Studies on human platelet protease activity.

SOLUBILIZED PROTEIN DERIVED FROM HUMAN PLATELETS WAS FRACTIONATED BY DEAE CELLULOSE COLUMN CHROMATOGRAPHY AND ANALYZED FOR PROTEASE ACTIVITY USING THREE SUBSTRATES: denatured bovine hemoglobin, alpha casein, and purified plasminogen-free human fibrinogen. A protein fraction was found with proteolytic activity which was heat labile and not attributable to plasmin. The activity was not potentiated by cysteine or inhibited by iodoacetamide. Studies of pH optima indicated a broad range of enzyme activity with peaks in both the acid and alkaline region. Cathepsin A activity was detected in the platelet protease fraction by hydrolysis of the synthetic substrate N-carbobenzoxy-alpha-L-glutamyl-L-tyrosine. Similar proteolytic activity was found when the proteins derived from isolated platelet granules were examined. The results indicate that human platelets possess potent intracellular proteolytic enzymes. The relationship of this proteolytic activity to the hemostatic process is discussed.