Evidence for three catalytic sites in heart sarcolemmal phospholipid N-methylation

Conversion of phosphatidylethanolamine to phosphatidylcholine through sequential transmethylation reactions was studied in rat cardiac sarcolemmal membrane prepared by the hypotonic shock-LiBr treatment method. Three catalytic sites were identified on the basis of their requirement for divalent cations and kinetic parameters. Site I (Km = 0.1 +/- 0.01 microM) required Mg2+ as a cofactor and had a very high affinity for [3H]-S-adenosyl-L-methionine (AdoMet). Sites II (Km = 2.9 +/- 0.3 microM) and III (Km = 112 +/- 6 microM) did not require any divalent cation and showed comparatively lower affinities for AdoMet. Developmental-related differences were found to exist among these three catalytic sites when phospholipid methylation was carried out in 1- to 12-week-old rats. The results suggest that three distinct methyltransferase sites are involved in cardiac sarcolemmal phospholipid methylation.