CHARACTERIZATION OF DISTINCT FORMS OF METHIONINE ADENOSYLTRANSFERASE IN NUCLEATED, AND MATURE HUMAN ERYTHROCYTES AND ERYTHROLEUKEMIC CELLS

Two peaks of methionine adenosyltransferase (MAT) activity from human erythrocytes were partially purified on a DEAE-cellulose column. Using anti-MAT antibodies, a 60 kDa form of MAT, referred to as p, was iden tified in peak I. Although p represented the major MAT protein in crud e erythrocyte extracts, the enzyme was very labile and accounted for o nly 6% of the total MAT activity. Peak II enzyme was stable, and consi sted of the previously described catalytic alpha (53 kDa) subunit and the beta subunit(38 kDa), both of which are found in activated human l ymphocytes and leukemic cells of lymphoid origin. Mature normal and po lycythemic erythrocytes contained predominantly p as the major MAT pro tein, while nucleated erythrocytes and reticulocytes contained predomi nantly the lambda (68 kDa), the major form found in resting human lymp hocytes. Human erythroleukemic cells (HEL 92.1.7) contained the alpha, alpha' and beta subunits of MAT, and in this regard was indistinguish able from MAT found in activated lymphocytes and leukemic cells of lym phoid origin (Jurkat). Since p was generated during the incubation of extracts from resting lymphocytes, which contain predominantly lambda, in the absence of protease inhibitors; the p form of MAT appears to b e derived from the lambda form by proteolytic cleavage. The data indic ate that distinct forms of MAT are present at different stages of eryt hrocyte maturation and reveal the presence of a new form of MAT with r educed activity compared to previously described forms.