This study examined effects of purified CAF and cathepsin D on bovine skeletal myofibrillar proteins and structure when incubated under postmortem-like conditions of pH and temperature;When CAF (purified from bovine cardiac muscle) was incubated with isolated myofibrils under near optimal conditions (pH 7.5 and 25(DEGREES)C), a rapid and significant release of soluble protein, including intact alpha-actinin, from the myofibrils was observed. SDS-PAGE analysis of treated sedimented myofibrils showed CAF digestion had degraded titin and, at least partly, tropomyosin, and troponins T and I. A trio of bands in the 30,000-dalton region appeared in the gels of the sedimented myofibrils. CAF treatment caused a significant increase in myofibril fragmentation;Lowered incubation pH or temperature reduced the effect of CAF on myofibrils. However, CAF effectively caused alterations in myofibrillar proteins and structure at pH 7.5 and 5(DEGREES)C, pH 6.5 and 15(DEGREES)C, or pH 5.5 and 25(DEGREES)C, suggesting that it may play a significant role in the tenderization process occurring in muscle postmortem;When cathepsin D (purified from bovine cardiac muscle) was incubated with myofibrils under conditions resembling those found in postmortem muscle, several alterations occurred. Cathepsin D at pH 5.5 and 37(DEGREES)C released a significant amount of soluble protein after 30 to 60 min of incubation. SDS-PAGE analysis of sedimented myofibrils showed degradation of myosin heavy chains and titin. A small amount of actin, tropomyosin, troponins T and I, and myosin light chains also were degraded. Myofibrils incubated with cathepsin D under these conditions (pH 5.5 and 37(DEGREES)C) and then subjected to a brief homogenization step showed no change in the degree of myofibril fragmentation, even though a significant narrowing of the A bands occurred. Raising the pH and/or lowering the temperature greatly reduced the effect of cathepsin D on myofibrillar proteins and structure. Cathepsin D had only a small effect at pH 6.5 and 37(DEGREES)C or at pH 5.5 and 25(DEGREES)C, suggesting that cathepsin D does not play a principal role in the tenderization process occurring in muscle postmortem.