Susceptibilities of various myofibrillar proteins to cathepsin B and morphological alteration of isolated myofibrils by this enzyme.

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-phthalaldehyde, 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 much greater than troponin greater than tropomyosin much greater than actin. alpha-Actinin was not degraded. Myosin heavy chain was degraded to several fragments with molecular weights of 175,000, 170,000, 160,000, and 145,000, whereas the light 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 three fragments with molecular weights of 30,000, 18,000, and 12,800. Tropomyosin was degraded slightly and its product had a molecular weight of 32,000. Actin was also 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: 1) disappearance of the Z-band in the early stage of incubation, 2) disappearance of the M-line following loss of the Z-band, and 3) decrease in the density of the A-band after swelling of the myofibrils.