Porcine microsomal vitamin D(3) 25-hydroxylase (CYP2D25). Catalytic properties, tissue distribution, and comparison with human CYP2D6.

The metabolic activation of the prohormone vitamin D(3) requires a 25-hydroxylation that has been reported to be catalyzed by both mitochondrial CYP27A and a microsomal vitamin D(3) 25-hydroxylase in the liver. CYP27A has been extensively studied, but its role as a physiologically important vitamin D(3) 25-hydroxylase has been questioned. The present paper reports that the microsomal vitamin D(3) 25-hydroxylase, purified from pig liver, converted vitamin D(3) into 25-hydroxyvitamin D(3) in substrate concentrations which are within the physiological range (apparent K(m) = 0.1 microm). The enzyme 25-hydroxylated vitamin D(3), 1 alpha-hydroxyvitamin D(3) and vitamin D(2) and also converted tolterodine, a substrate for human CYP2D6, into its 5-hydroxymethyl metabolite. Tolterodine inhibited the microsomal 25-hydroxylation, whereas quinidine, an inhibitor of CYP2D6, did not markedly inhibit the reaction. The primary structure of the microsomal vitamin D(3) 25-hydroxylase, designated CYP2D25, shows 77% identity with that of human CYP2D6. Northern blot and reverse transcription-polymerase chain reaction experiments revealed that CYP2D25 mRNA is expressed in higher levels in liver than in kidney and in small amounts in adrenals, brain, heart, intestine, lung, muscle, spleen, and thymus. Experiments with human liver microsomes and recombinantly expressed CYP2D6 strongly indicate that the microsomal 25-hydroxylation of vitamin D(3) in human liver is catalyzed by an enzyme different from CYP2D6.