Aromatase activity in the rabbit blastocyst.

Rabbit blastocysts were homogenized by sonication, and centrifuged at 105 000 g for 60 min. The pellet was resuspended and incubated in phosphate buffer containing [1 beta-3H]testosterone and a NADPH generating system. The amount of 3H2O produced was determined by liquid scintillation counting. Enzyme activity was calculated, after subtracting blank values obtained with boiled embryos, and expressed as pig testosterone aromatized per embryo per hour. Aromatase activity was undetectable to low on Day 5 and increased on Day 6 of pregnancy. There was a 10-fold increase in activity in Day-6 embryos cultured for 24 h, with a further 6-fold increase in activity in Day-6 embryos cultured for 48 h. The enzyme had an apparent Km of 0.77 microM and was completely inhibited by an aromatase inhibitor. The results clearly indicate that the rabbit blastocyst has an increasing capacity for aromatization of testosterone at about the time of implantation.