Studies on contractile proteins in ADP- and collagen-induced platelet aggregation

Human platelets were induced to aggregate by either ADP or collagen. Both aggregated and non-aggregated (resting) platelets were then washed twice, lyzed by osmotic shock in the presence of several protease inhibitors and fractionated. The cytoplasmic fraction (Fraction I) was separated from the rest of the cell by centrifugation, the pellet was suspended in 0.8 M KC1 and the extract (Fraction II) was separated from the particulate fraction (Fraction III). The relative amounts of three contractile proteins, myosin, actin and actin-binding protein were evaluated and the activities of myosin, K⁺-(EDTA)-activated ATPase, and of actomyosin, Mg²⁺-mediated ATPase, were determined in all fractions. Following ADP- or collagen-induced aggregation the specific activities of both myosin and actomyosin ATPases increased significantly in the non-soluble Fraction III.