In various extraadrenal organs, progesterone (P
4) is converted to deoxycorticosterone (DOC) by steroid 21-hydroxylation. To investigate the regulation of extraadrenal 21-hydroxylase activity by 17β-estradiol (E
2), the following two experiments were performed. Exp. 1). Three-week-old male SD rats were testectomized (TX) and injected with E
2 (1mg) or sesame-oil s.c. Sham rats were treated with sesame-oil. In these groups the serum concentration of DOC and corticosterone (B), microsomal 21-Hydroxylase activity and the expression of P450c21 mRNA of the liver and the adrenals were analyzed. Exp. 2). Isolated rat hepatocytes were cultured and stimulated by E
2, 10
-9-10
-5M. 21-Hydroxylase activity of these cells was analysed by the rate of production of DOC in the medium containing P
4. The results of experiment 1 showed that both serum DOC concentration and 21-hydroxylase activity in the liver microsomal fraction were increased by E
2 injection, but the expression of P450c21 mRNA was not detected in the liver even after E
2 injection. In experiment 2, the activity of steroid 21-hydroxylase in isolated rat hepatocytes was stimulated by E
2 in a dose dependent manner. These data provided evidence that: in rats the liver was one sites of the extraadrenal steroid 21-hydroxylase activity which had been stimulated by E
2. The results also suggested that this hepatic enzyme was a different enzyme from the steroid 21-hydroxylase P450c21 expressed in the adrenal gland.
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