[Inhibitory effects of calphobindins on phospholipase A2 and phospholipase C]

Calphobindins (CPBs) I, II and III which we isolated and characterized from placenta belong to a family of calcium and phospholipid binding proteins represented by lipocortins known to inhibit phospholipase A2(PLA2). The amino acid sequence of three CPBs shares much homology with lipocortins I and II, so we investigated the functions of CPBs in affecting the enzymatic activities of PLA2 and also phospholipase C(PLC). In our experiments CPBs I, II and III showed an apparent dose-dependent inhibition of PLA2 and PLC activities, and all three proteins had a more potent inhibitory effect than lipocortins. The inhibition was overcome by high phospholipid substrate concentrations, for example, in the presence of 2.9 x 10(-9)M PLA2 and 2.6 x 10(-7)M CPB I, the inhibition decreased from 85 to 9% as phosphatidylethanolamine was increased from 10 to 100 microM. A similar result was observed in the case of PLC also. The evidence strongly suggests that the inhibitory effect of CPBI results from binding of the proteins to the phospholipid substrate rather than from a direct action on the phospholipase itself.