Digestive enzyme distribution and activity in the digestive tracts of the rabbitfish, Siganus canaliculatus and the sea bass,Lates calcarifer were studied. Quantitative determinations of digestive enzymes in the guts of both fishes showed that they were capable of digesting carbohydrates and proteins in their diet. The carbohydrases, amylase, laminarinase, maltase, sucrase and trehalase were detected in the rabbitfish; their activities being mainly in the stomach, intestine and pyloriccaeca. Amylase, maltase, trehalase and chitinase activities were recorded in the gut of the sea bass, primarily in the intestine and the pyloriccaeca. Their activities were significantly lower than those in the rabbitfish. Proteases (pepsin, chymotrypsin, elastase, leucine aminopeptidase and trypsin) were found in both the rabbitfish and the sea bass. Pepsin activity however, was higher in the sea bass; while trypsin and chymotrypsin activities were higher in the rabbitfish. The activities of the various digestive enzymes in both fishes are discussed in relation to their feeding habits.
Some properties of the intestinal proteases of the rabbitfish were examined. At 25°C, both trypsin and chymotrypsin showed pH optima of 8.0. Leucine aminopeptidase, however, displayed maximum activity in the pH range, 7.0-9.0. Leucine aminopeptidase had the highest optimum temperature (60°C), and chymotrypsin, the lowest (30°C). The optimum temperature of trypsin was 55°C. The activation energy, Ea, was found to be 8.24 for trypsin and 8.50 kcal mol(-1) for chymotrypsin. The Ea for leucine aminopeptidase was 6.29 kcal mol(-1) above 40°C and 1.73 kcal mol(-1) below 40°C. Substrate concentration-velocity plots showed that all three enzymes followed Michaelis-Menten kinetics; the Km and Vmax were estimated for the three enzymes. The effects of various protease inhibitors on enzyme activity were also examined and confirmed the protease classes to which each enzyme belonged. The three proteases examined have similar properties to proteases in other fishes.
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