On the mechanism of action of cytochrome P-450. Oxidation and reduction of the ferrous dioxygen complex of liver microsomal cytochrome P-450 by cytochrome b5

The effects of cytochrome b5 on the decay of the ferrous dioxygen complexes of P-450LM2 and P-450LM4 from rabbit liver microsomes were studied by stopped-flow spectrophotometry. The P-450 (FeIIO2) complexes accept an electron from reduced cytochrome b5 and, in a reaction not previously described, donate an electron to oxidized cytochrome b5 to give ferric P-450. A comparison with the electron-transferring properties of ferrous P-450 under anaerobic conditions allowed determination of the limiting steps of the two reactions involving the oxygenated complex. The rate of decay of the dioxygen complex was increased in all cases with b5 present; however, with oxidized b5 a large increase in the rate was observed with P-450 isozyme 4 but not with isozyme 2, whereas the opposite situation was found when reduced b5 was used. The reactions between b5 and ferrous dioxygen P-450 were not at thermodynamic equilibrium under the conditions employed. From the results obtained, a model is proposed in which the ferrous dioxygen complex decomposes rapidly into another species differing from ferric P-450 in its spectral properties and from the starting complex in its electron-transferring properties. A scheme is presented to indicate how competition among spontaneous decay, cytochrome b5 oxidation, and cytochrome b5 reduction by the ferrous O2 complex may influence substrate hydroxylation.     

Thermodynamic studies of the protein-protein interactions between cytochrome P-450 and cytochrome b5. Evidence for a central role of the cytochrome P-450 spin state in the coupling of substrate and cytochrome b5 binding to the terminal hemoprotein

The interactions between purified rat hepatic microsomal cytochrome P-450 and the type I ligands benzphetamine and cytochrome b5 have been studied in the presence of phospholipid using difference spectrophotometry. Cytochrome b5 was shown to interact with cytochrome P-450 to form a tight 1:1 complex (Kd = 275 nM), in which the proportion of high spin cytochrome P-450 was increased from 7 to 30%. The presence of saturating cytochrome b5 was shown to cause a decrease in the apparent Kd for benzphetamine binding from 111 microM to 40 microM. Likewise, the presence of benzphetamine was shown to cause a decrease in the apparent dissociation constant for cytochrome b5 binding to cytochrome P-450 (Kd = 90 nM). The above interactions were resolved into the basic equilibria inter-relating the various ligation states of the hemoprotein in an energetically closed eight-state free energy coupling model and the relative magnitudes of the microequilibria were analyzed to determine the degree of coupling of the interactions between cytochrome P-450 and both benzphetamine and cytochrome b5. Consequently, the spin state changes in cytochrome P-450 induced by benzphetamine and cytochrome b5 binding were shown to arise because these ligands interact 7 and 4 times more tightly with high spin cytochrome P-450, respectively. Furthermore, the data revealed that these ligands interact at independent sites on cytochrome P-450. Thus the effects of cytochrome b5 upon benzphetamine binding and vice versa were rationalized simply in terms of an increase in the proportion of a high spin (high affinity) conformation of cytochrome P-450 brought about by pre-equilibration with the effector ligand, with the intrinsic binding affinities of the two ligands for the low or high spin states remaining relatively unaltered. The thermodynamic parameters associated with the interactions between cytochrome P-450 and cytochrome b5, determined from the temperature dependence of these interactions, revealed that these protein interactions are entropy driven and probably occur by a hydrophobic mechanism.     

Feminization of the hepatic microsomal cytochrome P-450 system in brook trout by estradiol, testosterone, and pituitary factors

The effects of estradiol, testosterone, and pituitary extract on hepatic microsomal enzymes were studied in sham-operated and gonadectomized immature brook trout. Estradiol reduced the specific content of cytochromes P-450 and b5 by 70% or more in both groups. Testosterone and pituitary extract also decreased the levels of total P-450 and b5, but to a lesser extent. These latter effects were not evident when the contents of P-450 and b5 were normalized per g liver. Immunoblot analysis with antibodies to P-450 forms from a teleost (scup) showed the presence of cross-reacting proteins in control fish, presumed counterparts to the scup forms. Levels of a trout counterpart to P-450A (a putative testosterone 6 beta-hydroxylase) were strongly suppressed in estradiol-treated fish. A trout P-450B counterpart was suppressed in estradiol-treated fish, but less strongly than was the P-450A counterpart. The trout orthologue of hydrocarbon-inducible P-450E (P-450IA1), the aryl hydrocarbon hydroxylase (AHH) catalyst, was undetectable in any group, consistent with very low levels of AHH activity in these fish. Estradiol or pituitary extract also decreased the levels of NADH-cytochrome b5 and NADPH-cytochrome P-450 reductase activities in sham-operated but not in total P-450 or b5, suggesting an influence of gonads in maintaining normal levels of the reductases. The results support a prominent suppressive role for estrogens in producing the general sex differences in microsomal enzymes in fish liver and indicate that there could be affects on several P-450 forms. However, the mechanism of hormone action in this regulation is uncertain. The results also suggest the existence of unidentified, possibly gonadal, factors which contribute to the regulation of reductases in teleost liver.     

The role of the hydrophobic fragment of cytochrome b5 in the interaction with cytochrome P-450

The interaction of highly purified liver microsomal cytochrome P-450 from phenobarbital-induced rabbits and cytochrome b5 has been investigated by the difference and second derivative difference spectroscopy. The addition of cytochrome b5 to cytochrome P-450 results in transition of cytochrome P-450 heme iron from low to high spin state. The interaction is accompanied by the changes in the second derivative spectrum of cytochrome P-450, which point to the participation of tryptophanyl residues in this process. The hydrophilic fragment of cytochrome b5 is unable to form a complex with cytochrome P-450 as judged by the absence of the difference spectrum and any changes in the second derivative UV-spectrum of cytochrome P-450. The evidence obtained indicates that the hydrophobic tail of the cytochrome b5 molecule responsible for its binding to membrane is also indispensable for forming a functional cytochrome P-450-cytochrome b5 complex.     

Highly homologous cytochromes P-450 and b5: a model to study protein-protein interactions in a reconstituted monooxygenase system.

Cytochrome b5 from mouse and rat liver formed a type I spectral complex with two murine cytochrome P-450 isozymes, the P450Coh and P450PBI. Mouse b5 stimulated the reactions catalyzed by reconstituted P450Coh and an equimolar amount of b5 to P450Coh was needed for maximal effect. In contrast, rat b5 inhibited P450Coh-mediated reactions progressively starting from 1:1 ratio of b5 to P-450. Neither b5 had any effect on reactions catalyzed by P45015 alpha, an isozyme highly homologous to P450Coh, but with a point mutation (Arg-129----Ser) at site considered important for P-450-b5 interactions. In case of P450PBI, neither b5 protein had any effect on the associated activities at b5: P-450 ratios below 1, and a progressive inhibition occurred when b5: P-450 ratio was above 1. The results were similar with either rat or mouse liver NADPH-cytochrome P-450 reductase used in reconstitution demonstrating that the critical differences take place in P-450-b5 interactions. Kinetic and spectral experiments revealed that the stimulatory and inhibitory effects of b5 on the enzymatic reactions were due to corresponding changes in the reaction velocity, and that b5 does not compete with the flavoprotein nor with the substrate for binding to P-450. These results indicate that the high spin shift of P-450 does not necessarily correlate with enhanced reaction rates. Also, the increase in the coupling efficiency of P450PBI may result from the increased affinity for substrate in the presence of b5. Sequenation of mouse b5 peptides generated with proteinases revealed three amino acid changes between the mouse and rat b5, two of which appeared at the hydrophobic domain necessary for the P-450-b5 interaction. This could explain the species specificity of b5 proteins in supporting the P-450-mediated reactions. This is the first time functionally important differences in the interaction of highly homologous cytochromes P-450 and b5 have been demonstrated. Isozymes P45015 alpha and P450Coh, and mouse and rat b5 could serve as an excellent model for further studies on the nature and significance of P-450-b5 interactions.     

Acetone-inducible cytochrome P-450: purification, catalytic activity, and interaction with cytochrome b5

A procedure was developed for the purification of an acetone-inducible form of cytochrome P-450 (P-450ac) to electrophoretical homogeneity from liver microsomes of acetone-treated rats. The P-450ac preparation containing 16.0 to 16.5 nmol P-450/mg protein moved as a single protein band with an estimated molecular weight of 52,000 upon gel electrophoresis in the presence of sodium dodecyl sulfate. The ferric P-450ac showed an absorption maximum at 394 nm at 25 degrees C, suggesting that it exists mainly in the high-spin form. It also existed in the low-spin form, especially at lower temperatures, as indicated by the absorption maximum in the 412-nm region. Upon reconstitution with NADPH: cytochrome P-450 reductase and phospholipid, P-450ac efficiently catalyzed both the demethylation and denitrosation of N-nitrosodimethylamine (NDMA) showing Vmax values of 23.8 and 2.3 nmol min-1 nmol P-450-1, respectively. The catalytic activity of P-450ac was greatly affected by cytochrome b5 which decreased the Km values of these reactions by a factor of 10 and increased the Vmax values. Cytochrome b5 appeared to interact with P-450 at a molar ratio of 1:1 and an intact cytochrome b5 structure was required for such interaction. Among the substrates studied, the demethylation of NDMA was affected the most by cytochrome b5 and showed the highest rate. P-450ac also catalyzed the oxygenation of N-nitrosomethylethylamine and aniline and the activity was enhanced slightly by cytochrome b5. Cytochrome b5 did not enhance the P-450ac-catalyzed metabolism of other drug substrates such as benzphetamine, aminopyrine, and ethylmorphine. P-450ac appeared to be similar in property to the previously studied rat P-450et (ethanol-inducible), rat P-450j (isoniazid-inducible), and rabbit P-450LM3a (ethanol-inducible). These P-450 species represent a new class of P-450 isozymes that are important in the metabolism of many endobiotics and xenobiotics.     

Induction of specific cytochrome P-450 isozymes by methylenedioxyphenyl compounds and antagonism by 3-methylcholanthrene

Two methylenedioxyphenyl compounds, isosafrole (5-propenyl-1,3-benzodioxole) and an analog, 5-t-butyl-1,3-benzodioxole (BD), differ markedly as inducers of cytochrome P-450 isozymes in rat liver microsomes. Isosafrole is a mixed-type inducer, inducing P-450b, P-450c, and P-450d. In contrast, BD is a phenobarbital-type inducer, increasing P-450b, but producing little or no increase in P-450c or P-450d. Similarly, isosafrole increases the amount of translatable mRNA for P-450b, c and d, while BD induces only the mRNA for P-450b. Dimethylation of the methylene bridge carbon of BD to give 2,2-dimethyl-5-t-butyl-1,3-benzodioxole (DBD) blocks the formation of NADPH-reduced Type III metabolite-P-450 complexes in vitro, and diminishes but does not abolish the ability of the compound to induce P-450b. Western blots of microsomes from isosafrole and BD-treated rat livers confirm that in contrast to isosafrole, BD does not induce P-450d or P-450c. However, the antibody to P-450d recognizes two new polypeptides (approximately 50K Mr) from sodium dodecyl sulfate-polyacrylamide gels of liver microsomes from BD-treated rats. These polypeptides are not observed in control, isosafrole, 3-methylcholanthrene (3-MC), or DBD-treated rats. They are intensified by coadministration of 3-MC with BD and may represent either modified isozyme-metabolite adducts or degradation products of P-450d. However, the polypeptides could not be generated in vitro by addition of BD to 3-MC-induced microsomes with NADPH under conditions which produced spectral metabolite complexes, or in a reconstituted system with P-450d. The two methylenedioxyphenyl compounds do not form stable metabolite complexes with the same P-450 isozymes. BD formed distinct spectral metabolite complexes in vitro with both P-450b and P-450c but not with P-450d in a reconstituted system. In contrast, isosafrole forms metabolite complexes with all three isozymes. Coadministration of 3-MC with BD blocked induction of P-450b by 80% and produced a similar repression of its translatable mRNA. This finding indicates that 3-MC type inducers not only induce certain cytochrome P-450 isozymes, but also repress synthesis of other isozymes.     

The role of cytochrome b5 in adrenal microsomal steroidogenesis.

The role of cytochrome b5 in adrenal microsomal steroidogenesis was studied in guinea pig adrenal microsomes and also in the liposomal system containing purified cytochrome P-450s and NADPH-cytochrome P-450 reductase. Preincubation of the microsomes with anti-cytochrome b5 immunoglobulin decreased both 17 alpha- and 21-hydroxylase activity in the microsomes. In liposomes containing NADPH-cytochrome P-450 reductase and P-450C21 or P-450(17) alpha,lyase, addition of a small amount of cytochrome b5 stimulated the hydroxylase activity while a large amount of cytochrome b5 suppressed the hydroxylase activity. The effect of cytochrome b5 on the rates of the first electron transfer to P-450C21 in liposome membranes was determined from stopped flow measurements and that of the second electron transfer was estimated from the oxygenated difference spectra in the steady state. It was indicated that a small amount of cytochrome b5 activated the hydroxylase activity by supplying additional second electrons to oxygenated P-450C21 in the liposomes while a large amount of cytochrome b5 might suppress the activity through the interferences in the interaction between the reductase and P-450C21.     

Differential expression and function of three closely related phenobarbital-inducible cytochrome P-450 isozymes in untreated rat liver

The levels of expression of cytochromes P-450b and P-450e (both inducible by phenobarbital (PB) and differing by only 14 of 491 amino acids) in liver microsomes from untreated male rats were separately quantitated by Western blotting with a polyclonal antibody raised against P-450b that is equally effective against P-450e (anti P-450b/e). A protein with mobility identical to P-450e was detected in all microsomal samples. Microsomes from uninduced livers of individual male rats from five different strains exhibited only minor interstrain and interindividual variability in the expression of P-450e (17 +/- 5 pmol P-450e/mg microsomal protein) with the exception of the Brown Norway strain (8.5 +/- 0.5 pmol P-450e/mg). Expression of P-450b varied widely from undetectable levels (less than 2 pmol/mg) in most Sprague-Dawley rats to about 50% of P-450e levels in Fischer and Brown Norway strains. Anti P-450b/e inhibited total metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) by uninduced microsomes, to an extent dependent on rat strain (15-30%), predominantly through inhibition of formation of 12-hydroxymethyl-7-methyl BA (12HOMMBA) (65-85%), the major metabolite of purified P-450e. A specific activity for P-450e-dependent DMBA metabolism was calculated from four sets of microsomes where the P-450b content was either undetectable or very low (0.7-1.0 nmol/nmol P-450e/min-1). Comparable calculated activities were, however, obtained from other untreated rat liver microsomes where P-450b levels were significant. Polymorphism in P-450b was detected but did not affect total P-450b expression or the sensitivity of DMBA metabolism to anti P-450b/e. A fourth band of greater mobility than P-450b (apparent Mr less than 50,000), was also recognized by anti P-450b/e. The intensity of this band did not vary among individual rats or among the different strains and therefore did not correlate with the sensitivity of microsomal DMBA metabolism to anti P-450b/e. A monoclonal antibody (MAb) against P-450b (2-66-3) recognized P-450's b, b2, and e on Western blots but did not react with this higher mobility band. MAb 2-66-3 and two other MAbs produced against P-450b inhibited 12-methylhydroxylation of DMBA by untreated rat liver microsomes to the same extent as anti P-450b/e. Following PB induction, P-450b was induced to about double the level of P-450e in most rat strains examined.(ABSTRACT TRUNCATED AT 400 WORDS)     

Zonal distribution of cytochromes P-450 and related enzymes of bovine adrenal cortex--quantitative assay of concentrations and total contents

The zona glomerulosa, zona fasciculata, zona reticularis, and medulla were separated from bovine adrenal glands and cytochromes P-450 and related enzymes in each zone were investigated immunochemically by Western blotting using antisera from chickens or rabbits against cytochromes P-450scc, P-450(11)beta, P-450s21, and b5, NADH-cytochrome b5 reductase, NADPH-cytochrome P-450 reductase, NADPH-adrenodoxin reductase, and adrenodoxin. Concentrations of cytochrome P-450(11)beta, NADPH-cytochrome P-450 reductase, and cytochrome b5 per milligram of protein of homogenate were higher in the zona glomerulosa than in the other zones; the levels of the other components were higher in the zona fasciculata. The total enzyme content of all components was the highest in the zona fasciculata. The amount of adrenodoxin was about 10 times that of NADPH-adrenodoxin reductase in each zone.     

Monoclonal antibodies distinguish among isozymes of the cytochrome P-450b subfamily

Polyclonal antibody has been shown previously to react identically with cytochromes P-450b and P-450e purified from Long Evans rats and a strain variant of cytochrome P-450b purified from Holtzman rats (P-450bH). In the present study, an array of 12 different monoclonal antibodies produced against cytochrome P-450b has been used to distinguish among these closely related phenobarbital-inducible rat hepatic cytochromes P-450. In immunoblots and enzyme-linked immunosorbent assays, 10 monoclonal antibodies bind to cytochromes P-450b, P-450e, and P-450bH; one monoclonal antibody (B50) recognizes cytochromes P-450b and P-450bH but not cytochrome P-450e; and one monoclonal antibody (B51) is specific for cytochrome P-450b. In addition, one monoclonal antibody (BEF29) reacts strongly with cytochrome P-450f, and another antibody (BEA33) reacts weakly with cytochrome P-450a. No cross-reactions with cytochromes P-450c, P-450d, and P-450g-P-450j were detected with any of the monoclonal antibodies in these assays. Six spatially distinct epitopes on cytochrome P-450b were identified, and differences in antibody reactivity provided evidence for three additional overlapping epitopes. Several monoclonal antibodies are potent inhibitors of testosterone and benzphetamine metabolism supported by cytochrome P-450b in a reconstituted system. B50 and BE52 do not inhibit metabolism of the two substrates by microsomes from untreated rats, but inhibit benzphetamine N-demethylation and testosterone metabolism to 16 alpha- and 16 beta-hydroxytestosterone as well as androstenedione formation 67-94% by microsomes from phenobarbital-treated rats. No other pathways of testosterone metabolism are inhibited by these monoclonal antibodies. The differential inhibition of microsomal metabolism of benzphetamine and testosterone by these monoclonal antibodies is a reflection of the content and inducibility of cytochromes P-450b and P-450e as well as other cytochrome P-450 isozymes.     

The identification of the heat-stable microsomal protein required for methoxyflurane metabolism as cytochrome b5

Methoxyflurane is an anesthetic whose metabolism by cytochrome P-450LM2 has been shown to be dependent upon a heat-stable microsomal protein (Canova-Davis, E., and Waskell, L. A. (1982) Biochem. Biophys. Res. Commun. 108, 1264-1270). Treatment of this protein with diethylpyrocarbonate, which modifies selected amino acids, caused a dose-dependent loss in its ability to effect the metabolism of methoxyflurane by purified cytochrome P-450LM2. This protein factor has been identified as cytochrome b5 by demonstrating that cytochrome b5 and the heat-stable factor coelute during cytochrome b5 purification. Neither ferriheme nor apocytochrome b5 was able to substitute for the activating factor, while cytochrome b5 reconstituted from apocytochrome b5 and heme exhibited an activity similar to that of native b5. Examination of the cytochrome b5 molecule by computer graphics suggested that diethylpyrocarbonate did not inactivate b5 by reacting with the anionic surface of the cytochrome b5 molecule. Maximal rates of methoxyflurane metabolism were obtained at a ratio of 1:1:1 of the three proteins, cytochrome P-450LM2:reductase:cytochrome b5. In summary, it has been demonstrated that the heat-stable protein, cytochrome b5, is obligatory for the metabolism of methoxyflurane by cytochrome P-450LM2. These data also suggest that cytochrome b5 may be acting as an electron donor to P-450LM2 in the O-demethylation of methoxyflurane.     

Differential time course of induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by Aroclor 1254

The time course of induction of rat liver microsomal cytochromes P-450a, P-450b + P-450e, P-450c, and P-450d and epoxide hydrolase has been determined in immature male rats administered a single large dose [1500 mumol (500 mg)/kg body wt] of the polychlorinated biphenyl mixture Aroclor 1254. Differential regulation of these xenobiotic-metabolizing enzymes was indicated by their characteristic patterns of induction. The rate of induction of cytochrome P-450a and epoxide hydrolase was relatively slow, and steady-state levels of these enzymes were maintained from approximately Days 9 to 15 after Aroclor 1254 treatment. In contrast, cytochrome P-450c was maximally induced 2 days after Aroclor 1254 treatment and remained at a constant level through Day 15. Steady-state levels of cytochrome P-450d, beginning 1 week after Aroclor 1254 treatment, were preceded by a fairly rapid rate of induction and possibly by a small decline from maximal levels observed around Days 4 to 5. Like those of the other cytochrome P-450 isozymes and epoxide hydrolase, the levels of cytochromes P-450b + P-450e were constant from Day 9 to 15 after Aroclor 1254 treatment. However, an unexpected but reproducible decline (approximately 25%) in total cytochrome P-450 content observed between Days 4 and 9 after Aroclor 1254 treatment principally reflected a dramatic and totally unanticipated decrease (approximately 45%) in the level of cytochromes P-450b + P-450e. This transient decline in the level of cytochromes P-450b + P-450e was not due to an unusual effect of a mixture of polychlorinated biphenyls, since identical results were obtained with two individual congeners, namely 2,3,4,5,4'-penta- and 2,3,4,5,3',4'-hexachlorobiphenyl, that induced the same isozymes as Aroclor 1254. In contrast, when rats were treated with 2,4,5,2',4',5'-hexachlorobiphenyl, which induces cytochromes P-450a and P-450b + P-450e and epoxide hydrolase but not cytochromes P-450c or P-450d, maximal levels of cytochromes P-450b + P-450e were attained on Day 4 and no decrease was observed over the next 11 days. These results suggest that there may be an interaction in the regulation of induction of certain individual cytochrome P-450 isozymes.     

Association of cytochrome b5 and cytochrome P-450 reductase with cytochrome P-450 in the membrane of reconstituted vesicles

A protein-protein association of cytochrome P-450 LM2 with NADPH-cytochrome P-450 reductase, with cytochrome b5, and with both proteins was demonstrated in reconstituted phospholipid vesicles by magnetic circular dichroism difference spectra. A 23% decrease in the absolute intensity of the Soret band of the magnetic CD spectrum of cytochrome P-450 was observed when it was reconstituted with reductase. A difference spectrum corresponding to a 7% decrease in absolute intensity was obtained when cytochrome b5 was incorporated into vesicles that already contained cytochrome P-450 and cytochrome P-450 reductase compared to a decrease of 13% in absolute intensity when cytochrome b5 was incorporated into vesicles that contained only cytochrome P-450. The use of the magnetic circular dichroism confirmed that protein-protein associations that have been detected by absorption spectroscopy between purified and detergent-solubilized proteins also exist in membranes. High ionic strength was shown to interrupt direct electron flow from cytochrome P-450 reductase to cytochrome P-450 but not the electron flow from reductase through cytochrome b5 to cytochrome P-450. Upon incorporation of cytochrome b5 into cytochrome P-450- and cytochrome P-450 reductase-containing vesicles, an increase of benzphetamine N-demethylation activity was observed. The magnitude of this increase was numerically identical to the residual activity of the reconstituted vesicles measured in the presence of 0.3 M KCl. It is concluded that there is a requirement for at least one charge pairing for electron transfer from reductase to cytochrome P-450. These observations are combined in a proposed mechanism of coupled reversible association reactions in the membrane.     

Regulation of cytochrome P-450b and P-450e mRNA expression in the developing rat. Hybridization to synthetic oligodeoxyribonucleotide probes

We conducted solution hybridization and Northern blot experiments utilizing synthetic 18'-mer oligodeoxyribonucleotides complementary to two major rat hepatic phenobarbital-inducible cytochrome P-450s, P-450b and P-450e, to assess their mRNA expression during rat development. At all ages studied, with one exception (i.e. in day 22 neonates), P-450b mRNA was not detected in control animals. However, traces of P-450e message were observed in control animals on day 22 and persisted to adulthood. Phenobarbital pretreatment caused marked increases in hepatic mRNA for both P-450s as early as 22 days after conception. No increases were observed in RNA isolated from phenobarbital-pretreated day 10 or 19 rats. In general, the inducible levels of P-450b and P-450e mRNA increased as a function of age. The age-dependent increase in responsiveness to phenobarbital was associated with an age-dependent decrease in the ratio of P-450b to P-450e mRNA levels. The levels of P-450b/P-450e varied from a ratio of 19 at day 22 of development to a ratio of 5 at day 62 of development. Maximal levels of phenobarbital-induced hepatic RNA for both isozymes occurred 24 days after birth (day 46 of development), at which time P-450b and P-450e mRNAs accumulated to levels 2.4- and 1.8-fold greater, respectively, than levels found in comparably induced adult rat liver. Northern blot analyses indicated that the major mRNA species hybridizing to either the P-450b or P-450e oligomers in all age groups studied was approximately 1.8 kilobases.     

Reduction of hexavalent chromium in a reconstituted system of cytochrome P-450 and cytochrome b5

The reduction of hexavalent chromium (Cr(VI] by the monooxygenase components was studied. Both a reconstituted system of cytochrome P-450 (P-450) and cytochrome b5 (b5) with NADPH was capable of reducing Na2CrO4 (30 microM) provided anaerobic atmosphere. The rates were 1.29 nmol Cr.min-1 nmol P-450(-1) and 0.73 nmol Cr.min-1 nmol b5(-1). Using NADH instead of NADPH gave very low reducing activities, confirming the enzymic nature of the P-450 dependent Cr(VI) reductase reaction. Oxygen, 22% (air) and 0.1% gave 89% and 69% inhibition of Cr(VI) reducing activity, respectively. Carbon monoxide (100%) caused an inhibition of about 37% and 44% for P-450 and b5, respectively. Externally added flavin mononucleotide (FMN) (3 microM) or Fe-ADP (10 microM) to the complete system stimulated the enzymatic reaction about 2-fold and 3-fold, respectively.     

Human liver microsomal cytochrome P-450 mephenytoin 4-hydroxylase, a prototype of genetic polymorphism in oxidative drug metabolism. Purification and characterization of two similar forms involved in the reaction

Two forms of cytochrome P-450 (P-450), designated P-450MP-1 and P-450MP-2, were purified to electrophoretic homogeneity from human liver microsomes on the basis of mephenytoin 4-hydroxylase activity. Purified P-450MP-1 and P-450MP-2 contained 12-17 nmol of P-450/mg of protein and had apparent monomeric molecular weights of 48,000 and 50,000, respectively. P-450MP-1 and P-450MP-2 were found to be very similar proteins as judged by chromatographic behavior on n-octylamino-Sepharose 4B, hydroxylapatite, and DEAE- and CM-cellulose columns, spectral properties, amino acid composition, peptide mapping, double immunodiffusion analysis, immunoinhibition, and N-terminal amino acid sequences. In vitro translation of liver RNA yielded polypeptides migrating with P-450MP-1 or P-450MP-2, depending upon which form was in each sample, indicating that the two P-450s are translated from different mRNAs. When reconsituted with NADPH-cytochrome-P-450 reductase and L-alpha-dilauroyl-sn-glyceryo-3-phosphocholine, P-450MP-1 and P-450MP-2 gave apparently higher turnover numbers for mephenytoin 4-hydroxylation than did the P-450 in the microsomes. The addition of purified rat or human cytochrome b5 to the reconstituted system caused a significant increase in the hydroxylation activity; the maximum stimulation was obtained when the molar ratio of cytochrome b5 to P-450 was 3-fold. Rabbit anti-human cytochrome b5 inhibited NADH-cytochrome-c reductase and S-mephenytoin 4-hydroxylase activities in human liver microsomes. In the presence of cytochrome b5, the Km value for S-mephenytoin was 1.25 mM with all five purified cytochrome P-450s preparations, and Vmax values were 0.8-1.25 nmol of 4-hydroxy product formed per min/nmol of P-450. P-450MP is a relatively selective P-450 form that metabolizes substituted hydantoins well. Reactions catalyzed by purified P-450MP-1 and P-450MP-2 preparations and inhibited by anti-P-450MP in human liver microsomes include S-mephenytoin 4-hydroxylation, S-nirvanol 4-hydroxylation, S-mephenytoin N-demethylation, and diphenylhydantoin 4-hydroxylation. Thus, at least two very similar forms of human P-450 are involved in S-mephenytoin 4-hydroxylation, an activity which shows genetic polymorphism.     

Interaction of cholesterol hydroxylating cytochrome P-450 with cytochrome b5

Some new relations between cytochrome P-450-dependent monooxygenases were discovered. Cytochrome b5, a representative of "microsomal" monooxygenases, was shown to form a highly specific complex with cytochrome P-450scc, a member of the "ferredoxin" monooxygenase family. This interaction is characterized by a dissociation constant, Kd, of 0.28 microM. The cytochrome P-450scc-cytochrome b5 complex may be cross-linked with water-soluble carbodiimide. Using proteolytic modification of cytochrome b5, it was shown that both hydrophilic and hydrophobic fragments of cytochrome b5 are involved in the interaction with cytochrome P-450scc. Cytochrome b5 immobilized via amino groups is an effective affinity matrix for cytochrome P-450scc purification. The role of some amino acid residues in cytochrome P-450scc interaction with cytochrome b5 was studied. The role and the nature of complexes in cytochrome P-450-dependent monooxygenases as well as interrelationships between "microsomal" and "ferredoxin" monooxygenases are discussed.     

Isoenzyme-specific phosphorylation of cytochromes P-450 and other drug metabolizing enzymes

A series of fourteen cytochrome P-450 isoenzymes was treated with three different protein kinases and found to divide into isoenzymes phosphorylated by both the cyclic AMP-dependent kinase and the calcium-phospholipid-dependent kinase (P-450 PB 3a and PB 2e), by none of these kinases (P-450 PB 1b, MC 1b, UT 1, and thromboxane synthase), and by either the cyclic AMP-dependent kinase (P-450 LM 2, PB 2d, and PB 3b) or the calcium-phospholipid-dependent kinase (P-450 PB 1a, PB 2a, MC 1a, LM 3c, and LM 4). Other components of the monooxygenase system, cytochrome P-450 reductase, cytochrome b5, cytochrome b5 reductase as well as microsomal epoxide hydrolase, were poor substrates for the kinases employed. On the other hand, glutathione transferases 1-2 and 4-4, but not 3-3, were relatively good substrates for the calcium-phospholipid-dependent kinase.     

Identification of the forms of cytochrome P-450 induced in rat liver by 2-acetylaminofluorene using immunoblotting and partial purification

The amounts of 5 different forms of cytochrome P-450 in liver microsomes from rats treated with 2-acetylaminofluorene were determined and compared with the corresponding patterns in microsomes from control, 3-methylcholanthrene- and phenobarbital-treated animals. 2-Acetylaminofluorene was found to increase the amount of cytochromes P-450b + e 10-fold and of cytochrome P-450d 3-fold, while there was a 54% increase in the level of cytochrome P-450 PB/PCN-E. Cytochrome P-450c was increased from a level too low to detect (less than 0.001 pmol/mg protein) to 0.019 pmol/mg protein. These findings were also confirmed by partial purification of cytochromes P-450b + e and c after 2-acetylaminofluorene treatment.