Action of ADP and ethanol on muscle proteins in vitro

The effects of ADP and ethanol on Ca2+ binding of troponin and the superprecipitation of actomyosin were studied. Ca2+ binding of troponin-tropomyosin complex bound to polystyrene particles (Lytron) was increased by ADP, and this increase was inhibited by ethanol. However, Ca2+ binding of the complex as measured by equilibrium dialysis and by the chelex resin method was not influenced by either ADP or ADP plus ethanol. Ca2+ binding of the thin filament, myosin-ghost myofibrils and myofibrils was also not inhibited. Superprecipitation of actomyosin was augmented in the presence of ADP, and the enhancement was inhibited by ethanol. However, this effect of ADP or ethanol was not observed in the presence of an inhibitor of myokinase, p1, p5-di (adenosine-5') pentaphosphate (Ap5A). In the presence of Ap5A, superprecipitation of actomyosin was enhanced when small amounts of ATP (10 microM) and ADP (100 microM) were added 5 min prior to the addition of 2 mM ATP to start the reaction. The enhancement of superprecipitation of actomyosin by ADP may be caused by nuclei produced by a low concentration of ATP which is produced from ADP by contaminating myokinase activity. These data suggest that ADP and ethanol influence Ca2+ binding of the troponin-tropomyosin complex bound to a solid phase, but their effects on superprecipitation may not necessarily reflect muscle contraction in vivo.