EFFECTS OF DIETARY PROTEIN/STARCH AND LIPID/STARCH RATIOS ON DIGESTIVE ENZYMES AND OXIDATIVE STRESS MARKERS IN EUROPEAN SEA BASS Dicentrarchus labrax

As increasing alternative ingredients are included in aquaculture diets, there is a need to determine the effects these materials have on fish physiological processes. For 18 weeks, two independent and simultaneous trials were carried out to determine the effects of three dietary protein/starch (P/S trial: 1.8, 1.4 and 1.0) and four lipid/starch ratios (L/S trial: 3.1, 2.2, 1.5 and 0.9) in isoenergetic diets, on digestive enzymes and oxidative status of European sea bass. After the growth period, intestinal pH, digestive enzymes anticipatory activity (total alkaline protease-TPA and lipase activities), lipid peroxidation (MDA concentration) and oxidative defense enzymes (superoxide dismutase-SOD, catalase-CAT and glutathione peroxidase-GPx activities) were measured.

 TPA anticipatory activity was affected by total protein content in P/S trial and by high fiber inclusion in L/S trial. Lipase activity presented no significant differences among dietary treatments in any of the trials. Zymography analysis showed activity of 6 bands, but it is for the first time reported in this work the appearance of 24, 21 and 15 kDa  bands with chymotrypsin-like activity . Higher TPA activities appear to be related to band appearance in zymography. Lipid peroxidation was affected by protein content and L/S ratio, and enzymatic defenses against oxidative stress (SOD, CAT and GPx) by protein content. Although there were differences in digestive enzyme anticipatory activity and oxidative stress markers among treatments, sea bass in this study were able to compensate through other mechanisms, as there were no significant differences among performance indicators (Growth, FCR and SGR). Overall, diet 1.8 in P/S trial and diets 1.5 and 0.9 in L/S trial, performed better.