Digestive proteases and in vitro protein digestibility in bagrid catfish Mystus nemurus (Cuvier and Valenciennes 1840)

Abstract

Digestive proteases in the stomach and intestine of bagrid catfish Mystus nemurus juveniles and in vitro protein digestibility of fishmeal (FM), soya bean meal (SBM) and squid meal (SM) were determined. Postprandial data demonstrated highest protease activity 2 and 5 hr in the stomach and intestine, respectively, after feeding. Acidic-like protease in the stomach exhibited an optimum activity at pH 2, while alkaline proteases dominated the intestine, with optimum activities between pH 9 and 11. Investigations using specific commercial inhibitors confirmed the presence of pepsin in the stomach. Chymotrypsin, trypsin and metalloproteases were present in the intestine; however, the results indicated a possible involvement of other serine proteases. The highest inhibition of intestinal protease activity of bagrid catfish occurred during incubation with SM, followed by SBM and FM. Fishmeal had the highest relative protein digestibility (RPD), compared with SBM and SM, regardless of enzyme system used. Crude enzyme extract and the 4E system resulted in the highest RPD followed by 3E and 1E systems. The current results suggest that bagrid catfish adopt the typical protein digestion pathway of a carnivorous fish of endoprotease and exoprotease activities from the stomach to the intestine. Pepsin acts in the stomach, while trypsin, chymotrypsin, metalloproteases and other serine proteases are involved in the intestine. This study also recommends the use of crude intestinal enzyme extract for digestibiliy studies, instead of commercial enzyme systems, to better identify the suitable protein sources for bagrid catfish.