ABNORMAL SYNTHESIS OF N-METHYLATED PHOSPHOLIPIDS DURING CALCIUM PARADOX OF THE HEART

Phosphatidylethanolamine (PtdEtn) N-methyltransferase activity that sy nthesizes phosphatidylcholine (PtdCho) via formation of methylated int ermediates (phosphatidyl-N-monomethylethanolamine, PtdEtnMe and phosph atidyl-N,N-dimethylethanolamine, PtdEtnMe(2)) was comparatively studie d in rat heart sarcolemmal (SL), sarcoplasmic reticular (SR) and mitoc hondrial fractions during Ca2+ paradox, Perfusion (5 min) with Ca2+-fr ee medium followed by reperfusion (5 min) with Ca2+ containing medium produced a marked rise in resting tension without any recovery of cont ractile force, Methyltransferase catalytic sites I, II and III which s ynthesize PtdEtnMe, PtdEtnMe(2) and PtdCho, respectively, were assayed by measuring the [H-3] methyl group incorporation from 0.055, 10 and 150 mu M S-adenosyl-L-[H-3-methyl] methionine into membrane PtdEtn mol ecules. Five minutes of perfusion with Ca2+-free medium did not affect either SL or SR N-methyltransferase systems, Ca2+-readmission for 1 t o 5 min induced a selective, time-dependent depression of SL site II a nd SR site I methyltransferase activities. Individual N-methylated pho spholipids specifically formed at the two sites reflected these change s. The above abnormalities were differently influenced by the duration (1-5 min) of Ca2+-free perfusion and were characterized by different kinetic alterations. The mitochondrial methylation system was not affe cted under Ca2+ paradox, The results suggest that reduced synthesis of SL N-methylated phospholipids may contribute to the contractile dysfu nction observed in Ca2+ paradox.