Muscle proteases from mackerel and milkfish were purified to electrophoretical homogeneity by concanavalin A-Sepharose and Sephadex G-100 chromatographies. Both proteases appear to be an aspartic protease, cathepsin D (EC 3.4.23.5). The molecular weights of the purified cathepsin D’s from mackerel and milkfish were 51,000 and 54,000, estimated by Sephadex G-100, and 59,000 and 61,000 by SDS–PAGE, respectively. Both cathepsin D’s were completely inhibited by pepstatin, but not affected by leupeptin, N-ethylmaleimide, dithiothreitol, or glutathione. s-Mercaptoethanol, iodoacetic acid, p-chloromercuri-benzoate, phenylmethylsulfonyl fluoride, and sodium dodecyl sulfate partially or completely inhibited both cathepsin D’s. Na+ and K+ partially activated the cathepsin D from milkfish. Both cathepsin D’s were inhibited by Mg2+, Sr2+, Fe2+, and H2+, but activated by Ca2+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+. The pI and optimal temperature of the cathepsin D’s from mackerel and milkfish were 5.04 and 4.91, 45°, and 5...