Depressed myosin ATPase activity in hearts of myopathic hamsters: Dissociation from neutral protease activity

Myosin was isolated from the hearts of 210-day-old genetically myopathic Syrian hamsters (Bio 53:58) and its properties compared to the myosin from random bred controls of the same age. The Ca²⁺ ATPase and the total-SH content did not differ significantly in the two groups. Myosin from myopathic animals showed a loss of LC₂. Analyses of myofibrils and actomyosin revealed that the LC₂ was present in these preparations from the myopathic animals and the loss of LC₂ was taking place during the purification of myosin. Despite the presence of LC₂ in the myofibrils and actomyosin, the corresponding ATPase activities were depressed in the myopathic hearts. The loss of LC₂ in myopathic cardiac myosin was found to be due to high levels of neutral protease activity co-purifying with myosin. The protease fraction from myopathic hearts degraded the cardiac LC₂ of control hamster and dog cardiac myosin, however, the Ca⁺⁺ ATPase remained unchanged.