Inhibition of platelet factor XIIIa-catalyzed reactions by calmodulin

Calmodulin was found to exhibit an inhibitory effect on platelet factor XIIIa-catalyzed incorporation of pseudodonor amines into dimethylcasein, platelet actin and myosin. The inhibitory action of calmodulin on the calcium-dependent enzyme reactions was analogous to the effects of EGTA and parvalbumin on these reactions. The extent of inhibition of factor XIIIa activity was a function of calmodulin concentration when factor XIII and Ca2+ concentrations were held constant. These results indicate that calmodulin inhibits platelet factor XIIIa-catalyzed reactions by sequestering calcium.