Regulation of S-adenosylmethionine synthetase activity in cultured human lymphocytes

S-Adenosylmethionine (AdoMet), inorganic pyrophosphate (PPi) and inorganic phosphate (Pi) are potent product inhibitors of AdoMet synthetase and have been postulated to play a role in increasing AdoMet levels and turnover in peripheral blood mononuclear cells (PBM) after stimulation with phytohemagglutinin (PHA). Measurements of these metabolites in PHA-stimulated PBM showed the expected 2- to 3-fold increases in AdoMet after 8 h, and smaller increases in PPi and Pi. Since the kinetic model requires substantial decreases in PPi and Pi in response to PHA, product inhibition cannot explain the observed changes in AdoMet metabolism in this system. A 2.5-fold increase in AdoMet synthetase catalytic activity was found in crude extracts of PBM within 8 h of PHA-stimulation and probably accounts for increased cellular levels and utilization of AdoMet. Immunochemical analyses with a monoclonal antibody specific for the alpha/alpha' subunits of human lymphocyte AdoMet synthetase showed that these increases in catalytic activity were not associated with increases in immunoreactive protein. The ratio of catalytic activity to immunoreactivity in stimulated cells was 4-fold higher than in unstimulated controls and almost identical to that found in extracts from the human B-lymphocyte line WI-L2. Unstimulated PBM appear to contain substantial amounts of AdoMet synthetase alpha/alpha' subunit with reduced or absent catalytic activity, which can be activated by PHA-stimulation.