Progesterone (Diagram)

-How is biosynthesis of progesterone compartmentalized and regulated
 within the cell?

The rate-limiting step in steroid hormone biosynthesis in all tissues is the transport across the mitochondrial membrane by StAR to make cholesterol available to the P 450 side-chain cleavage enzyme. LH increases expression of both StAR and P450scc in the theca cells and Luteinized granulosa cells of the ovary but only of StAR in the leydig cells of the testes.

ACTH increases expression of both StAR and P 450scc in the zona fasciculata and zona reticularis of the adrenal cortex. Angiotensin 2 increases the expression of both StAR and P450scc in the zona glomerulosa of the adrenal cortex. P450scc converts cholesterol to pregnenolone in the mitochondria and pregnenolone then diffuses out of the mitochondria and into the SER, where it is converted to progesterone by 3 beta HSD.

Expression of 3 beta HSD is increased by LH in the theca cells and luteinized granulosa cells of the ovary. In women, plasma progesterone originates from both the adrenal cortex and the ovaries whereas in males all progesterone in plasma originates from the testes.