Fig. 2.

Estrogen synthesis. Cholesterol is transported into the mitochondria by steroidogenic acute regulatory protein (StAR), then converted into pregnenolone by P450Scc (20,22 desmolase). Pregnenolone is released in the cytosol where it is converted into progesterone by 3β-HSD Δ4,5 isomerase. Pregnenolone is alternatively converted by various reactions in the testis into androstenediol, which is then converted to testosterone by 3β-HSD Δ4,5 isomerase. Progesterone may be converted via sequential pathways into androstenedione then testosterone, which are then converted by aromatase into E1 and 17β-E2, respectively. 17β-E2 is transformed into E1 by 17β-HSD-2, and reversibly from E1 to E2 by 17β-HSD-1.