The present paper reports the study on a lyase in rat liver which decomposes S-adenosylmethionine into 5'-methylthioadenosine and alpha-amino-gamma-butyrolactone. The partial purification of this enzyme and some of its properties are discussed. S-adenosyl(5')-3-methylthiopropylamine, the decarboxylated analog of AdoMet, exerts a non-competitive inhibition of the enzyme whereas is inactive as substrate. The reported results demonstrate the relevance of the carboxyl group of AdoMet for the mechanism of the enzymatic hydrolysis of the sulfonium compound.