CHARACTERIZATION OF FISH MUSCLE PROTEASES USING RADIO‐LABELED PROTEIN SUBSTRATES

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Abstract
Cathepsin D and alkaline protease in the kidney and skeletal muscle tissues of Atlantic croaker were studied using either 14C‐casein or 14C‐hemoglobin. In general, the alkaline protease is more stable than cathepsin D upon heat treatment and the alkaline protease from muscle is more heat stable than that of kidney tissue. Both cathepsin D and alkaline protease showed specificity toward the different protein substrates. Under the defined conditions, it is possible to use a single substrate (casein or hemoglobin) for, the study of both proteases. The pH activity curves using both substrates suggest that at physiological pH, both acid and alkaline proteases may contribute to the proteolytic activity observed during holding or processing of fish products at lower temperatures (45–60°C). It was demonstrated that pepstatin may be effectively used to selectively inhibit the catheptic activity so that the relative contribution of alkaline protease to the degradation of the fish protein may be evaluated.