The synthesis of methionine from 5'-methylthioadenosine was examined in vitro using cell-free homogenates of rat liver. We had previously reported that methionine synthesized by this system contained carbons from the ribose moiety of 5'-methylthioadenosine (Backlund, P. S., Jr., and Smith, R. A. (1981) J. Biol. Chem. 256, 1533-1535). The present study examines some of the steps involved in this conversion. 2-Keto-4-methylthiobutyrate was identified as one of the products of 5'-[methyl]14C]methylthioadenosine when incubated in vitro with rat liver homogenates. Partial purification of the enzyme system by gel exclusion chromatography to remove small molecular weight substrates and cofactors revealed that synthesis of 2-keto-4-methylthiobutyrate requires magnesium and is inhibited by calcium. 5-Methylthioribose 1-phosphate can replace 5'-methylthioadenosine as a substrate for methionine synthesis under these conditions, but 5-methylthioribose cannot. 5-Methylthioribonic acid was prepared and was found not to be an intermediate compound in this pathway. The final step in this pathway for methionine synthesis is then the transamination of the keto acid, catalyzed by glutamine and asparagine transaminases.