The effect of pharmacologic inhibition of phospholipid methylation on human platelet function

Human platelets are capable of synthesizing their major membrane phospholipid, phosphatidylcholine, by a methylation pathway. This involves the sequential transfer of methyl groups from S-adenosyl-L- methionine (AdoMet) to phosphatidylethanolamine, and in the process, AdoMet is converted to S-adenosylhomocysteine (AdoHcy). The activity of this methylation pathway is decreased upon stimulation of platelets by various agonists. We inhibited methylation reactions pharmacologically to see whether this inhibition plays any role in the process of platelet activation. Two inhibitors of AdoHcy hydrolase, 3-deaza- adenosine and 3-deaza-(+/-)aristeromycin (500 microM each), were effective in increasing platelets levels of AdoHcy and preventing turnover of AdoMet. Also, these compounds were equipotent in inhibiting platelet phospholipid methylation. However, while 3-deaza-adenosine potentiated platelet aggregation and 14C-serotonin release induced by epinephrine or adenosine diphosphate (ADP) (p less than 0.01), 3-deaza- aristeromycin had no such effect. Neither compound affected platelet responses to thrombin or collagen. Inhibition of methylation reactions was not the only biochemical effect of 3-deaza-adenosine since it also blunted significantly the elevation of platelet cyclic adenosine monophosphate (AMP) levels induced by prostaglandin E1 (p less than 0.02). Therefore, these studies demonstrate that inhibition of platelet phospholipid methylation, per se, has no discernable effect on the function of human platelets. The methylation pathway, though active in platelets, does not appear to be primarily involved in membrane events responsible for platelet activation.