The results on the activity of lysosomal hydrolases (proteases (cathepsins) and nucleases) in salmon (Salmo salar L.) underyearlings (0+), 1+ and 2+ parr, and smolts (2+, 3+) from the Indera River (White Sea drainage basin) are reported, testifying their possible involvement in the processes accompanying the growth and development of juveniles.
The highest levels of the enzymes’ activity found in 0+ fish were probably related to the demand for “building blocks” for the biosynthetic processes of protein accumulation necessary for the subsequent structural changes in growing juveniles switching from endogenous nutrition to a mixed diet. Further on, in 1+ & 2+ parr and 2+ & 3+ smolts, the activity of cathepsin D (lysosomal proteolytic enzyme with a catabolic function) decreased somewhat in the liver and was virtually undetectable in skeletal muscles. These changes in the activity of cathepsin D are consistent with the indeterminacy of growth processes in fish, which is based on the prevalence of synthetic processes over catabolic ones. The relatively high activity of cathepsin B (lysosomal cysteine protease with a regulatory function) and nucleases (RNase and DNase) in older salmon smolts (3+) most likely reflects the role of these enzymes in forming the smolts’ fitness for foraging migration to the sea.
The revealed changed in the activity of lysosomal hydrolases in salmon juveniles indicates there is a correlation between the activity of the enzymes and the fish age, showing that lysosomal enzymes are involved in protein metabolism rearrangements accompanying the processes of growth and development in salmonids.