Phospholipid methylation in canine myocardium: kinetic characteristics and the effect of endotoxin administration.
The kinetic characteristics and the effect of endotoxin administration on the enzymatic methylation of phospholipids in dog heart microsomes were studied using S-adenosyl-L-[methyl-3H]methionine as a methyl donor. Kinetic studies in control dogs reveal that the stepwise methylation of phosphatidylethanolamine to phosphatidylcholine was catalyzed by three different enzymes. Methyltransferase I catalyzed the methylation of phosphatidylethanolamine to phosphatidyl-N-monomethylethanolamine, had a very low Km (approximately 1.5 microM) for S-adenosylmethionine, and a pH optimum of 6.5, and it was stimulated by Mg2+ and Ca2+. Methyltransferase II catalyzed the methylation of phosphatidyl-N-monomethylethanolamine to phosphatidyl-N,N-dimethylethanolamine, had a low Km (8-12 microM) for S-adenosylmethionine, and a pH optimum of 8.5, and it was stimulated by low concentrations (less than 1 mM) of Ca2+ but was unaffected by Mg2+. Methyltransferase III catalyzed the formation of phosphatidylcholine from phosphatidyl-N,N-dimethylethanolamine, had a high Km (approximately 33 microM) for S-adenosylmethionine, and a pH optimum of 9.5, and it was unaffected by Mg2+ or Ca2+. Experiments with trypsin digestion indicate that methyltransferases I and III were partially embedded while methyltransferase II was completely exposed to the surface of the membrane. Endotoxin administration (2 and 4 hr) decreased the Km and Vmax by 30 to 36% and 24 to 37.7%, respectively, for S-adenosylmethionine. Since the enzymatic methylation of phospholipids has been implicated to play an important role in the regulation of membrane structure and function, the endotoxin-induced decreases in the Km and Vmax of phospholipid-methylating enzymes in dog heart microsomes may contribute to the development of myocardial dysfunction in endotoxin shock.
Liu
M S
MS
Department of Physiology, St. Louis University School of Medicine, Missouri 63104.
Yang
Y
Y
D000818:Animals; D004285:Dogs; D004731:Endotoxins / Q000494:pharmacology; D005260:Female; D006321:Heart / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008297:Male; D008745:Methylation; D009206:Myocardium / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism*; D012436:S-Adenosylmethionine / Q000378:metabolism; D012772:Shock, Septic / Q000378:metabolism*
D004731:Endotoxins; D010743:Phospholipids; D012436:S-Adenosylmethionine