Decreased lipid order induced by microsomal cytochrome P-450 and NADPH-cytochrome P-450 reductase in model membranes: fluorescence and electron spin resonance studies

Membrane topology of cytochrome P450 2B4 in Langmuir-Blodgett monolayers

Using Langmuir-Blodgett monolayers of both phosphatidylethanolamines and phosphatidylcholines as membrane mimics, we have examined the topology of cytochrome P450 2B4 anchoring. The interaction of wild-type P450 2B4 with phosphatidylethanolamine monolayers can be characterized as a biphasic reaction, with the initial fast phase explained by the specific insertion of membrane-spanning segments of the protein into the monolayer. Injection of cytochrome b5 (b5) beneath dipalmitoyl-phosphatidylcholine monolayers also resulted in biphasic kinetics. Regardless of the nature of the lipid employed, neither a truncated cytochrome P450 2B4 (P450 2B4 Delta2-27) lacking the amino-terminal hydrophobic residues widely believed to be the major transmembrane segment nor a soluble b5 fragment (Deltab5) lacking its carboxy terminus anchor exhibit the fast-phase behavior characteristic of specific insertion. To further characterize the membrane topology of P450 2B4, its insertion area in DPPE monolayers was measured and analyzed with use of the Gibbs equation for adsorption at an interface. The mean molecular insertion area derived from isotherms of P450 2B4 in a DPPE monolayer at a pressure of 19 mN/m, 680 +/- 95 A2 is large enough to accommodate two to four transmembrane helices. The large insertion area and the fact that the truncated cytochrome retains as much as 30% of its membrane localization when expressed in Escherichia coli (Pernecky, S. J., Larson, J. R., Philpot, R. M., and Coon, M. J. (1993) Proc. Natl. Acad. Sci. USA 90, 2651-2655) suggest that this cytochrome is not deeply embedded but that other regions, in addition to the amino-terminal 26 residues, may be involved in the interaction of cytochrome P450 with the membrane.

Characterization of the secondary structure and membrane interaction of the putative membrane anchor domains of prostaglandin I2 synthase and cytochrome P450 2C1

Prostaglandin I2 synthase (PGIS) produces prostaglandin I2 (PGI2) which has opposite actions on platelet aggregatory and vasoconstrictive properties compared to thromboxane A2 (TXA2) produced from the same substrate by another P450 enzyme, thromboxane A2 synthase (TXAS). PGIS and TXAS have only 16% amino acid sequence identity. Hydropathy analysis suggests that the putative NH2-terminal membrane anchor domain of PGIS is similar to many other membrane-bound microsomal P450s, which are believed to be anchored by a single transmembrane segment, and thus different from the TXAS anchor, which appears to have two transmembrane segments. To characterize the membrane anchor function of the PGIS NH2-terminal region, we have used the peptidoliposome reconstitution assay to identify the membrane anchor segment in the PGIS NH2-terminal domain and compared it with the anchor segment of P450 2C1. Four peptides, mimicking putative NH2-terminal membrane anchor segments of PGIS and P450 2C1, containing residues 1-28 (PGIS-LP1 and P450 2C1-LP1) or residues 25-54 (PGIS-LP2 and P450 2C1-LP2), were synthesized and their ability to insert in a lipid bilayer was evaluated. The results indicated that both LP1 peptides of PGIS and P450 2C1 became bound to the lipid bilayer, whereas both LP2 peptides did not bind the lipid. The two LP1 peptides were further characterized as to their conformation using CD spectroscopy. Helical structure induced in these peptides by addition of trifluoroethanol, dodecylphosphocholine, or incorporation into liposomes indicated that these segments tend to adopt a helical structure in a hydrophobic environment and thus could function as membrane anchor segments. These results support the hypothesis that PGIS and TXAS interact with the endoplasmic reticulum membrane in different ways, in which the NH2-terminal anchor domain of PGIS, as with P450 2C1, appears to have a single transmembrane segment.

Peroxidation and phospholipase A2 hydrolytic susceptibility of liposomes consisting of mixed species of phosphatidylcholine and phosphatidylethanolamine

The relationship between lipid peroxidation and phospholipase A2 (PLA2) hydrolytic activity was studied using unilamellar vesicles (liposomes) as model membranes. Hydrolytic specificity was examined using vesicles prepared with pure bovine heart phosphatidylcholine (PC), bovine heart phosphatidylethanolamine (PE), or mixtures of these phospholipids, using two preparative procedures, i.e., sonication or extrusion. Lipid peroxidation was induced by incubating vesicles with cumene hydroperoxide and hematin at 37 degrees C. Determinations of the extent of peroxidation by means of diene conjugate content derived from second derivative spectra or by polarographic measurement of oxygen consumption rates provided a basis for comparing the extent of peroxidation of each phospholipid species to their subsequent hydrolysis by PLA2 (from Crotalus adamanteus). The extent of hydrolysis was determined through the release of arachidonic acid from either PC or PE. The PE distribution among the outer vs. inner leaflet of the membrane bilayer was nearly equal in sonicated vesicles, whereas most of the phospholipid was incorporated into the inner leaflet in extruded vesicles. The proportion of PE found in the inner leaflet progressively increased as the ratio of PE to PC increased in both sonicated and extruded vesicle preparations. Lipid peroxidation had no effect on PE distribution under the conditions examined. There was a clear preference for PC peroxidation for all vesicle compositions tested and PC was preferentially hydrolyzed by PLA2. This effect is proposed to result from a perturbation of membrane structure following peroxidation with assimilation of PC into PLA2-susceptible domains whereas PE peroxidation and hydrolysis is less affected in mixed PC/PE vesicles. Lipid peroxidation imposes an additional hydrolytic susceptibility over the effects exerted through the mixing of these phospholipids which is based on structural changes rather than formation of specific substrates for PLA2.

A fluorescence study of the interactions of benzo[a]pyrene, cytochrome P450c and NADPH-cytochrome P450 reductase

Fluorescence quenching of benzo[a]pyrene (BP) by cytochrome P450c was used to probe this substrate-enzyme binding interaction. Addition of NADPH-cytochrome P450 reductase, an essential electron carrier during P450 catalysis, resulted in increased quenching and thus strengthened binding of BP to P450c. This shows that the role of reductase extends beyond that of an electron transfer agent to influence substrate binding. Fluorescence titration measurements revealed that reductase and P450c formed a complex with an apparent KD of 13.7 +/- 0.9 nM. Reductase had no effect in the presence of an anti-P450c monoclonal antibody which inhibits BP hydroxylation, which suggests that this monoclonal antibody binds P450c near its reductase binding region.

Chemical modification of rat liver microsomal cytochrome P-450: study of enzymic properties and membrane topology

Isolated rat liver cytochrome P-450IIB1 was alkylated and acetylated at primary amino groups, and the position of the modified amino acids in the protein was identified. Alkylation of up to nine amino groups did not disturb the interaction of reconstituted P-450 and NADPH-cytochrome-P-450 reductase in a way that hydroxylation of benzphetamine was altered, whereas deethylation of 7-ethoxycoumarin was gradually reduced in parallel with impaired 7-ethoxycoumarin binding. Acetylation of four lysine residues completely inhibited binding and metabolism of 7-ethoxycoumarin but not of benzphetamine. These results suggest the presence of different substrate binding sites on P-450. Exhaustive proteolysis of modified P-450 in proteoliposomes liberated all but the N-terminal modified peptide and 85 to 90% of the cytochrome's mass from intact proteoliposomes. These findings further support our previously proposed model of P-450 topology (Vergères, G., Winterhalter, K.H. and Richter, C. (1989) Biochemistry 28, 3650-3655), in which P-450 is anchored to the membrane with the N-terminal peptide only, the N-terminal methionine facing the lumenal interior.

Selectivity of lipid-protein interaction with myelin proteolipids PLP and DM-20. A fluorescence anisotropy study

The two main myelin proteolipids, PLP (30 kDa) and DM-20 (25 kDa), differ by an internal deletion in DM-20. The deleted fragment, of 35 amino acids (116-150), corresponds to the major hydrophilic domain of PLP. Fluorescence anisotropy experiments using diphenylhexatriene as a fluorescent probe were performed to detect the phase separation induced by these two proteolipids in multilamellar vesicles of binary composition. We found that in vesicles composed of 30% L-alpha-PS and 70% DPPC, the PLP boundary layer contained about 18 motionally restricted phospholipids, almost exclusively L-alpha-PS. On the contrary, the DM-20 boundary layer contained only 14 to 15 phospholipids, with a composition no different from that of the bulk vesicle. In mixtures of DMPG and DPPC, the selectivity of PLP for the acidic phospholipid DMPG was maintained, but was lower than that observed for L-alpha-PS. We assume that this selectivity of PLP stems mainly from electrostatic interactions between the charged residues of the 116-150 fragment, deleted in DM-20, and the acidic phospholipids. These results suggest that fragment 116-150 may play a specific role in the interaction of PLP with the lipid bilayer of the myelin membrane.

Biophysical consequences of lipid peroxidation in membranes

This article reviews the biophysical consequences of lipid peroxidation in biological membranes. In the lipid domain, lipid peroxidation (a) causes an increase in the order and "viscosity" of the membrane bilayer, particularly at the depth around acyl-carbon 12, (b) changes the thermotropic phase behaviour, (c) decreases the electrical resistance, and (d) facilitates phospholipid exchange between the two monolayers. Upon lipid peroxidation membrane proteins are crosslinked, and their rotational and lateral mobility is decreased. Studies with microsomal cytochrome P-450 suggest protein aggregation but not the increased lipid order to be the major cause of protein immobilization in peroxidized membranes.