Solubilization and reconstitution of kidney 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylases from vitamin D-replete pigs

Regulation of cytochrome P450 by posttranslational modification

Cytochrome P450s are a family of enzymes represented in all kingdoms with expression in many species. Over 3,000 enzymes have been identified in nature. Humans express 57 putatively functional enzymes with a variety of critical physiological roles. They are involved in the metabolic oxidation, peroxidation, and reduction of many endogenous and exogenous compounds including xenobiotics, steroids, bile acids, fatty acids, eicosanoids, environmental pollutants, and carcinogens [Nelson, D. R., Kamataki, T., Waxman, D. J., Guengerich, F. P., Estabrook, R. W., Feyereisen, R., Gonzalez, F. J., Coon, M. J., Gunsalus, I. C., Gotoh, O. (1993) The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature. DNA Cell Biol. 12(1):1-51.] The development of numerous diseases and disorders including cancer and cardiovascular and endocrine dysfunction has been linked to P450s. Several levels of regulation, including transcription, translation, and posttranslational modification, participate in maintaining the proper function of P450s. Modifications including phosphorylation, glycosylation, nitration, and ubiquitination have been described for P450s. Their physiological significance includes modulation of enzyme activity, targeting to specific cellular compartments, and tagging for proteasomal degradation. Knowledge of P450 posttranslational regulation is derived from studies with relatively few enzymes. In many cases, there is only enough evidence to suggest the occurrence and a possible role for the modification. Thus, many P450 enzymes have not been fully characterized. With the introduction of current proteomics tools, we are primed to answer many important questions regarding regulation of P450 in response to a posttranslational modification. This review considers regulation of P450 in a context that describes the potential role and physiological significance of each modification.

Enzymatic properties of mouse 25-hydroxyvitamin D3 1 alpha-hydroxylase expressed in Escherichia coli

Renal 25-hydroxyvitamin D3 1 alpha-hydroxylase cDNA cloned from the kidneys of mice lacking the vitamin D receptor was expressed in Escherichia coli JM109. As expected, the bacterially-expressed enzyme catalyzes the 1 alpha-hydroxylation of 25-hydroxyvitamin D3 with a Michaelis constant, K(m), value of 2.7 microM. Unexpectedly, the enzyme also hydroxylates the 1 alpha-position of 24,25-dihydroxyvitamin D3 with a K(m) of 1.3 microM, and a fourfold higher Vmax/K(m) compared with the 25-hydroxyvitamin D3 hydroxylase activity, suggesting that 24,25-dihydroxyvitamin D3 is a better substrate than 25-hydroxyvitamin D3 for 1 alpha-hydroxylase. In addition, the enzyme showed 1 alpha-hydroxylase activity toward 24-oxo-25-hydroxyvitamin D3. However, it showed only slight activity towards 23,25-dihydroxyvitamin D3 and 24-oxo-23,25-dihydroxyvitamin D3, and no detectable activity towards vitamin D3 and 24,25,26,27-tetranor-23-hydroxyvitamin D3. These results suggest that the 25-hydroxyl group of vitamin D3 is essential for the 1 alpha-hydroxylase activity and the 24-hydroxyl group enhances the activity, but the 23-hydroxyl group greatly reduced the activity. Another remarkable finding is that living recombinant E. coli cells can convert the substrates into the 1 alpha-hydroxylated products, suggesting the presence of a redox partner of 1 alpha-hydroxylase in E. coli cells.

Measurement of vitamin D3 metabolites in smelter workers exposed to lead and cadmium

OBJECTIVES

To investigate the effects of lead and cadmium on the metabolic pathway of vitamin D3.

METHODS

Blood and urinary cadmium and urinary total proteins were measured in 59 smelter workers occupationally exposed to lead and cadmium. In 19 of these workers, the plasma vitamin D3 metabolites, (25-hydroxycholecalciferol (25 OHD3), 24R, 25-dihydroxycholecalciferol (24R,25(OH)2D3) and 1 alpha,25-dihydroxycholecalciferol (1 alpha, 25(OH)2D3)) were measured together with blood lead. Vitamin D3 metabolites were measured by radioimmunoassay, (RIA), lead and cadmium by atomic absorption spectrophotometry, and total proteins with a test kit.

RESULTS

Ranges for plasma 25(OH)D3, 24R,25(OH)2D3 and 1 alpha,25(OH)2D3 were 1.0-51.9 ng/ml, 0.6-5.8 ng/ml, and 0.1-75.7 pg/ml, respectively. Ranges for blood lead were 1-3.7 mumol/l, (21-76 micrograms/dl), blood cadmium 6-145 nmol/l, and urinary cadmium 3-161 nmol/l. Total proteins in random urine samples were 2.1-32.6 mg/dl. Concentrations of lead and cadmium in blood showed no correlation (correlation coefficient -0.265) but there was a highly significant correlation between blood and urinary cadmium. Concentrations for 24R,25(OH)2D3 were depressed below the normal range as blood and urinary cadmium increased, irrespective of lead concentrations. High cadmium concentrations were associated with decreased plasma 1 alpha,25(OH)2D3 when lead concentrations were < 1.9 mumol/l and with above normal plasma 1 alpha,25(OH)2D3 when lead concentrations were > 1.9 mumol/l, Kruskal-Wallis analysis of variance (K-W ANOVA) chi 2 = 10.3, p = 0.006. Plasma 25(OH)D3 was negatively correlated with both urinary total proteins and urinary cadmium, but showed no correlation with plasma 24R,25(OH)2D3, 1 alpha,25(OH)2D3, blood lead, or blood cadmium.

CONCLUSION

Continuous long term exposure to cadmium may result in a state of equilibrium between blood and urinary cadmium. Cadmium concentrations in blood could be predicted from the cadmium concentration of the urine, (regression coefficient +0.35 SE 0.077). Exposure to cadmium alone decreased the concentrations of 1 alpha,25(OH)2D3 and 24R,25(OH)2D3, whereas exposure to both cadmium and lead increased the concentrations of 1 alpha,25(OH)2D3. It has been suggested that cadmium and lead interact with renal mitochondrial hydroxylases of the vitamin D3 endocrine complex. Perturbation of the vitamin D metabolic pathway by cadmium may result in health effect, such as osteoporosis or osteomalacia, risks which are possibly increased in the presence of lead.

Assay of 25-hydroxyvitamin D3 1 alpha-hydroxylase in rat kidney mitochondria

An assay method for 25-hydroxyvitamin D3 1alpha-hydroxylase [calcidiol, NADPH: oxygen oxidoreductase (1-hydroxylating), EC 1.14. 13.13] in rat kidney is described. The mitochondrial and nuclear fraction was solubilized effectively with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate(Chaps). By subsequent ultracentrifugation of the solubilized suspension the effect of inhibitory factor(s) in mammals was removed. The enzyme was then assayed by the reconstitution method using saturated amounts of adrenodoxin and NADPH-adrenodoxin reductase. Products were analyzed by HPLC, monitoring absorbance at 265 nm. The enzyme activity depended on not only pH of the medium but also the kind of buffers. N,N-Bis(2-hydroxyethyl)glycine was the best buffer. At 30 degrees C, the reaction velocity was linear at least up to 10 min, by which time enough amounts of the product needed for analysis were formed. The enzyme activity was linear to a protein concentration up to 0.8 mg of protein/ml. Under the best assay conditions established, the maximal velocity of enzyme in the rachitic rat was 12.9 pmol of product/min/mg of protein, which was 30- to 1000-fold higher than those reported by other authors with the enzyme of rachitic rat. Michaelis constant was 1.8 microM. Specific activity with the enzyme of normal rat was 0.25 pmol of product/min/mg.

A possible role for CYP27 as a major renal mitochondrial 25-hydroxyvitamin D3 1 alpha-hydroxylase

A mitochondrial cytochrome P450 fraction catalyzing 1 alpha- and 27-hydroxylation but not 24-hydroxylation of 25-hydroxyvitamin D3 was purified from pig kidney. The ratio between the 1 alpha- and 27-hydroxylase activities was the same in all purification steps including a side fraction. Attempts to separate the 1 alpha- and 27-hydroxylase activities were unsuccessful. A monoclonal antibody directed against purified pig liver CYP27 recognized a protein of the same apparent M(r) and immunoprecipitated both the 1 alpha- and 27-hydroxylase activities towards 25-hydroxyvitamin D3 in the purified kidney enzyme fraction as well as in a solubilized, crude cytochrome P450 extract considered to represent the major part of the 25-hydroxyvitamin D3 hydroxylases in kidney mitochondria. Taken together, the results from the purification and the experiments with CYP27 antibody, substrate inhibition, and recombinant expressed human liver CYP27 strongly indicate that CYP27 is able to catalyze 1 alpha-hydroxylation but not 24-hydroxylation of 25-hydroxyvitamin D3 in kidney. In conclusion, the results provide evidence for a role for CYP27 as a major renal mitochondrial 25-hydroxyvitamin D3 1 alpha-hydroxylase.

Purification from pig kidney of a microsomal cytochrome P450 catalyzing 1 alpha-hydroxylation of 25-hydroxyvitamin D3

A cytochrome P450 catalyzing 1 alpha-hydroxylation of 25-hydroxyvitamin D3 was purified from pig kidney microsomes. The enzyme preparation showed one protein band on gel electrophoresis with apparent M(r) of 52,500 and a specific cytochrome P450 content of 10.7 nmol/mg of protein. The 25-hydroxyvitamin D3 1 alpha-hydroxylase copurified with the vitamin D3 25-hydroxylase during purification. A cytochrome P450 catalyzing 1 alpha-hydroxylation was purified also from liver microsomes. The apparently homogeneous enzyme showed the same catalytic properties and apparent M(r) as the kidney enzyme. The results of the present communication demonstrate the presence in kidney of a previously unknown microsomal 1 alpha-hydroxylase in addition to the assumed specific mitochondrial 1 alpha-hydroxylase. The possibility that microsomal 1 alpha-hydroxylation in pig kidney and liver is catalyzed by the previously described porcine microsomal vitamin D 25-hydroxylase(s) is discussed.

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.

Human 25-hydroxyvitamin D 24-hydroxylase cytochrome P450 subunit maps to a different chromosomal location than that of pseudovitamin D-deficient rickets

We have cloned part of the human 25-OHD 24-hydroxylase cytochrome P450 (P450cc24) cDNA. The characterized sequence consists of 776 bp of the coding and 720 bp of the 3'-untranslated region interrupted by an intron. In the coding region we found 79.8% similarity in DNA and 87.5% in deduced amino acid sequences between human and rat, with no similarity in the 3'-untranslated region. By Southern blot hybridization of DNA from human-hamster somatic cell hybrids and by in situ immunofluorescence hybridization, we mapped P450cc24 to human chromosome 20q13.1. This location of P450cc24 is different from that of pseudovitamin D-deficient rickets (PDDR), previously assigned to chromosome 12q14 by linkage analysis, thus excluding it as a target of the PDDR mutation. Since it is likely that PDDR is caused by a mutation in the 25-OHD 1 alpha-hydroxylase P450 subunit (P450cc1 alpha) our results do not support the hypothesis that the two cytochromes are encoded by a single gene.

Rat kidney 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylases: evidence for two distinct gene products

Total RNA was isolated from kidneys of Sprague-Dawley rats. Oligo (dT)-primed single-stranded cDNA was obtained by the reverse transcriptase reaction from which a 285 bp cDNA probe coding for 25-hydroxyvitamin D3-24-hydroxylase [25(OH)D3-24-OHase] was generated by the polymerase chain reaction. Northern blotting performed with kidney poly (A)+ RNA isolated from rats (1) fed a standard diet, (2) depleted in D3 and hypocalcemic, and (3) fed a standard diet and injected intraperitoneally with 50,000 IU of vitamin D3 for 5 days showed that the transcript for 24-OHase was weakly expressed in control, and highly induced in vitamin D3-treated animals. No transcript could be elicited in vitamin D-depleted hypocalcemic animals in which 25(OH)D3-1 alpha-OHase was maximally induced. The data show that 24-OHase is independently regulated of 1 alpha-OHase, strongly suggestive of the enzymes being encoded by two distinct genes.

Characterization and regulation of the vitamin D hydroxylases

The metabolism of vitamin D is regulated by three major cytochrome P450-containing h hydroxylases-the hepatic 25-hydroxylase, the renal 1α-hydroxylase, and the renal and intestinal 24-hydroxylase. In the liver, the 25-hydroxylation reaction is catalyzed by microsomal and mitochondrial cytochrome P450cc25. The microsomal P450 accepts electrons from the NADPH-cytochrome P450 reductase, and the mitochondrial P450 accepts electrons from NADPH-ferredoxin reductase and ferredoxin. In the kidney, the 1α- and 24-hydroxylation reactions are catalyzed by mitochondrial cytochromes P450cc1α and P450cc24, respectively. The 24-hydroxylase is also found in vitamin D target tissues such as the intestine. The rat hepatic mitochondrial P450cc25 and the rat renal mitochondrial P450cc24 have been purified, and their cDNAs have been cloned and sequenced. 1,25-Dihydroxyvitamin D, the active metabolite of vitamin D, markedly stimulates renal P450cc24 mRNA and 24-hydroxylase activity in the intact animal and in renal cell lines. This stimulation occurs via a receptor-mediated mechanism requiring new protein synthesis. Despite the availability of a clone, no studies have yet been reported of the regulation of hepatic P450cc25 at the mRNA level. The study of one of the most important enzymes in vitamin D metabolism, the renal 1α-hydroxylase which produces the active metabolite, awaits the definitive cloning of the cDNA for the P450cc1α.

Vitamin D hydroxylases

There are three mixed function oxidases which catalyze hydroxylations of vitamin D and its derivatives. These include the hepatic mitochondrial or microsomal vitamin D3-25-hydroxylase and the two renal mitochondrial enzymes which further hydroxylate 25-hydroxyvitamin-D3 (25-OH-D3) to form 24R,25-dihydroxyvitamin D3 (24,25(OH)2D3) and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], the primary steroid hormonal derivative of vitamin D3. All three enzymes are cytochrome P450 dependent. The two renal mitochondrial enzymes are regulated, usually in a reciprocal fashion. The intracellular signalling systems involved in this regulation include 1,25(OH)2D3 itself and both protein kinases A and C. Recent progress has been made in the purification and cloning of the vitamin D3-25-hydroxylase and the 25-OH-D3-24-hydroxylase. When the 25-OH-D3-1-hydroxylase is purified and cloned, efforts which have thus far been frustrated by its low abundance, fertile new ground for the study of the regulation of vitamin D metabolism at the molecular level will be opened up.

Kinetic properties of 25-hydroxyvitamin D- and 1,25-dihydroxyvitamin D-24-hydroxylase from chick kidney

1,25-Dihydroxyvitamin D3 induces both 25-hydroxyvitamin D3- and 1,25-dihydroxyvitamin D3- 24-hydroxylase activities. However, whether 24-hydroxylation of these substrates is catalyzed by a single enzyme is unknown. We have examined the substrate specificity of the enzyme using the solubilized and reconstituted chick renal mitochondrial 24-hydroxylase enzyme system. The soluble enzyme catalyzes 24-hydroxylation of both substrates. The apparent Km of the 24-hydroxylase for 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 were 1.47 and 0.14 microM, respectively. Kinetic studies demonstrated that 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 act as competitive inhibitors with respect to each other. 1,25-Dihydroxyvitamin D3 inhibited the production of 24,25-dihydroxyvitamin D3 with an apparent Ki of 0.09 microM and 25-hydroxyvitamin D3 inhibited the production of 1,24,25-trihydroxyvitamin D3 with an apparent Ki of 3.9 microM. These results indicate that chick 24-hydroxylase preferentially hydroxylates 1,25-dihydroxyvitamin D3 and support the idea that the 24-hydroxylation of these substrates is catalyzed by a single enzyme.

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.

Characteristics of the 25-hydroxyvitamin D3- and 1,25-dihydroxyvitamin D3-24-hydroxylase(s) from HL-60 cells

1,25-Dihydroxyvitamin D3 induces the human promyelocyte leukemia cell line, HL-60, to differentiate into macrophages/monocytes via a steroid-receptor mechanism. This system is a relevant one for an investigation of the molecular mechanism of 1,25-dihydroxyvitamin D3. We have now examined the effect of 1,25-dihydroxyvitamin D3 on the induction of 1,25-dihydroxyvitamin D3- and 25-hydroxyvitamin D3-24-hydroxylase activities in HL-60 cells. The hydroxylase activities were measured by a periodate-based assay, which was validated by comparison with well-established HPLC analysis. HPLC analysis also suggested that 1,25-dihydroxyvitamin D3 induces a 23-hydroxylase in addition to the 24-hydroxylase. 1,25-Dihydroxyvitamin D3- and 25-hydroxyvitamin D3-24-hydroxylase activities were stimulated as early as 4 h after the addition of 10(-7) M 1,25-dihydroxyvitamin D3 and became maximal by 24 h. 1,25-Dihydroxyvitamin D3 stimulated both activities in a dose-dependent manner up to 10(-6) M. The Km of 24-hydroxylase for 1,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 were 2 x 10(-8) M and 5.2 x 10(-7) M, respectively. Cycloheximide (5 microM) inhibited 1,25-dihydroxyvitamin D3-mediated stimulation of 24-hydroxylase activity. Other differentiation inducers, such as retinoic acid and phorbol ester, did not induce either activity. 1,25-Dihydroxyvitamin D3-24-hydroxylase in HL-60 mitochondria was solubilized with 0.6% cholate and reconstituted with NADPH, beef adrenal ferredoxin, and beef adrenal ferredoxin reductase, each component being essential for 24-hydroxylase activity. These results strongly suggest that the 24-hydroxylase in HL-60 cells is a three-component cytochrome P450-dependent mixed-function oxidase.

Amino-terminal sequence homology of two chick kidney mitochondrial proteins immunoisolated with monoclonal antibodies to the cytochrome P450 of 25-hydroxyvitamin D3-1 alpha-hydroxylase

We demonstrate the unique capability of monoclonal antibodies for the specific immunodetection and characterization of two antigenic proteins occurring in normal chick kidney mitochondrial extracts. The antigens were adsorbed to cyanogen bromide-activated Sepharose gel coupled to monoclonal antibodies (MAbs) of the IgM class raised against cytochrome P450(1) alpha, which inhibit equally the 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylase catalytic activities (Mandel et al. 1990 J Clin Lab Immunol, in press). The two identified antigenic proteins are polypeptides with apparent molecular weights of 57,000 and 55,000 daltons. The 1 alpha-hydroxylase cytochrome P450 has been shown to have a molecular weight of 57,000 daltons (Mandel et al. 1990 Biochim Biophys Acta 1034:239-246). The optimal antigen:gel ratio for maximal antigen binding as cytochrome P450 heme, which was determined spectrally, was found to be 1.3 nmol cytochrome P450 per g MAb-coupled Sepharose. At this ratio the total binding capacity of the gel was 1 nmol cytochrome P450 per g Sepharose. The two polypeptides were desorbed with 0.1% Emulgen 911 in 25% glycerol at pH 3.0 and separated by SDS-gel electrophoresis. The amino-terminal sequences of the two antigens were determined by automated Edman degradation with an on-line analyzer of PTH derivatives. The sequences in both antigens were 100% homologous. Complete amino acid composition analysis also revealed that their amino acid compositions were highly similar. These findings suggest that the smaller protein may be a proteolytic cleavage product of the 1 alpha-hydroxylase P450 cytochrome and may represent a putative 24-hydroxylase antigen.

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.

Induction of 25-OH-vitamin D3 24- and 23-hydroxylase activities in partially purified renal extracts from pigs given exogenous 1,25-(OH)2D3

A renal mitochondrial cytochrome P 450 preparation from pigs treated with exogenous 1,25-(OH)2D3 was reconstituted with an NADPH-generating system, adrenodoxin and adrenodoxin reductase. The reconstituted system catalyzed the conversion of the substrate, 25-OH-D3, to metabolites comigrating with authentic 23,25-(OH)2D3 and 24,25-(OH)2D3 in both straight- and reverse-phase high-performance liquid chromatography systems, which achieve separation of these metabolites from each other as well as from other vitamin D metabolites. The putative 23,25-(OH)2D3 product was resistant to periodate treatment, while the 24,25-(OH)2D3 product was sensitive, providing additional evidence for the identity of the products. Although induction of 24-hydroxylase activity has been studied using renal homogenates from several species, only recently have techniques become available to study the activity of the enzyme in a solubilized and reconstituted form. Using these techniques, the present study shows that production of 24,25-(OH)2D3 was increased more than 80-fold with 1,25-(OH)2D3 treatment compared with untreated controls, an effect much greater than that previously observed with homogenates. In addition, production of both 23,25-(OH)2D3 and 24,25-(OH)2D3 varied with substrate concentration and was consistent with a monooxygenase-linked enzyme reaction.

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.

Separation of the cytochromes P-450 in pig kidney mitochondria catalyzing 1 alpha-, 24- and 26-hydroxylations of 25-hydroxyvitamin D3

The cytochromes P-450 in pig kidney mitochondria catalyzing 1 alpha-, 24- and 26-hydroxylations of 25-hydroxyvitamin D3 have been separated. The cytochrome P-450 fractions required NADPH, mitochondrial ferredoxin and ferredoxin reductase for catalytic activity. The present report demonstrates that different forms of cytochrome P-450 are involved in 1 alpha-, 24- and 26-hydroxylations of 25-hydroxyvitamin D3 and provides a basis for further purification and characterization of these enzymes.

Reciprocal post-translational regulation of renal 1 alpha- and 24-hydroxylases of 25-hydroxyvitamin D3 by phosphorylation of ferredoxin. mRNA-directed cell-free synthesis and immunoisolation of ferredoxin

We have used a cell-free rabbit reticulocyte translational system programmed with polyadenylated [poly(A)+] RNA prepared from chick kidney tissue to study the synthesis of nascent ferredoxin, a class of iron-sulphur-containing proteins functional in the renal mitochondrial 1 alpha- and 24-hydroxylases of 25-hydroxyvitamin D3. The synthesis of ferredoxin was monitored by determining [35S]methionine incorporation into ferredoxin and quantified by SDS/PAGE and autoradiography after immunoprecipitation from the total translation products. Compared with normal controls, vitamin D deprivation caused a significant increase in the net synthesis of nascent ferredoxin with an Mr of 12,000-13,000. [3H]Orotate incorporation as uridine into kidney poly(A)+ RNA was stimulated by aminophylline, a potent inducer of 25-hydroxyvitamin D3 24-hydroxylase; however, the amount of nascent ferredoxin synthesis was the same as in normal controls. Also, partially purified chick kidney mitochondrial cyclic AMP-stimulated protein kinase catalysed the phosphorylation of ferredoxin in vitro. The catalytic activity of the ferredoxin in 1 alpha- and 24-hydroxylations of 25-hydroxyvitamin D3 in reconstituted systems consisting of cytochrome P-450 and ferredoxin reductase was altered with ferredoxin phosphorylation. The phosphorylation caused inhibition of the 1 alpha-hydroxylase activity while at the same time it stimulated the 24-hydroxylase. Authentic 1 alpha,25- and 24,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 were used as standards to monitor the separation of the enzymic products by h.p.l.c. using methanol/water (4:1, v/v) as solvent. These results indicate that, in the absence of vitamin D or its metabolites in the deficient state, the synthesis of ferredoxin necessary for the 1 alpha-hydroxylase is accentuated, whereas the stimulation of the 24-hydroxylase requires the phosphorylation of existing ferredoxin without a net gain in its synthesis. This would suggest a post-translational regulation of the 1 alpha- and 24-hydroxylases. A model delineating the various aspects of this study is presented.

Antigenic and catalytic disparity in the distribution of cytochrome P-450-dependent 25-hydroxyvitamin D3-1 alpha- and 24-hydroxylases

Chick 25-hydroxyvitamin D3-1 alpha-hydroxylase, a cytochrome P-450 monooxygenase with a molecular weight of 57 kDa, can be isolated as described by Mandel et al. (1990 b). Under normal physiological circumstances, it occurs exclusively in kidney mitochondria. An isozyme of the 1 alpha-hydroxylase, known as the 24-hydroxylase, which uses the same substrate to yield an isomeric product, is also a cytochrome P-450 monooxygenase, has a molecular weight of 55 kDa, and like-wise occurs in kidney mitochondria. The amino-terminal sequences of the first 10 residues of the two isozymes are 100% homologous. Monoclonal antibodies of the IgM class raised against the 1 alpha-hydroxylase, which quantitatively discriminate against other P-450 cytochromes of mitochondrial or microsomal origin, recognize and interact with the 24-hydroxylase as an antigen. In the present study we show that the intestine, which is the only non-renal tissue with demonstrable 24-hydroxylase activity, gives a positive peroxidase-antiperoxidase immunohistochemical reaction using the monoclonal antibodies against the 1 alpha-hydroxylase. The reactions revealed that the antigen in the kidney is restricted to the cortical proximal tubular cells while in the intestine, the antigen is localized in the enterocytes of the villi. In kidney medullary or intestinal crypt cells, or in liver, heart and lung tissues where 1 alpha-hydroxylase or 24-hydroxylase activity could not be detected using cell or tissue homogenates, the immunohistochemical reactions were also negative.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of 25-hydroxyvitamin D3 24-hydroxylase from rat kidney mitochondria

25-Hydroxyvitamin D3 24-hydroxylase was purified to an electrophoretically homogeneous state (Mr = 53,000) from kidney mitochondria of female rats treated with vitamin D3.