Ethanolamine plasmalogen methylation by rabbit myocardial membranes.

Enzymatic methylation of alkenylacylglycerophosphoethanolamine 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-3H]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 formed/mg per h (mean +/- S.D.), even when normalized for ethanolamine substrate concentration (5.7 +/- 1.6 vs. 1.8 +/- 0.4 pmol CH3 incorporated/mumol diradylglycerophosphoethanolamine). Rabbit myocardial phospholipid methyltransferase activity is optimal at basic pH for each substrate, is moderately stimulated by added Ca2+ or Mg2+, and is completely inhibited by S-adenosylhomocysteine. An apparent Km 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.