Transsulfuration in mammals: fetal and early development of methionine-activating enzyme and its relation to hormonal influences.

The development of activity of methionine-activating enzyme was studied in four organs of the rat. Three different patterns were observed: (a) in the liver, specific activity began to increase in late fetal life and reached a maximum 2 days after birth; (b) in the small intestine, specific activity began to rise in the 2nd wk after birth and reached a maximum at age 18 days; and (c) in the brain and kidney, specific activity did not change markedly from the earliest stage of fetal development studied to adult life. Hydrocortisone increased hepatic methionine-activating enzyme activity as much as 55% in the young rat. However, adrenalectomy in the newborn rat did not prevent the postnatal rise in hepatic methionine-activating enzyme activity, nor did adrenalectomy at age 10 days prevent the developmental rise of intestinal activity at age 18 days. Conjugated estrogens partially inhibited both the neonatal rise in hepatic methionine-activating enzyme activity and the rise in activity after adrenal steroid injection. Injection of L-methionine did not increase hepatic methionine-activating enzyme activity in the developing or adult rat.