Treatment of human platelets with trypsin, thrombin, or collagen inhibits the pertussis toxin-induced ADP-ribosylation of a 41-kDa protein.

Permeabilization of human platelets with saponin (15-25 micrograms/ml) allows the determination of the ADP-ribosylation of a 41-kDa protein by pertussis toxin. The ADP-ribosylated protein is present in the particulate fraction. ADP-ribosylation of the 41-kDa protein increases for 20 min; it is not affected by indomethacin, prostacyclin, and 1,2-diacylglycerols but is inhibited by 1 mM Ca2+ and phorbol esters. Treatment of platelets with trypsin, thrombin, or collagen before saponin addition precludes subsequent pertussis toxin-induced ADP-ribosylation of the 41-kDa protein. The effect of trypsin or thrombin is blocked by soybean trypsin inhibitor and leupeptin. Trypsin proteolytically cleaves the ADP-ribosylated 41-kDa protein to an ADP-ribosylated fragment slightly smaller than 20 kDa. The results suggest that a modification of a guanine nucleotide-binding regulatory protein is associated with the actions of trypsin, thrombin, and collagen on platelet activation.