Degradation of myofibrillar proteins by cathepsins B and D

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Abstract
The procedure of Barrett for isolating cathepsins B and D from human liver was modified for use with rat liver and skeletal muscle. The purified enzymes appeared to be similar to those reported in other species. Sephadex G-75 chromatography of concentrated rat muscle extract resolved 2 peaks of cathepsin B inhibitory activity, corresponding to MW of 12,500 and 62,000. The degradation of purified myofibrillar proteins by cathepsins, B and D was clearly demonstrated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. After incubation with enzyme, the polypeptide bands representing the substrates decreased in intensity and lower MW products appeared. Cathepsins B and D, purified from either rat liver or skeletal muscle, degraded myosin, purified from either rabbit or rat muscle. Soluble denatured myosin was degraded more extensively than insoluble native myosin. Degradation by cathepsin B was inhibited by lack of reducing agent, or by myoglobin, iodoacetic acid and leupeptin, but not by pepstatin. The same potential modifiers were applied to cathepsin D and only pepstatin produced inhibition. Rat liver cathepsin B had a pH optimum of 5.2 on native rabbit myosin. The pH optimum of cathepsin D was 4.0, with a shoulder of activity about 1 pH unit above the optimum. Rat liver cathepsins B and D were demonstrated to degrade rabbit F-actin at pH 5.0, and were inhibited by leupeptin and pepstatin, respectively. The degradation of myosin and actin by cathepsin D was more extensive than that by cathepsin B.