The Cholesterol Side‐Chain Cleavage Enzyme System in Mitochondria of Human Term Placenta

Abstract

The cholesterol side‐chain cleavage enzyme system of human term placental mitochondria has been studied. Native mitochondria were capable of converting [4‐ 14 C]cholesterol to [4‐ 14 C]‐progesterone in the presence of NADPH and oxygen. Disruption of mitochondrial structure resulted in an inhibition of the enzymic activity. However, active enzyme could be extracted from the disrupted mitochondria by high speed centrifugation, the endogenous inhibitor remaining associated with the particulate material. The nature of the endogenous inhibitor has been determined. It appears to involve the mitochondrial phospholipid, especially lecithin. Investigations with authentic phospholipids confirm these studies. Separation of the enzyme system from the bulk of the mitochondrial material permitted investigation of the characteristics of the enzyme. Light reversibility studies of the carbon monoxide inhibition of the enzymic activity showed the involvement of cytochrome P450 in the enzyme system. Fractionation of the enzyme using gel exclusion and ion—exchange chromatography revealed the involvement of a heam iron protein (presumably cytochrome P450), a component having NADPH‐diaphorase activity (flavoprotein) and a non‐haem iron ‘protein in the enzyme system.