Multiplicity of progesterone 21-hydroxylase activities associated with constitutive forms of rabbit liver cytochrome P-450

Constitutive cytochromes P-450 have been solubilized from untreated outbred New Zealand White rabbit liver microsomes. Gradient phosphate buffer elution of DEAE-cellulose columns partially resolved six P-450 fractions. Progesterone 21-hydroxylase activity was reconstituted with several fractions and inhibited by an antibody towards P-450 Form 1. One fraction (LM3b) preferentially catalysed the 6 beta- and 16 alpha-hydroxylation of progesterone. SDS-PAGE indicated the presence of proteins with mobilities closely related to Form 1 in several fractions that were separated from this isozyme by DEAE-cellulose chromatography. These results suggest that several constitutive P-450 fractions may contribute to the regiospecific 21-hydroxylation of progesterone.