Actin from Heart Muscle: Studies on Amino Acid Composition.

• The differences that exist between the physiological behavior of cardiac and skeletal muscle may be due, at least in part, to differences in their contractile proteins. For example, the relative weakness of cardiac muscle' 2 may be a reflection of a lesser contractile force developed by cardiac actomyosin.4 In view of these, and other observations, studies directed toward the detection of chemical differences between actins from the two tissues have been carried out. Previous investigations in this laboratory have indicated that the extraction of cardiac actin can be accomplished using some of the same techniques that have proved successful with skeletal actin, and that the molecular weight of actin from dog hearts does not differ significantly from that of rabbit skeletal actin. In the present report, sedimentation behavior, starch gel electrophoresis patterns, amino acid contents, and peptide patterns obtained after tryptic digestion ("fingerprints") of dog cardiac and skeletal actin preparations are compared. No significant differences between these actin preparations have been found, indicating a high degree of similarity, if not identity, of these proteins. This suggests that the difference in behavior of the two types of muscle may reside elsewhere.