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A possible explanation for the decrease in myosin Ca2+-dependent ATPase activity as rat heart cells age in culture is presented. The subunit structure and enzyme kinetics of myosin from adult and neonatal rat hearts and from rat heart cells of young and old cultures are compared. These studies indicate that the loss in Ca-ATPase activity of myosin from older cultures was an intrinsic property of the myosin itself. Myofibrillar fractions from the indicated four sources showed no qualitative or quantitative differences in electrophoretic patterns. Myosin from older cultures was more sensitive to alkaline denaturation than was myosin from younger cultures, as indicated by its more accelerated loss of K+(EDTA)-dependent ATPase activity after 10 min of incubation at pH 10. Furthermore, myosin from older cultures was more temperature-sensitive, as indicted by a more rapid loss of Ca-ATPase with decrease in assay temperature. It is suggested that there is either a change in conformation of myosin molecules at or near the active site of the enzyme or alternatively there is a change in light chain 1-light chain 2 and/or light-chain-heavy-chain interaction(s) in the myosin molecules under study.

A comparative study of myosin and its subunits in adult and neonatal-rat hearts and in rat heart cells from young and old cultures