Mono S cation-exchange chromatography (A) and SDS-PAGE (B) patterns of cathepsin B from horse mackerel muscle. (A) The enzyme solution (1.37 mg protein) was applied to a column (1.7 mL), equilibrated with 50 mM acetate buffer (pH 4.5) containing 2 mM 2-mercaptoethanol, and eluted with a linear gradient of NaCl at the concentration of 0 to 0.5 M in the same buffer, and then eluted with a step wise of NaCl at the concentration of 0.5 to 1.0 M in the same buffer; (B) The purified cathepsin B fraction (18 µg protein) was applied to SDS-PAGE under reducing conditions on a 10% gel and stained by CBB. Arrowheads indicate protein bands of cathepsin B polypeptides. M, molecular weight protein standards were loaded. Protein standards was composed of carbonic anhydrase (M.W. 31,000), trypsin inhibitor (M.W. 21,500), lysozyme (M.W. 14,400), and aprotinin (M.W. 6500).