Tropomyosin-troponin-induced changes in the partitioning of free energy release of actomyosin-catalyzed ATP hydrolysis as measured by ATP-phosphate exchange.

ATPase activity and ATP-Pi exchange of unregulated (without tropomyosin-troponin) and regulated (with tropomyosin-troponin) acto-HMM were measured in media containing 0.2 mg/ml actin, HMM, and (when present) tropomyosin-troponin, 2 mM MgCl2, 10 mM KCl, 2 mM NaN3, 10 mM Pi (pH 7.0), 3 MM ATP. The following mean values for ATPase activity and for the rate of incorporation of Pi into ATP (each per mg HMM and per min) were obtained: unregulated acto-HMM 0.33 mumol Pi and 0.33 nmol Pi, regulated acto-HMM 0.54 mumol Pi and 1.06 nmol Pi. The ratio of Pi incorporation rate to ATPase activity was 1.01 x 10(-3) for unregulated and 2.02 x 10(-3) for regulated acto-HMM. From these ratios and from the overall free energy change of ATP hydrolysis it was calculated that under the prevailing experimental conditions in unregulated acto-HMM 62% and in regulated acto-HMM 66% of the free energy change of ATP hydrolysis occurs after the release of phosphate from actomyosin. It is probably this part of the free energy change that is used by the muscle for the performance of work.