Studies on cardiac myofibrillar adenosine triphosphatase.

It has long been known that myosin and actin can be obtained from heart as well as from skeletal muscle and that, in combining them to form actomyosin, they are interchangeable with their skeletal counterparts (1). The contractile properties of glycerolextracted cardiac strips and cardiac actomyosin (myosin B) threads and bands have also been demonstrated (2, 3). The molecular parameters of cardiac and skeletal myosin have been found to be identical by Gergely and Kohler (4), whereas Olson and Ellenbogen (5) have reported differences. Cardiac myosin functions as an adenosine triphosphatase but its activity is only about one-third that of skeletal myosin. Cardiac myosin, furthermore, is more resistant to digestion by trypsin and chymotrypsin (6). Myofibrils are an attractive model for the study of muscle: they are free of some of the complicating features of intact musple-nervous elements, membranes, sarcosomes-yet contain the structural proteins, actin and myosin, in, presumably, the same relation in which they exist in intact muscle; their small diameter makes it possible to avoid the diffculties inherent in a long diffusional path leading to an adenosine triphosphate-free core (7, 8). Perry and Grey (9), in their study of the ATPase activity of skeletal myofibrils, brought out two striking features. First, ATP concentrations exceeding that of the Mg present inhibited the ATPase activity; the inhibition was counteracted by a small amount of Ca++. The authors interpreted these results in terms of the Mg-ATP complex’s being the true substrate and of free ATP’s being an inhibitor. This view has been disputed by Geske et ~2. (10) who stressed the possibility of a direct interaction of Mg++ with the enzyme, depending on the concentration of free Mg++ ions. Second, ethylenediaminetetraacetate inhibited the ATPase activity even in the presence of excess Mg, but, not with Ca++ as the activator; moreover, when both Mg++ and Ca++ were present, no inhibition occurred. In view of the similarities and differences between the skeletal and cardiac systems referred to above, it appeared of interest to study some aspects of the ATPase activity of cardiac myofibrils.