Study of Proteases (Gastric, Intestine and Pancreas) Enzyme Activities of Rainbow Trout, Oncorhynchus Mykiss, Fed Partial Substitution of Dietary Fish Meal with Rice Bran Protein Concentrate

Consistent composition of high quality diets is extremely important for aquaculture. Digestion and absorption of the nutrients depend directly on activity of digestive enzymes, in particular the protease enzymes in carnivores. The present study investigated the effects of substituting fish meal (FM) with rice bran protein concentrate (RBPC) on digestive enzyme activity of rainbow trout (Oncorhynchus mykiss). The activities of gastric (pepsin), pancreatic (trypsin and chymotrypsin) and brush border intestine (alkalinephosphatase and amino peptidase) enzymes were examined in specimens that were fed the experimental diets, i.e. FM meal was replaced by RBPC at three levels (10, 25 and 35%). One diet that had no replacement was considered as control. In fish fed vegetable diets, there was no significant difference between groups fed RBPC and the control in the specific activity of gastric (pepsin) and pancreatic (Trypsin and Chymotrypsin) enzymes (P > 0.05). Results of intestine enzymes showed that there were significant differences between specimens in the control group and those received higher levels of replacement. The alkalinephosphatase (AP) activity was at the lowest level in RBPC-35 group. No significant difference was detected between the control and RBPC-10 groups in the activity of aminopeptidase (AM) but the activity decreased with increase of level of replacement. Reduced intestinal enzyme activity in high levels of replacement, despite the lack of significant difference between gastric and pancreatic enzymes activity, may be due to simpler protein structure of rice bran protein concentrate compared with that of the fish meal, or nutritional inhibitors may have reduced the activity of digestive enzymes. However, in spite of these differences in intestinal enzymes activity, there was no significant difference in growth characteristics of fishes between 25 % replacement and the control group. It seems that this species is able to adapt to partial substitution of fish meal by RBPC, however, the mechanism for compensating the decrease of specific activity in the enterocytes of rainbow trout is still needed to be investigated.