PERSISTING IN-VITRO ACTIN MOTILITY AT NANOMOLAR ADENOSINE-TRIPHOSPHATE LEVELS - COMPARISON OF SKELETAL AND CARDIAC MYOSINS

We have previously demonstrated in vitro actin movement at nanomolar a denosine triphosphate (ATP) levels using heavy meromyosin from skeleta l muscle. In the present work we tested whether the motility at nonomo lar ATP-concentrations could be supported by cardiac myosin as well. A ctomyosin (skeletal actin and bovine ventricular myosin) was pretreate d in the in vitro motility assay with 1 mM ATP; subsequently, the ATP level was reduced by multiple rigor-solution washes. By the final rigo r-solution wash, the ATP concentration, monitored by the luciferin-luc iferase assay, dropped to the order of 100 nM. Even at this low ATP le vel actin-filament movement remained in evidence. This was in marked c ontrast to the situation where ATP concentration was gradually increas ed from zero; in the latter, filament movement began only as ATP level s exceeded 1-2 mu M. The difference indicates that potential energy is stored during the initial ATP treatment, and utilized later as the fr ee ATP falls below micromolar levels. Although the velocity of cardiac myosin-supported movement was only one fourth of that of skeletal myo sin, both myosins supported actin movement down to similar ATP concent rations. The similarity in response of the two myosins to ATP implies a similar degree of potential energy storage. Given the significantly different specific ATPase activities, however, it appears that the mec hanism of potential energy storage and release involves factors differ ent from those involved in the release of chemical energy by the myosi n ATPase.