Studies on Myofibrils from the Stored Muscle

Adenosine triphosphatase (ATPase) activity of myofibrils isolated from fresh muscle and the muscle stored at 4°C have been measured. An increase in Mg-activated ATPase activity of myofibrils was caused by lengthened homogenization. With the progress of aging of muscle, Mg-activated ATPase activity of myofibrils increased remarkably. When myofibrils from pre-rigor and rigor muscle in 0.16 m KCl were treated with 0.6 m KCl-18 mm Tris-maleate solution (pH 7.0), Mg-activated ATPase activity of myofibrils at low ionic strength increased markedly. However, the Mg-activated ATPase activity of the myofibril isolated from the muscle stored at 4°C for 8 days (8-myofibril) increased slightly after the similar treatment. The dependence of myofibrillar ATPase activity on KCl concentration became greater with the progress of aging of muscle. These results may show that, as long as ATPase activity and the dependence of ATPase activity on KCl concentration are concerned, 8-myofibril is the most similar to the isolated actomyosin among myofibrils, although actomyosin in muscle may exist in a different form from that in solution. It is suggested that, with the progress of aging, the structural alteration of myofibril occurred and the myofibril became more susceptible to ATP-induced transformation.