Abstract
Synthesis of phosphatidylcholine (PC) by S-adenosyl-L-methionine (AdoMet)-dependent methylation of phosphatidylethanolamine (PE) has been recently characterized in rat heart sarcolemma obtained by hypotonic shock-LiBr treatment method. The present study, employing different procedures for the isolation of purified cardiac sarcolemmal membranes in rat, confirms the existence of three catalyctic sites which are specifically involved in the sequential methyltransfer reactions from PE to PC. Other subcellular organelles such as sarcoplasmic reticulum (microsomes) and mitochondria showed methyltransferase activity which was absent in myofibrils and in cytosolic fraction. The kinetic pattern of methyltransferase activity in both microsomes and mitochondria was comparable to that obtained in sarcolemma. In addition, the characteristics of three catalytic sites are identified by the synthesis of phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N-dimethylethanolamine and PC in these subcellular organelles were similar to those of sarcolemma. The results are consistent with the view that methyltransferase activity is localized in different membrane systems of the myocardium.