Isozymes of S-adenosylmethionine synthetase from rat liver: isolation and characterization

S-Adenosylmethionine synthetase exists in at least two distinct forms, alpha- and beta-forms, in adult liver. The beta-form was purified to homogeneity from the soluble fraction of rat liver with a yield of about 10%. An antiserum directed against the purified beta-form from rat liver was prepared by injecting the purified enzyme into a rabbit. Ouchterlony double diffusion analysis and immunochemical titrations revealed that the isozymes, alpha- and beta-forms, are identical. Thus, the alpha-form was isolated from rat liver as a single protein using immunoaffinity chromatography against the beta-form. The molecular weights of the beta- and alpha-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 filtration. These results indicate that the beta-form consisted of two subunits of 48,000 daltons and the alpha-form of four subunits of 48,000 daltons. The sedimentation coefficient was calculated to be 5.5S for the beta-form and 8.0S for the alpha-form.