Characterization of an isozyme of S-adenosylmethionine synthetase from rat liver

An isozyme of S-adenosylmethionine synthetase has been purified to homogeneity by ammonium sulfate fractionation, DEAE-cellulose column chromatography, and gel filtration on a Sephadex G-200 column. The purified enzyme is very unstable and has a molecular weight of 120 000 consisting of two identical subunits. Amino acid analysis on the purified enzyme showed glycine, glutamate, and aspartate to be the most abundant and the aromatic amino acids to be the least abundant. It possesses tripolyphosphatase activity which can be stimulated five to six times by S-adenosylmethionine (20-40 microM). The findings support the conclusion that an enzyme-bound tripolyphosphate is an obligatory intermediate in the enzymatic synthesis of S-adenosyl-methionine from ATP and methionine.