Dietary glycine supplementation enhances syntheses of creatine and glutathione by tissues of hybrid striped bass(Morone saxatilis ♀ × Morone chrysops ♂) fed soybean meal-based diets

Background We recently reported that supplementing glycine to soybean meal-based diets is necessary for the optimum growth of 5-to 40-g(Phase-I) and 110-to 240-g(Phase-II) hybrid striped bass(HSB), as well as their intestinal health. Although glycine serves as an essential substrate for syntheses of creatine and glutathione(GSH) in mammals(e.g., pigs), little is known about these metabolic pathways or their nutritional regulation in fish. This study tested the hypothesis that glycine supplementation enhances the activities of creatine-and GSH-forming enzymes as well as creatine and GSH availabilities in tissues of hybrid striped bass(HSB; Morone saxatilis♀ × Morone chrysops♂).Methods Phase-I and Phase-II HSB were fed a soybean meal-based diet supplemented with 0%, 1%, or 2% glycine for 8 weeks. At the end of the 56-d feeding, tissues(liver, intestine, skeletal muscle, kidneys, and pancreas) were collected for biochemical analyses.Results In contrast to terrestrial mammals and birds, creatine synthesis occurred primarily in skeletal muscle from all HSB. The liver was most active in GSH synthesis among the HSB tissues studied. In Phase-I HSB, supplementation with 1% or 2% glycine increased(P < 0.05) concentrations of intramuscular creatine(15%–19%) and hepatic GSH(8%–11%), while reducing(P < 0.05) hepatic GSH sulfide(GSSG)/GSH ratios by 14%–15%, compared with the 0-glycine group; there were no differences(P > 0.05) in these variables between the 1% and 2% glycine groups. In Phase-II HSB, supplementation with 1% and 2% glycine increased(P < 0.05) concentrations of creatine and GSH in the muscle(15%–27%) and liver(11%–20%) in a dose-dependent manner, with reduced ratios of hepatic GSSG/GSH in the 1% or 2% glycine group. In all HSB, supplementation with 1% and 2% glycine dose-dependently increased(P < 0.05) activities of intramuscular arginine:glycine amidinotransferase(22%–41%) and hepatic γ-glutamylcysteine synthetase(17%–37%), with elevated activities of intramuscular guanidinoacetate methyltransferase and hepatic GSH synthetase and GSH reductase in the 1% or 2% glycine group. Glycine supplementation also increased(P < 0.05) concentrations of creatine and activities of its synthetic enzymes in tail kidneys and pancreas, and concentrations of GSH and activities of its synthetic enzymes in the proximal intestine.Conclusions Skeletal muscle and liver are the major organs for creatine and GSH syntheses in HSB, respectively. Dietary glycine intake regulates creatine and GSH syntheses by both Phase-I and Phase-II HSB in a tissue-specific manner. Based on the metabolic data, glycine is a conditionally essential amino acid for the growing fish.