DEPTH-RELATED ENZYMIC ACTIVITIES IN MUSCLE, BRAIN AND HEART OF DEEP-LIVING PELAGIC MARINE TELEOSTS

The activities [units-1 wet wt tissue] of lactate dehydrogenase [LDH; EC 1.1.1.27], pyruvate kinase [PK; EC 2.7.1.40], malate dehydrogenase [MDH; EC 1.1.1.37] and isocitrate dehydrogenase [IDH; EC 1.1.1.42] in white skeletal muscle, brain and heart of 24 pelagic teleost fishes were determined. For several species, citrate synthase [CS; EC 4.1.3.7] activities were examined in white skeletal muscle. In muscle, the activities of all the enzymes decrease exponentially with increasing minimum depth of occurrence of the species; this decrease closely parallels the decrease in respiratory rate found previously for these same species. The decline in enzymic activity with increasing minimum depth of occurrence is much greater than the decline in body protein content of the whole fishes, suggesting a disproportionately rapid fall in enzyme concentration in comparison to contractile and structural protein concentrations. The similar reductions in activities of both glycolytic (LDH and PK) and citric acid cycle (CS, MDH and IDH) enzymes with depth indicate that both standard and active metabolisms of deeper-dwelling species are reduced relative to shallower-dwelling forms. There is no suggestion of increased anaerobic capacity with depth or in relation to species'' occurrence in the oxygen minimum layer. In brain and heart, there is no significant decrease in enzymic activity with increasing minimum depth of occurrence. These 2 tissues have similar capacities for metabolism in most fishes when comparisons are based on enzymic activity per gram wet weight of tissue.