Abstract
A mitochondrial cytochrome P450 fraction catalyzing 1α- and 27-hydroxylation but not 24-hydroxylation of 25-hydroxyvitamin D 3 was purified from pig kidney. The ratio between the 1α- and 27-hydroxylase activities was the same in all purification steps including a side fraction. Attempts to separate the 1α- 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 1α- and 27-hydroxylase activities towards 25-hydroxyvitamin D 3 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 D 3 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 1α-hydroxylation but not 24-hydroxylation of 25-hydroxyvitamin D 3 in kidney. In conclusion, the results provide evidence for a role for CYP27 as a major renal mitochondrial 25-hydroxyvitamin D 3 1α-hydroxylase.
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