Abstract

Studies on the disposition of extracellular S-adenosylhomocysteine by isolated rat hepatocytes have shown that S-adenosyl-L-homocysteine is not taken up by cells, but binds to acceptor(s) on the cell surface. The Scatchard plots for the binding of S-adenosylhomocysteine to hepatocytes and purified rat liver membranes at 0 degrees were nonlinear, and consistent with high-affinity components with Kd values of 0.4 microM and 0.7 microM, respectively. About 60% of the S-adenosylhomocysteine that was bound to cells and purified membranes dissociated rapidly from its binding sites. The rapid initial phase was followed by a second slow phase obeying first-order kinetics, corresponding to a dissociation rate constant of 0.09 min-1. S-Tubercidinylhomocysteine and unlabeled S-adenosylhomocysteine were potent inhibitors of the binding of S-[14C]adenosylhomocysteine, whereas S-3-deazaadenosylhomocysteine, S-adenosylmethionine, and S-adenosyl-D-homocysteine were less effective. A fraction of the S-adenosylhomocysteine that was bound to rat hepatocytes was displaced by low concentrations of sinefungin and its metabolite, A9145C, but these compounds were weak inhibitors of S-adenosylhomocysteine binding to purified membranes. 5'-Deoxy-5'-S-isobutylthioadenosine showed slight inhibitory activity against S-adenosylhomocysteine binding to both cells and purified membranes. In conclusion, the equilibrium binding, dissociation rate kinetics, and displacement curves in the presence of S-adenosylhomocysteine analogues show that S-adenosylhomocysteine binds to a heterogeneous population of binding sites of intact hepatocytes and purified liver plasma membranes.