Aromatization of steroids by mitochondrial preparations from human term placenta

Abstract

Structural and functional aspects of the association of aromatizing activity with placental mitochondrial preparations were examined. Total mitochondrial preparations, previously characterized with respect to their capacity for oxidative phosphorylation and for progesterone synthesis, converted androst-4-ene-3, 17-dione to estrogens with specific activities averaging one-half those of corresponding microsomal fractions. Heavy mitochondria separated by differential centrifugation showed specific activities 16% of microsomal activities. Metabolically active mitochondria, selectively isolated by calcium phosphate loading combined with sucrose density gradient fractionation, also displayed significant aromatizing activity despite considerable inactivation by the experimental procedures. An exogenous NADPH-generating system supported maximum rates of mitochondrial aromatization. No stimulation of aromatization was induced when mitochondrial permeability barriers were disrupted by 11 mM CaCl 2 although cholesterol side-chain cleavage activity was doubled by this treatment. Mitochondrial dehydrogenase systems required exogenous NADP + for effective support of aromatization. Under these conditions, Ca 2+ stimulated aromatization when malate served as metabolic substrate but had no effect on the support of aromatization by citrate or isocitrate. Thus, aromatizing activity of placental mitochondria appears to be firmly associated with or localized in the external mitochondrial compartment and to be less directly integrated with internal metabolic processes for NADPH generation than is the mixed-function oxidase system for cholesterol side-chain cleavage.