Assessing seasonal changes in the efficiency of protein digestion in gastric and agastric fish using in vitro models

The objective of this study was to evaluate whether the efficiency of protein digestion in selected freshwater fish species exhibits functional adaptations to seasonal fluctuations in water temperature, mediated by variations in the proteolytic capacity to hydrolyze substrates. To test this hypothesis, individuals of two species-the Prussian carp (Carassius gibelio) and the European perch (Perca fluviatilis)-were collected during two different seasons (spring and summer) in the Chany Lake basin (Western Siberia, Russia). Digestive enzyme extracts were employed in in vitro hydrolysis assays using a standard protein substrate (hemoglobin), under varying experimental conditions (hydrolysis time, pH, and temperature). The extent of protein digestion was quantified by measuring the total release of free amino acids (AA). The patterns of AA release identified during in vitro simulated protein digestion under specific condition differed markedly between species, with five distinct phases in perch and three in Prussian carp. The results revealed significant effects of both species and sampling season as well as studied factors on the overall extent of protein hydrolysis. Our results further indicate that the relative contribution of the acid stage to total protein hydrolysis was greater at low temperatures (5 °C) and intestinal pH (6.5), and decreased progressively with increasing temperature and pH. Under simulated spring conditions, the percentage of total amount of AA release during the acid stage was approximately twice that observed under summer conditions. These findings suggest a potential functional adaptation of digestive proteases to seasonal temperature variations, likely aimed at optimizing protein digestion under changing environmental conditions.