Control of platelet actomyosin activity: Effect of ADP on superprecipitation and ATPase activity of human platelet actomyosin ☆

Abstract 1. 1. Platelet actomyosin superprecipitation performed in 0.09 M to 0.12 M KCl showed a biphasic curve with clearing and turbidity increase after addition of Mg-ATP. 2. 2. Addition of ADP produced inhibition of the duration and extent of the clearing phase ( K m of 4 · 10 −5 M), and activation of the superprecipitation phase ( K m of 1.96 · 10 −5 M). 3. 3. The effect of ADP was observed with no significant increase of ATPase activity and was apparently specific for ADP and not for GDP, UDP, or IDP when superprecipitation was initiated only with Mg-ATP. 4. 4. The effect of ADP was observed when superprecipitation occurred in the presence of EGTA. These data suggest that ADP may form part of a control mechanism of platelet contractile activity.