title: Metabolism of S-adenosyl-L-methionine in intact human erythrocytes.
abstract: Freshly isolated human erythrocytes contain S-adenosyl-L-methionine (AdoMet) at a concentration of about 3.5 mumol/l cells. When such cells are incubated in a medium containing 30 microM L-methionine, 18 mM D-glucose and 118 mM sodium phosphate (pH 7.4), intracellular AdoMet levels continuously decrease to a value of about 0.1 microM after 24 h. This occurs in spite of the fact that the cellular concentrations of the substrates for the AdoMet synthetase reaction, ATP and L-methionine, remain relatively constant. In a search for incubation conditions that lead to stable levels of AdoMet in incubated cells, we have developed a sodium-Hepes-buffered medium which includes 1 mM adenine and a stoichiometric excess of MgCl2 over its ligand, phosphate. The inclusion of magnesium ion (and a reduction in phosphate) appears to increase intracellular free Mg2+, which is required for full activity of the erythrocyte AdoMet synthetase. Even in the presence of MgCl2, however, the AdoMet pool level can drop 4-6-fold within the first 2 h of incubation. We present evidence that suggests that this initial fall in the cellular AdoMet level may be due to the activation of AdoMet-dependent protein carboxyl methyltransferase, an enzyme which accounts for a large fraction of the total cellular AdoMet utilization. Adenine, or related compounds in the medium may prevent this activation, although the mechanism of this action is not clear at present.
mesh_terms: D000225:Adenine / Q000494:pharmacology; D002478:Cells, Cultured; D003470:Culture Media; D000242:Cyclic AMP / Q000378:metabolism; D004912:Erythrocytes / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D012436:S-Adenosylmethionine / Q000097:blood*
chemical_list: D003470:Culture Media; D012436:S-Adenosylmethionine; D000242:Cyclic AMP; D008274:Magnesium; D000225:Adenine