Muscle Cathepsins in Three Species of Pacific Sole

The cathepsins in saline muscle extracts of three species of Pacific sole were investigated. A buffered hemoglobin solution (pH 3.0) served as substrate for the enzyme assay. No apparent change in catheptic activity in muscle homogenates of English sole was observed during 25 days of frozen (−26 °C) storage. Partial characterization of the cathepsins indicated a pH activity optimum of 3.0–3.5 for rex and petrale sole but a slightly higher PH optimum of 3.2–3.8 for English sole. The three species showed a temperature optimum of 45 °C for hemoglobin splitting. A wide variation in catheptic activity within the species was observed for rex sole while English and petrale sole showed more uniform catheptic activity. The mean activities for the three species varied significantly (P < 0.05). Results of sensory evaluation of muscles containing high and low levels of catheptic activity from the three species of sole did not support the assumption that cathepsins have a substantial influence upon the quality of marine food fish.