Phospholipid methylation and human platelet function

Abstract

Lysed human platelet preparations were shown to methylate phosphatidylethanolamine to the mono-, di- and trimethyl products. At a low concentration of S-adenosylmethionine (SAM). 2 μM, the major product formed was phosphatidylmonomethylethanolamine (PNE). There was a broad pH optimum from 9 to 11 for the formation of PNE. The combined formation of phosphatidyldimethylethanolamine (PNNE) and phosphatidylcholine (PC) showed a peak of activity at pH 8.2 followed by a broad shoulder up to pH 12. At a higher concentration of SAM (200 μM). the predominant product formed was PC; the pH dependence for formation of the combination of PNNE and PC under these conditions showed a broad peak from 9 to 11. Inhibitors of SAM-dependent methylations. S-adenosylhomocysteine (SAH) and 3-deazaadenosine (3-DZA), blocked phospholipid methylation in lysed platelets and whole cells respectively. 3-DZA, did not inhibit platelet responses to thrombin, ADP, epinephrine, collagen, arachidonic acid and the calcium ionophore A23187 as measured by serotonin release, platelet aggregation, malondialdehyde formation, clot retraction, or arachidonic acid release at high agonist concentrations, and appeared to potentiate responses at low concentrations. Prostaglandin-induced inhibition of platelet function was unaffected by 3-DZA. Thrombin stimulation of platelets failed to produce the changes in methylation of phospholipids that have been associated with receptor activation of other cell types. Phospholipid methylation is not required for intial activation of platelets but the inhibition of this methylation may activate platelet function at low levels of agonists.