Inhibition by fatty acids of the cholesterol side-chain cleavage in the course of progesterone biosynthesis in the mitochondrial fraction of human term placenta.

1. Saturated and unsaturated fatty acids inhibit the progesterone biosynthesis from cholesterol in the human placenta mitochondrial fraction. 2. Inhibition of the progesterone biosynthesis by fatty acids could be relieved by ATP, CoA and carnitine. 3. The inhibitory effect of fatty acids is due to their action on the cholesterol side-chain mixed-function oxidase system but not on the NADPH-regenerating system.