Comparison of Tropomyosins from Cardiac and Skeletal Muscle.

• Characterization of the myocardial contractile proteins has been undertaken in an attempt to define biochemical mechanisms responsible for the physiological dissimilarities between cardiac and skeletal muscle. These functional differences include the lesser tension generated by cardiac muscle,' different actions of inotropic agents,' and the relative slowness of onset of the active state of cardiac muscle when compared to its skeletal counterpart. The relative weakness of cardiac muscle is manifested in the behavior of glycerol-extracted cardiac fibers ° and may be related to the lower adenosinetriphosphatase activities of cardiac actomyosin and cardiac myosin." Physicochemical studies of cardiac myosin indicate a molecular weight similar to that of skeletal myosin 8> "• n although different amino acid sequences are suggested by immunological dissimilarities 12 and different effects of trypsin. Comparison of cardiac and skeletal actins has failed to reveal detectable differences in physicochemical properties, amino acid compositions, and the number and reactions of sulfhydryl (SH) groups. Tropomyosin,* the third of the more abun-