Regulatory effects of ATP and calcium on the myofibrillar ATPase of frog sartorius muscle

The ATPase activity of frog sartorius myofibrils has been studied at 1.5°C using different concentrations of ATP and calcium. The progressive activation of the ATPase activity at Ca-concentrations between pCa 8 and pCa 4 is paralleled by increases in Ca-binding. Similar to the findings of Weber and Bremel (1972) on rabbit psoas myofibrils more calcium is bound at pCa 5 – 7 in presence of 10 μM ATP than at 2 mM ATP. The observation, that in presence of 2 mμM N-ethyl maleimide/mg myofibrillar protein Ca-binding is essentially abolished at the lower calcium levels and becomes reduced by 30 – 40% at pCa 4 – 6, has been explained in terms of a Ca-binding site on the myosin. Using carbon-14-labelled ATP it could be demonstrated that the lower ATPase activity at pCa 7 or pCa 9 is associated with an increase in nucleotide binding, which is much reduced at a pCa of 4. However, removal of calcium from the medium does not increase the number of nucleotide binding sites as has been reported for rabbit myofibrils. A kinetic interpretation of the ATPase and ligand binding studies is offered.