Purification from rabbit and rat liver of cytochromes P-450 involved in bile acid biosynthesis

Bile acids and their receptors in regulation of gut health and diseases

It is well established that bile acids play important roles in lipid metabolism. In recent decades, bile acids have also been shown to function as signaling molecules via interacting with various receptors. Bile acids circulate continuously through the enterohepatic circulation and go through microbial transformation by gut microbes, and thus bile acids metabolism has profound effects on the liver and intestinal tissues as well as the gut microbiota. Farnesoid X receptor and G protein-coupled bile acid receptor 1 are two pivotal bile acid receptors that highly expressed in the intestinal tissues, and they have emerged as pivotal regulators in bile acids metabolism, innate immunity and inflammatory responses. There is considerable interest in manipulating the metabolism of bile acids and the expression of bile acid receptors as this may be a promising strategy to regulate intestinal health and disease. This review aims to summarize the roles of bile acids and their receptors in regulation of gut health and diseases.

CYP7B1-mediated metabolism of 5alpha-androstane-3alpha,17beta-diol (3alpha-Adiol): a novel pathway for potential regulation of the cellular levels of androgens and neurosteroids

The current study presents data indicating that 5alpha-androstane-3alpha,17beta-diol (3alpha-Adiol) undergoes a previously unknown metabolism into hydroxymetabolites, catalyzed by CYP7B1. 3alpha-Adiol is an androgenic steroid which serves as a source for the potent androgen dihydrotestosterone and also can modulate gamma-amino butyric acid A (GABA(A)) receptor function in the brain. The steroid hydroxylase CYP7B1 is known to metabolize cholesterol derivatives, sex hormone precursors and certain estrogens, but has previously not been thought to act on androgens or 3alpha-hydroxylated steroids. 3alpha-Adiol was found to undergo NADPH-dependent metabolism into 6- and 7-hydroxymetabolites in incubations with porcine microsomes and human kidney-derived HEK293 cells, which are high in CYP7B1 content. This metabolism was suppressed by addition of steroids known to be metabolized by CYP7B1. In addition, 3alpha-Adiol significantly suppressed CYP7B1-mediated catalytic reactions, in a way as would be expected for substrates that compete for the same enzyme. Recombinant expression of human CYP7B1 in HEK293 cells significantly increased the rate of 3alpha-Adiol hydroxylation. Furthermore, the observed hydroxylase activity towards 3alpha-Adiol was very low or undetectable in livers of Cyp7b1(-/-) knockout mice. The present results indicate that CYP7B1-mediated catalysis may play a role for control of the cellular levels of androgens, not only of estrogens. These findings suggest a previously unknown mechanism for metabolic elimination of 3alpha-Adiol which may impact intracellular levels of dihydrotestosterone and GABA(A)-modulating steroids.

Fifty years of advances in bile acid synthesis and metabolism

There are two major pathways that mammalian cells use to supply themselves with cholesterol, one involving the synthesis of sterols from acetyl-CoA and the other the metabolism of cholesterol-rich lipoprotein particles via receptor-mediated endocytosis. There also are several pathways that mammalian cells use to break down cholesterol, and these disposal pathways are equal in physiological importance to the supply pathways. A major catabolic route involves conversion of cholesterol into conjugated bile salts, a transformation mediated by 16 or more liver enzymes. This review highlights findings in cholesterol catabolism from the last five decades with special emphasis on advances in bile acid synthesis, transport, and regulation.

Metabolism of a novel side chain modified Delta8(14)-15-ketosterol, a potential cholesterol lowering drug: 28-hydroxylation by CYP27A1

The synthetic inhibitors of sterol biosynthesis, 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one and 3beta-hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one, are of interest as potential cholesterol lowering drugs. Rapid metabolism of synthetic 15-ketosterols may lead to a decrease, or loss, of their potency to affect lipid metabolism. 3beta-Hydroxy-5alpha-cholest-8(14)-en-15-one is reported to be rapidly side chain oxygenated by rat liver mitochondria. In an attempt to reduce this metabolism, the novel side chain modified 15-ketosterol 3beta-Hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one was synthesized. We have examined the metabolism by recombinant human CYP27A1 of this novel side chain modified 3beta-hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one and compared the rate of metabolism with that of the previously described 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one. Both sterols were found to be efficiently metabolized by recombinant human CYP27A1. None of the two 15-ketosterols was significantly metabolized by microsomal 7alpha-hydroxylation. Interestingly, CYP27A1-mediated product formation was much lower with the side chain modified 3beta-hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one than with the previously described 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one. A surprising finding was that this novel side chain modified sterol was metabolized mainly in the C-28 position by CYP27A1. The data on 28-hydroxylation by human CYP27A1 provide new insights on the catalytic properties and substrate specificity of this enzyme. The finding that 3beta-hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one with a modified side chain is metabolized at a dramatically slower rate than the previously described 15-ketosterol with unmodified side chain may be important for future development of synthetic cholesterol lowering sterols.

The pharmacological exploitation of cholesterol 7alpha-hydroxylase, the key enzyme in bile acid synthesis: from binding resins to chromatin remodelling to reduce plasma cholesterol

Mammals dispose of cholesterol mainly through 7alpha-hydroxylated bile acids, and the enzyme catalyzing the 7alpha-hydroxylation, cholesterol 7alpha-hydroxylase (CYP7A1), has a deep impact on cholesterol homeostasis. In this review, we present the study of regulation of CYP7A1 as a good exemplification of the extraordinary contribution of molecular biology to the advancement of our understanding of metabolic pathways that has taken place in the last 2 decades. Since the cloning of the gene from different species, experimental evidence has accumulated, indicating that the enzyme is mainly regulated at the transcriptional level and that bile acids are the most important physiological inhibitors of CYP7A1 transcription. Multiple mechanisms are involved in the control of CYP7A1 transcription and a variety of transcription factors and nuclear receptors participate in sophisticated regulatory networks. A higher order of transcriptional regulation, stemming from the so-called histone code, also applies to CYP7A1, and recent findings clearly indicate that chromatin remodelling events have profound effects on its expression. CYP7A1 also acts as a sensor of signals coming from the gut, thus representing another line of defence against the toxic effects of bile acids and a downstream target of agents acting at the intestinal level. From the pharmacological point of view, bile acid binding resins were the first primitive approach targeting the negative feed-back regulation of CYP7A1 to reduce plasma cholesterol. In recent years, new drugs have been designed based on recent discoveries of the regulatory network, thus confirming the position of CYP7A1 as a focus for innovative pharmacological intervention.

Flutamide metabolism in four different species in vitro and identification of flutamide metabolites in human patient urine by high performance liquid chromatography/tandem mass spectrometry

A new metabolic scheme of flutamide is proposed in this article. Some patients treated with flutamide, a nonsteroidal antiandrogen, have developed severe hepatic dysfunction. Toxic metabolites have been proposed to be responsible for these negative effects. In this study, the qualitative aspects of the in vitro metabolism of flutamide in liver microsomes from human, dog, pig, and rat were evaluated. A direct comparison of the flutamide metabolism in liver and prostate microsomes from pig was made, and the in vivo metabolism of flutamide was investigated in urine from orally treated prostate cancer patients. Liquid chromatography/tandem mass spectrometry was used for analysis. The mass spectrometer was equipped with an electrospray interface and operated in the negative ion mode. In liver microsomes from pig, dog, and rat, extensive hydroxylation of flutamide occurred. One, two, or three hydroxy groups were attached, and isomeric forms were detected for both monohydroxylated and trihydroxylated drug. In pig liver microsomes, isomers of a third metabolite, hydroxylated 4-nitro-3-(trifluoromethyl)-aniline, were also found after incubation with either flutamide or 2-hydroxyflutamide. In human liver microsomes, the pharmacologically active 2-hydroxyflutamide was the only metabolite detected. Several phase I metabolites as well as four intact phase II metabolites could be recovered from the urine samples. For the first time in humans, glucuronic acid conjugates of hydroxylated 4-nitro-3-(trifluoromethyl)-aniline, and mono- and dihydroxylated flutamide were identified, together with hydroxylated 4-nitro-3-(trifluoromethyl)-aniline conjugated with sulfate. In addition, one mercapturic acid conjugate of hydroxylated flutamide, probably formed from flutamide via a reactive intermediate, was detected.

Molecular basis for pseudo vitamin D-deficiency rickets in the Hannover pig

The molecular basis for pseudo vitamin D deficiency rickets (PDDR) in the Hannover pig model was determined in the current study. Consistent with the inability of Hannover PDDR pigs to maintain ambient levels of 1,25-dihydroxyvitamin D (i.e., 1,25D), the bioactivation enzyme cytochrome P450C1 (or CYP27B1) was determined to contain coding-region deletions that rendered the enzyme ineffective due to frame-shift mutations and expression of a premature termination codon. Expression levels of P450C1mRNA were up-regulated in response to the low-1,25D high-parathyroid hormone state of the PDDR animals. In a complementary manner, cytochrome P450C24 mRNA was not detectable in PDDR pigs. Two different deletions were detected within the Hannover pig strain in which the P450C1 coding region contained either 173 bp or 329 bp deletions that resulted in the expression of non-sense products beginning within the I-helix region and extending through the truncated C-terminal domains. The boundaries for the deletion segments aligned with derived mRNA processing sites. This observation was consistent with an mRNA processing error as the causative factor for the coding-region deletions. Based upon the expression of a non-functional P450C1 enzyme, the Hannover pig model for PDDR was determined to be identical to the human disease in which enzyme-inhibitory mutations are the molecular basis for the calcium disorder.

Oxysterol 7 alpha-hydroxylase activity by cholesterol 7 alpha-hydroxylase (CYP7A)

A 7 alpha-hydroxylation is necessary for conversion of both cholesterol and 27-hydroxycholesterol into bile acids. According to current theories, cholesterol 7 alpha-hydroxylase (CYP7A) is responsible for the former and oxysterol 7 alpha-hydroxylase (CYP7B) for the latter reaction. CYP7A is believed to have a very high substrate specificity whereas CYP7B is active toward oxysterols, dehydroepiandrosterone, and pregnenolone. In the present study, 7 alpha-hydroxylation of various oxysterols in liver and kidney was investigated. Surprisingly, human cholesterol 7 alpha-hydroxylase, CYP7A, expressed as a recombinant in Escherichia coli and COS cells, was active toward 20(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol. This enzyme has previously been thought to be specific for cholesterol and cholestanol. A partially purified and reconstituted cholesterol 7 alpha-hydroxylase enzyme fraction from pig liver showed 7 alpha-hydroxylase activity toward the same oxysterols as metabolized by expressed recombinant human and rat CYP7A. The 7 alpha-hydroxylase activity toward 20(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol in rat liver was significantly increased by treatment with cholestyramine, an inducer of CYP7A. From the present results it may be concluded that CYP7A is able to function as an oxysterol 7 alpha-hydroxylase, in addition to the previously known human oxysterol 7 alpha-hydroxylase, CYP7B. These findings may have implications for oxysterol-mediated regulation of gene expression and for pathways of bile acid biosynthesis. A possible use of 20(S)-hydroxycholesterol as a marker substrate for CYP7A is proposed.

DNA-mediated transport of the intermediate filament protein vimentin into the nucleus of cultured cells

A number of characteristic properties of intermediate filament (IF) proteins, such as nucleic acid-binding activity, affinity for histones and structural relatedness to transcription factors and nuclear matrix proteins, in conjunction with the tight association of IFs with the nucleus, suggest that these proteins might also fulfill nuclear functions in addition to their structure-organizing and -stabilizing activities in the cytoplasm. Yet, cytoplasmic IF proteins do not possess nuclear localization signals. In a search for carriers capable of transporting the IF protein vimentin into the nucleus, complexes of FITC-vimentin with various DNAs were microinjected into the cytoplasm of cultured cells and the intracellular distribution of the protein was followed by confocal laser scanning microscopy. The single-stranded oligodeoxyribonucleotides oligo(dG)25, oligo[d(GT)12G] and oligo[d(G3T2A)4G] proved to be excellent nuclear carriers for vimentin. However, in fibroblasts, fluorescence-labeled vimentin taken up by the nuclei remained undetectable with affinity-purified, polyclonal anti-vimentin antibody, whereas it was readily identifiable in the nuclei of microinjected epithelial cells in this way. Moreover, when FITC-vimentin was preinjected into fibroblasts and allowed to assemble into the endogenous vimentin filament system, it was still transferred into the nucleus by post-injected oligo(dG)25, although to a lesser extent. Superhelical circular DNAs, like pBR322, SV40 and mitochondrial DNA, were also characterized by considerable capacities for nuclear vimentin transport; these transport potentials were totally destroyed by relaxation or linearization of the DNA molecules. Nevertheless, certain linear double-stranded DNA molecules with a high affinity for vimentin IFs, such as repetitive telomere and centromere or mobile long interspersed repeat (LINE) DNA, could carry FITC-vimentin into the nucleus. This was also true for a 375 bp extrachromosomal linear DNA fragment which occurs in the cytoplasm of mouse tumor cells and which is capable of immortalizing human lymphocytes. On the basis of these results, it appears very likely that cellular and viral products of reverse transcription as well as other extrachromosomal DNAs, which are circular, superhelical and apparently shuttling between the cytoplasm and the nucleus (eccDNA), are constantly loaded with vimentin in vimentin-positive cells. Since such DNAs are considered as markers of genomic instability, it is conceivable that vimentin directly participates as an architectural, chromatin-modifying protein in recombinatorial processes set off by these DNAs in the nucleus.

Renal and hepatic 1 alpha-hydroxylation of 25-hydroxyvitamin D3 in piglets suffering from pseudo vitamin D-deficiency rickets, type I

The piglets examined suffer from rickets and have symptoms similar to those of classic pseudo vitamin D-deficiency rickets, type I (PVDRI), including plasma concentrations of 1 alpha, 25-dihydroxyvitamin D3 considerably lower than in healthy control piglets. It has been suggested that the rachitic piglets have a defective renal 1 alpha,25-dihydroxyvitamin D3 production. The present study shows that partially purified mitochondrial and microsomal cytochrome P450 from kidney and liver of both rachitic and control animals is able to catalyze 1 alpha-hydroxylation of 25-hydroxyvitamin D3. The renal mitochondrial 1 alpha-hydroxylase activity was higher in the rachitic piglets whereas the renal microsomal 1 alpha-hydroxylase activity was decreased. The immunodetectable levels in kidney of a mitochondrial 1 alpha-hydroxylase (CYP27) and a microsomal 1 alpha-hydroxylase (vitamin D3 25-hydroxylase) were correlated with the 1 alpha-hydroxylase activities. The results suggest that the renal microsomal 1 alpha-hydroxylase is affected by the rachitic condition. It is concluded that the primary genetic defect of systemic 1 alpha,25-dihydroxyvitamin D3 deficiency in the rachitic PVDRI piglets does not reside in a defective function or absence of renal mitochondrial 25-hydroxyvitamin D3 1 alpha-hydroxylase. From this, it may also be concluded that PVDRI in man and pig appear to be two different forms of the disease.

Biochemical characterization of the 7alpha-hydroxylase activities towards 27-hydroxycholesterol and dehydroepiandrosterone in pig liver microsomes

Microsomal cytochrome P-450 catalyzing the 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone was partially purified from pig liver. This enzyme fraction also catalyzed 7alpha-hydroxylation of 25-hydroxycholesterol and pregnenolone but did not 7alpha-hydroxylate cholesterol or testosterone. Studies with extrahepatic tissues have suggested the possibility of one common enzyme responsible for the 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone. A series of experiments was performed to study if there are one or several enzymes 7alpha-hydroxylating these steroids in the liver. The activities towards the two substrates copurified but the ratio between 27-hydroxycholesterol and dehydroepiandrosterone 7alpha-hydroxylation varied considerably in different purification steps and between different preparations. The enzyme inhibitors disulfiram, N-bromosuccinimide, ketoconazole, metyrapone and alpha-naphthoflavone affected the activities in a similar way. Dehydroepiandrosterone inhibited 27-hydroxycholesterol 7alpha-hydroxylation whereas 27-hydroxycholesterol had almost no inhibitory effect on dehydroepiandrosterone 7alpha-hydroxylation. Experiments to examine the nature of inhibition by dehydroepiandrosterone indicated that the two steroids did not compete for the same active site. The results of this study do not rule out the possibility of one single enzyme catalyzing 7alpha-hydroxylation of the two steroids. However, taken together the data suggest that hepatic microsomal 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone involves at least two, probably closely related, enzymes. (c) 1998 Elsevier Science B. V.

Purification of a 3beta-hydroxy-delta5-C27-steroid dehydrogenase from pig liver microsomes active in major and alternative pathways of bile acid biosynthesis

A 3beta-hydroxy-Delta5-C27-steroid dehydrogenase active in bile acid biosynthesis was purified from pig liver microsomes by solubilization with sodium cholate and by chromatography on DEAE-Sepharose, aminohexyl-Sepharose, and blue Sepharose. The last step in the purification procedure was preparative isoelectric focusing in a Rotofor cell. The final enzyme preparation showed only one protein band upon SDS-polyacrylamide gel electrophoresis. The isoelectric point was estimated to about 7.0 and the apparent Mr was 36,000. The purified enzyme catalyzed the conversion of 7alpha-hydroxycholesterol, 7alpha,25-dihydroxycholesterol, 7alpha, 27-dihydroxycholesterol, and 3beta,7alpha-dihydroxy-5-cholestenoic acid into the corresponding 3-oxo-Delta4 compounds. The enzyme was inactive with C19 and C21 steroids as substrates. The enzyme was also inactive with C27 steroids having the 7-hydroxy group in beta- instead of alpha-position. The Km was found to be 0.30 and 0.32 microM with 7alpha-hydroxycholesterol and 7alpha, 27-dihydroxycholesterol as substrates, respectively. NAD+ was the preferred cofactor. A monoclonal antibody raised against the 3beta-hydroxy-delta5-C27-steroid dehydrogenase was prepared. After coupling to Sepharose, the antibody was able to bind the dehydrogenase and to decrease the conversion of 7alpha-hydroxycholesterol into 7alpha-hydroxy-4-cholest-3-one by more than 90%. The N-terminal amino acid sequence was determined and found to be similar but not identical with those of known 3beta-hydroxy-Delta5-steroid dehydrogenases active in steroid hormone biosynthesis. Thus, the purified enzyme active toward C27 steroids in bile acid biosynthesis appears to represent a novel type of 3beta-hydroxy-delta5-steroid dehydrogenase.

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.

7 alpha hydroxylation of 25-hydroxycholesterol in liver microsomes. Evidence that the enzyme involved is different from cholesterol 7 alpha-hydroxylase

Rat, pig and human liver microsomes were found to catalyze 7 alpha-hydroxylation of 25-hydroxycholesterol. In contrast to cholesterol 7 alpha-hydroxylase activity, the 7 alpha-hydroxylase activity towards 25-hydroxycholesterol in rat liver was not stimulated by cholestyramine treatment. After transfection with cDNA for human cholesterol 7 alpha-hydroxylase, COS cells showed a significant activity towards cholesterol but not towards 25-hydroxycholesterol. During purification of cholesterol 7 alpha-hydroxylase from pig liver microsomes, about 99% of the 7 alpha-hydroxylase activity towards 25-hydroxycholesterol and 27-hydroxycholesterol was clearly separated from 7 alpha-hydroxylase activity for cholesterol. The small amount of 25-hydroxycholesterol 7 alpha-hydroxylase activity retained in a partially purified preparation of cholesterol 7 alpha-hydroxylase was not inhibited by addition of cholesterol, indicating that the oxysterol binding site is different from the cholesterol binding site, presumely due to the presence of two different enzymes. It is concluded that different enzymes are involved in 7 alpha-hydroxylation of cholesterol and 7 alpha hydroxylation of side-chain-oxidized cholesterol in rat, pig and human liver. Inhibition experiments with a partially purified fraction of the oxysterol 7 alpha-hydroxylase from pig liver gave results consistent with the contention that the same enzyme is responsible for 7 alpha hydroxylation of both 25-hydroxycholesterol and 27-hydroxycholesterol. It has been suggested that cholesterol 7 alpha-hydroxylase can preferentially use oxysterols, in particular 25-hydroxycholesterol, as substrates and by this means inactivate important physiological regulators of cholesterol homeostasis. Such a mechanism would explain the unique property of the liver to resist down-regulation of the low-density-lipoprotein receptor [Dueland, S., Trawick, J.D., & Davies, R.A. (1993) J. Biol. Chem. 267, 22695-22698]. The present results do not support the contention that the important coupling between cholesterol 7 alpha-hydroxylase activity, the low-density-lipoprotein receptor activity and hydroxymethylglutaryl coenzyme A reductase activity in liver cells is due to inactivation of 25-hydroxycholesterol or 27-hydroxycholesterol by the action of cholesterol 7 alpha-hydroxylase.

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.

Purification and characterization of a vitamin D3 25-hydroxylase from pig liver microsomes

A cytochrome P-450 which catalyses 25-hydroxylation of vitamin D3 has been purified to apparent homogeneity from pig liver microsomes. The specific content of cytochrome P-450 was 12 nmol.mg of protein-1, and the preparation showed a single band with an apparent M(r) of 50,500 upon SDS/PAGE. A monoclonal antibody raised against the vitamin D3 25-hydroxylase reacted strongly with the purified 25-hydroxylating cytochrome P-450 from pig kidney microsomes [Bergman & Postlind (1990) Biochem. J. 270, 345-350]. The liver enzyme showed structural and functional properties very similar to those of the kidney enzyme. The two enzymes differed with respect to only one of the first 16 N-terminal amino acids. The vitamin D3 25-hydroxylase in pig liver microsomes exhibited a turnover and an apparent Km for 25-hydroxylation of vitamin D3 which were of the same order of magnitude as those of a well-characterized male-specific 25-hydroxylating cytochrome P-450 in rat liver microsomes. The two enzymes differed structurally. The pig liver enzyme was, in contrast to the rat liver enzyme, not sex-specific, and did not catalyse 16 alpha-hydroxylation of testosterone. These properties of the 25-hydroxylase in rat liver microsomes have led to questions on the role of microsomal 25-hydroxylation of vitamin D3. It is concluded that studies on microsomal 25-hydroxylation with the rat may be misleading. The results of the present study show that the pig appears to be a representative species for evaluation of vitamin D3 hydroxylases in other mammals, including man.

Testing and characterizing enzymes and membrane-bound carrier proteins acting on amphipathic ligands in the presence of bilayer membrane material and soluble binding protein. Application to the uptake of oleate into isolated cells

1. A multiphasic modelling approach [Heirwegh, Meuwissen, Vermeir & De Smedt (1988) Biochem. J. 254, 101-108] is applied to systems containing poorly water-soluble amphipathic reactants, membrane material, soluble binding protein and acceptor protein (enzyme or membrane-bound carrier protein). 2. The field of application is constrained by the assumptions (i) that the amount of acceptor-bound substrate is small compared with the total amount and (ii) that all preceding chemical reactions and steps of mass transport are rapid compared with the chemical change monitored. 3. Initial-rate formulae for systems in which an acceptor interacts with unbound or protein-bound ligand are given. The saturation curves are near-hyperbolic or sigmoidal, depending both (i) on the form of ligand (unbound or protein-bound) acted upon by the acceptor and (ii) on whether the assays are performed at constant concentration of soluble binding protein Cp or at constant substrate/binding-site molar ratio RS. 4. Several diagnostic features permit unequivocal distinction between acceptor action on unbound or protein-bound substrate. In the former case, saturation curves, run at the same constant concentration of one of several binding proteins of increasing binding affinity, will show progressively increasing inhibition, the shape changing from near-hyperbolic at Km' less than K1' to sigmoidal at Km' greater than K1'.Km' is the effective Michaelis constant of the acceptor and K1' the effective dissociation constant of the binding sites of the soluble protein (for the sites with the higher binding affinity, if several classes of binding site are present on the protein). Alternatively, the maximum velocity obtained at constant RS less than or equal to 1 should increase hyperbolically with RS/(1-RS) for a binding protein with a single class of binding site. The formula that applies when the binding protein contains two classes of independent binding site is also available. When the acceptor acts on protein-bound ligand, the maximum velocity obtained at constant binding-protein concentration, Cp, increases hyperbolically with Cp. 5. Application of these and additional criteria to initial-rate data on the uptake of oleate into isolated cells supports a mechanism of carrier-mediated uptake of the unbound ligand and allows one to clarify some observations that hitherto had been poorly explained. 6. The influence of soluble binding protein on the reaction and substrate specificities of ligand/acceptor interaction is also discussed. 7. In its present state, data treatment for 'double binding-protein systems' generally requires separate determination of the binding parameters of the soluble binding protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Bile acid biosynthesis

To conclude, the last several years have seen a resurgence of interest in the biosynthesis of bile acids. This focus has come about due to the central roles that these molecules play in cholesterol and fat metabolism and due to recent advances in their chemistry, biochemistry, and molecular biology. The application of probes generated by these methodologies has begun to generate novel insight into bile acid metabolism, regulation, and genetics. The next several years should be equally exciting.

7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid by cytochrome P-450 in pig liver microsomes

Pig liver microsomes were found to catalyze the 7 alpha-hydroxylation of several potential bile acid precursors besides cholesterol. 26-Hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid were all efficiently converted into the 7 alpha-hydroxylated products. Two cytochrome P-450 fractions showing 7 alpha-hydroxylase activity could be isolated. One fraction catalyzed 7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid but was inactive towards cholesterol. The other fraction catalyzed 7 alpha-hydroxylation of cholesterol in addition to the other substrates. 26-Hydroxycholesterol in equimolar concentration did not inhibit the cholesterol 7 alpha-hydroxylase activity of this fraction. It is concluded that liver microsomes contain a cytochrome P-450 catalyzing 7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid. The results indicate that this cytochrome P-450 is different from that catalyzing 7 alpha-hydroxylation of cholesterol.

25-Hydroxyvitamin D3-1 alpha-hydroxylase in porcine hepatic tissue: subcellular localization to both mitochondria and microsomes

In vitro studies were performed to assess the ability of hepatic homogenates, mitochondria, and microsomes to 1 alpha-hydroxylate 25-hydroxyvitamin D3 [25(OH)D3]. Addition of 25(OH)D3 to either hepatic mitochondria or microsomes caused a concentration-dependent increase in the production of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. Hepatic homogenates also produced purported 1,25(OH)2D3, although at a much reduced efficiency as compared with hepatic mitochondria or microsomes. Purported 1,25(OH)2D3 synthesized by hepatic mitochondria or microsomes was identified by its mobility on several high-performance liquid chromatographic systems and, ultimately, by its ability to interact with the bovine thymus 1,25(OH)2D3 receptor protein. Production of 1,25(OH)2D3 by hepatic mitochondria and microsomes was dependent on time of incubation, protein content, and pH of incubation medium, and it required an adequate source of reducing equivalents. Generation of 1,25(OH)2D3 by these organelles could be totally blocked by the cytochrome P-450 inhibitor ketoconazole. The microsomal 1 alpha-hydroxylase could not be saturated even at the highest concentration (240 microM) of 25(OH)D3 used. The mitochondrial 1 alpha-hydroxylase, however, displayed saturation at approximately 40 microM 25(OH)D3. Eadie-Hofstee reciprocal plot analysis of the hepatic mitochondrial 1 alpha-hydroxylase gave a Km of 17 microM 25(OH)D3 and a Vmax of 481 pg of 1,25(OH)2D3 per min per mg of protein. Because of its inability to achieve substrate saturation, meaningful kinetic parameters could not be calculated for the hepatic microsomal 1 alpha-hydroxylase. These data demonstrate the liver to be an even more dynamic organ than was previously believed with respect to vitamin D metabolism in that the liver has the potential to produce 1,25(OH)2D3 in situ by at least two separate mechanisms.

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.

Purification of a cytochrome P-450 from pig kidney microsomes catalysing the 25-hydroxylation of vitamin D3

Cytochrome P-450 catalysing 25-hydroxylation of vitamin D3 was purified from pig kidney microsomes. The enzyme fraction contained 7 nmol of cytochrome P-450/mg of protein and showed only one protein band with an apparent Mr of 50,500 upon SDS/polyacrylamide-gel electrophoresis. The purified cytochrome P-450 catalysed 25-hydroxylation of vitamin D3 up to 1,000 times more efficiently, and 25-hydroxylation of 1 alpha-hydroxyvitamin D3 up to 4000 times more efficiently, than the microsomes. The cytochrome P-450 required microsomal NADPH-cytochrome P-450 reductase for catalytic activity. Mitochondrial ferredoxin and ferredoxin reductase could not replace microsomal NADPH-cytochrome P-450 reductase. The enzyme preparation showed no detectable 25-hydroxylase activity towards vitamin D2 or 1 alpha-hydroxylase activity towards 25-hydroxyvitamin D3. CO inhibited the 25-hydroxylation by more than 85%. Mannitol, hydroquinone, catalase and superoxide dismutase did not affect the 25-hydroxylation. The possible role of the kidney microsomal cytochrome P-450 in the metabolism of vitamin D3 is discussed.

25-hydroxylation of vitamin D3 in rat liver: roles of mitochondrial and microsomal cytochrome P-450

A mitochondrial cytochrome P-450 fraction, which catalyzed 25-hydroxylation of vitamin D3 much more efficiently than intact mitochondria was isolated from livers of male and female rats. For comparison, a microsomal cytochrome P-450 fraction was isolated by the same procedures. The mitochondrial cytochrome P-450 from female rats catalyzed 25-hydroxylation as efficiently as the same material from male rats. The microsomal 25-hydroxylation was male specific. The 25-hydroxylase activity in intact mitochondria and the 25-hydroxyvitamin D3 concentration in serum were similar in male and female rats. There was no correlation between the 25-hydroxylase activity in microsomal cytochrome P-450 and the 25-hydroxyvitamin D3 concentration in serum.

The role of sterol carrier protein2 and other hepatic lipid-binding proteins in bile-acid biosynthesis

The ability of different lipid-binding proteins in liver cytosol to affect enzyme activities in bile-acid biosynthesis was studied in whole microsomes (microsomal fractions) and mitochondria and in purified enzyme systems. Sterol carrier protein2 stimulated the 7 alpha-hydroxylation of cholesterol and the 12 alpha-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha-diol in microsomes and the 26-hydroxylation of cholesterol in mitochondria 2-3-fold. It also stimulated the oxidation of 5-cholestene-3 beta, 7 alpha-diol into 7 alpha-hydroxy-4-cholesten-3-one in microsomes. The stimulatory effect of sterol carrier protein2 was much less with purified cholesterol 7 alpha- and 26-hydroxylase systems than with microsomes and mitochondria. No stimulatory effect of sterol carrier protein2 was observed with purified 12 alpha-hydroxylase and 3 beta-hydroxy-delta 5-C27-steroid oxidoreductase. Sterol carrier protein (fatty-acid-binding protein), 'DEAE-peak I protein' [Dempsey, McCoy, Baker, Dimitriadou-Vafiadou, Lorsbach & Howards (1981) J. Biol. Chem. 256, 1867-1873], ligandin (glutathione transferase B) and serum albumin had no marked stimulatory effects in either crude or in purified systems. The results suggest that sterol carrier protein2 facilitates the introduction of the less-polar substrates in bile-acid biosynthesis to the membrane-bound enzymes in crude systems in vitro. The broad substrate specificity appears, however, not to be consistent with a specific regulatory function for sterol carrier protein2 in bile-acid biosynthesis.

Modulation of reconstituted cholesterol 7 alpha-hydroxylase by phosphatase and protein kinase

Cholesterol 7 alpha-hydroxylase activity was completely inhibited by incubation with alkaline phosphatase in a reconstituted enzyme system containing a cytochrome P-450, NADPH-cytochrome P-450 reductase and phospholipid. On the other hand, cAMP-dependent protein kinase stimulated cholesterol 7 alpha-hydroxylase activity by 2.5-fold. The modulation of cholesterol 7 alpha-hydroxylase activity was dependent on the amount of phosphatase or kinase added. The phosphatase inhibited enzyme activity was partially reversed by the treatment with protein kinase. These experiments indicate that the reconstituted cholesterol 7 alpha-hydroxylase activity is reversibly regulated by phosphorylation/dephosphorylation mechanism.