A cytochrome P450 catalyzing 1α-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 1α-hydroxylase copurified with the vitamin D3 25-hydroxylase during purification. A cytochrome P450 catalyzing 1α-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 1α-hydroxylase in addition to the assumed specific mitochondrial 1α-hydroxylase. The possibility that microsomal 1α-hydroxylation in pig kidney and liver is catalyzed by the previously described porcine microsomal vitamin D 25-hydroxylase(s) is discussed.
