The effect of cathepsin D from muscle and spleen on bovine myofibrils has been examined under postmortem pH conditions using sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis, scanning electron microscopy (SEM) and electron spin resonance spectroscopy (ESR). Cathepsin D causes a degradaticy of 2 disks of the myofibrils and has a relatively selective action on the myofibrillar proteins in comparison to the plant derived enzyme papain. The heavy chain of myosin (200,000 daltons) was degraded to fragments of about 170,000, 150,000, and 80,000 daltons at 25°C. Degradation becomes more extensive at 37°C. ESR studies on spin-labeled myofibrils indicated that the proteolytic attack of cathepsin D occurred more distal to the globular region of the myosin when compared to papain or trypsin. Although there appears to be little proteolytic effect on o-actinin or actin, changes in the gel electrophoresis pattern below 42,000 daltons indicate alterations of the regulatory complex and myosin light chains.

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