Quantitative Changes in Ovarian 17α-Hydroxylase/C 17,20 -Lyase and Aromatase Activities during the Estrous Cycle of the Hamster

Abstract

Aromatase activity of the microsomal fraction of ovarian homogenates, measured by a tritium exchange assay using androstenedione (A-dione) as substrate, did not change during the 4-day estrous cycle of the hamster. In contrast, 17α-hydroxylase, measured by a tritium exchange assay with progesterone as substrate, and particularly C17,20-lyase activity, evaluated by acetic acid production from progesterone, drastically decreased on the afternoon of proestrus (Day 4). The latter two activities remained low on Days 1 and 2 but increased dramatically on Day 3 and the morning of Day 4. The serum concentration of A-dione and 17-hydroxyprogesterone reached a peak at 1600 hr of proestrus and then decreased rapidly as the enzyme activities decreased. A-dione levels were undetectable on Day 1 but 17-hydroxyprogesterone levels remained elevated through Day 2. The results are consistent with the view that 17α-hydroxylase and 07,20-lyase are activities of a single cytochrome P-450, as has been shown for testis and adrenal. Increases in hydroxylase and lyase activities occur concomitantly with decreases in the serum concentrations of progesterone, suggesting that the latter steroid may play a role in controlling these enzymes.