Mode of action of rabbit skeletal muscle cathepsin B towards myofibrillar proteins and the myofibrillar structure.

1. The mode of degradation of myofibrillar proteins and the structural changes in myofibrils due to the action of cathepsin B highly purified from rabbit skeletal muscle were studied. 2. Cathepsin B degraded myosin heavy chain, actin and troponin T, but not alpha-actinin, tropomyosin, troponin I or troponin C among myofibrillar proteins. 3. Cathepsin B optimally degraded myosin heavy chain, actin and troponin T at around pH 5. Degradation of myosin heavy chain produced 6 fragments, 180,000, 150,000, 87,000, 81,000, 75,000 and 69,000 Da, respectively. Actin was hydrolyzed into fragments of 41,000, 38,000 and 30,000 Da. Troponin T was degraded into fragments of 21,000, 12,000 and 10,000 Da. 4. Cathepsin B caused the fragmentation of myofibrils and disturbance of the lateral arrangement of myofibrils. 5. Cathepsin B partly disintegrated the Z-line and the M-line, and induced disordering of the arrangement of filaments in the I-band.