Comparison of the subunit compositions and ATPase activities of myosin in the myocardium and conduction system

Abstract

The light chains and ATPase activities of specialized and ordinary muscle myosin from bovine heart were compared. The specialized heart muscle was composed of the A-V node, bundle of His, and right and left bundle branches. The yield of myosin from the specialized heart muscle was approximately 20% of that from the ordinary heart muscle. On sodium dodecyl sulfate (SDS)-polyacrylamide gel (10%) electrophoresis, ordinary heart myosin gave two bands of low molecular weight (light chains) as reported previously, whereas specialized heart myosin gave three bands. By comparison of the electrophoretic mobilities of these light chains with those of proteins of known molecular weight, the molecular weights of the two myosin light chains from ordinary heart muscle were estimated as 25 000 (LC 1) and 18 000 (LC 2), and those of the three bands of specialized heart myosin as 25 000 (LC 1), 22 500 (LC 1′) and 18 000 (LC 2). There was approximately 3.5 mol of myosin light chains/mol of myosin in both tissue (Ordinary heart muscle: LC 1 1.7 mol, LC 2 1.8 mol; Specialized heart muscle: LC 1 0.9 mol, LC 1′ 1.1 mol, LC 2 1.5 mol). The specific activities of myosin K +-EDTA- and Ca 2+-activated ATPase ( V max values in μmol phosphate/mg/min) in the two types of heart myosin were similar. These results suggest the existence of a new type of cardiac myosin isoenzyme in specialized heart muscle.