Some Properties and Postmortem Changes of Cysteine Protease Inhibitors from Rabbit Skeletal Muscles

Rabbit skeletal muscles were demonstrated to contain several inhibitors of cysteine proteases, cathepsins B and L, and papain. Sephadex G-100 column chromatography exhibited two inhibitor peaks at the positions corresponding to 9, 500 and 52, 000 daltons, which commonly inhibited the above three cysteine proteases. There were two other peaks (corresponding to 21, 000 and 28, 000 daltons) inhibiting papain, one peak (25, 000 daltons) inhibiting cathepsin B and one peak (36, 000 daltons) inhibiting cathepsin L. Muscle extracts contain an unknown factor which can degrade the inhibitors during incubation at pH 4.0 and 37°C. This factor is not cathepsin D. Total inhibitor activities slightly changed on 9-day storage of rabbit skeletal muscle at 4°C, while the extractable activity of cathepsin L increased and that of cathepsin B slightly decreased.