The abilities of cathepsin B purified from liver to degrade purified myofibrillar proteins, myosin, actin, troponin, tropomyosin and .alpha.-actinin from rabbit skeletal muscle were studied. The amino acids or peptides liberated from these proteins by cathepsin B were determined quantitatively by fluorometry with o-phthaladehyde, and qualitatively by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. At a molar ratio of cathepsin B to substrate of 1 : 100, the order of susceptibilities was myosin .mchgt. troponin > tropomyosin .mchgt. actin. .alpha.-Actinin was not degraded. Myosin H-chain was degraded to several fragments with MW of 175,000, 170,000, 160,000 and 145,000. The L-chains were scarcely degraded. Cathepsin B degraded troponin-T rapidly, and troponin-I more slowly, but did not degrade troponin-C. Troponin-T and troponin-I were degraded to 3 fragments with MW of 30,000, 18,000 and 12,800. Tropomyosin was degraded slightly and its product had a MW of 32,000. Actin was degraded only slowly and no liberated product could be detected. Morphological changes in myofibrils prepared from glycerinated psoas muscle of rabbit during incubation with cathepsin B were observed. Three notable phenomena were observed: disappearance of the Z-band in the early stage of incubation, disappearance of the M-line following loss of the Z-band and decrease in the density of the A-band after swelling of the myofibrils.
