Participation of cathepsin L into extensive softening of the muscle of chum salmon caught during spawning migration.

The muscle of some chum salmon caught during spawning migration shows very high autolytic activity i.e. proteolytic activity and extensive softening. The process of the muscle softening during storage was closely correlated to the levels of autolytic activity and activities of cathepsins B and L. Proteolysis proceeded extensively in muscles when its autolytic activity was very high. The muscle softening was accelerated when the muscle underwent freeze-thaw. Inhibition ex-periments showed that most of the auto1ytic activity at pH 6.5 was attributable to the lysosmal cys-teine proteases, such as cathepsins B and/or L. An SDS-PAGE analysis indicated that the de-gradation pattern of myofibrillar protein of the softened muscle was almost identical to that of myofibrils treated with purified cathepsin L. The tentative conclusion, therefore, is that cathepsin L must be the most probable enzyme responsible for the muscle autolysis and causes the extensive muscle softening which is often observed in the catch of matured chum salmon.