Actin from Heart Muscle: Sulfhydryl Groups.

• A comparison of a number of chemical features of actin prepared from cardiac and skeletal muscle has been undertaken in an attempt to evaluate the role that this contractile protein may play in determining specific physiological responses by the two types of muscle. Analyses of physicochemical properties 1 and amino acid compositions failed to reveal significant differences between these proteins, although minor dissimilarities of amino acid sequence, which could be of functional significance, were not excluded. In extending this comparison, the reactions of the sulfhydryl groups (SH) of cardiac and skeletal actins have been investigated because these groups play an important role in polymerization ~ and provide a sensitive indication of changes in the protein occurring as a result of binding of nucleotides and bivalent cations." Comparison of the number and reactivities of the SH groups of cardiac and skeletal actin preparations failed to reveal