The effect of ADP on the superprecipitation of human platelet thrombosthenin

Thrombosthenin, an actomyosin like protein from platelets, undergoes superprecipitation at low ionic strength in the presence of Mg ATP. Unlike muscle actomyosin, turbidity increase (620 nm, 27°) of 0.2-0.5 mg/ml thrombosthenin could not be correlated with ATP hydrolysis. The early turbidity increase seen after addition of Mg ATP to thrombosthenin reflected true superprecipitation because the volume of precipitate decreased while its protein content remained unchanged. This suggested that the stimulus for superprecipitation was the appearance of a product, ADP, rather than utilization of a substrate, ATP. ADP augmented both the rate and extent of this early turbidity increase. Within the range, 50-150 μM ADP, turbidity increased with increasing ADP concentration, while ADP in absence of Mg ATP had no effect. ADP action was not seen above and below 90-120 mM KCl. The findings suggest that the well known physiological effects of ADP on platelets may be mediated at least in part by its direct effect on platelet contractile proteins.