Proteolysis of myosin and troponin in human myocardium of elderly subjects.

Actomyosin was prepared from human myocardium and its protein composition was examined by SDS-polyacrylamide gel electrophoresis. For some preparations, particularly actomyosin isolated from elderly subjects, a high molecular weight (HMW) band (identified as a breakdown product of myosin heavy chain) appeared, while the troponin-T subunit decreased. Myofibril associated protease (MFP) activity showed no significant difference as a function of proteolysis. In agreement with the proteolysis of troponin-T and myosin, the Ca2+ sensitivity of Mg2(+)-ATPase activity decreased while the extent of the stimulation of Ca2(+)-ATPase by N-ethylmaleimide remained unchanged. This type of proteolysis would affect the Ca2(+)-dependent regulation of muscle contraction but not the contractility per se.