Study on the Sex Difference in Rat Liver Histidase

The mechanism of expression of the sex difference in adult rat liver histidase was studied. When female rats were fed a 0 or 91% casein diet, the difference between the histidase activities of ovariectomized and intact rats was not significantly different. However, the difference in the activities was significant (p = 0.01) in animals on a 25% casein diet. The sex difference in histidase activity was also only expressed in animals on diet containing a normal level of protein. Hypophysectomy elevated the enzyme activity in male, but not in female rats. Injection of growth hormone (GH) or thyroxine (T4) suppressed the histidase activity in hypophysectomized rats of both sexes.Estradiol-17-β did not induce the enzyme activity of hypophysectomized female rats. This suggests that estrogen does not induce the enzyme activity directly, but that its effect is mediated by other hypophysial hormones. Estrogen tended to restore the histidase activity in hypophysectomized female rats when this was inhibited by GH (p = 0.05) but not when it was inhibited by T4. Injection of estrogen into thyroidectomizedovariectomized rats induced the histidase activity, but after treatment of these animals with reserpine this inductive effect of estrogen was no longer observed. From these results, a hypophysial factor(s) including GH appears to be important for expression of the sex difference of the histidase activity in rat liver. (Endocrinology90: 207, 1972)