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Isoforms of purified methyltransferase from human blood platelets
A membrane-bound protein with N-methyltransferase activity, associated with phospholipid metabolism, has been isolated from purified human blood platelet plasma membranes. The activity of this enzyme has been detected in crude platelet preparations. However, the nature and properties of this enzyme and its purification from normal human blood platelets have not been investigated, hence, the aim of this study was to purify, characterise the enzyme from human blood platelets and determine its possible role in phospholipid transmethylation. The plasma membranes were purified by velocity and sucrose gradient centrifugation and solubilized in buffer containing 0.5% Triton-X 100. The partially purified extracts were further purified using a combination of ion-exchange chromatographic procedures, preparative IEF and preparative SDS-PAGE. The preparative rotorfor electrofocusing, using pH 3-10 ampholyte at 40EC, yielded activities at both acidic and basic pHs of approximately 3.5 and 8.5, respectively. The pH 8.5 activity yielded predominantly a single 67 KDa band on SDS-PAGE gel and the pH 3.5 activity also gave a similar band and a few lighter bands at 35 KDa and 50 KDa. The acidic and basic fractions catalysed the transfer of methyl groups from S-adenosyl inethionine to PE with a predominant formation of PME (ca 90%). The fraction purified by preparative SDS-PAGE also yielded a major methylating activity that eluted between 60-70 KDa. This latter protein was washed free of SDS and its activity measured; it, too yielded predominaly the monomethylated PE. These data are consistent with the hypothesis that several SAM-dependent isozymes also exist in human blood platelets that convert PE to PC.
JOURNAL OF THE GHANA SCIENCE ASSOCIATION Volume 2 No. 3 (2000) pp. 98-102