Methionine metabolism in mammals: synthesis of S-adenosylhomocysteine in rat tissues

We have developed a sensitive double-isotope assay to measure the enzymatic synthesis of S-adenosylhomocysteine from adenosine and l-homocysteine. The enzyme is present in all rat tissues except the small intestinal mucosa. In the liver, the enzyme is found in the postmicrosomal supernatant. We studied some of the kinetic properties of the liver enzyme. At high concentration, both substrates inhibit product formation. S-Adenosylmethionine also inhibits the reaction but other metabolites of methiouine do not. The specific activity of the enzyme in liver increases in animals fed a high protein ration. Treatment with hydrocortisone and estradiol also resulted in an increase in the specific activity while administration of thyroxine led to a decrease.