Effects of Ca2+ on Ethanolaminephosphotransferase and Cholinephosphotransferase in Rabbit Platelets

The effects of Ca2+ on ethanolaminephosphotransferase [EC 2.7.8.1] and choline-phosphotransferase [EC 2.7.8.2] activities in rabbit platelet membranes were studied using endogenous diglyceride and CDP-PH]ethanolamine or CDP-[14C]choline as substrates. Both transferases required Mn2+, Co2+, or Mg2+ as a metal cofactor and the optimal concentrations of the metals for both activities were about 5, 10, and 5 mM, respectively. When 5 mM Mg2+ was used as a cofactor, both transferase activities were inhibited by a low concentration of Ca2+ (half maximal inhibition at approx. 15 μm). In the presence of 5 mM Mn2+, however, approx. 5 mM Ca2+ was required to produce half maximal inhibition. The Ca2+-induced inhibition was reversible and the rate of the inhibition was not affected either by the concentrations of the CDP-compound or by exogenously added diacylglycerol. The relationship between Ca2+ and both Mg2+ and Mn2+ on the transferase activities was competitive. 45Ca2+ binding (and/or uptake) to the platelet membranes was inhibited by Mn2+, Mg2+, and Co2+, in a concentration-dependent manner. However, the inhibitory effects of the three metal ions on the total Ca2+ binding (and/or uptake) did not correlate with the activation of both transferase activities by the three metal ions in the presence of Ca2+. These results suggest that both transferase activities are regulated by low concentrations of Ca2+ in the presence of optimal concentrations of Mg2+, and that the inhibition is mediated directly by Ca2+, which interacts with a specific metal cofactor binding site(s) of the transferases.