title: Effects of Ca2+ on ethanolaminephosphotransferase and cholinephosphotransferase in rabbit platelets.
abstract: The effects of Ca2+ on ethanolaminephosphotransferase [EC 2.7.8.1] and cholinephosphotransferase [EC 2.7.8.2] activities in rabbit platelet membranes were studied using endogenous diglyceride and CDP-[3H]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 microM). 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.
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000201:enzymology*; D002118:Calcium / Q000494:pharmacology*; D002462:Cell Membrane / Q000201:enzymology; D003035:Cobalt / Q000378:metabolism; D003565:Cytidine Diphosphate / Q000031:analogs & derivatives / Q000378:metabolism; D003566:Cytidine Diphosphate Choline / Q000378:metabolism; D002798:Diacylglycerol Cholinephosphotransferase / Q000097:blood*; D004982:Ethanolaminephosphotransferase / Q000097:blood*; D004983:Ethanolamines / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D010770:Phosphotransferases / Q000097:blood*; D011817:Rabbits
chemical_list: D004983:Ethanolamines; C006933:CDP ethanolamine; D003035:Cobalt; D008345:Manganese; D003566:Cytidine Diphosphate Choline; D003565:Cytidine Diphosphate; D010770:Phosphotransferases; D004982:Ethanolaminephosphotransferase; D002798:Diacylglycerol Cholinephosphotransferase; D002118:Calcium