5'-S-Methylthioadenosine is converted to methionine in mammalian systems, microorganisms and plants. 5'-S-Methylthioadenosine is first converted to 1-phospho-5-S-methylthioribofuranoside (1-PMTR) which is then converted to 2-keto-4-S-methylthiobutyrate, the precursor of methionine. We have now investigated the conversion of 1-PMTR to the keto acid. This conversion requires at least three protein fractions designated A, B, and C. Fraction A catalyzes an isomerization of 1-PMTR to form 1-phospho-5-S-methylthioribulose. The identification of this compound is based in part on the products obtained after NaIO4 oxidation, i.e. S-methylthioacetaldehyde, formate, and phosphoglycolic acid. When fractions A and B are added to 1-PMTR, two additional compounds, designated II and III, were detected. No O2 was consumed in the formation of compounds II and III. These compounds are, therefore, at the oxidation state of 5-S-methylthioribose. Compound II is phosphorylated as evidenced by its electrophoretic behavior before and after alkaline phosphatase treatment. Addition of fraction C to compounds II and III leads to O2 consumption and to the conversion of these compounds to 2-keto-4-S-methylthiobutyrate. Thus, compounds II and III are precursors of the keto acid. These compounds have not been fully characterized.