Cleavage of cytoskeletal proteins by two forms of Ca2+ activated neutral proteases in human platelets.

Two forms of calcium activated neutral proteases (CANPs) with different affinity to Ca2+ were partially purified from the soluble fraction of human platelets; one (mu-CANP*) required 6 muM Ca2+ and the other (m-CANP*) did 900 muM Ca2+ for the respective half maximal activity. Human platelets were found to contain mu-CANP predominantly in contrast to our previous study in bovine platelets, in which mu- and m-CANP were equally distributed. Among platelet protein preparations examined for possible endogenous substrates of platelet CANPs, actin binding protein (ABP) and 230 K protein were proteolysed completely by mu- or m-CANP in the presence of the respective optimal concentration of Ca2+. As far as mu-CANP is concerned, this is the first demonstration of proteolysis of endogenous substrates, which implies possible involvement of mu-CANP in stimulus-linked platelet reaction coupled with an increase in intracellular Ca2+ to micromolar concentration. Furthermore, microtubules associated proteins (MAPs) of bovine brain was proteolysed by mu- or m-CANP of human platelet in the similar manner, indicating functional ubiquity of CANPS.