Molecular and enzymatic properties of cardiac myosin A as compared with those of skeletal myosin A.

Previous studies have suggested that the molecular and enzymatic parameters of cardiac myosin are quite similar to those of its skeletal counterpart (I). Thus, the weight average molecular weight, iWW, weight intrinsic viscosity, [v], and intrinsic sedimentation constant, siO, W, are comparable with the corresponding data on the skeletal protein (2). Also, the enzymatic adenosine triphosphatase activity of cardiac myosin was found to possess some features similar to those of the skeletal enzyme. However, a distinct difference was observed in the value of the maximal velocity (V,,,) of cardiac ATPase as compared with the skeletal enzyme. In order to elucidate the cause of this difference, more detailed studies of molecular and enzymatic properties of cardiac myosin were undertaken. In this paper, further similarities in molecular and enzymatic properties of cardiac myosin were established. In its secondary structure, native cardiac myosin was shown to be not significantly different from its skeletal counterpart, as revealed by optical rotatory dispersion measurements. Cardiac myosin was found to be dissociable by aqueous guanidine hydrochloride into subunits similar in size to those obtained with skeletal myosin under comparable dissociation conditions. The kinetic data on cardiac myosin ATPase point to the existence of similarities between the two enzymes. However, the comparatively low value of Vmax and the slightly lower value of k (reciprocal of Km) with the cardiac enzyme probably reflect some differences in the structure of the 2 molecules, and this is demonstrated further by the different behavior of the cardiac molecule toward attack by the proteolytic enzyme, trypsin. Since the secondary structural characteristics of the two proteins are similar, it is concluded that the observed differences in enzymatic properties may reside in slight alteration of the tertiary structure of the cardiac molecule, probably in the “hydrophobic regions.”