Abstract
Enzymatic methylation of alkenylacylglycerophosphoethanolmine to form alkenylacylglycerophosphocholine was observed in rabbit myocardial membranes, and was compared to the corresponding methylation sequence for diacyl substrates. Membranes were incubated with S- adenosyl- l -[ methyl- 3 H] methionine and assayed for incorporation of radioactivity into selected lipids. The rate of incorporation of methyl groups into diacylglycerophosphocholine exceeded that for alkenylacylglycerophosphocholine, 12.0 ± 3.6 vs. 3.9 ± 0.7 pmol product fonned/ mg per h ( mean ± S. D.), even when normalized for ethanolamine substrate concentration (5.7 ±1.6 vs. 1.8 ± 0.4 pmol CH 3 incorporated/μmol diradylglycerophosphoethanolamine). Rabbit myocardial phospholipid methyltransferase activity is optimal at basic pH for each substrate, is moderately stimulated by added Ca 2+ or Mg 2+ and is completely inhibited by S-adenosylhomocysteine. An apparent K m of 0.2 mM for S-adenosylmethionine applies to diacyl- and alkenylacylglycerophosphocholine formation; at low concentrations of methyl donor (0.003 mM), the monomethylated products accumulate.
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