Binding of cytochrome b5 to cholesterol-containing phosphatidylcholine vesicles

Cytochrome b₅ was found to bind readily to sonicated vesicles containing as much as 0.8 mol cholesterol per mol egg phosphatidylcholine. This observation conflicts with the suggestion of Enomoto and Sato ((1977) Biochim. Biophys. Acta 466, 136–147) that cholesterol prevents binding of this protein to erythrocyte membranes.     

Modification of phospholipid structure results in greater stability of liposomes in serum

Prevous studies have revealed that the replacement of the C-2 ester group in phosphatidylcholine by the carbamyloxy function renders the resulting lipids, without affecting the properties of the liposomes, resistant to hydrolysis by phospholipase A₂ (Gupta, C.M. and Bali, A. (1981) Biochim. Biophys. Acta 663, 506–515). As an extension of this work, the effect of serum on the stability of liposomes, prepared from $\text{1-}\text{palmitoyl-2-heptadec}\text{-10-cis-}\text{enylcarbamyloxyphosphatidylcholine}$ (carbamylphosphatidylcholine), has been examined. The stability has been measured in terms of (a) bilayer permeability to solutes, and (b) the lipid transfer to serum proteins, Replacement of egg phosphatidylcholine in liposomes by the carbamyl analog prevented serum-induced leakage of the entrapped solutes and also inhibited the lipid (phospholipid and cholesterol) transfer. Manipulation of the cholesterol content of the liposomes had no effect on the stability. These observations indicate that the interaction of serum proteins with liposomes probably involves a highly specific binding of the proteins to the liposome surface.     

Asymmetric binding of cytochrome b5 to the membrane of human erythrocyte ghosts

The intact, amphipatic form of cytochrome $\text{b}{}_5$ could bind to unsealed ghosts, but not to resealed ghosts, suggesting that the cytochrome could bind only to the inner (cytoplasmic) surface of the ghost membrane. This was further confirmed by the finding that the cytochrome could bind to closed, inside-out vesicles prepared from the ghosts. This asymmetric binding was not due to the exclusive localization of sialic acid and sugar chains on the outer surface of the ghosts membrane, because the cytochrome could not bind to ghosts even after enzymatic removal of these components. Although liposomes consisting of phosphatidylcholine or both phosphatidylcholine and sphingomyelin could effectively bind the cytochrome, this binding capacity was progressively decreased as increasing amount of cholesterol was included in the composition of phosphatidylcholine liposomes. Removal of cholesterol from resealed ghosts by incubation with egg phosphatidylcholine liposomes resulted in the binding of cytochrome $\text{b}{}_5$ to the outer surface of the treated ghosts. The possibility is discussed that the asymmetric binding is due to preferential localization of cholesterol in the outer leaflet of the lipid bilayer that constitutes the ghost membrane.     

Localization of the active center of microsomal cytochrome P-450

To solve the problem of localization of the active center of cytochrome P-450 in microsomal membranes, new bifunctional compounds (I-IV), which contain pyridine radical, aliphatic chain of variable length and diphosphonic acid ("floating" molecules) have been applied. These compounds inhibit oxidation and binding of the substrates of cytochrome P-450 (aminopyrine and aniline), inhibition being of a competitive character. Measurements of distribution coefficients between water and membranes of microsomes and liposomes from egg phosphatidylcholine evidence that the microsomal proteins are necessary for providing effective interaction of I-IV with microsomal membrane. The 1H-NMR method has demonstrated compounds to be incorporated into lipid bilayer so that the non-polar part is in the inner membrane volume. The results obtained confirm our previous conclusion (Krainev A.G., Weiner L.M., Alferyev I.S., Slynko N.M. (1985) Biochim. Biophys. Acta, 818, 96-104) about localization of the active center of microsomal cytochrome P-450 at the depth of approximately 18 A from the hydrophilic surface of a membrane.     

The involvement of the lipid phase transition in the plasma-induced dissolution of multilamellar phosphatidylcholine vesicles

Unsonicated liposomes prepared from dimyristoyl phosphatidylcholine were nearly completely dissolved during a 3 h incubation with rat plasma at or close to the phase-transition temperature of 24°C. At 37 or 15°C virtually no liposomal disintegration was observed even after 24 h of incubation. The liposomal solubilization, which was monitored by turbidity measurements or by determination of phospholipid sedimentability, was accompanied by the formation of a phospholipid-protein complex similar or identical to the one we previously reported to be formed from sonicated liposomes of egg phosphatidylcholine (Scherphof, G., Roerdink, F., Waite, M. and Parks, J. (1978) Biochim. Biophys. Acta 542, 296–307). Unsonicated multilamellar liposomes made of egg phosphatidylcholine were completely resistant to the dissolving potency of plasma when incubated at 37°C. Liposomes from equimolar mixtures of dimyristoyl and dipalmitoyl phosphatidylcholine were only degraded by plasma in the temperature range between 30 and 35°C at which temperature this cocrystallizing phospholipid mixture undergoes a phase transition. However, even at these temperatures the rate of dissolution of this mixture was significantly lower than of dimyristoyl phosphatidylcholine at 24°C. In the dissolving process of this mixture a slight preference for the lower-melting component was observed.The ability of cholesterol to completely abolish the susceptibility of dimyristoyl phosphatidylcholine liposomes to plasma at a 1:2 molar ratio of cholesterol to phospholipid substantiates the essential role of the phase transition in the process of liposome solubilization.When liposomes of the monotectic mixtures dimyristoyl and distearoyl phosphatidylcholine or dilauroyl and distearoyl phosphatidylcholine were incubated with plasma at temperatures in between those at which the constituent lipids undergo a phase change in the mixture, the liposomes were slowly disolved. Under those conditions a selective removal of the lipids in the liquid-crystalline phase was observed.It is concluded that for the plasma-induced dissolution of unsonicated liposomes, which is most probably achieved by interaction with (apo)lipoproteins, the presence of phase boundaries is required in much the same way as was first reported for phospholipases by Op den Kamp, J.A.F., de Gier, J. and Van Deenen, L.L.M. (1974) Biochim. Biophys. Acta 345, 253–256).     

Light-induced oxygen reduction as a probe of electron transport between respiratory and photosynthetic components in membranes of Rhodopseudomonas capsulata

Membrane vesicles prepared from photosynthetically grown cells of Rhodopseudomonas capsulata strain M7 are able to perform light-induced oxygen uptake. In contrast, chromatophores from mutant strain M6, lacking the cytochrome b₂₆₀-containing pathway of oxygen reduction, are completely devoid of oxygen uptake induced by light. Therefore, cytochrome b₂₆₀ (cyt b₂₆₀), previously demonstrated to be the alternative oxidase in photosynthetic and aerobic membranes of R. capsulata (La Monica, R. F., and Marrs, B. L. (1976) Biochim. Biophys. Acta449, 431–439; Zannoni, D., Melandri, B. A., and Baccarini-Melandri, A. (1976) Biochim. Biophys. Acta449, 386–400), also appears to be involved in light-induced oxygen uptake. Light-driven oxygen reduction activity is inhibited by high concentrations of cyanide (5 × 10^?3m) and by carbon monoxide in accord with the sensitivity of cyt b₂₆₀ to these inhibitors reported in aerobic membranes (Zannoni, D., Melandri, B. A., and Baccarini-Melandri, A. (1976) Biochim. Biophys. Acta423, 413–430). Further evidence for a direct involvement of the cyt b₂₆₀-containing pathway in light-induced oxygen uptake has been obtained by comparing membranes from semiaerobically grown cells to those from photosynthetic cells of R. capsulata M7. The substrate-dependent dark oxidase activity associated with the cyt b₂₆₀-containing pathway is 4.6 times higher in semiaerobic than in photosynthetic membranes, and a parallel enhancement of light-induced oxygen uptake is observed. The data presented are in agreement with and extend previous results on the composition and function of the respiratory and photosynthetic apparatus, supporting an association of cytochrome b₂₆₀ with both the aerobic and photosynthetic systems present in membranes of R. capsulata. These findings clearly demonstrate that respiratory electron carriers have access to electrons flowing from the photosynthetic reaction center, i.e., the two systems are “electrically connected” in membrane fragments from this organism.     

Action of phospholipases A2 on phosphatidylcholine bilayers. Effects of the phase transition, bilayer curvature and structural defects

We examined the action of porcine pancreatic and bee-venom phospholipase A2 towards bilayers of phosphatidylcholine as a function of several physical characteristics of the lipid-water interface. 1. Unsonicated liposomes of dimyristoyl phosphatidylcholine are degraded by both phospholipases in the temperature region of the phase transition only (cf. Op den Kamp et al. (1974) Biochim. Biophys. Acta 345, 253--256 and Op den Kamp et al. (1975) Biochim. Biophys. Acta 406, 169--177). With sonicates the temperature range in which hydrolysis occurs is much wider. This discrepancy between liposomes and sonicates cannot be ascribed entirely to differences in available substrate surface. 2. Below the phase-transition temperature the phospholipases degrade dimyristoyl phosphatidylcholine single-bilayer vesicles with a strongly curved surface much more effectively than larger single-bilayer vesicles with a relatively low degree of curvature. 3. Vesicles composed of egg phosphatidylcholine can be degraded by pancreatic phospholipase A2 at 37 degrees C, provided that the substrate bilayer is strongly curved. The bee-venom enzyme shows a similar, but less pronounced, preference for small substrate vesicles. 4. In a limited temperature region just above the transition temperature of the substrate the action of both phospholipases initially proceeds with a gradually increasing velocity. This stimulation is presumably due to an increase of the transition temperature, effectuated by the products of the phospholipase action. 5. Structural defects in the substrate bilayer, introduced by sonication below the phase-transition temperature (cf. Lawaczeck et al. (1976) Biochim. Biophys. Acta 443, 313--330) facilitate the action of both phospholipases. The results lead to the general conclusion that structural irregularities in the packing of the substrate molecules facilitate the action of phospholipases A2 on phosphatidylcholine bilayers. Within the phase transition and with bilayers containing structural defects these irregularities represent boundaries between separate lipid domains. The stimulatory effect of strong bilayer curvature can be ascribed to an overall perturbation of the lipid packing as well as to a change in the phase-transition temperature.     

Mode of binding of cytochrome b5 to phospholipid bilayers in lamellar structure

X-ray diffraction studies were made on the multilamellar systems produced by incubation of phospholipid bilayers and the membrane protein, cytochrome b₅, or non-membrane proteins (albumin, ovalbumin and β-lactoglobulin A) at pH 8.1 in aqueous 5 mM CaCl₂ solutions.Detergent-extracted cytochrome b₅ (soluble aggregate) forms two types of lamellar phase with dipalmitoyl phosphatidylcholine bilayers, depending upon the incubation temperature. One type, which has a repeat distance of 114Å, is formed above 34°C, where the binding of cytochrome b₅ to the bilayers is hydrophobic. The other type, with a repeat distance of 153 Å, is formed below 34°C, where the binding is electrostatic. It is also suggested that cytochrome b₅ is monomeric in the former phase but remains aggregated in the latter phase.When dimyristoyl phosphatidylcholine is used, the boundary temperature for the two types shifts to 12°C. These boundary temperatures coincide with the thermal pretransition points of hydrated dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine, respectively.Trypsin-treated cytochrome b₅ (monomeric) and the three non-membrane proteins exhibit only binding of the electrostatic type to the bilayers, independently of the incubation temperature. The observed repeat distances suggest that in these cases two layers of protein molecules are incorporated between the bilayers.     

Glycolipids are not extracted from phospholipid bilayers by binding to ferritin-lectin conjugates

A radioactively-labelled glycosphingolipid, asialo-G_M1, has been incorporated into phosphatidylcholine multilamellar vesicles. After incubation with ferritin-Ricinus communis agglutinin 60 (RCA 60) conjugate at different temperatures, the vesicles were separated from the conjugate by discontinuous density gradient ultracentrifugation. Measurement of the distribution of the radioactively-labelled asialo-G_M1 in the pelleted conjugate fraction and freeze-etch electron microscopy of the vesicle fraction indicate that the decrease in labelling of vesicles by ferritin-RCA 60 conjugate with increasing temperatures (Tillack, T.W., Wong, M., Allietta, M. and Thompson, T.E. (1982) Biochim. Biophys. Acta 691, 261–273) reflects a decrease in apparent binding affinity rather than an ability of the conjugate to extract glycolipid from the phospholipid bilayer after binding.     

A cytolysin, theta-toxin, preferentially binds to membrane cholesterol surrounded by phospholipids with 18-carbon hydrocarbon chains in cholesterol-rich region.

We have previously suggested the existence of two distinctive states of cholesterol in erythrocyte and lymphoma cell membranes as revealed by high- and low-affinity binding sites for theta-toxin of Clostridium perfringens [Ohno-Iwashita, Y., Iwamoto, M., Mitsui, K., Ando, S., & Nagai, Y. (1988) Eur. J. Biochem. 176, 95-101; Ohno-Iwashita, Y., Iwamoto, M., Ando, S., Mitsui, K., & Iwashita, S. (1990) Biochim. Biophys. Acta 1023, 441-448]. To understand factor(s) which determine membrane cholesterol heterogeneity, we analyzed toxin binding to large unilamellar liposomes composed of cholesterol and phospholipids (phosphatidylcholine/phosphatidylglycerol = 82:18, mol/mol). Liposomes containing phospholipids with 18-carbon hydrocarbon chains at both positions 1 and 2 of the glycerol have both high- and low-affinity toxin-binding sites with Kd values similar to those of intact erythrocytes, whereas liposomes with hydrocarbon chains containing 16 or fewer carbons at either position 1 or 2 have only low-affinity toxin-binding sites. The cholesterol/phospholipid ratio, in addition to the length of phospholipid hydrocarbon chain, also determines the number of toxin-binding sites, indicating that at least these two factors determine the topology of membrane cholesterol by creating distinctively different affinity sites for the toxin. Since theta-toxin binding detects specific populations of membrane cholesterol that are not detectable by the measurements of susceptibility to cholesterol oxidase and cholesterol desorption from membranes, the toxin could provide a unique probe for studying the organization of cholesterol in membranes.     

A spectral shift in cytochrome a induced by calcium ions

Ca²⁺ induces a red shift in the absorption spectrum of ferrocytochrome a when added to uncoupled mitochondria, sub-mitochondrial particles or isolated cytochrome aa₃. The shift is identical within experimental error to the previously reported energy-linked shift in intact mitochondria (Wikström, M. K. F. (1972), Biochim. Biophys. Acta 283, 385–390). One mol of calcium produces the shift in one mol of cytochrome a, the K_D being approx. 20–30 μM. The calcium-induced shift is readily reversed by chelating agents such as EDTA, ethyleneglycol-bis-(μ-aminoethyl ether)N,N′-tetraacetic acid (EGTA) and ATP and is insensitive to uncoupling agents and inhibitors of calcium transport (La³⁺ and ruthenium red). It is shown that the binding site for calcium that is responsible for the spectral shift is located on the outside of the permeability barrier of the mitochondrial cristae membrane.It is proposed that calcium simulates the energy-linked shift in cytochrome a by binding to a site of cytochrome aa₃ that is occupied by protons in energized mitochondria and that is located at the external surface of the mitochondrial membrane.     

The structure of melittin in lipid bilayer membranes

0.15 M inorganic phosphate dramatically increased the α-helix content of melittin in aqueous solution.When melittin interacted with egg yolk phosphatidylcholine liposomes in the absence of inorganic phosphate, it was converted to an α-helix rich form, as postulated by Dawson et al. (Dawson, C.R., Drake, A.F. Helliwell, J. and Hider, R.C. (1978) Biochim. Biophys. Acta 510, 75–86).     

The structure of melittin in lipid bilayer membranes.

0.15 M inorganic phosphate dramatically increased the alpha-helix content of melittin in aqueous solution. When melittin interacted with egg yolk phosphatidylcholine liposomes in the absence of inorganic phosphate, it was converted to an alpha-helix rich form, as postulated by Dawson et al. (Dawson, C.R., Drake, A.F. Helliwell, J. and Hider, R.C. (1978) Biochim. Biophys. Acta 510, 75--86).     

ESR spectroscopy demonstrates that cytochrome b559 remains low potential in Ca2+-reactivated,salt-washed PSII particles

Cytochrome b₅₅₉ in various Photosystem II preparations was studled by using low temperature ESR spectroscopy. This technique was used because it is able to distinguish high from low potential forms of the cytochrome owing to the g-value differences between these species. Moreover, by using low temperature irradiation to oxidize cyt b₅₅₉ we have avoided the use of redox mediators. Previous work (Ghanotakis DF., Topper J.N. and Yocum, C.F. (1984) Biochim. Biophys. Acta 767, 524–531) demonstrated that reduction and extraction of manganese of the oxygen evolving complex, which might be expected to alter the redox properties of cyt b₅₅₉, occurs when certain PSII preparations are exposed to reductants. The ESR data presented here show that a mixture of high potential and lower potential cyt b₅₅₉ species is observed in the oxygen evolving Photosystem II complex. Treatment of PSII membranes with 0.8 M Tris converts the high potential form(s) to those of lower potential. Exposure of the membranes to 2M NaCl shifts a significant amount of high potential cyt b₅₅₉ to lower potential form(s); addition of CaCl₂ reconstituted oxygen evolution activity but did not restore cyt b₅₅₉ to its high potential form(s).Abbreviations Chl chlorophyll - cyt cytochrome - DCBQ 2,5-dichloro-benzoquinone - DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone - ESR electron spin resonance - OEC oxygen evolving complex - PS photosystem Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement     

The binding of cytochrome b5 to phosphatidylcholine vesicle

Cytochrome b₅ was isolated from rabbit liver by a detergent procedure and by a proteolytic procedure. Only cytochrome b₅ isolated by the detergent procedure would bind to phosphatidylcholine vesicles and the cytochrome b₅ was not removed by 1 M KCl. The E_o′ and visible absorption spectrum of the cytochrome b₅ and its rate of reduction by NADH plus NADH-cytochrome b₅ reductase did not change appreciably upon binding. These data indicate that cytochrome b₅ is bound to phospholipid by a hydrophobic interaction which leaves the heme portion in the aqueous environment.     

Cholesterol affects spectrin-phospholipid interactions in a manner different from changes resulting from alterations in membrane fluidity due to fatty acyl chain composition

We previously showed that erythrocyte and brain spectrins bind phospholipid vesicles and monolayers prepared from phosphatidylethanolamine and phosphatidylserine and their mixtures with phosphatidylcholine (Review: A.F. Sikorski, B. Hanus-Lorenz, A. Jezierski, A. R. Dluzewski, Interaction of membrane skeletal proteins with membrane lipid domain, Acta Biochim. Polon. 47 (2000) 565). Here, we show how changes in the fluidity of the phospholipid monolayer affect spectrin-phospholipid interaction. The presence of up to 10%-20% cholesterol in the PE/PC monolayer facilitates the penetration of the monolayer by both types of spectrin. For monolayers constructed from mixtures of PI/PC and cholesterol, the effect of spectrins was characterised by the presence of two maxima (at 5 and 30% cholesterol) of surface pressure for erythroid spectrin, and a single maximum (at 20% cholesterol) for brain spectrin. The binding assay results indicated a small but easily detectable decrease in the affinity of erythrocyte spectrin for FAT-liposomes prepared from a PE/PC mixture containing cholesterol, and a 2- to 5-fold increase in maximal binding capacity (B_max) depending on the cholesterol content. On the other hand, the results from experiments with a monolayer constructed from homogenous synthetic phospholipids indicated an increase in Δπ change with the increase in the fatty acyl chain length of the phospholipids used to prepare the monolayer. This was confirmed by the results of a pelleting experiment. Adding spectrins into the subphase of raft-like monolayers constructed from DOPC, SM and cholesterol (1/1/1) induced an increase in surface pressure. The Δπ change values were, however, much smaller than those observed in the case of a natural PE/PC (6/4) monolayer. An increased binding capacity for spectrins of liposomes prepared from a “raft-like” mixture of lipids could also be concluded from the pelleting assay. In conclusion, we suggest that the effect of membrane lipid fluidity on spectrin-phospholipid interactions is not simple but depends on how it is regulated, i.e., by cholesterol content or by the chemical structure of the membrane lipids.     

Studies on the mechanism of electron transport in the bc1-segment of the respiratory chain in yeast. II. The binding of antimycin to mitochondrial particles and the function of two different binding sites

1. In mitochondrial particles antimycin binds to two separate specific sites with dissociation constants $\text{K}{}_{\mathrm{<mtext>d</mtext><mtext>1</mtext>}}\text{≦ 4 · 10}{}^{\mathrm{?13}}\text{M}$ and $\text{K}{}_{\mathrm{<mtext>d</mtext><mtext>2</mtext>}}\text{= 3 · 10}{}^{\mathrm{?9}}\text{M}$, respectively.2. The concentrations of the two antimycin binding sites are about equal. The absolute concentration for each binding site is about 100 – 150 pmol per mg of mitochondrial protein.3. Antimycin bound to the stronger site mainly inhibits NADH- and succinate oxidase. Binding of antimycin to the weaker binding site inhibits the electron flux to exogenously added cytochrome c after blocking cytochrome oxidase by KCN.4. Under certain conditions cytochrome b and c₁ are dispensible components for antimycin-sensitive electron transport.5. A model of the respiratory chain in yeast is proposed which accounts for the results reported here and previously. (Lang, B., Burger, G. and Bandlow, W. (1974) Biochim. Biophys. Acta 368, 71–85).     

Involvement of cytochrome b5 and a cyanide-sensitive monooxygenase in the 4-demethylation of 4,4-dimethyl-zymosterol by yeast microsomes

According to Ohba et al. (Ohba M., Sato R., Yoshida Y., Nishino T. and Katsuki H. (1978) Biochem. Biophys. Res. Commun. 85, 21–27), yeast microsomes catalyze the removal of three methyl groups attached to the C-4 and C-14 positions of [1,7,15,22,26,30-¹⁴C]lanosterol (4,4,14α-trimethyl-5α-cholesta-8, 24-dien-3β-ol) in the presence of NADPH, NAD⁺ and molecular oxygen, concomitant with the liberation of ¹⁴CO₂ derived from C-30 (one of the two methyl groups at the C-4 position). In this process the methyl group at the C-14 position is first removed in a cyanide-insensitive reaction and then the two methyl groups at the C-4 position are removed by a cyanide-sensitive enzyme system. In this study it was found that the ¹⁴CO₂ formation from the ¹⁴C-labeled lanosterol was inhibited by antibodies to yeast cytochrome b₅ and by palmitoyl-CoA, a substrate of the cytochrome b₅-containing fatty acyl-CoA desaturase system of yeast microsomes. However, neither the antibodies nor palmitoyl-CoA inhibited the conversion of lanosterol to 4,4-dimethyl zymosterol (4,4-dimethyl-5α-cholesta-8,24-dien-3β-ol). It is concluded that cytochrome b₅ and a cyanide-sensitive enzyme are involved in the 4-demethylation of 4,4-dimethylzymosterol, but not the 14α-demethylation of lanosterol, by yeast microsomes. It is suggested that a cyanide-sensitive enzyme acts as the terminal 4-demethylase and cytochrome b₅ transfers reducing equivalents from NADPH to the terminal enzyme, as in the case of fatty acyl-CoA desaturation. The cyanide sensitivity of the 4-demethylation was, however, much greater than that of the desaturation.