Changing 17β-Hydroxysteroid Dehydrogenase Activity in Preimplantation Rat and Mouse Embryos 1

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Abstract
The 17.beta.-hydroxysteroid dehydrogenase (17.beta.-HSD) activity in rat and mouse preimplantation embryos was determined by measuring the interconversion of estradiol (E2) and estrone (E1). Rat and mouse embryos were cultured In medium containing 450 nmM [3H]E1 or -E2 and the amount of [3H]E1 and -E2 in the medium at the end of the 1 h was determined. In both species 17.beta.-HSD activity was detectable from the 1-cell stage (Day 1) onward. In the rat, 17.beta.-HSD effected primarily E2 .fwdarw. E1 conrvesion, wtih the activity decreasing from Day 1-Day 5. In the mouse, primarily E1 .fwdarw. E2 conversion from Day 1 to the morning of day 4, then E2 .fwdarw. E1 increased sharply to range the E1 .fwdarw. E2 rate in the evening of Day 4 an surpassed the E1 .fwdarw. E2 rates the next morning. Apparently, 17.BETA.-HSD is active throughout the entire preimplantation period. The enyzme activity evidently changes during preimplantation development. The rat and mouse preimplantation embryo could regulate the E1-to-E2 ratio in the embryos and in their environment.