Studies on the substrate-binding sites of liver microsomal cytochrome P-448.

The interaction of substrates of the microsomal mixed-function oxidases with cytochromes P-450 and P-448 was investigated by using liver microsomes from rats pretreated with phenobarbital or 3-methylcholanthrene, and with purified forms of the cytochromes isolated from rabbit liver. The two forms of the cytochrome have different substrate specificities; cytochrome P-450 has one type 1 substrate-binding site that can accommodate a large variety of substrates, but in contrast cytochrome P-448 may possess two type 1 substrate-binding sites, one of which is different to that of cytochrome P-450 in that it shows a specificity for substrates such as safrole and 9-hydroxy-ellipticine. These findings explain why the two forms of the cytochrome have different substrate specificities and play contrasting roles in the activation and deactivation of xenobiotics.     

Structural requirements for substrates of cytochromes P-450 and P-448

Distinct and different molecular structural features are manifested by substrates, inhibitors and inducers of the two families of liver microsomal enzymes, the phenobarbital-induced cytochromes P-450 and the 3-methylcholanthrene-induced cytochromes P-448. In a theoretical study based on molecular orbital calculations and molecular graphics, it is established that cytochrome P-448 substrates contain fused aromatic or heteroaromatic rings giving rise to overall molecular planarity with relatively small molecular depth. In contrast, substrates of the cytochromes P-450 have greater conformational freedom and an ability to bind at more than one point of attachment, as a result of possession of certain characteristic functions, namely, a carbonyl and/or amine moiety coupled with an iso-propyl group, or similar function of equivalent shape and hydrophobicity. The implications are that the binding sites of cytochromes P-448 contain a number of hydrophobic aromatic amino acid residues orientated so as to allow occupation by similar substrates containing co-planar aromatic rings, whereas those of the phenobarbital-induced cytochromes P-450 contain hydrophilic amino acid residues capable of hydrogen bonding to greater than C = O moieties and at least one leucine or valine residue, as these contain the complementary isopropyl function. The corollary of these findings is the possibility of prediction of the toxicity of new chemicals on the basis of their molecular dimensions.     

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 interaction of aliphatic analogs of methylene-dioxyphenyl compounds with cytochromes P-450 and P-420.

The spectra resulting from the interaction of a series of substituted dioxolanes with microsomal cytochromes P-450 or P-420, as well as purified cytochrome P-450, were measured. With the exception of dioxolane, 4-methyldioxolane and 4-ethyldioxolane, these compounds interacted with ferric cytochrome P-450 to give complexes exhibiting type I optical difference spectra, and, after incubation with NADPH, spectra with peaks at about 430 nm. These complexes, as well as those formed from dioxolanes in the presence of cumene hydroperoxide, inhibit the binding of CO to the cytochrome. Consideration of the known chemistry of dioxolanes, together with recent advances in the understanding of double Soret spectra, lead to a possible explanation for the differences between the spectra of dioxolanes and their aromatic analogs, the methylenedioxyphenyl compounds.     

Liver microsomal cytochromes P-450 and azoreductase activity.

Hepatic microsomal azoreductase activity with amaranth (3-hydroxy-4[(4-sulfo-1-naphthalenyl)azo]-2,7-naphthalenedisulfonic acid trisodium salt) as a substrate is proportional to the levels of microsomal cytochrome P-450 from control or phenobarbital-pretreated rats and mice or cytochrome P-448 from 3-methylchol-anthrene-pretreated animals. In the "inducible" C57B/6J strain of mice, 3-methylcholanthrene and phenobarbital pretreatment cause an increase in cytochrome P-448 and P-450 levels, respectively, which is directly proportional to the increase of azoreductase activity. However, in the "noninducible" DBA/2J strain of mice, only phenobarbital treatment causes the increase both in cytochrome P-450 levels and azoreductase activity, while 3-methylcholanthrene has no effect. These experiments suggest that the P-450 type cytochromes are responsible for azoreductase activity in liver microsomes.     

Metabolic conditions determining the composition and catalytic activity of cytochrome P-450 monooxygenases in Candida tropicalis.

In the microsomal fraction of Candida tropicalis cells, two distinct monooxygenases were detected, depending on the growth conditions. The distinction of the two monooxygenases was evident from: (i) the absorption maxima in the reduced CO difference spectra of the terminal oxidases (cytochromes P-450 and P-448); (ii) the contents of the monooxygenase components (cytochromes P-450/P-448, NADPH-cytochrome c (P-450) reductase, and cytochrome b5) and (iii) the catalytic activity of the complete system (aliphatic hydroxylation and N-demethylation activity). The occurrence of the respective monooxygenases could be related to the carbon source (n-alkanes or glucose). Oxygen limitation led to a significant increase of cytochrome P-450/P-448 content, independent of the carbon source utilized by the cells. An improved method for the isolation of microsomes enabled us to demonstrate the presence of cytochrome P-448 in glucose-grown cells.     

Purification and partial characterization of hepatic microsomal cytochrome P-450s from phenobarbital- and 3-methylcholanthrene-treated rats.

Hepatic microsomal cytochrome P-450 and P-448 have been purified from phenobarbital (PB)- and 3-methylcholanthrene (MC)-treated rats, by modifications of Imai and Sato's procedures )1974). The purified preparations of cytochrome P-450 and P-448 were homogeneous judging from their specific contents (17 and 16 nmol per mg protein, respectively) and the results of SDS-polyacrylamide gel electrophoresis and Ouchterlony immunodiffusion analyses. These two cytochromes are different in their physico-chemical and immunological properties, and their substrate specificities. In reconstituted systems containing the purified cytochrome and NADPH-cytochrome P-450 reductase, ethoxycoumarin deethylation and benzo(a)pyrene hydroxylation catalyzed by cytochrome P-450 and P-448 were completely inhibited by the homologous antibody, while essentially no effect was observed with heterologous conbinations of antigen and antibody. In contrast, the benzphetamine demethylation activities of cytochrome P-450 and P-448 were markedly inhibited by the heterologous antibody as well as by the homologous one. These results suggest that the two cytochromes are immunologically different but have some antigenic determinants in common. Drug metabolizing activities of microsomes from PB- and MC-treated rats were inhibited by the antibodies, essentially as expected from the results with the reconstituted systems. The remaining activities in the presence of excess concentrations of the antibody, however, were higher in MC-microsomes treated with anti P-448 antibody than in PB microsomes treated with anti P-450 antibody. These results suggest that cytochrome P-448 molecules may be so localized in the microsomal membrane that the membrane structure may hinder the access of the antibody to the antigenic determinant.     

Aryl hydrocarbons as inducers of cytochrome P-450 in Acinetobacter calcoaceticus

Summary The inducibility of cytochrome P-450 in Acinetobacter calcoaceticus by some compounds known as typical inducers of hepatic cytochromes P-450 was investigated. Besides biphenyl also indene and phenanthrene are inducers, whereas compounds of the so-called phenobarbital type are not. Biphenyl appears to be the most effective inducer with regard to the yield of cytochrome P-450/mg of cell protein. By addition of the compounds in the vapour phase an induction of the protein by naphthalene could be demonstrated. The results are indicative of the existence of bacterial cytochromes P-450 that resemble hepatic cytochromes.     

The role of vitamin K3 in the hydroxylation of benz(a)pyrene in rat liver mitochondria after induction with 3-methylcholanthrene and phenobarbital

The "fast" phase reduction of microsomal cytochromes P-450 and P-448 and their benz(a)pyrene (BP) hydroxylase activity was investigated as a function of menadione concentrations. Within a narrow concentration range (1.5-3 microM) menadione activates cytochrome P-448 reduction and the BP hydroxylase activity. At higher concentrations menadione inhibits cytochromes P-450 and P-448 reduction and BP hydroxylation with participation of the both cytochromes. These data suggest that menadione molecules present in membrane lipids serve as an additional electron carrier to cytochrome P-448, the active site of which is embedded into lipids. The activating effect is unobserved is case of cytochrome P-450 with an active site localized in the aqueous phase. The number of different BP metabolites formed at low (3 microM) menadione concentrations in the microsomes of rats induced with 3-methylcholanthrene (MC) and phenobarbital (PB) was compared. In PB-induced microsomes the amount of 7,8-dihydrodiol rises whereas the total content of BP metabolites decreases. Contrariwise, in MC-induced microsomes the synthesis of all BP metabolites is augmented. Menadione has a very weak effect on the ratio of different BP metabolites in PB- and MC-microsomes, but strongly inhibits the formation of more polar metabolites. This results in a marked reduction of the number of "dangerous" BP diolepoxides.     

Interaction between cytochrome P-448 and NADP-cytochrome P-450 reductase in reconstituted microsomal membranes

The regularities of changes in the functional activity of the microsomal monooxygenase system reconstituted by self-assembly from intact rat liver microsomes solubilized with 4% sodium cholate were studied at variable levels of NADPH-cytochrome P-450 reductase and the 3-methylcholanthrene-induced form of cytochrome P-450. Using antibodies against cytochrome P-448, the role of cytochrome P-448 in the overall reaction of benzopyrene hydroxylation induced in the microsomal membrane by a set of molecular forms of cytochrome P-450 was investigated. The effect of NADPH-cytochrome P-450 reductase and cytochrome P-448 incorporation into reconstituted microsomal membranes on benzpyrene metabolism suggests that in intact microsomal membranes benzopyrene metabolism induced by different forms of cytochrome P-450, with the exception of P-448, is limited by reductase is not the limiting component; however, cytochrome P-448 reveals its maximum activity at the cytochrome to reductase optimal molar ratio of 5:1; above this level, the catalytic activity of cytochrome P-448 is lowered.     

Catalytic activity of isolated cytochromes P-450d and P-450c

Cytochrome P-450d was isolated from isosafrol-induced rat liver microsomes by affinity chromatography on 1.8-diaminooctyl-Sepharose 4B and chromatography on hydroxylapatite using a linear potassium phosphate gradient (45-250 mM). The enzyme has a molecular mass of 54 kDa, CO-maximum 448 nm is characterized by a high spin state; the rate of 4-aminobiphenyl hydroxylation is 54 nmol/min/nmol of cytochrome P-450d (37 degrees C), those, of 7-ethoxyresorufin O-deethylation and benz (a) pyrene oxidation are 1 nmol/min/nmol of cytochrome P-450d (22 degrees C) and 2 nmol/min/nmol of cytochrome P-450d (37 degrees C), respectively. The properties of cytochrome P-450d were compared to those of cytochrome P-450c isolated from 3-methylcholanthrene-induced rats. The yield of these cytochromes under the conditions used (10% P-450d from isosafrol-induced microsomes and 15% P-450c from 3-methylcholanthrene-induced microsomes) was relatively high. Antibodies to cytochromes P-450d and P-450c were obtained. Using rocket immunoelectrophoresis the percentage of these hemoprotein forms in 3-methylcholanthrene-induced (P-450d-20%, P-450c-70%) and isosafrol-induced rat liver microsomes (P-450d-50%, P-450c-15%) was determined.     

Regulation of three forms of cytochrome P-450 and epoxide hydrolase in rat liver microsomes. Effects of age, sex, and induction

A procedure for the preparation of monospecific antibody directed against rat liver microsomal cytochrome P-45-a is described. This antibody, together with monospecific antibodies to cytochromes P-450b and P-450c, has been used to show that these three forms of cytochrome P-450 are distinct and share no common antigenic determinants. These antibodies (a) give single immunoprecipitin bands with detergent-solubilized microsomes; (b) do not cross-react with the purified heterologous antigens in Ouchterlony double diffusion analyses; (c) have no effect on catalytic activity of the heterologous antigens but completely inhibit the enzymatic activity of the homologous antigens; and (d) remove only the homologous antigen from detergent-solubilized microsomes when covalently bound to a solid support. With radial immunodiffusion assay, we have quantitated these three forms of cytochrome P-450 in liver microsomes after treatment of rats with seven different inducers of cytochrome P-450. The levels of these cytochrome P-450 isozymes vary independently and are also regulated by the age and sex of the animal. The antibodies have also been used to assess the contribution of cytochromes P-450a, P-450b, and P-450c in the metabolism of xenobiotics by rat liver microsomes. A large proportion of benzo(a)pyrene metabolism and testosterone 16 alpha-hydroxylation in microsomes from untreated rats is not catalyzed by cytochromes P-450a, P-450b, and P-450c. Epoxide hydrolase, another microsomal enzyme involved in the metabolism of xenobiotics, was also quantitated by radial immunodiffusion after prior treatment of rats with microsomal enzyme inducers. The inductions of epoxide hydrolase varies independently of the induction of cytochromes P-450a, P-450b, and P-450c.     

Immunochemical evidences that hexachlorobenzene induces two forms of cytochrome P-450 in the rat liver microsomes

Induction by hexachlorobenzene (HCB) of the liver microsomal system of metabolism of xenobiotics has been studied in comparison with the inductions by phenobarbital (PB) and 3-methylcholanthrene (MC). It has been shown that HCB increases the content of cytochrome P-450 in the microsomes. Like PB, HCB induces the activities of aminopyrine- and benzphetamine-N-demethylases. At the same time HCB increases also the activities of benzpyrenehydroxylase and 7-ethoxyresorufin-O-deethylase, which are characteristic of the MC-induction. However, sodium dodecyl sulphate (SDS)-electrophoresis on polyacrylamide gel has revealed that HCB, similar to PB, induces protein with Mr = 52 000 (cytochrome P-450), but not the protein with Mr = 56 000, which is the main isoenzyme of cytochrome P-450 in MC-microsomes (P-448). Using specific antibodies to isolated cytochromes P-450 and P-448 (anti-P-450 and anti-P-448) it has been found by rocket immunoelectrophoresis that in HCB-treated microsomes 20% of the total cytochrome P-450 consist of PB-form and about 10% comprise cytochrome P-488. It has also been found that anti-P-448 totally inhibit 7-ethoxyresorufin-O-deethylase activity of HCB-microsomes while anti-P-450 was inactive. The data presented give direct proof that HCB exemplifies an individual chemical compound which is able to initiate the synthesis of both PB-form and MC-form of the cytochrome P-450.     

Anthraflavic acid is a potent and specific inhibitor of cytochrome P-448 activity

Consideration of the computer-optimised dimensions of anthraflavic acid indicates that it is essentially a planar molecule with a large area/depth ratio, that would preferentially interact with the polycyclic aromatic hydrocarbon-induced family of cytochrome P-450 proteins (cytochromes P-448). Anthraflavic acid was a potent inhibitor of the O-deethylations of ethoxycoumarin and ethoxyresorufin, both catalysed primarily by cytochromes P-448, in Arochlor-1254-induced hepatic microsomes. Similarly anthraflavic acid markedly inhibited the mutagenicity of 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-I) in the Ames test. In contrast, it has no effect on the dealkylation of pentoxyresorufin, a reaction catalysed primarily by the phenobarbital-induced cytochromes P-450, and NADPH-dependent reduction of cytochrome c. It is concluded that anthraflavic acid is a potent and specific inhibitor of cytochrome P-448 activity.     

Monoclonal antibodies against a high spin form of rat cytochrome P-448

Ten monoclonal antibodies reactive with a high spin form of rat cytochrome P-448 (P-448-H) were obtained from hybridoma clones established by a fusion between P3X63Ag8.653 mouse myeloma cells and spleen cells of a BALB/c mouse hyperimmunized with the cytochrome. One monoclonal antibody recognized an epitope characteristic for P-448-H. Five monoclonal antibodies were cross-reactive with a low spin form of rat cytochrome P-448, but not with cytochrome P-450. Reactivity of these monoclonal antibodies with microsomes of rats pretreated with drug metabolizing inducers and Western blots of the microsomal cytochrome P-450 components are also demonstrated.     

Retinoic acid metabolism by a system reconstituted with cytochrome P-450

Feeding rats with a diet containing a hundred times the normal amount of vitamin A resulted, within 2 to 3 weeks, in an increase in total hepatic microsomal cytochrome P-450 content. This was associated, in isolated microsomes, with an enhanced conversion of all-trans-retinoic acid to polar metabolites, including a two- to threefold increased production of 4-hydroxy- and 4-oxo-retinoic acid, whether expressed per microsomal protein or per cytochrome P-450. Unlike effects of other inducers (e.g., phenobarbital or methylcholanthrene), activities of benzphetamine, aminopyrine, and ethylmorphine demethylases or benzopyrene hydroxylase were not increased. Furthermore, the CO-reduced difference spectral peak was shifted towards 449 nm. On sodium dodecyl sulfate-gel electrophoresis, one band was increased with electrophoretic mobility identical to that of cytochrome P-450f, a recently isolated new form which has a CO-reduced difference spectral peak at 448 nm. In a system reconstituted with NADPH-cytochrome P-450 reductase, NADPH, and phospholipid, purified cytochromes P-450f and b were discovered to promote conversion of retinoic acid to polar metabolites, including 4-hydroxy-retinoic acid.     

Induction of multiple cytochrome P-450 species in housefly microsomes--SDS-gel electrophoresis studies.

1. Microsomal fractions isolated from various housefly strains have been characterized with respect to multiple forms of cytochrome P-450 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. 2. Susceptible NAIDM houseflies were pretreated with known inducers of cytochrome P-450, and their microsomal electrophoretic profiles were compared to control NAIDM microsomes, using as standards partially purified cytochrome P-450s from noninduced NAIDM houseflies. 3. Tentatively, at least five different species of cytochrome P-450 may exist in the NAIDM housefly strain. 4. A comparison of the microsomal electrophoretic profile of different housefly strains also indicates the presence of at least two additional cytochrome P-450 species. 5. Induction with alpha-pinene and phenobarbital was expressed by a shift of the maximum absorbance at 452 nm in the CO-difference spectrum to lower wavelengths in the NAIDM strain; whereas, beta-naphthoflavone, although increasing the amount of cytochrome P-450, did not change the wavelength of maximum absorbance. 6. Cytochromes of the P-452 type appear to predominate in the susceptible NAIDM strain, while cytochromes of the P-450 and P-448 types predominate in resistant strains.     

Inhibition of uroporphyrinogen decarboxylase by halogenated biphenyls in chick hepatocyte cultures. Essential role for induction of cytochrome P-448.

Uroporphyrinogen decarboxylase (EC 4.1.1.37) activity was assayed in cultures of chick-embryo hepatocytes by the changes in composition of porphyrins accumulated after addition of excess 5-aminolaevulinate. Control cells accumulated mainly protoporphyrin, whereas cells treated with 3,4,3',4'-tetrachlorobiphenyl or 2,4,5,3',4'-pentabromobiphenyl accumulated mainly uroporphyrin, indicating decreased activity of the decarboxylase. 3-Methylcholanthrene and other polycyclic-hydrocarbon inducers of the P-448 isoenzyme of cytochrome P-450, did not affect the decarboxylase in the absence of the biphenyls. Induction of P-448 was detected as an increase in ethoxyresorufin de-ethylase activity. Pretreatment of cells with methylcholanthrene decreased the time required for the halogenated biphenyls to inhibit the decarboxylase. The dose response of methylcholanthrene showed that less than 40% of the maximal induction of cytochrome P-448 was needed to produce the maximum biphenyl-mediated inhibition of the decarboxylase. In contrast, induction of the cytochrome P-450 isoenzyme by propylisopropylacetamide had no effect on the biphenyl-mediated decrease in decarboxylase activity. Use of inhibitors of the P-450 and P-448 isoenzymes (SKF-525A, piperonyl butoxide and ellipticine) supported the concept that only the P-448 isoenzyme is involved in the inhibition of the decarboxylase by the halogenated biphenyls. The effect of preinduction with methylcholanthrene to enhance inhibition of the decarboxylase was also shown by the increased rate at which porphyrin accumulated from endogenously synthesized 5-aminolaevulinate after treatment of cells with the combination of propylisopropylacetamide and the biphenyls. Antioxidants, chelators of iron, and chromate affected the decrease in decarboxylase activity only if they prevented the induced increase in cytochrome P-448. We conclude that the P-448 and not the P-450 isoenzyme of cytochrome P-450 plays an obligatory role in the inhibition of uroporphyrinogen decarboxylase caused by halogenated biphenyls.     

The induction of cytochrome P-448 dependent benzo(a)pyrene hydroxylase in Saccharomycescerevisiae

When grown in high concentrations of glucose, the yeast $\text{Saccharomyces}\text{cerevisiae}$ produces a microsomal cytochrome P-450 monooxygenase system which is capable of hydroxylating benzo(a)pyrene. The addition of benzo(a)pyrene to the yeast during growth causes only a small increase in cytochrome P-448 levels but results in a dramatic improvement in the apparent kinetics of benzo(a)pyrene hydroxylation as measured by a decrease in the Michaelis constant and an increase in maximal velocity. Dimethylnitrosamine, phenobarbital and 3-methylcholanthrene also induce this enzyme to various degrees. Yeast pretreatment with β-naphthoflavone did not affect this enzyme, yet pretreatment with lanosterol resulted in a decreased affinity for benzo(a)pyrene. The addition of benzo(a)pyrene to yeast growing at low glucose concentration does not induce cytochrome P-448. The implications of these findings with regard to the presence of multiple forms of cytochromes $\text{P-448}\text{P-450}$ in yeast are briefly discussed.     

Comparison of cytochrome P-450 forms induced by phenobarbital and 1,4-bis[3,5-dichloropyridyloxy)]benzene in the mouse and rat liver

A form of cytochrome P-450 (P-450PB) with a molecular weight of 53.5-54.0 kD possessing a high benzphetamine-N-demethylase activity (100-120 nmol formaldehyde/min/nmol cytochrome) was isolated from liver microsomes of phenobarbital-induced C57Bl/6 mice. This cytochrome P-450 form is immunologically identical to its rat liver counterpart-P-450b (Mr = 52 kD) which is also characterized by a high rate of benzphetamine-N-demethylation. It was shown that 1.4-bis[2-(3.5-dichloropyridyloxy])benzene (TCPOBOP) induces in mouse liver the synthesis of the monoxygenase form whose substrate specificity and immunologic properties are identical to those of cytochromes P-450PB and P-450b. The immunochemically quantitated content of this form makes up to 20% of the total P-450 pool in liver microsomes of phenobarbital- or TCPOBOP-induced mice. Immunochemical analysis of microsomes with the use of antibodies to cytochromes P-450PB and P-450b revealed the presence on the electrophoregrams of phenobarbital-induced rat liver microsomes of two immunologically identical forms of cytochrome P-450, i.e., P-450b and P-450e (the latter had a low ability to benzphetamine N-demethylation). Liver microsomes of phenobarbital- or TCPOBP-induced mice gave only one precipitation band corresponding to cytochrome P-450PB.