lysosomes but absent from the plasma membrane. Internalization of LDL bound photosensitizer via ApoB/E receptor mediated pathway was demonstrated by overexpression experiments. A pH decrease from 7.4 to 6.9 did not affect the intracellular distribution of Ce6, but significantly increased its overall cellular uptake.

Structural and physico-chemical determinants of the interactions of macrocyclic photosensitizers with cells

New therapies have been developed using reactive oxygen species produced by light-activation of photosensitizers (PS). Since the lifetime of these species is extremely short and their diffusion in space is limited, the photo-induced reactions primarily affect the cell organelles labeled by the PS. In addition to the development of molecules with the best optical and photosensitizing properties, considerable research has been done to understand the physico-chemical parameters governing their subcellular localization. In this review, we examine these parameters to establish the structure/efficacy relationships, which allow specific targeting of PS. We examine the effect of subcellular localization on the cellular response to photosensitization processes. We discuss the determinants of subcellular localization, including the hydrophobic/hydrophilic balance, the specific charge effects and the dynamics of PS' transfer through membranes. Specific targeting can also be achieved with molecular structures able to recognize cellular or intracellular receptors, and this is also dealt with in this paper.

NMR of molecules interacting with lipids in small unilamellar vesicles

Detailed characterization of protein, peptide or drug interactions with natural membrane is still a challenge. This review focuses on the use of nuclear magnetic resonance (NMR) for the analysis of interaction of molecules with small unilamellar vesicles (SUV). These phospholipid vesicles are often used as model membranes for fluorescence or circular dichroism experiments. The various NMR approaches for studying molecule-lipid association are reviewed. After a brief survey of the SUV characterization, the use of heteronuclear NMR (phosphorous, carbon, fluorine) is described. Applications of proton NMR through transferred nuclear Overhauser effect to perform structural determination of peptide are presented. Special care is finally given to the influence of the kinetic of the interactions for the proton NMR of bound molecules in SUV, which can constitute a good model for the study of dynamical processes at the membrane surface.

Proton NMR detection of porphyrins and cytochrome C in small unilamellar vesicles: role of the dissociation kinetic constant

The molecular tumbling of small unilamellar vesicles is not fast enough to enable the detection of (1)H NMR signals of molecules associated with phospholipids. We show that relatively fast kinetic exchange of the interacting molecules is able to induce a strong decrease of the residual homonuclear dipolar coupling, allowing the acquisition of sharp signals. At low molecule/lipids molecular ratio, this can be lead to signal broadening due to exchange at intermediate rates on the NMR chemical timescale. However, proton resonances can be easily detected when sufficient lipids are added to prevent the occurrence of any free compounds in solution. This is demonstrated, using lipid signal suppression, in the case of paramagnetic porphyrin derivatives as well as diamagnetic hematoporphyrin. Since several peptides and proteins are expected to be associated with lipids having relatively fast dynamics, this study addresses, as a first example, the interaction of cytochrome c.

Dynamics of interactions of photosensitizers with lipoproteins and membrane-models: correlation with cellular incorporation and subcellular distribution

The incorporation and subcellular localization of photosensitizers are critical determinants of their efficiency. Here, we correlate these properties with the interactions of photosensitizers with membrane-models and low density lipoproteins (LDL) in acellular systems. Focus was given on dynamics aspects. Two amphiphilic photosensitizers, deuteroporphyrin (DP) and aluminum phthalocyanine sulfonated on two adjacent isoindole units (AlPcS2a) were selected. The phthalocyanine was bound to LDL with an overall association constant around 5 x 10(7)M(-1). Biphasic association kinetics was indicative of two types of sites. The release of the phthalocyanine into the bulk aqueous medium occurred within less than a second. A similar behavior was found previously for deuteroporphyrin although its affinity was somewhat higher (5.5 x 10(8)M(-1)). Both compounds were previously characterized by high affinity for membrane-models and quick exchange with the bulk solution. However, they strongly differed by their rate of transfer through the lipid bilayer, in the range of seconds for the porphyrin, several hours for the phthalocyanine. In the case of the porphyrin, fluorescence microscopy on human fibroblasts showed diffuse labeling with no significant modification of the distribution upon vectorization by LDL. In contrast, the phthalocyanine was localized in intracellular vesicles. Vectorization by LDL favored lysosomal localization although little effect was found on the overall uptake as shown by extraction experiments. The role of lipoproteins in the cellular localization of photosensitizers is significantly more important for photosensitizers not freely diffusing through bilayers. The dynamics of the interactions of photosensitizers with membranes appears as an important determinant of their subcellular localization.

Dynamics of pH-dependent self-association and membrane binding of a dicarboxylic porphyrin: a study with small unilamellar vesicles

Steady-state and stopped-flow measurements of the absorbance and fluorescence of aqueous solutions were performed to characterize the pH-dependent ionization and aggregation states of deuteroporphyrin. Porphyrin self-association promoted by neutralization of the carboxylic groups takes place within a few milliseconds impeding characterization of the monomer ionization states. Extrapolation at infinite dilution of the values obtained from steady-state measurements yielded the pKs of the carboxylic groups (6.6, 5.3) and inner nitrogens (4.1, 2.3). The kinetics of interactions of the porphyrin with unilamellar fluid state dioleoylphosphatidylcholine vesicles was examined in a large pH range, with focus on the entry step. From alkaline pH to a value of 6.5, the entrance rate is maximal (1.69 x 10(6) M(-1) s(-1) versus phospholipid concentration). It decreases to 2.07 x 10(5) M(-1) s(-1) at lower pH with an apparent pK of 5.39. This effect appears to be related to the formation of porphyrin dimer rather than to the protonation of inner nitrogen. In keeping with previous data, these results support the concept of a pH-mediated selectivity of carboxylic porphyrins for tumor. They also indicate that the propensity of these molecules to self-associate at low pH could yield to some retention in acidic intracellular vesicles of the endosome/lysosome compartment.

Equilibrium and kinetic studies of the interactions of a porphyrin with low-density lipoproteins

Low-density lipoproteins (LDL) play a key role in the delivery of photosensitizers to tumor cells in photodynamic therapy. The interaction of deuteroporphyrin, an amphiphilic porphyrin, with LDL is examined at equilibrium and the kinetics of association/dissociation are determined by stopped-flow. Changes in apoprotein and porphyrin fluorescence suggest two classes of bound porphyrins. The first class, characterized by tryptophan fluorescence quenching, involves four well-defined sites. The affinity constant per site is 8.75 x 10(7) M(-1) (cumulative affinity 3.5 x 10(8) M(-1)). The second class corresponds to the incorporation of up to 50 molecules into the outer lipidic layer of LDL with an affinity constant of 2 x 10(8) M(-1). Stopped-flow experiments involving direct LDL porphyrin mixing or porphyrin transfer from preloaded LDL to albumin provide kinetic characterization of the two classes. The rate constants for dissociation of the first and second classes are 5.8 and 15 s(-1); the association rate constants are 5 x 10(8) M(-1) s(-1) per site and 3 x 10(9) M(-1) s(-1), respectively. Both fluorescence and kinetic analysis indicate that the first class involves regions at the boundary between lipids and the apoprotein. The kinetics of porphyrin-LDL interactions indicates that changes in the distribution of photosensitizers among various carriers could be very sensitive to the specific tumor microenvironment.

Interaction of dicarboxylic metalloporphyrins with liposomes. The effect of pH on membrane binding revisited

The acid-base properties of Zn-hematoporphyrin IX (ZnHP) and Zn-mesoporphyrin IX (ZnMP) and the effect of pH on their binding to liposomes have been studied. The ionization constants for the two carboxylate groups of ZnHP were calculated by principal component analysis and are 5.7 +/- 0.1 and 6.9 +/- 0.05. The neutral species and the mono- and dianionic forms all bind to liposomes, but a strong pH effect on the binding constant was observed for both the investigated compounds. We also observed a decrease in the binding of the two anionic species when the membranes carried a negative charge. These results indicate that the porphyrins partition into the membrane with their carboxylic moieties near the lipid-water interface so that their deprotonation, leading to a charged molecule, does not prevent the insertion of the tetrapyrrole ring into the lipid environment of neutral liposomes.

Emerging strategies in microbial haem capture

Gram-negative pathogenic bacteria have evolved novel strategies to obtain iron from host haem-sequestering proteins. These include the production of specific outer membrane receptors that bind directly to host haem-sequestering proteins, secreted haem-binding proteins (haemophores) that bind haem/haemoglobin/haemopexin and deliver the complex to a bacterial cell surface receptor and bacterial proteases that degrade haem-sequestering proteins. Once removed from haem-sequestering proteins, haem may be transported via the bacterial outer membrane receptor into the cell. Recent studies have begun to define the steps by which haem is removed from bacterial haem proteins and transported into the cell. This review describes recent work on the discovery and characterization of these systems. Reference is also made to the transport of haem in serum (via haemoglobin, haemoglobin/haptoglobin, haemopexin, albumin and lipoproteins) and to mechanisms of iron removal from the haem itself (probably via a haem oxygenase pathway in which the protoporphyrin ring is degraded). Haem protein-receptor interactions are discussed in terms of the criteria that govern protein-protein interactions in general, and connections between haem transport and the emerging field of metal transport via metallochaperones are outlined.

Kinetic and equilibrium studies of incorporation of di-sulfonated aluminum phthalocyanine into unilamellar vesicles

The interactions of cis-di-sulfonated aluminum phthalocyanine (PcS(2)Al) with dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles have been investigated by fluorescence spectroscopy. At pH 7.0, PcS(2)Al incorporates into the vesicles with a high affinity constant (2.7x10(6) M(-1), in terms of phospholipid concentration). The fluorescence changes following rapid mixing of PcS(2)Al with vesicles are biphasic. The first phase is attributed to the entry of PcS(2)Al into the vesicles, as deduced from the linear dependence of the rate upon lipid concentration. More surprisingly, this rate is strongly pH dependent with a marked maximum around pH 7.3, a result interpreted in terms of the coordination state of the aluminum ion in aqueous solutions. At this pH, a hydroxide ion neutralizes the residual positive charge of the metal ion that remains unbalanced after coordination by the phthalocyanine cycle. A water molecule is likely to complete the metal coordination sphere. Only this form, PcAl(+)(OH(-))(OH(2)), with an uncharged core is quickly incorporated into the vesicles. The protonation of OH(-) or the deprotonation of the coordinated H(2)O leading to a positively or negatively charged core, respectively, account for the observed pH effect. Studies on the effect of cholesterol addition and exchange of PcS(2)Al between vesicles and albumin all indicate the absence of transfer of the phthalocyanine between the vesicle hemileaflets, a result expected from the presence of the two negatively charged sulfonated groups at the ring periphery. Instead, the slower kinetic phase is likely due to the movement of the phthalocyanine becoming more buried within the outer leaflet upon the loss of the water molecule coordinated to the aluminum ion.