endoplasmic reticulum membrane
Pregn-5-ene-3,20-dione isomerizes to progesterone. This reaction is catalyzed by the isomerase activity of 3 beta-HSD, associated with the endoplasmic reticulum membrane. The active form of the enzyme is a homodimer. The enzyme occurs in two isoforms that are similar in their biochemical properties but differ in their tissue expression: type I (HSD3B1) is found in placenta and skin, while type II (HSD3B2) is found in the adrenal glands and gonads.
The hormone lutropin (LH) triggers ovulation and development of the corpus luteum, that in turn increases production of progesterone.