Camp. Biochem. fhysiol. Vol. lOOA, No. 3, pp. 759-762, 1991 0300-9629/91 $3.00 + 0.00 Printed in Great Britain 0 1991 Pergamon Press plc zyxwvu NUTRITIVE AND METABOLIC UTILIZATION OF PROTEINS WITH HIGH GLUTAMIC ACID CONTENT BY THE RAINBOW TROUT (ONCORHYNCHUS MYKISS) F.J. MOYANO,*G.CARDENETE~ and M. DE LA HIGUERAt *Departamento Biologia Animal, Ecologia y Genitica. E.U. Politecnica, Univ. Granada, 04120, Almeria, Spain; and, TDepartamento Biologia Animal, Ecologia y Genbtica, Fat. Ciencias, Univ. Granada, 18071, Granada, Spain (Received 3 1 January 1991) Abstract-l. The use of high levels of vegetable proteins in trout feeding resulted in an increased specific GDH activity. Nevertheless, deamination ability was not as high as expected in relation to the level of glutamic acid, a major substrate of such an enzyme, existing in the utilized protein sources. 2. Transaminase activities, GOT and GPT, showed no clear response to protein quality of the diets. In the case of GPT, lower values were measured when using corn gluten meal in diets, which is probably related to a very good nutritive utilization. 3. The use of GDH and GOT as enzyme indicators of the utilization of dietary protein by the trout does not seem to he very useful due to their susceptibility of being influenced by quite different nutritional factors. INTRODUCTION The use of alternative protein sources to fish meal in fish feeding has become an important research line in the nutrition of these animals. Among the veg- etable protein sources commonly assayed and later utilized, leguminous seeds have played a major role due to their high protein contents and quite good protein quality. Until now, soya bean has been the seed most widely utilized in the elaboration of fish feeds (Smith, 1977; Lovell, 1984; Reinitz et al., 1978). But recent investigations have demonstrated the potential interest of using the lupin (Lupinus sp.), another leguminous seed well established in Mediterranean countries (De la Higuera et af., 1988; Viola et al., 1989). In the same sense, the use of corn gluten meal has proved its suitability as protein supply in fish rations (Ketola, 1982; Alexis et al., 1985). Taking into account such information and considering the scarcity of studies relating changes in protein quality of fish feeds to protein metabolism (Dean et al., 1985) we have evaluated the result of using the forementioned vegetable protein sources in fish feeding both from a nutritive point of view and by means of some enzymatic (GDH, GOT, GPT) determinations. That can be a way of testing the suitability of using such enzymes as indicators of dietary protein quality. MATERIALSANDMETHODS Animals and diets Ten lots of nearly 45 rainbow trout (Oncorhynchus mykiss) 30g initial average weight, were fed ad lib. twice daily for 8 weeks on 45% d.m. protein diets including soya bean meal (SBM), lupin seed meal (LSM) or corn gluten meal (CGM) proteins at substitution levels of 20, 30 and 40% of fish meal protein. Approximate analysis of such protein sources is displayed in Table 1. A diet including fish meal as sole protein source was used as reference diet. Mixes were formulated to be approximately isoenergetic in gross energy and isonitrogenous in crude protein (Table 2). Amino-acid profile of the protein sources assayed was determined by HPLC (Table 3). Fish were maintained in polyethylene tanks, with a continuous flow (1 l/min) of dechlorinated water at 15°C. The fish in each tank were bulk weighed at the beginning of the experiment and at 2-week intervals until the end of the 8-week experimental period. Food consumption was recorded daily. At the end of the experimental period, after starvation for 24 hr, five animals of each group were sampled and, together with an initial sample of 10 fish, used for body composition analyses. Protein content was determined by the Kjeldahl method (N x 6.25). For enzymatic determinations, fish were also killed after starvation for 24 hr by a sharp blow to the head and livers were immediately removed, weighed and frozen at -20°C until analysis. Tissues of three fish were pooled and manually homogenized in 10~01. of ice-cold buffer (100 mM Tris-HCl, 250 mM sucrose, pH 7.6) to form a sample. Three samples of each lot were obtained at the end of the experimental period. In GDH determinations, homogenates were sonicated (3Osec interval for 3 min, 5OOOHz) to rupture mitochondrial membranes. All the homogenates were centrifuged (3O,OOOg, 30 min, 4°C) and supernatants utilized for enzyme assays. Activity determi- nations were carried out by measuring NADH oxidation at 340 nm (Uvikon 810-P spectrophotometer, 25’C). GDH activity was measured after 18 hr dialysis against 50 vol. of Tris-sucrose buffer. Table I. Approximate composition (X) of the protein sources utilized in the experimental diets; soya bean meal (SBM), htpin seed meal (LSM), corn gluten meal (CGM) and fish meal (FM) SBM LSM CGM FM Moisture 13.1 13.1 8.1 6.9 Crude protein 48.2 40.9 74.2 71.5 Ether extract 3.9 12.7 2.6 11.4 Crude fibre 7.6 15.4 N-free extract 36.7 26.3 22.2 1 Ash 3.6 4.7 0.9 17.1 CBPA IW,s-Q 759