Purification and characterization of cholesterol 7 alpha-hydroxylase cytochrome P-450 of untreated rabbit liver microsomes

Cytochrome P-450 which catalyzes the 7 alpha-hydroxylation of cholesterol was purified from liver microsomes of untreated rabbits. The minimum molecular weight of the cytochrome P-450 was estimated to be 48,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The preparation contained 7 nmol of cytochrome per mg of protein. The oxidized form of the P-450 showed absorption maxima at 568, 535, and 417 nm, which are characteristic of a low spin hemoprotein, while the reduced form showed maxima at 545 and 413 nm. The carbon monoxide complex of the reduced form showed maxima at 550 and 447 nm. The cholesterol 7 alpha-hydroxylase system of untreated rabbit liver microsomes was reconstituted with the purified P-450, NADPH-cytochrome P-450 reductase, and cytochrome b5. The P-450 catalyzed the 7 alpha-hydroxylation of cholesterol 500 times more efficiently than the starting microsomes. The reconstituted hydroxylase system showed a substantial salt dependency. In the presence of cytochrome b5 the activity was maximum at 0.4 M KCl (4.55 nmol product formed/mg of protein per min), whereas in the absence of cytochrome b5 the activity was marginal (0.65 nmol product formed/mg of protein per min) and inhibited by KCl. Thus, cytochrome b5 stimulated the hydroxylase activity by one order of magnitude. These results indicate that cytochrome b5 is an essential component of the cholesterol 7 alpha-hydroxylase system of untreated rabbit liver microsomes.     

Isolation and characterization of a constitutive form of rabbit liver microsomal cytochrome P-450

A heretofore unrecognized form of cytochrome P-450 was purified from rabbit liver microsomes with an average yield and purity similar to that of other highly purified forms of cytochrome P-450. Several properties of this cytochrome are contrasted with those of form 2, the major phenobarbital-inducible cytochrome P-450, form 4, the major 2,3,7,8-tetrachlorodibenzo-p-dioxin-inducible cytochrome, and form 6, a cytochrome that is selectively induced in liver microsomes by 2,3,7,8-tetrachlorodibenzo-p-dioxin during the perinatal period. Thes four forms can be distinguished by virtue of their molecular weights as determined using polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, by their respective peptide fingerprints, and by the monospecificity of their antisera. Since the enumerated properties are thought to reflect the primary structure of the cytochromes and since the observed differences are extensive, we suggest that these four forms are not derived from a common protein precursor.     

Isolation of a cytochrome P-450 that catalyzes the 25-hydroxylation of vitamin D3 from rat liver microsomes

A molecular species of cytochrome P-450 that catalyzes the 25-hydroxylation of cholecalciferol (P-450cc25) was purified from rat liver microsomes on the basis of its catalytic activity. The purification procedure consisted of polyethylene glycol fractionation, and column chromatographies on octylamino Sepharose 4B, hydroxylapatite, DEAE-Sepharose CL-6B, and CM-Sepharose CL-6B. The specific cytochrome P-450 content of the final preparation was 17.0 nmol/mg of protein. The enzymatic activity was reconstituted with the purified cytochrome P-450, NADPH-cytochrome P-450 reductase, an NADPH-generating system, and dilauroylglyceryl-3-phosphorylcholine, the specific activity obtained being 3.7 nmol/min/mg of protein, which was 4,000 times as high as that in microsomes. The apparent molecular weight of the P-450cc25 was 50,000, based on the results of sodium dodecyl sulfate polyacrylamide gel electrophoresis. The absorption spectra of the oxidized form of the enzyme showed a Soret band at 416 nm, which is typical of the low spin state of cytochrome P-450, and alpha and beta bands at 570 and 536 nm, respectively. The Soret peak of the reduced cytochrome P-450-CO complex was at 450 nm. The purified enzyme not only catalyzed the 25-hydroxylation of cholecalciferol but also showed hydroxylation activity toward a variety of substrates, i.e. 1 alpha-hydroxycholecalciferol (at 25), testosterone (at 2 alpha and 16 alpha) and dehydroepiandrosterone (at 16 alpha). Amino terminal sequence of the purified cytochrome P-450 was determined by the manual sequence method to be H2N-Met-Asp-Pro-Val-leu-Val-Leu-Val-. The antibody elicited against the purified enzyme in a rabbit inhibited the cholecalciferol 25-hydroxylation activity by more than 90% with a concentration of 2 mg of immunoglobulin per nmol of cytochrome P-450.     

Purification and properties of P-450LM3b, a constitutive form of cytochrome P-450, from rabbit liver microsomes

This laboratory has previously reported the occurrence in rabbit liver microsomes of a non-inducible form of cytochrome P-450, designated P-450lm_3b because of its electrophoretic mobility relative to that of phenobarbital-inducible P-450lm₂ and 5,6-benzoflavone-inducible P-450lm₄. In the present study, P-450lm_3b was purified to electrophoretic homogeneity and a specific content of over 19 nmol per mg of protein by chromatographic procedures carried out in the presence of detergents. The isolated cytochrome has a minimal molecular weight of 52,000 and exhibits absorption maxima at 418, 537, and 571 nm in the oxidized state, 412 and 547 nm in the reduced state, and 451 and 555 nm as the CO complex. In a reconstituted system containing NADPH-cytochrome P-450 reductase and phosphatidylcholine, P-450lm_3b has relatively high activity in the hydroxylation of testosterone in the 6β and 16α positions as well as significant activity toward a number of other substrates tested. The NADPH oxidase activity of P-450lm_3b is less than half that of P-450lm₂ and lm₄.     

Induction of the ethanol-inducible form of rabbit liver microsomal cytochrome P-450 by inhibitors of alcohol dehydrogenase

Cytochrome P-450 LMeb was purified from liver microsomes obtained from rabbits treated with either benzene or imidazole and was shown to have identical N-terminal amino acid sequence as that of cytochrome P-450 LM3a. The amino acid compositions of the proteins were indistinguishable. Quantitation of P-450 LMeb in various types of microsomes using radial immunodiffusion, revealed that pyrazole- or imidazole-treatment of the animals caused a 2-3-fold induction of the enzyme, accompanied by 2-3-fold increases of the rates of ethanol and aniline oxidation.     

Evidence that isoniazid and ethanol induce the same microsomal cytochrome P-450 in rat liver, an isozyme homologous to rabbit liver cytochrome P-450 isozyme 3a

Cytochrome P-450j has been purified to electrophoretic homogeneity from hepatic microsomes of adult male rats administered ethanol and compared to the corresponding enzyme from isoniazid-treated rats. The enzymes isolated from ethanol- and isoniazid-treated rats have identical chromatographic properties, minimum molecular weights, spectral properties, peptide maps, NH2-terminal sequences, immunochemical reactivities, and substrate selectivities. Both preparations of cytochrome P-450j have high catalytic activity in aniline hydroxylation, butanol oxidation, and N-nitrosodimethylamine demethylation with turnover numbers of 17-18, 37-46, and 15 nmol product/min/nmol of P-450, respectively. A single immunoprecipitin band exhibiting complete identity was observed when the two preparations were tested by double diffusion analysis with antibody to isoniazid-inducible cytochrome P-450j. Ethanol- and isoniazid-inducible rat liver cytochrome P-450j preparations have also been compared and contrasted with cytochrome P-450 isozyme 3a, the major ethanol-inducible isozyme from rabbit liver. The rat and rabbit liver enzymes have slightly different minimum molecular weights and somewhat different peptide maps but similar spectral, catalytic, and immunological properties, as well as significant homology in their NH2-terminal sequences. Antibody to either the rat or rabbit isozyme cross-reacts with the heterologous enzyme, showing a strong reaction of partial identity. Antibody against isozyme 3a specifically recognizes cytochrome P-450j in immunoblots of induced rat liver microsomes. Aniline hydroxylation catalyzed by the reconstituted system containing cytochrome P-450j is markedly inhibited (greater than 90%) by antibody to the rabbit protein. Furthermore, greater than 85% of butanol or aniline metabolism catalyzed by hepatic microsomes from ethanol- or isoniazid-treated rats is inhibited by antibody against isozyme 3a. Results of antibody inhibition studies suggest that cytochrome P-450j is induced four- to sixfold by ethanol or isoniazid treatment of rats. All of the evidence presented in this study indicates that the identical cytochrome P-450, P-450j, is induced in rat liver by either isoniazid or ethanol, and that this isozyme is closely related to rabbit cytochrome P-450 isozyme 3a.     

Carbon tetrachloride-induced lipid peroxidation dependent on an ethanol-inducible form of rabbit liver microsomal cytochrome P-450

Treatment of rats with ethanol or rabbits with either imidazole or pyrazole, agents known to induce the ethanol-inducible form of liver microsomal cytochrome P-450 (P-450 LMeb), caused, compared to controls, 3-25-fold enhanced rates of CCl4-dependent lipid peroxidation or chloroform production in isolated liver microsomes. No significant differences were seen when the rate of CCl4-dependent lipid peroxidation was expressed relative to the amount of P-450 LMeb in the various types of microsomal preparations. In reconstituted membranous systems, this type of P-450 was a 100-fold more effective catalyst of CCl4 metabolism than either of the cytochromes P-450 LM2 or P-450 LM4. It is proposed that the induction of this isozyme provides the explanation on a molecular level for the synergism seen of ethanol on CCl4-dependent hepatotoxicity.     

Allosteric regulation of the 16 alpha-hydroxylation of progesterone as catalyzed by rabbit microsomal cytochrome P-450 3b

A variety of compounds that can arise from the metabolism of progesterone in vivo stimulate the 16 alpha-hydroxylation of progesterone as catalyzed by highly purified, reconstituted preparations of cytochrome P-450 3b obtained from rabbit strain IIIVO/J. In general, reduction of the 20-keto moiety of progesterone or related compounds increases the extent of stimulation. Reduction of the 3-keto group also results in increased stimulatory activity in most cases. The resulting 3 beta-hydroxy derivatives are more active than the corresponding 3 alpha-isomers. In a similar fashion, 5 beta-pregnanes exhibit greater activity than the corresponding 5 alpha-pregnanes. The effect of these allosteric effectors is saturable at relatively low concentrations when compared to other positive effectors of P-450-mediated metabolism. These compounds increase the apparent ratio of Vmax/Km without altering the amount of reductase required for half-maximal activity when reconstituted with the cytochrome. In contrast, many of these compounds do not affect or inhibit the 6 beta-hydroxylation of progesterone catalyzed by a subform of P-450 3b that is expressed in New Zealand White rabbits but not in strain IIIVO/J. Many of the compounds investigated here are metabolites of progesterone and, therefore, may modulate P-450 3b-mediated metabolism during pregnancy.     

Isolation and sequence analysis of three cloned cDNAs for rabbit liver proteins that are related to rabbit cytochrome P-450 (form 2), the major phenobarbital-inducible form

We have isolated from rabbit liver three cDNA clones of 1400-1800 base pairs that hybridize selectively to RNA from animals treated with phenobarbital. The nucleotide sequences of the cDNAs have been determined. In the protein coding region the nucleotide sequences of two of the cDNAs are 88% homologous, and the third cDNA is about 72-74% homologous to the other two. All three are 55-60% homologous to rat liver cytochrome P-450b cDNA. The amino acid sequences derived from the cDNA sequences are about 50% homologous to those of rat liver cytochrome P-450b and rabbit liver cytochrome P-450 (form 2). The degree of homology differs substantially in different regions of the protein. The hydrophobicity profiles of these five mammalian cytochromes P-450 are very similar and contain up to eight regions of hydrophobicity that are long enough to span a membrane. These results indicate that these three cDNAs code for rabbit liver cytochromes P-450 which are different from any rabbit liver cytochrome P-450 for which amino acid sequence information is published. These cDNAs are part of a family of genes that are related to rabbit liver cytochrome P-450 (form 2) and rat liver cytochrome P-450b which are the major phenobarbital-inducible forms. The divergence of amino acid sequence between the rat and rabbit forms and the divergence of nucleotide sequences of silent sites in the two most closely related rabbit forms suggest that cytochromes P-450 have a relatively high rate of amino acid divergence compared to many other vertebrate proteins.     

Immunochemical similarity of P-450 HFLa, a form of cytochrome P-450 in human fetal livers, to a form of rat liver cytochrome P-450 inducible by macrolide antibiotics

A protein immunochemically related to P-450 HFLa, a form of cytochrome P-450 purified from human fetal livers, was detected in rat liver microsomes. The content of the immunoreactive protein in rat liver microsomes was increased by treatments with phenobarbital, pregnenolone 16 alpha-carbonitrile (PCN), erythromycin, erythromycin estolate, and oleandomycin but not with 3-methylcholanthrene, imidazole, ethanol, isosafrole, josamycin, midecamycin, or miocamycin. The activity of erythromycin N-demethylase correlated with the content of the immunoreactive protein in rat liver microsomes (r = 0.72). In addition, anti-P-450 HFLa IgG inhibited erythromycin N-demethylase in liver microsomes from erythromycin- or oleandomycin-pretreated rats. Furthermore, the content of the immunoreactive protein highly correlated with that of P-450 PB-1, which is distinct from Waxman's terminology, and is one of the forms of PCN-inducible cytochrome P-450s (r = 0.95). From these results and the results reported so far, it seems possible that P-450 HFLa is one of the forms of cytochrome P-450 inducible by glucocorticoids.     

Isolation and properties of cytochrome P-450 from rabbit liver microsomes]

A purified low-spin form of cytochrome P-450 was isolated from phenobarbital-induced rabbit liver microsomes. The preparation was functionally active and free from cytochromes b5 and P-420 and phospholipids. The specific content of the cytochrome was 18 nmoles per mg of protein. At the molecular weight of the hemoprotein of 50,000, it corresponds to 90% of purification. The purified hemoprotein binds substrates of type II and some substrates of type I. The complexes formed reveal spectral properties, similar to those for the complexes of these substrates with the microsomal form of cytochrome P-450.     

Inhibition of cholesterol 7 alpha-hydroxylase by an antibody to a male-specific form of cytochrome P-450 from subfamily P450IIC.

The absence of antibodies to cholesterol 7 alpha-hydroxylase (EC 1.14.13.17), the rate-determining enzyme for bile acid synthesis, has significantly compromised studies on this protein. Nine antibodies raised against proteins from the cytochrome P-450 gene families (P450I, P450IIA, P450IIB, P450IIC and P450III) were tested as inhibitors of 7 alpha-hydroxylase activity. An antibody raised against a male-predominant P-450 (PB2a, P450h) from the P450IIC gene subfamily was an effective inhibitor of activity in liver microsomal fractions from rat, mouse and hamster. The inhibition could be reversed by the addition of PB2a antigen, indicating structural similarity between cholesterol 7 alpha-hydroxylase and proteins within the P450IIC subfamily. Western blot analysis of hepatic microsomal fractions with the PB2a antibody gave three bands, two of which, like cholesterol 7 alpha-hydroxylase, did not inhibit sexual dimorphism. The intensity of one of the bands (apparent Mr 54,000) correlated with changes observed in activity due to diet [Spearman correlation of 0.800 (P less than 0.01)]. These findings suggest that cholesterol 7 alpha-hydroxylase is a form of P-450 which shares structural similarity with cytochromes P-450 in the P450IIC gene subfamily and that its feedback regulation by bile acid involves protein induction rather than simply post-translational modification.     

Evidence that rabbit cytochrome P-450 K is encoded by the plasmid pP-450 PBc2

The complete coding sequence for P-450 PBc2 was inserted into a T7-phage promoter system, and a capped cRNA was generated using T7 RNA polymerase. The P-450 PBc2 cRNA was translated in a rabbit reticulocyte lysate. The in vitro translation product was indirectly immunoprecipitated by the monoclonal antibodies 2F5 and 3C3 that recognize P-450 K. SDS-polyacrylamide gel electrophoresis revealed that the translated protein product exhibits the same relative electrophoretic mobility as P-450 K. The N-terminal amino acid sequence was determined to be MDLVVVLGL-LS-LLLLSL- for P-450 K immunopurified from rabbit kidney using the monoclonal antibody 2F5. This sequence agrees with the predicted amino acid sequence derived from the P-450 PBc2 cDNA. These results indicate that P-450 K or a protein closely related to P-450 K is encoded by the plasmid pP-450 PBc2.     

A rabbit liver constitutive form of cytochrome P450 responsible for amphetamine deamination

A cytochrome P450 isozyme responsible for amphetamine deamination was purified from hepatic microsomes of untreated rabbits. The purification procedures consisted of a set of column chromatographies with omega-aminooctyl-Sepharose 4B, DEAE-cellulose, CM-Sephadex C-50, and hydroxyapatite. The deamination activity was determined by measuring the formation of phenylacetone after derivatization to the p-nitrobenzyloxim by HPLC. This isozyme, which was designated P450APD, showed a monomeric molecular weight of 51,000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and exhibited an absorption maximum of reduced CO complex at 451 nm. On the basis of the specificity toward testosterone metabolism and the N-terminal amino acid sequence, P450APD was attributed to a member of P450 class IIC subfamily, which is identical or closely related to LM3b (D. R. Koop and M. J. Coon (1979) Biochem, Biophys, Res. Commun. 91, 1075-1081), form 3b (E. F. Johnson (1980) J. Biol. Chem. 255, 304-309), and other similar preparations. Antibody against the P450APD inhibited about 80% of the amphetamine deamination activity in rabbit hepatic microsomes as well as in the reconstitution system of this P450. The present results support that P450APD is the major P450 isozyme responsible for amphetamine deamination in rabbit liver.     

Immunolocalization of cholesterol side-chain-cleavage cytochrome P-450 and 17 alpha-hydroxylase cytochrome P-450 in bovine ovarian follicles

Follicles were collected from cows and processed for electron microscopy and for immunofluorescent staining at the light microscope level. Key regulatory steroidogenic enzymes cholesterol side-chain-cleavage cytochrome P-450 (P-450scc) and 17 alpha-hydroxylase cytochrome P-450 (P-45017 alpha) were immunolocalized using specific IgG fractions raised against these enzymes. In larger follicles in which the theca interna had differentiated, positive staining for cytochromes P-450scc and P-450(17) alpha was observed in the cells of the theca interna. Electron microscopic examination showed that these cells were rich in endoplasmic reticulum, mainly rough, and had moderate numbers of mitochondria with tubular and lamellar cristae. Positive staining was also present in the theca of follicles undergoing atresia. Positive staining for cytochrome P-450(17) alpha was not observed in the membrana granulosa but cytochrome P-450scc was present in the membrana granulosa in some follicles, particularly in the larger antral follicles. By contrast, positive staining for both enzymes was not observed in stroma, surface epithelium or in small preantral follicles in which the theca interna had not differentiated. These results indicate good agreement between the type(s) of steroidogenic enzyme(s) present in tissues and the type(s) of steroid hormone(s) produced. It is concluded that regulation of steroid hormone production involves, at least in part, regulation of the levels of steroidogenic enzymes.