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Methionine and adenosyltransferase (ATP: L-methionine S-adenosyltransferase, EC 2.5. 1.6) is the specific enzyme responsible for the formation of S-adenosylmethionine, a major donor of methyl groups in mammalian tissues ( 1 ) . It plays a role in the regulation of the metabolism of single carbon units (2) and other intermediates (3).Our previous studies (4-6) showed that the transferase activity in rat liver was significantly influenced by such nutritional and hormonal factors as adrenal glucocorticoids and dietary protein, indicating a close association with carbohydrate and protein metabolism. The actions of hormones and diets are known to be interrelated (7-10). However, the effects of hormones upon enzymes are not always predictable from their similar or antagonistic properties ( 1 1-1 5).Since the pancreatic hormones, insulin (16) and glucagon (1 7, 18) have profound effects on carbohydrate and protein metabolism, we investigated their roles in the regulation of hepatic methionine adenosyltransferase in rats, the liver being the organ with a high concentration of the enzyme ( 19). The results indicate that insulin deficiency brought about a marked increase in the hepatic transferase activity by an adrenalmediated process which appears to require a continued synthesis of the enzyme protein, whereas administration of excess glucagon has little effect on this enzyme. While the present work was in progress, an exploratory study of the effects of alloxan and glucagon

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