Characterization of the liver mitochondrial cytochrome P-450 catalyzing the 26-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol

Enzymatic activation in vitamin D signaling - Past, present and future

Vitamin D signaling is important in regulating calcium homeostasis essential for bone health but also displays other functions in cells of several tissues. Disturbed vitamin D signaling is linked to a large number of diseases. The multiple cytochrome P450 (CYP) enzymes catalyzing the different hydroxylations in bioactivation of vitamin D₃ are crucial for vitamin D signaling and function. This review is focused on the progress achieved in identification of the bioactivating enzymes and their genes in production of 1α,25-dihydroxyvitamin D₃ and other active metabolites. Results obtained on species- and tissue-specific expression, catalytic reactions, substrate specificity, enzyme kinetics, and consequences of gene mutations are evaluated. Matters of incomplete understanding regarding the physiological roles of some vitamin D hydroxylases are critically discussed and the authors will give their view of the importance of each enzyme for vitamin D signaling. Roles of different vitamin D receptors and an alternative bioactivation pathway, leading to 20-hydroxylated vitamin D₃ metabolites, are also discussed. Considerable progress has been achieved in knowledge of the vitamin D₃ bioactivating enzymes. Nevertheless, several intriguing areas deserve further attention to understand the pleiotropic and diverse activities elicited by vitamin D signaling and the mechanisms of enzymatic activation necessary for vitamin D-induced responses.

Comparative regulation of hepatic sterol 27-hydroxylase and cholesterol 7alpha-hydroxylase activities in the rat, guinea pig, and rabbit: effects of cholesterol and bile acids

The regulation of the classic and alternative bile acid synthetic pathways by key hepatic enzyme activities (microsomal cholesterol 7alpha-hydroxylase and mitochondrial sterol 27-hydroxylase, respectively) was examined in bile acid depletion and replacement and cholesterol-feeding experiments with rats, guinea pigs, and rabbits. The bile acid pool was depleted by creating a bile fistula (BF) and collecting bile for 2 to 5 days, and it was replaced by intraduodenal infusion of the major biliary bile acids (taurocholic acid [TCA], glycochenodeoxycholic acid [GCDCA], and glycocholic acid [GCA] in the rat, guinea pig, and rabbit, respectively) at rates equivalent to the measured hepatic flux of the bile acids. To study the effects of cholesterol, the animals were fed for 7 days on a basal diet with and without 2% cholesterol. Cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase activities, measured by isotope incorporation assays, were related to bile acid output and composition and hepatic cholesterol concentrations. Intraduodenal infusion of bile acids increased the output of the tested bile acids, but did not significantly change hepatic cholesterol concentrations and had no effect on sterol 27-hydroxylase activity. Neither bile acid depletion nor replacement affected sterol 27-hydroxylase activity when three different substrates (cholesterol, 5beta-cholestane-3alpha,7alpha-diol, and 5beta-cholestane-3alpha,7alpha,12alpha-triol) were tested. In contrast, feeding 2% cholesterol increased hepatic cholesterol concentrations in rats, guinea pigs, and rabbits threefold, twofold, and eightfold, respectively, and increased hepatic mitochondrial sterol 27-hydroxylase activity (conversion of cholesterol to 27-hydroxycholesterol) in all three animal models. The stimulation and feedback inhibition of cholesterol 7alpha-hydroxylase activity by bile acid depletion and replacement were observed in all three animal models, whereas the effect of cholesterol feeding was species-dependent (cholesterol 7alpha-hydroxylase activity increased in the rat, did not change in the guinea pig, and was inhibited in the rabbit). Thus, in contrast to sterol 27-hydroxylase, which was upregulated by cholesterol but not affected by bile acid depletion and replacement in all three animal models, cholesterol 7alpha-hydroxylase activity was controlled consistently and inversely by the hepatic flux of bile acids, but was species-dependent in its response to a 1-week feeding with 2% cholesterol.

Biochemical characterization of a truncated form of CYP27A purified from rabbit liver mitochondria

During purification of CYP27A from rabbit liver mitochondria, a cytochrome P450 of different molecular size was co-isolated. The latter enzyme has an apparent M(r) 51,000 which is slightly lower than that of CYP27A. The 51,000-M(r) protein was found to be present in mitochondria from liver, small intestine, kidney, and spleen but not in lung, testis, heart, or brain mitochondria. Determination of the N-terminal sequence revealed that the 51,000-M(r) protein is a truncated form of CYP27A lacking the first 12 residues. The truncated enzyme was less efficient than the full-length CYP27A in the 27-hydroxylation of C(27)-sterols and much less efficient in the 25-hydroxylation of 1alpha-hydroxyvitamin D(3). The K(m) values for cholesterol and 5beta-cholestane-3alpha,7alpha,12alpha-triol were about the same with both enzymes whereas the K(m) for 1alpha-hydroxyvitamin D(3) was much higher with the truncated CYP27A. The results strongly indicate that the 51,000-M(r) protein is formed via proteolytic processing of CYP27A by endogenous protease(s) in some of the tissues examined. The truncation at the N terminus markedly impairs the ability of CYP27A to use 1alpha-hydroxyvitamin D(3) as substrate and to catalyze 25-hydroxylation in the bioactivation of vitamin D(3).

Expression, purification, and enzymatic properties of recombinant human cytochrome P450c27 (CYP27)

A large number of microsomal P450s have been expressed in Escherichia coli in quantities sufficient for structure/function analysis. However, only one mitochondrial P450 has been successfully overexpressed, that being cholesterol side chain cleavage cytochrome P450 (P450scc). We report here overexpression, purification, and characterization of a second mitochondrial P450, human sterol C-27 hydroxylase (P450c27). The conditions used for expression are very similar to those applied for P450scc, although a quite different purification protocol was necessary to achieve highly purified P450c27. The catalytic properties of purified recombinant human P450c27 resemble those of purified, endogenous rat and rabbit P450c27, regarding both specificity and turnover numbers. Like endogenous P450c27 from rat and rabbit liver, human recombinant P450c27 is only functional in the presence of adrenodoxin and adrenodoxin reductase and shows no activity in the presence of the microsomal P450 reductase. We conclude that P450c27 is most likely not the 1alpha-hydroxylase of 25-hydroxyvitamin D3, contrary to a previous suggestion (Axen, E., Postlind, H., Sjöberg, H., and Wikvall, K. (1994) Proc. Natl. Acad. Sci. USA 91, 10014-10018) because this activity of P450c27 (28 pmol/min/nmol P450) seems far too low to be physiologically relevant. This activity is 10(3) times lower than the 27-hydroxylase activity toward 5beta-cholestane-3alpha,7alpha,12alpha-triol and 40 times lower than the 27-hydroxylation of cholesterol by this enzyme. The development of this expression system and purification procedure creates the potential for structure/function analysis of P450c27, including possible crystallization of this important enzyme.

Cytochrome P450 CYP27-catalyzed oxidation of C27-steroid into C27-acid

Rabbit liver cytochrome P450 CYP27 has been previously shown to catalyze the complete conversion of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid. This study compares some properties of the reactions involved, the 27-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol and the further oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. The Km was the same for the two substrates, whereas the Vmax was three times higher for 27-hydroxylation than for the oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. Ketoconazole inhibited both reactions, whereas disulfiram did not. Carbon monoxide inhibited the 27-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol but not the further oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. There was no difference in sensitivity to varying oxygen concentrations between the two reactions. The present study shows that CYP27 also converts, although less efficiently, 5 beta-cholestane-3 alpha,7 alpha-diol into 3 alpha,7 alpha-dihydroxy-5 beta-cholestanoic acid and cholesterol into 3 beta-hydroxy-5-cholestanoic acid. The rate of oxidation of cholesterol into C27-acid was very low--less than 1% of that with the other C27-steroids.

Characterisation of taurochenodeoxycholic acid 6 alpha-hydroxylase from pig liver microsomes

A fraction of cytochrome P-450 catalysing an efficient 6 alpha-hydroxylation of taurine-conjugated 3 alpha,7 alpha-dihydroxy-5 beta- cholanoic acid (taurochenodeoxycholic acid) was partially purified from pig liver microsomes. The specific content of cytochrome P-450 was 6 nmol/mg protein and the preparation showed two major protein bands upon SDS/PAGE. These two bands were isolated after SDS/PAGE and protein blotting. The protein band with a molecular mass of 53 kDa had an N-terminal amino acid sequence and internal sequences resembling that of the cytochrome P-450 4A subfamily (CYP 4A). Polyclonal antibodies raised against this protein were able to, after SDS/PAGE and immunoblotting, detect the protein in microsomal fractions as well as in the purified cytochrome P-450 fraction. Furthermore, addition of these antibodies to a reconstituted system containing the cytochrome P-450 fraction, inhibited 6 alpha-hydroxylation of taurochenodeoxycholic acid by up to 90%. Experiments with irrelevant antibodies did not show inhibition of 6 alpha-hydroxylation. The purified cytochrome P-450 fraction catalysed in addition omega- and omega-1 hydroxylation of lauric acid and 6 alpha-hydroxylation of 3 alpha-hydroxy-5 beta-cholanoic acid (lithocholic acid). However, these hydroxylase activities were rather low compared to 6 beta-hydroxylation of taurochenodexycholic acid. The enzyme fraction did not show hydroxylase activities towards cholesterol and 5 beta-cholestane-3 alpha,7 alpha-diol. These results indicate that 6 alpha-hydroxylation of taurochenodeoxycholic acid is catalysed by a specific species of cytochrome P-450 that, according to N-terminal amino acid sequence as well as catalytic properties, could be a member of the CYP 4A subfamily.

Liver mitochondrial cytochrome P450 CYP27 and recombinant-expressed human CYP27 catalyze 1 alpha-hydroxylation of 25-hydroxyvitamin D3

A cytochrome P450 catalyzing 1 alpha-hydroxylation of 25-hydroxyvitamin D3 was purified from pig liver mitochondria. It also catalyzed 27-hydroxylation of 25-hydroxyvitamin D3 and 25-hydroxylation of vitamin D3. The ratio between the 1 alpha-, 27-, and 25-hydroxylase activities remained essentially constant during the purification. Substrates for sterol 27-hydroxylase CYP27 inhibited and a monoclonal antibody raised against CYP27 immunoprecipitated the 1 alpha-, 27-, and 25-hydroxylase activities. Apparently homogeneous preparations of CYP27 from pig and rabbit liver mitochondria catalyzed 1 alpha-hydroxylation. Human liver mitochondrial CYP27 was expressed from its cDNA in Escherichia coli. The nucleotide sequence encoding the N terminus of CYP27 was modified in the first eight codons to achieve expression in E. coli. The purified recombinant-expressed CYP27 reconstituted with the electron-transferring system of adrenal mitochondria catalyzed 1 alpha-hydroxylation of 25-hydroxyvitamin D3. Expression of unmodified CYP27 cDNA in simian COS cells confirmed the 1 alpha-hydroxylase activity toward 25-hydroxyvitamin D3.

Selective inhibition of mitochondrial 27-hydroxylation of bile acid intermediates and 25-hydroxylation of vitamin D3 by cyclosporin A

It was demonstrated recently that cyclosporin A blocks bile acid synthesis in cultured rat and human hepatocytes by specific inhibition of chenodeoxycholic acid formation. The site of inhibition was found to be the 27-hydroxylation of cholesterol catalysed by the liver mitochondrial 27-hydroxylase [Princen, Meijer, Wolthers, Vonk and Kuipers (1991) Biochem J. 275, 501-505]. In this paper the mechanism by which cyclosporin A blocks mitochondrial 27-hydroxylation was further investigated. It is shown that cyclosporin A inhibited 27-hydroxylation of bile acid intermediates, depending on their polarity. In isolated rat liver mitochondria, 27-hydroxylation of cholesterol was dose-dependently blocked by the drug, giving half-maximal inhibition at 4 microM, whereas 27-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol was not affected. A similar observation was made using electrophoretically homogeneous cytochrome P-450(27) isolated from rabbit liver mitochondria, excluding the possibility that cyclosporin A interfered with transport of substrates into the mitochondrion. Kinetic studies showed that inhibition of the 27-hydroxylation of cholesterol by cyclosporin A was of a non-competitive type. The drug also inhibited the 25-hydroxylase activity towards vitamin D3, catalysed by the same enzyme preparation, to the same extent as 27-hydroxylation of cholesterol. These results suggest that cyclosporin A may interfere with binding of cholesterol, but not of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol, to the active site of the enzyme. These data provide an explanation for the selective inhibition of chenodeoxycholic acid synthesis.

24-, 25- and 27-hydroxylation of cholesterol by a purified preparation of 27-hydroxylase from pig liver

Pig liver mitochondria were found to catalyze 27-, 25- and 24-hydroxylation of cholesterol at relative rates of about 1:0.2:0.04. An apparently homogeneous preparation of pig liver mitochondrial cytochrome P-450-27 was found to catalyze the same three hydroxylations at about the same relative rates when reconstituted with adrenodoxin and adrenodoxin reductase. The 24-hydroxycholesterol formed was shown to consist of one of the two possible stereoisomers. When using specifically deuterium-labeled substrates a significant isotope effect was observed in the case of 24-hydroxylation (KH/KD > 10), but not 25-hydroxylation (KH/KD = 1.1), or 27-hydroxylation (KH/KD = 1.1). The difference between the 24-hydroxylation and the other two hydroxylations may be due to different interactions between cholesterol and the same enzyme, with a resulting difference with respect to the rate-limiting step in the reaction. The physiological significance of the mitochondrial 24-hydroxylation is discussed.

The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature

We provide here a list of 221 P450 genes and 12 putative pseudogenes that have been characterized as of December 14, 1992. These genes have been described in 31 eukaryotes (including 11 mammalian and 3 plant species) and 11 prokaryotes. Of 36 gene families so far described, 12 families exist in all mammals examined to date. These 12 families comprise 22 mammalian subfamilies, of which 17 and 15 have been mapped in the human and mouse genome, respectively. To date, each subfamily appears to represent a cluster of tightly linked genes. This revision supersedes the previous updates [Nebert et al., DNA 6, 1-11, 1987; Nebert et al., DNA 8, 1-13, 1989; Nebert et al., DNA Cell Biol. 10, 1-14 (1991)] in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene and cDNA, we recommend that the italicized root symbol "CYP" for human ("Cyp" for mouse), representing "cytochrome P450," be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen should precede the final number in mouse genes. "P" ("p" in mouse) after the gene number denotes a pseudogene. If a gene is the sole member of a family, the subfamily letter and gene number need not be included. We suggest that the human nomenclature system be used for all species other than mouse. The mRNA and enzyme in all species (including mouse) should include all capital letters, without italics or hyphens. This nomenclature system is identical to that proposed in our 1991 update. Also included in this update is a listing of available data base accession numbers for P450 DNA and protein sequences. We also discuss the likelihood that this ancient gene superfamily has existed for more than 3.5 billion years, and that the rate of P450 gene evolution appears to be quite nonlinear. Finally, we describe P450 genes that have been detected by expressed sequence tags (ESTs), as well as the relationship between the P450 and the nitric oxide synthase gene superfamilies, as a likely example of convergent evolution.

Characterization of mitochondrial cytochromes P-450 from pig kidney and liver catalysing 26-hydroxylation of 25-hydroxyvitamin D3 and C27 steroids

The properties of cytochrome P-450 from pig kidney mitochondria, catalysing 26-hydroxylation of 25-hydroxyvitamin D3 and C27 steroids [Postlind & Wikvall (1989) Biochem. Biophys. Res. Commun. 159, 1135-1140; Postlind (1990) Biochem. Biophys. Res. Commun. 168, 261-266], were compared with those of a 26-hydroxylating cytochrome P-450 from pig liver mitochondria. The liver enzyme was purified to a cytochrome P-450 content of 7.4 nmol/mg of protein and showed only one protein band with an apparent Mr of 53,000 upon SDS/PAGE. The cytochrome P-450 catalysed 26-hydroxylation of 25-hydroxyvitamin D3, cholesterol and 5 beta-cholestane-3 alpha, 7 alpha-diol at rates of 361, 1090 and 2065 pmol/min per nmol of cytochrome P-450. A monoclonal antibody against the purified liver mitochondrial cytochrome P-450 26-hydroxylase (cytochrome P-450(26] was prepared. After coupling to Sepharose, the antibody was able to bind to cytochrome P-450(26) from liver as well as from kidney mitochondria and to immunoprecipitate the 26-hydroxylase activity towards 25-hydroxyvitamin D3 and cholesterol when assayed in a reconstituted system. After SDS/PAGE and immunoblotting with the antibody, the cytochrome P-450(26) was detected in the purified liver and kidney preparations. These results indicate that similar species of cytochrome P-450 catalyse 26-hydroxylation of 25-hydroxyvitamin D3 and C27 steroids in liver and kidney mitochondria. The results with the monoclonal antibody together with the finding that cholesterol competitively inhibits the 26-hydroxylation of 25-hydroxyvitamin D3 further indicate that 26-hydroxylation of 25-hydroxyvitamin D3 and cholesterol is catalysed by the same species of cytochrome P-450 in each tissue. The N-terminal amino acid sequence of cytochrome P-450(26) in kidney mitochondria resembled that of pig kidney microsomal 25-hydroxylase active in 25-hydroxylation of vitamin D3 and C27 steroids, whereas the sequence of pig liver mitochondrial cytochrome P-450(26) differed from those of rabbit and rat liver mitochondrial 26-hydroxylases as well as from those of other hitherto isolated mammalian cytochromes P-450.

A cDNA encoding a rat mitochondrial cytochrome P450 catalyzing both the 26-hydroxylation of cholesterol and 25-hydroxylation of vitamin D3: gonadotropic regulation of the cognate mRNA in ovaries

A cDNA expression library prepared from rat liver RNA was screened with a polyclonal antibody specific for mitochondrial vitamin D3 25-hydroxylase and a cDNA for rabbit liver mitochondrial cytochrome P450c26 (CYP 26), yielding cDNA clones with identical sequences. The deduced amino acid sequence derived from a 1.9-kb full-length cDNA was 73% identical to that of rabbit cytochrome P450c26. A monoclonal antibody was used to demonstrate that the product of the 1.9-kb cDNA clone was targeted to the mitochondrial compartment when expressed in COS cells. Mitochondrial membranes containing the expressed protein showed both vitamin D3 25-hydroxylase and cholesterol 26-hydroxylase activities when reconstituted with ferredoxin reductase and ferredoxin, demonstrating that the same P450, designated as P450c26/25, can catalyze both reactions. Northern blot analysis revealed that the P450c26/25 cDNA hybridizes with a 2.4-kb RNA from rat liver and unstimulated ovaries. Treatment of rats with pregnant mare's serum gonadotropin resulted in a fivefold increase in the 2.4-kb mRNA as well as the appearance of a 2.1-kb mRNA species in the ovaries. Our findings document the presence of a regulated bifunctional mitochondrial cytochrome P450 capable of catalyzing the 25-hydroxylation of vitamin D3 and the 26-hydroxylation of cholesterol.

Oxidation of 5 beta-cholestane-3 alpha,7 alpha, 12 alpha-triol into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid by cytochrome P-450(26) from rabbit liver mitochondria

The mitochondrial cytochrome P-450(26), previously shown to catalyze 26-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol, was found to convert this substrate also into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid. The formation of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid increased with increasing incubation time and enzyme concentration. Addition of NAD+ to the incubation mixture did not increase the formation of the acid. Incubation with 5 beta-cholestane-3 alpha,7 alpha,12 alpha,26-tetrol, cytochrome P-450(26), ferredoxin, ferredoxin reductase and NADPH resulted in one major product, 3 alpha,7 alpha, 12 alpha-trihydroxy-5 beta-cholestanoic acid. The cytochrome P-450 required both ferredoxin, ferredoxin reductase and NADPH for activity. NADPH could not be replaced by NAD+ or NADP+.