Metabolism of substrates incorporated into phospholipid vesicles by mouse 25-hydroxyvitamin D3 1alpha-hydroxylase (CYP27B1).

CYP27B1 catalyzes the 1alpha-hydroxylation of 25-hydroxyvitamin D3 to 1alpha,25-dihydroxyvitamin D3, the hormonally active form of vitamin D3. To further characterize mouse CYP27B1, it was expressed in Escherichia coli, purified and its activity measured on substrates incorporated into phospholipid vesicles, which served as a model of the inner mitochondrial membrane. 25-Hydroxyvitamin D3 and 25-hydroxyvitamin D2 in vesicles underwent 1alpha-hydroxylation with similar kinetics, the catalytic rate constants (k(cat)) were 41 and 48mol/min/mol P450, respectively, while K(m) values were 5.9 and 4.6mmol/mol phospholipid, respectively. CYP27B1 showed inhibition when substrate concentrations in the membrane were greater than 4 times K(m), more pronounced with 25-hydroxyvitamin D3 than 25-hydroxyvitamin D2. Higher catalytic efficiency was seen in vesicles prepared from dioleoyl phosphatidylcholine and cardiolipin than for dimyristoyl phosphatidylcholine vesicles. CYP27B1 also catalyzed 1alpha-hydroxylation of vesicle-associated 24R,25-dihydroxyvitamin D3 and 20-hydroxyvitamin D3, and 25-hydroxylation of 1alpha-hydroxyvitamin D3 and 1alpha-hydroxyvitamin D2, but with much lower efficiency than for 25(OH)D3. This study shows that CYP27B1 can hydroxylate 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 associated with phospholipid membranes with the highest activity yet reported for the enzyme. The expressed enzyme has low activity at higher concentrations of 25-hydroxyvitamin D in membranes, revealing that substrate inhibition may contribute to the regulation of the activity of this enzyme.

Expression of 25-hydroxyvitamin D-1-alpha-hydroxylase mRNA in individuals with colorectal cancer.

Vitamin D prevents proliferation, promotes differentiation, and induces apoptosis of colon cells, and reduced intake or insufficiency of the vitamin in the body are associated with increased risk of colorectal cancer. Results of previous studies have suggested that mRNA that codes for 25-hydroxyvitamin D-1-alpha-hydroxylase (1 alpha OHase), which converts 25-hydroxyvitamin D to its active metabolite, might be up regulated in human colon carcinomas. We used real-time reverse transcription PCR assays to measure absolute 1 alpha OHase mRNA concentrations in the colonic mucosa of 44 individuals without cancer, and in paired healthy colon and cancerous colon samples taken from 27 individuals with the disease, to ascertain whether or not such up regulation takes place. Our results suggest that concentrations of 1 alpha OHase mRNA in tumour samples and in healthy colon samples from individuals without cancer are similar, but that concentrations are significantly lower in the paired, phenotypically healthy mucosa of individuals with cancer.

Molecular cloning of (25-OH D)-1 alpha-hydroxylase: an approach to the understanding of vitamin D pseudo-deficiency.

Pseudovitamin D-deficiency rickets (PDDR) is the first identified inborn error of vitamin D metabolism. Its clinical course is similar to that of nutritional rickets due to simple vitamin D deficiency. The treatment of choice is replacement therapy with calcitriol [1,25(OH)2D3]. PDDR is inherited as a simple autosomal recessive trait. The PDDR locus has been mapped to chromosome 12q13-q14. The molecular defect underlying the 25-hydroxyvitamin D-1 alpha-hydroxylase enzyme dysfunction has remained elusive due to the lack of sequence information for the gene encoding the cytochrome P450 moiety of the enzyme. We have used a probe derived from the rat 25-hydroxyvitamin D-24-hydroxylase sequence to identify and clone the 1 alpha-OHase cDNA. The candidate gene was transiently expressed in P19 embryonal carcinoma cells. Only those cells that were transfected with the candidate cDNA in the sense orientation were able to produce a compound that co-eluted with the 1 alpha, 25 vitamin D3 standard. Mass spectrometry analysis confirmed the identity of the produced metabolite. A human genomic clone was isolated from a chromosome 12 cosmid library and subsequently mapped to human chromosome 12q13.1-q13.3. To address the putative biological function of 24,25-dihydroxyvitamin I) 24,25(OH)2D, we also engineered a null mutation in the 24-OHase gene in embryonic stem cells (ES). Animals heterozygous for the engineered mutation are normal and fertile. One half of the homozygous animals die before weaning. Breeding of surviving females gives an F2 generation in which bone development is abnormal at sites of intramembranous ossification. Growthplate maturation and endochondral ossification appeared to proceed normally. The results show that a complete absence of vitamin D metabolites hydroxylated in position 24 during embryogenesis leads to abnormal bone structure and suggests a key role for 24,25(OH)2D in the developmental regulation of intramembranous ossification.

Purification and characterization of 25-hydroxyvitamin D3 1alpha-hydroxylase from rat kidney mitochondria.

We purified extensively 25-hydroxyvitamin D3 1alpha-hydroxylase (calcidiol, NADPH: oxygen oxidoreductase (1-hydroxylating), EC 1.14.13.13) from kidney mitochondria of rachitic rats and disclosed its peculiar properties as a P450. The final preparation was identified as a 55 kDa protein having an intense absorption at 417 nm characteristic of P450. The specific activity was 4.8 nmol/min/mg of protein indicating a 350-fold purification. Specific content of P450 was 1.1 nmol/mg of protein and turnover number was 4.4 min-1.

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.

Evidence for 54-kD protein in chicken kidney as a cytochrome P450 with a high molecular activity of 25-hydroxyvitamin D3 1 alpha-hydroxylase.

Conversion of 25-hydroxyvitamin D3 (25(OH)D3) to the active vitamin D3, 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) is catalyzed by 25(OH)D3, 1 alpha-hydroxylase(1 alpha-hydroxylase). It has been suggested that this enzyme is cytochrome P450 (P450). We purified 1 alpha-hydroxylase 430-fold from cholate-solubilized kidney mitochondria of vitamin D-deficient chickens by utilizing hydrophobic and ion-exchange column chromatographies. Enzymatic activity was assessed by measuring on HPLC the formation of 1 alpha,25(OH)2D3 from 25(OH)D3 in the assay mixture containing NADPH, adrenodoxin reductase, adrenodoxin as a reducing system. The purified enzyme showed a CO-difference spectrum characteristic of P450. The molecular activity of this preparation was calculated to be 8.7 pmol/min/pmol P450. This value was higher by more than 87-fold than those reported so far. The present preparation was found to contain several proteins on SDS-PAGE. Among them, only the 54-kD protein became undetectable when kidney mitochondria from normal and vitamin D-replete chickens, where 1 alpha-hydroxylase activities were 15 and 0% of that found in vitamin D-deficient chicken, respectively, were used as the starting enzyme sources. Furthermore, the band intensity of the 54-kD protein accounted for the spectrophotometrically determined amount of P450 in the preparation. These results suggest that the 54-kD protein is 1 alpha-hydroxylase.

Immunopurified 25-hydroxyvitamin D 1 alpha-hydroxylase and 1,25-dihydroxyvitamin D 24-hydroxylase are closely related but distinct enzymes.

The chick kidney mitochondrial cytochrome P-450 1,25-dihydroxyvitamin D3 24-hydroxylase was partially purified by sequential polyethylene glycol precipitation, aminohexyl-Sepharose 4B, and hydroxylapatite chromatography. The specific activity of the final preparation, when reconstituted with NADPH, adrenodoxin, and adrenodoxin reductase, was 245 pmol/min/mg of protein or 0.56 pmol/min/pmol of P-450. The specific cytochrome P-450 content was 0.45-0.73 nmol/mg of protein. BALB/c mice immunized with this preparation developed serum polyclonal antibodies to the 24-hydroxylase, as demonstrated by immunoprecipitation. Splenic lymphocytes from an immunized mouse were fused with myeloma NSI/1-Ag-4-1 cells, and hybridomas secreting monoclonal antibodies to the 24-hydroxylase were detected by immunoprecipitation. The hybridoma lines were cloned by limiting dilution and further characterized as IgG1, IgG3, and IgM subclasses. In one-dimensional immunoblots of soluble 24-hydroxylase preparations, the monoclonal antibodies revealed a single band with an apparent molecular weight of 59,000. The monoclonal antibodies did not cross-react with cytochrome P-450s from other species but immunoprecipitated and immunoblotted a soluble chick renal mitochondrial 25-hydroxyvitamin D3 1 alpha-hydroxylase preparation, demonstrating the close similarity of these two hydroxylases. These antibodies were coupled to Sepharose CL-4B and used to isolate to homogeneity the two enzymes from chick kidney mitochondria. Amino-terminal sequences and amino acid composition data demonstrate that these enzymes are different but homologous.

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.

Assay and properties of rat yolk sac 25-hydroxyvitamin D3 1 alpha-hydroxylase.

An in vitro assay has been developed for the rat yolk sac 25-hydroxyvitamin D3 1 alpha-hydroxylase (1 alpha-hydroxylase). The subcellular location and some properties of the enzyme are described. 1,25-Dihydroxyvitamin D3 produced from incubations of yolk sac homogenates was extracted, purified by Sephadex LH-20 chromatography and straight- and reverse-phase high-performance liquid chromatography (HPLC), and measured by a competitive binding assay using chick intestinal receptor. The reaction is linear with time for up to 45 min at a substrate concentration of 80 microM and 4-6 mg/mL microsomal protein. The enzyme, located in the microsomes, requires molecular oxygen and NADPH. Metyrapone (1 X 10(-3) M) was found to inhibit 1-hydroxylation, but a 90% carbon monoxide-10% oxygen atmosphere did not, leaving open the question of involvement of cytochrome P-450. Diphenyl-p-phenylenediamine, a lipid peroxidase inhibitor, inhibited 1 alpha-hydroxylation.

Solubilization and reconstitution of chick renal mitochondrial 25-hydroxyvitamin D3 24-hydroxylase.

Chick kidney mitochondrial 25-hydroxyvitamin D3 24-hydroxylase has been solubilized with sodium cholate and reconstituted with NADPH, beef adrenal ferredoxin, and beef adrenal ferredoxin reductase, each component being essential for maximal 24-hydroxylase activity. The product 24(R),25-dihydroxyvitamin D3 was identified by cochromatography with synthetic compound on straight-phase and reversed-phase high-performance liquid chromatography and by periodate oxidation. The enzyme has an apparent Km for 25-hydroxyvitamin D3 of 0.67 microM. At 1 microM 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3 production is linear with time for up to 15 min and with protein concentrations of up to 2 mg/mL. The antioxidant diphenyl-p-phenylenediamine (1.3 X 10(-4) M) has no effect on this reaction. Reconstituted 24-hydroxylase activity is enhanced by the addition of NaCl and KCl up to 100 mM, with higher concentrations having an inhibitory effect. 1 alpha-Hydroxylase is not present in this preparation from vitamin D replete chicks. The similarities of this reconstituted system to the 25-hydroxyvitamin D3 1 alpha-hydroxylase and the adrenal systems suggest that the 25-hydroxyvitamin D3 24-hydroxylase is also a cytochrome P-450 type mixed-function oxidase.

Isolation of chick renal mitochondrial ferredoxin active in the 25-hydroxyvitamin D3-1alpha-hydroxylase system.

An iron-sulfur protein has been isolated from chick kidney mitochondria and purified (200-fold as determined enzymatically by its NADPH-cytochrome c reductase activity in the presence of adrenodoxin reductase) on DEAE-cellulose and gel filtration on Sephadex G-100. The purified protein showed an absorption peak at 411 nm with a shoulder at 460 nm. The electron paramagnetic resonance spectrum was typical of a ferredoxin-type iron-sulfur protein with g values: gx=gy-1.94 and gz=2.02. The molecular weight was estimated by gel filtration to be 12,500. When tested against anti-adrenodoxin gamma-globulin, the protein showed a precipitin line that fused completely with that of adrenodoxin. Based on these findings it is concluded that this protein is an iron-sulfur protein quite similar to adrenal ferredoxin. In the presence of adrenoxodin reductase, NADPH, and carbon monoxide, the purified renal ferredoxin was found to be active in the reduction of cytochrome P-450 solubilized from chick kidney mitochondria. It was also effective in the reconstituted 25-hydroxyvitamin D3-1alpha-hydroxylase composed of the cytochrome P-450 from rachitic chick kidneys and adrenodoxin reductase. A ferredoxin reductase isolated from chick kidney mitochondria could replace adrenodoxin reductase in the reconstituted system. These results strongly support a previous conclusion that the kidney mitochondrial 25-hydroxyvitamin D3-1alpha-hydroxylation system consists of a renal ferredoxin reductase (presumably a flavoprotein), renal ferredoxin, and cytochrome P-450.