Characterization of Cardiac Myosin from the Dog

Previous studies of the physicochemical properties of cardiac myosin have led to estimates of molecular weight ranging from 225,000 to 7F;O,OOO (l-4).Comparable values for the molecular weight of skeletal myosin, ranging from 400,000 to 620,000, have been obtained by different investigators employing the Archibald method (5-S).The higher values did not agree with results calculated from light scattering and hydrodynamic measurements (g-11).On account of these large discrepancies, a comparison of molecular parameters of the two proteins entailed some uncertainty.Gergely and Kohler (a), as well as Davis, Carrol, Trapasso, and Yankopoulos(3), reached the conclusion that the molecular weights of skeletal and cardiac myosin were identical, i.e. approximately 500,000 for both proteins.Brahms and Kay (4) found a molecular weight of 750,000 for cardiac myosin from Archibald studies and considered this to be within experimental error of the value of 620,000 obtained by Kielley and Harrington (7) for skeletal myosin.Ellenbogen, Iyengar, Stern, and Olson (1) found molecular weights in the range of 225,000 to 270,000 for cardiac myosin.They considered this protein to be essentially different from any previously characterized myosin and suggested a monomer-dimer relationship between the cardiac and skeletal protein.This conclusion attracted particular attention, since it formed the basis of a hypothesis which attempted to explain the pathogenesis of congestive cardiac failure in molecular terms (12, 13).In contrast to the large discrepancies in regard to molecular weights, there seems to be unanimous agreement that the specific adenosinetriphosphatase activity of cardiac myosin is much lower than its skeletal counterpart (1, 4,(14)(15)(16)(17).Previous estimations of organ differences in molecular and enzymic properties of cardiac and skeletal myosin were based on studies with dog cardiac and rabbit skeletal myosin.This communication presents largely a reinvestigation of previous studies which have led to conflicting results.Molecular and enzymic properties, as well as electron microscopic appearance of dog cardiac myosin, were compared with skeletal myosins of dog and rabbit.EXPERIMENTAL PROCEDURE Isolation of Myosin-The procedures described by Ellenbogen et al. (1) were employed for obtaining the hearts, with minor modifications for the isolation of myosin.All preparative procedures were performed at 1-4" in the cold room.Minced cardiac muscle was extracted with 3 volumes of 0.3 M KCI-0.075