Obligatory role of cytochrome b5 in the microsomal metabolism of methoxyflurane

Modulation of CYP2C9 activity and hydrogen peroxide production by cytochrome b5

Cytochromes P450 (CYP) play a major role in drug detoxification, and cytochrome b₅ (cyt b5) stimulates the catalytic cycle of mono-oxygenation and detoxification reactions. Collateral reactions of this catalytic cycle can lead to a significant production of toxic reactive oxygen species (ROS). One of the most abundant CYP isoforms in the human liver is CYP2C9, which catalyzes the metabolic degradation of several drugs including nonsteroidal anti-inflammatory drugs. We studied modulation by microsomal membrane-bound and soluble cyt b5 of the hydroxylation of salicylic acid to gentisic acid and ROS release by CYP2C9 activity in human liver microsomes (HLMs) and by CYP2C9 baculosomes. CYP2C9 accounts for nearly 75% of salicylic acid hydroxylation in HLMs at concentrations reached after usual aspirin doses. The anti-cyt b5 antibody SC9513 largely inhibits the rate of salicylic acid hydroxylation by CYP2C9 in HLMs and CYP2C9 baculosomes, increasing the K_M approximately threefold. Besides, soluble human recombinant cyt b5 stimulates the Vmax nearly twofold while it decreases nearly threefold the Km value in CYP2C9 baculosomes. Regarding NADPH-dependent ROS production, soluble recombinant cyt b5 is a potent inhibitor both in HLMs and in CYP2C9 baculosomes, with inhibition constants of 1.04 ± 0.25 and 0.53 ± 0.06 µM cyt b5, respectively. This study indicates that variability in cyt b5 might be a major factor underlying interindividual variability in the metabolism of CYP2C9 substrates.

Mechanisms of Cytochrome P450-Catalyzed Oxidations

Enzymes are complex biological catalysts and are critical to life. Most oxidations of chemicals are catalyzed by cytochrome P450 (P450, CYP) enzymes, which generally utilize mixed-function oxidase stoichiometry, utilizing pyridine nucleotides as electron donors: NAD(P)H + O₂ + R → NAD(P)⁺ + RO + H₂O (where R is a carbon substrate and RO is an oxidized product). The catalysis of oxidations is largely understood in the context of the heme iron-oxygen complex generally referred to as Compound I, formally FeO³⁺, whose basis was in peroxidase chemistry. Many X-ray crystal structures of P450s are now available (≥ 822 structures from ≥146 different P450s) and have helped in understanding catalytic specificity. In addition to hydroxylations, P450s catalyze more complex oxidations, including C-C bond formation and cleavage. Enzymes derived from P450s by directed evolution can even catalyze more unusual reactions, e.g. cyclopropanation. Current P450 questions under investigation include the potential role of the intermediate Compound 0 (formally Fe^III-O₂ ^-) in catalysis of some reactions, the roles of high- and low-spin forms of Compound I, the mechanism of desaturation, the roles of open and closed structures of P450s in catalysis, the extent of processivity in multi-step oxidations, and the role of the accessory protein cytochrome b ₅. More global questions include exactly how structure drives function, prediction of catalysis, and roles of multiple protein conformations.

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Impaired 17,20-Lyase Activity in Male Mice Lacking Cytochrome b5 in Leydig Cells

Androgen and estrogen biosynthesis in mammals requires the 17,20-lyase activity of cytochrome P450 17A1 (steroid 17-hydroxylase/17,20-lyase). Maximal 17,20-lyase activity in vitro requires the presence of cytochrome b₅ (b5), and rare cases of b5 deficiency in human beings causes isolated 17,20-lyase deficiency. To study the consequences of conditional b5 removal from testicular Leydig cells in an animal model, we generated Cyb5^flox/flox:Sf1-Cre (LeyKO) mice. The LeyKO male mice had normal body weights, testis and sex organ weights, and fertility compared with littermates. Basal serum and urine steroid profiles of LeyKO males were not significantly different than littermates. In contrast, marked 17-hydroxyprogesterone accumulation (100-fold basal) and reduced testosterone synthesis (27% of littermates) were observed after human chorionic gonadotropin stimulation in LeyKO animals. Testis homogenates from LeyKO mice showed reduced 17,20-lyase activity and a 3-fold increased 17-hydroxylase to 17,20-lyase activity ratio, which were restored to normal upon addition of recombinant b5. We conclude that Leydig cell b5 is required for maximal androgen synthesis and to prevent 17-hydroxyprogesterone accumulation in the mouse testis; however, the b5-independent 17,20-lyase activity of mouse steroid 17-hydroxylase/17,20-lyase is sufficient for normal male genital development and fertility. LeyKO male mice are a good model for the biochemistry but not the physiology of isolated 17,20-lyase deficiency in human beings.

Effects of membrane mimetics on cytochrome P450-cytochrome b5 interactions characterized by NMR spectroscopy

Mammalian cytochrome P450 (P450) is a membrane-bound monooxygenase whose catalytic activities require two electrons to be sequentially delivered from its redox partners: cytochrome b5 (cytb5) and cytochrome P450 reductase, both of which are membrane proteins. Although P450 functional activities are known to be affected by lipids, experimental evidence to reveal the effect of membrane on P450-cytb5 interactions is still lacking. Here, we present evidence for the influence of phospholipid bilayers on complex formation between rabbit P450 2B4 (CYP2B4) and rabbit cytb5 at the atomic level, utilizing NMR techniques. General line broadening and modest chemical shift perturbations of cytb5 resonances characterize CYP2B4-cytb5 interactions on the intermediate time scale. More significant intensity attenuation and a more specific protein-protein binding interface are observed in bicelles as compared with lipid-free solution, highlighting the importance of the lipid bilayer in stabilizing stronger and more specific interactions between CYP2B4 and cytb5, which may lead to a more efficient electron transfer. Similar results observed for the interactions between CYP2B4 lacking the transmembrane domain (tr-CYP2B4) and cytb5 imply interactions between tr-CYP2B4 and the membrane surface, which might assist in CYP2B4-cytb5 complex formation by orienting tr-CYP2B4 for efficient contact with cytb5. Furthermore, the observation of weak and nonspecific interactions between CYP2B4 and cytb5 in micelles suggests that lipid bilayer structures and low curvature membrane surface are preferable for CYP2B4-cytb5 complex formation. Results presented in this study provide structural insights into the mechanism behind the important role that the lipid bilayer plays in the interactions between P450s and their redox partners.

A model of the membrane-bound cytochrome b5-cytochrome P450 complex from NMR and mutagenesis data

Microsomal cytochrome b5 (cytb5) is a membrane-bound protein that modulates the catalytic activity of its redox partner, cytochrome P4502B4 (cytP450). Here, we report the first structure of full-length rabbit ferric microsomal cytb5 (16 kDa), incorporated in two different membrane mimetics (detergent micelles and lipid bicelles). Differential line broadening of the cytb5 NMR resonances and site-directed mutagenesis data were used to characterize the cytb5 interaction epitope recognized by ferric microsomal cytP450 (56 kDa). Subsequently, a data-driven docking algorithm, HADDOCK (high ambiguity driven biomolecular docking), was used to generate the structure of the complex between cytP4502B4 and cytb5 using experimentally derived restraints from NMR, mutagenesis, and the double mutant cycle data obtained on the full-length proteins. Our docking and experimental results point to the formation of a dynamic electron transfer complex between the acidic convex surface of cytb5 and the concave basic proximal surface of cytP4502B4. The majority of the binding energy for the complex is provided by interactions between residues on the C-helix and β-bulge of cytP450 and residues at the end of helix α4 of cytb5. The structure of the complex allows us to propose an interprotein electron transfer pathway involving the highly conserved Arg-125 on cytP450 serving as a salt bridge between the heme propionates of cytP450 and cytb5. We have also shown that the addition of a substrate to cytP450 likely strengthens the cytb5-cytP450 interaction. This study paves the way to obtaining valuable structural, functional, and dynamic information on membrane-bound complexes.

The interaction of microsomal cytochrome P450 2B4 with its redox partners, cytochrome P450 reductase and cytochrome b(5)

Cytochrome P450 2B4 is a microsomal protein with a multi-step reaction cycle similar to that observed in the majority of other cytochromes P450. The cytochrome P450 2B4-substrate complex is reduced from the ferric to the ferrous form by cytochrome P450 reductase. After binding oxygen, the oxyferrous protein accepts a second electron which is provided by either cytochrome P450 reductase or cytochrome b(5). In both instances, product formation occurs. When the second electron is donated by cytochrome b(5), catalysis (product formation) is ∼10- to 100-fold faster than in the presence of cytochrome P450 reductase. This allows less time for side product formation (hydrogen peroxide and superoxide) and improves by ∼15% the coupling of NADPH consumption to product formation. Cytochrome b(5) has also been shown to compete with cytochrome P450 reductase for a binding site on the proximal surface of cytochrome P450 2B4. These two different effects of cytochrome b(5) on cytochrome P450 2B4 reactivity can explain how cytochrome b(5) is able to stimulate, inhibit, or have no effect on cytochrome P450 2B4 activity. At low molar ratios (<1) of cytochrome b(5) to cytochrome P450 reductase, the more rapid catalysis results in enhanced substrate metabolism. In contrast, at high molar ratios (>1) of cytochrome b(5) to cytochrome P450 reductase, cytochrome b(5) inhibits activity by binding to the proximal surface of cytochrome P450 and preventing the reductase from reducing ferric cytochrome P450 to the ferrous protein, thereby aborting the catalytic reaction cycle. When the stimulatory and inhibitory effects of cytochrome b(5) are equal, it will appear to have no effect on the enzymatic activity. It is hypothesized that cytochrome b(5) stimulates catalysis by causing a conformational change in the active site, which allows the active oxidizing oxyferryl species of cytochrome P450 to be formed more rapidly than in the presence of reductase.

The many roles of cytochrome b5

Four distinct suggestions have been made to explain the mechanism of the cytochrome b(5)-imposed positive modifier action of the cytochrome P450 monooxygenase reaction. The first mechanism involves a direct input of an electron into the monooxygenase cycle. This is the second of the two electrons necessary for activation of molecular oxygen, and appears to be a rate-limiting step in the monooxygenase reaction. P450 monooxygenases all appear to be uncoupled to varying extents, releasing superoxide and hydrogen peroxide instead of oxidized substrate. A second mechanism suggests that cytochrome b(5) acts as a positive modifier of the monooxygenase by decreasing the extent of uncoupling of the monooxygenase reaction. The implication is that a slow input of the second electron allows uncoupling of a superoxide anion instead of formation of two-electron reduced oxygen. Faster input of the second electron via cytochrome b(5) would result in formation of more of the activated oxygen that reacts with substrate to form product. A third suggestion involves formation of a two-hemoprotein complex between cytochrome b(5) and cytochrome P450 that allows acceptance of two electrons from NADPH-cytochrome P450 reductase. Uncomplexed cytochrome P450 accepts an electron from the reductase, dissociates from it, binds oxygen, and re-associates with the reductase to accept another electron. Complexation with cytochrome b(5) enhances the rate of formation of the active oxygen by obviating the need for two interactions with reductase. The fourth mechanism has cytochrome b(5) serving as an effector without a reduction-oxidation role in the monooxygenation reaction. This effector function may be to enhance the breakdown of the oxygenated hemoprotein to products or to facilitate flow of electrons through the system.

Role of cytochrome b5 in the oxidative metabolism of polychlorinated biphenyls catalyzed by cytochrome P450

1. The role of cytochrome b5 in the cytochrome P450-dependent hydroxylation of tetrachlorobiphenyl (TCB) isomers was examined using a reconstituted system consisting of CYP2B1 and CYP1A1 and rat liver microsomes. 2. By addition of cytochrome b5 to the reconstituted system containing CYP2B1, the 3-hydroxylation of 2,5,2,'5'- and 2,5,3',4'-TCB was increased about six-fold, but the 3- and 5-hydroxylation of 2,4,3',4'-TCB was decreased by about 50% 3. All hydroxylations of 3 ,4,3',4'-,2,5, 3,4'- and 2,4,3',4'-TCBs were decreased by addition of cytochrome b5 to the reconstituted system containing CYPlA1. 4. In stoichiometry measurements, changes in NADPH oxidation and coupling efficiency by addition of cytochrome b5 was observed and these differed according to the position of chlorine atoms of TCBs and cytochrome P450 isoforms used in the systems.

Involvement of cytochrome b5 in the metabolism of tetrachlorobiphenyls catalyzed by CYP2B1 and CYP1A1

The role of cytochrome b5 in the cytochrome P450 (CYP)-dependent hydroxylation of tetrachlorobiphenyl (TCB) isomers was examined using a reconstituted mixed function oxygenase (MFO) system containing purified CYP2B1 or 1A1, and rat liver microsomes. Hydroxylations of 2,2',5,5'- and 3,3',4,4'-TCBs were catalyzed mainly by CYP2B1 and 1A1, respectively, in the reconstituted MFO system and those of 2,3',4',5- and 2,3',4,4'-TCBs were mediated by both cytochrome P450 systems. The activity toward 2,2',5,5'- and 2,3',4',5-TCB was significantly increased 6.5- and 5.5-fold, respectively, by addition of cytochrome b5 in the reconstituted MFO system containing of CYP2B1. Either hydroxylation activity toward 2,3',4,4'-TCB with the CYP2B1 system was very low or decreased by addition of cytochrome b5. These results suggest that the involvement of cytochrome b5 to the hydroxylation of TCBs is dependent on the TCB congener being metabolized, and the cytochrome P450 isoform involved in its metabolism.

The stoichiometry of the cytochrome P-450-catalyzed metabolism of methoxyflurane and benzphetamine in the presence and absence of cytochrome b5

The complete stoichiometry of the metabolism of the cytochrome b5 (cyt b5)-requiring substrate, methoxyflurane, by purified cytochrome P-450 2B4 was compared to that of another substrate, benzphetamine, which does not require cyt b5 for its metabolism. Cyt b5 invariably improved the efficiency of product formation. That is, in the presence of cyt b5 a greater percentage of the reducing equivalents from NADPH were utilized to generate substrate metabolites, primarily at the expense of the side product, superoxide. With methoxyflurane, cyt b5 addition always resulted in an increased rate of product formation, while with benzphetamine the rate of product formation remained unchanged, increased or decreased. The apparently contradictory observations of increased reaction efficiency but decrease in total product formation for benzphetamine can be explained by a second effect of cyt b5. Under some experimental conditions cyt b5 inhibits total NADPH consumption. Whether stimulation, inhibition, or no change in product formation is observed in the presence of cyt b5 depends on the net effect of the stimulatory and inhibitory effects of cyt b5. When total NADPH consumption is inhibited by cyt b5, the rapidly metabolized, highly coupled (approximately equal to 50%) substrate, benzphetamine, undergoes a net decrease in metabolism not counterbalanced by the increase in the efficiency (2-20%) of the reaction. In contrast, in the presence of the slowly metabolized, poorly coupled (approximately equal to 0.5-3%) substrate, methoxyflurane, inhibition of total NADPH consumption by cyt b5 was never sufficient to overcome the stimulation of product formation due to an increase in efficiency of the reaction.

Isoflurane and cytochrome b5 stimulation of 2-chloro-1,1-difluoroethene metabolism by reconstituted rat CYP2B1 and CYP2C6

Isoflurane stimulates the metabolism of 2-chloro-1,1-difluoroethene (CDE) in liver microsomes from phenobarbital-treated rats or rabbits. The P450 isozymes involved and the mechanism by which such stimulation occurs have not been clarified. The present study examined the effects of isoflurane and cytochrome b5 on CDE metabolism in reconstituted systems containing purified rat CYP2B1 or CYP2C6. Under similar incubation conditions, CYP2B1 defluorinated CDE at approximately five times the rate of CYP2C6. Isoflurane was a potent stimulator of CDE metabolism, increasing it nearly 5-fold when catalyzed by CYP2B1, but only 2-fold when catalyzed by CYP2C6. Isoflurane had no stimulatory effect on benzphetamine metabolism by CYP2B1 or CYP2C6. Cytochrome b5 was not required for isoflurane-facilitated CDE metabolism; however, the addition of cytochrome b5 to CYP2B1 increased CDE metabolism 71 and 44%, in the absence and presence of isoflurane, respectively. In reconstituted CYP2B1, isoflurane generated a type I difference spectrum of approximately twice the magnitude of CDE and stimulated NADPH consumption more so than CDE. The same quantity of NADPH was consumed when CDE was present with isoflurane as compared with isoflurane alone. These data support the hypothesis that isoflurane stimulates CDE metabolism by a mechanism involving increased P450 reduction via direct isoflurane interaction with P450.

The carboxyl terminus of the membrane-binding domain of cytochrome b5 spans the bilayer of the endoplasmic reticulum

Preliminary studies (Vergères, G., and Waskell, L. (1992) J. Biol. Chem. 267, 12583-12591) have suggested that the carboxyl-terminal membrane-binding domain of cytochrome b5 traverses the membrane and that the carboxyl terminus is in the lumen of the endoplasmic reticulum. In order to confirm and extend these studies, additional experiments were conducted. The gene coding for rat cytochrome b5 was transcribed and the resulting mRNA was translated in vitro in a rabbit reticulocyte lysate in the presence of microsomes. The binding and topology of cytochrome b5 were investigated by treating microsomes containing the newly incorporated cytochrome b5 with carboxypeptidase Y and trypsin. Our studies indicate that cytochrome b5 is inserted both co- and post-translationally into microsomes in a topology in which the membrane-binding domain spans the bilayer with its COOH terminus in the lumen. Cytochrome b5 is also incorporated into microsomes pretreated with trypsin in a topology indistinguishable from the one resulting from the insertion of the protein into untreated microsomes, reconfirming that cytochrome b5 does not use the signal recognition particle-dependent translocation machinery. Our results do not allow a distinction to be made between a spontaneous insertion mode or some other trypsin-resistant receptor-mediated mechanism. A role for Pro115 in the middle of the membrane-binding domain of cytochrome b5 was also examined by mutating it to an alanine and subsequently characterizing the ability of the mutant protein to be incorporated into membranes. The mutant protein inserted more slowly in vitro into microsomes as well as into pure lipid bilayers by a factor of 2 to 3.

Cytochrome b5, its functions, structure and membrane topology

The first part of the present communication reviews recent advances in our understanding of the known physiological functions of cytochrome b5. In addition, one section is devoted to a description of a recently discovered function of cytochrome b5, namely its involvement in the synthesis of the oncofetal antigen N-glycolylneuraminic acid. The second part of the article summarizes site-directed mutagenesis studies, primarily conducted in the author's laboratory, in both the catalytic heme-binding and membrane-binding domain of cytochrome b5. These studies have shown that: 1) the membrane binding domain of cytochrome b5 spans the bilayer; 2) cytochrome b5 lacking 19 COOH-terminal amino acids does not bind to membrane bilayers; and 3) specific amino acids in the membrane binding domain have been mutated and shown not to be essential for the function of cytochrome b5 with its redox partners.

Changes in hepatic xenobiotic-metabolising enzymes in mouse liver following infection with Leishmania donovani

Infection of mice with Leishmania donovani resulted in decreased activities of several liver enzymes involved in the metabolism of xenobiotics. Microsomal membranes from infected livers contained reduced amounts of cytochromes P450 and b5 and NADPH-cytochrome P450 reductase. Several cytochrome P450 isoenzymes (P450-PB1, P450-PB3, P450-PCN and P450-UT1) and P450-mediated reactions (aminopyrine demethylase, aniline hydroxylase, benzphentamine demethylase and ethoxycoumarin deethylase) were affected similarly. The metabolism of two carcinogens (nitrosodimethylamine and 7,12-dimethylbenz[a]anthracene) by liver microsomal membrane preparations was also reduced. Leishmania infection caused an increase of cytosolic epoxide hydrolase and microsomal epoxide hydrolase and NADH-cytochrome b5 reductase were unaffected. The results suggest that Leishmania-infected animals are likely to have altered responses to exogenous toxins compared to uninfected animals.

Human colon tumor cell line LS174T drug metabolizing system

The effects of culture variables on the specific content and activity of various enzymes of the drug metabolizing system were assessed in colon tumor cell line LS174T. The NADH reduced cytochrome b5 (cyt b5)4 spectrum of these cells was similar to rat liver cyt b5. When released from the membrane by trypsin and concentrated, the cyt b5 was found to cross react with rabbit antibody to rat liver cyt b5 and human liver cyt b5. The enzyme activities were found stable over limited cell passages with control values of 0.03 and 0.13 mumol/min/mg protein for NADPH and NADH cytochrome c (cyt c) reducing activity, 0.05 nmol cyt b5 and 0.013 nmol cytochrome P450 per milligram of microsomal protein. Phenobarbital/hydrocortisone showed a consistent, but not always significant increase in the NADPH and NADH cyt c reduction and benzanthracene an increase in the NADH cyt c reducing activity and cyt b5 content. Griseofulvin lowered the NADH cyt c reducing activity. Delta-aminolevulinic acid (0.5 mM) caused a significant decrease in the specific activity of all enzymes, as judged by a student's t test, with a p less than 0.001.

Methoxyflurane acts at the substrate binding site of cytochrome P450 LM2 to induce a dependence on cytochrome b5

Rabbit cytochrome P450 isozyme 2 requires cytochrome b5 to metabolize the volatile anesthetic methoxyflurane but not the substrate benzphetamine [E. Canova-Davis and L. Waskell (1984) J. Biol. Chem. 259, 2541-2546]. To determine whether the requirement for cytochrome b5 for methoxyflurane oxidation is mediated by an allosteric effect on cytochrome P450 LM2 or cytochrome P450 reductase, we have investigated whether this anesthetic can induce a role for cytochrome b5 in benzphetamine metabolism. Using rabbit liver microsomes and antibodies raised in guinea pigs against rabbit cytochrome b5, we found that methoxyflurane did not create a cytochrome b5 requirement for benzphetamine metabolism. Methoxyflurane also failed to induce a role for cytochrome b5 in benzphetamine metabolism in the purified, reconstituted mixed function oxidase system. Studies of the reaction kinetics established that in the absence of cytochrome b5, methoxyflurane and benzphetamine are competitive inhibitors, and that in the presence of cytochrome b5, benzphetamine and methoxyflurane are two alternate substrates in competition for a single site on the same enzyme. These results all indicate that the methoxyflurane-induced cytochrome b5 dependence of the mixed function oxidase cytochrome P450 LM2 system is a direct result of the interaction between methoxyflurane and the substrate binding site of cytochrome P450 LM2 and suggest the focus of future studies of this question.

Chemical modification of cytochrome b5, cytochrome c and myoglobin with diethylpyrocarbonate

Cytochrome b5 is required for the cytochrome P-450 LM2 catalyzed oxidation of the anesthetic methoxyflurane. The ability of cytochrome b5 to support methoxyfluorane oxidation is affected by treatment with diethylpyrocarbonate, a reagent that at neutral pH is relatively specific for histidine residues. This inactivation of cytochrome b5 is reversed with hydroxylamine, which also suggests but does not prove histidine involvement. The studies reported in this paper were undertaken to determine whether histidine modification was involved in the decrease in effectiveness of cytochrome b5, or whether the inactivation could be attributed to modification of another amino acid. Our experiments demonstrate that diethylpyrocarbonate inactivates detergent-solubilized cytochrome b5 by modifying the axial histidines and displacing the heme. Because of the unexpected ease with which diethylpyrocarbonate displaced the heme from cytochrome b5, this same process was investigated in two other hemoproteins, cytochrome c and myoglobin. Diethylpyrocarbonate could not dissociate the heme from cytochrome c, whereas the heme was lost from myoglobin even more readily than from cytochrome b5.

Modification of trypsin-solubilized cytochrome b5, apocytochrome b5, and liposome-bound cytochrome b5 by diethylpyrocarbonate

The interactions of diethylpyrocarbonate (DEP) with the various forms of cytochrome b5 were studied to gain a better understanding of the factors that influence the extent of modification of the axial histidines of cytochrome b5. Very low concentrations of DEP were able to decrease the heme binding capacity of apocytochrome b5. Moreover, it was shown that two additional histidines, presumed to be the axial ligands (His 39 and 63), were modified in the apo but not the holo form of a given preparation of cytochrome b5. Trypsin-solubilized bovine cytochrome b5 was resistant to the effects of DEP. A 200-fold molar excess of DEP displaced only 15% of the heme in the trypsin-solubilized protein in contrast to an 84% displacement of the heme in the detergent-solubilized protein. However, detergent-solubilized cytochrome b5 which had been incorporated into phospholipid vesicles exhibited the same reactivity with DEP as did the trypsin-solubilized protein. This is attributed to the fact that the two resistant preparations of cytochrome b5 are monomeric in their respective environments while detergent-solubilized cytochrome b5 is known to exist as an octamer in aqueous solutions. Our studies suggest that dissociation of the octamer to the monomer results in a conformational change that decreases the reactivity of the axial ligands of the hydrophilic heme-containing domain of cytochrome b5. Examination of the cytochrome b5 molecule by computer graphics indicates that a tunnel leads from the surface of the molecule to axial histidine 63 and that axial histidine 39 is buried.

Purification and characterization of the dog hepatic cytochrome P-450 isozyme responsible for the metabolism of 2,2',4,4',5,5'-hexachlorobiphenyl

The biochemical basis for the marked difference in the rate of the hepatic metabolism of 2,2',4,4',5,5'-hexachlorobiphenyl (245-HCB) by Beagle dogs and Sprague-Dawley rats has been investigated. Control dog liver microsomes metabolize this substrate 15 times faster than control rat liver microsomes. Upon treatment with phenobarbital (PB), at least two cytochrome P-450 isozymes are induced in the dog, and the hepatic microsomal metabolism of 245-HCB is increased on both a per nanomole P-450 basis (twofold) and a per milligram protein basis (fivefold). One of the PB-induced isozymes, PBD-2, has been purified to a specific content of 17-19 nmol/mg protein and to less than 95% homogeneity, as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In a reconstituted system containing cytochrome b5, this isozyme shows an activity toward 245-HCB which is greater than threefold that seen in intact liver microsomes from PB-induced dogs. A reconstituted system containing the major isozyme induced by PB in the rat (PB-B) metabolizes 245-HCB at 1/10 the rate observed with purified PBD-2. Antibody inhibition studies have shown that PBD-2 accounts for greater than 90% of the hepatic microsomal metabolism of 245-HCB in control and PB-induced dogs, while PB-B only accounts for about half of the metabolism of this compound by microsomes obtained from PB-treated rats. Immunoblot analysis has revealed that the level of PBD-2 in dog liver microsomes increases nearly sixfold with PB treatment, and this increase correlates well with the fivefold increase in the rate of hepatic microsomal metabolism of 245-HCB by dogs. Together these data support a primary role for isozyme PBD-2 in the hepatic metabolism of 245-HCB in control and PB-induced dogs. In addition, these results suggest that, in contrast to rats, dogs can readily metabolize 245-HCB as a result of the presence of a cytochrome P-450 isozyme with efficient 245-HCB metabolizing activity.