Base-exchange reactions of the phospholipids in cardiac membranes

Canine cardiac microsomes were shown to incorporate the nitrogenous bases, serine, ethanolamine, and choline, into their respective phospholipids by the energy-independent, Ca2+-stimulated base-exchange reactions. The optimal Ca2+ concentration was 2.5 mM. Metal ions other than Ca2+ either inhibited or had no effect on the activities. La3+ and Mn2+ were both potent inhibitors. The pH optimum for the reactions at 2.5 mM Ca2+ was approx. 7.8 and depended upon Ca2+ concentration. Apparent Km values at 2.5 mM Ca2+ were 0.06 mM for L-serine, 0.13 mM for ethanolamine and 0.49 mM for choline. The kinetic and metal ion inhibition studies suggest that the choline-exchange reaction is a separate process from the serine and ethanolamine reactions. The ATP-stimulated Ca2+ binding system of the cardiac membranes was not related to the base-exchange reactions; however, the energy-independent Ca2+ binding to the membranes appears to be related to the exchange reactions.