Isozymes of S-Adenosylmethionine Synthetase from Rat Liver: Isolation and Characterization1

Abstract

S-Adenosylmethionine synthetase exists in at least two distinct forms, α- and β-forms, in adult liver. The β-form was purified to homogeneity from the soluble fraction of rat liver with a yield of about 10%. An antiserum directed against the purified β-form from rat liver was prepared by injecting the purified enzyme into a rabbit. Ouchterlony double diffusion analysis and immunochemical titrα-tions revealed that the isozymes, α- and β-forms, are identical. Thus, the α-form was isolated from rat liver as a single protein using immunoaffinity chromatography against the β-form. The molecular weights of the β- and α-forms were determined to be 48,000 each by sodium dodecyl sulfate disc gel electrophoresis, and about 100,000, and 200,000, respectively, by Sephacryl S-200 gel fitration. These results indicate that the β-form consisted of two subunits of 48,000 daltons and the α-form of four subunits of 48,000 daltons. The sedimentation coefficient was calculated to be 5.5S for the β-form and 8.0S for the α-form.