The response of digestive proteases to abrupt salinity decrease in the euryhaline sparid Sparus aurata L.

The response of the digestive proteases to abrupt salinity change was studied in juvenile gilthead sea bream (Sparus aurata) for 15 days after transfer from 33‰ to 21‰. Salinity decrease affected significantly neither the activity of total acid proteases in stomach, nor the activities of total alkaline proteases and major serine proteases - trypsin and chymotrypsin - in the alkaline part of the intestine. The activity of the major proteases was significantly different between the alkaline segments of the intestine, with the posterior intestine presenting the highest activities followed by the pyloric caeca. This distribution pattern remained unaffected by salinity decrease. Notably, salinity change led to significant alterations in elastase and carboxypeptidase activity. The changes were more prominent in the upper part of the intestine (pyloric caeca and anterior intestine) than in the posterior intestine. In pyloric caeca significant alteration of carboxypeptidase A and B activities was observed, elastase changes were confined to anterior intestine together with alterations in carboxypeptidase B activity, while in posterior intestine the changes were restricted to carboxypeptidase A activity. The results are discussed in relation to the osmoregulatory action of the intestinal segments and dietary protein digestion.