Effects of Dietary Tryptophan on Growth, Protein Degradation, and Antioxidant Enzyme Activity in Juvenile Meagre (Argyrosomus regius)

Tryptophan has been shown to affect fish feed intake and growth performance. Moreover, it is the precursor of several bioactive molecules such as serotonin, which can be converted into melatonin. Melatonin is a potent antioxidant that directly neutralises free radicals and reduces oxidative stress. Diets rich in tryptophan may contribute to reduced oxidative stress, potentially through its role as a precursor of serotonin and melatonin. In this study, three diets containing different contents of tryptophan: 0.5 (Trip1), 0.6 (Trip2) and 0.8% (Trip3), were tested in triplicate in 112-day-old meagre with an initial weight of 32.6 ± 3.4 g and 14.4 ± 0.5 cm length for 56 days. Although the results showed no significant differences for growth and FCR between treatments, there was a tendency toward increased growth and a decrease in FCR in meagre fed higher levels of tryptophan. The main protein degradation systems in the liver and white muscle were evaluated. The activity of the tested proteases in the muscle was unaffected by dietary tryptophan levels. A decrease in oxidative stress was also observed as the level of tryptophan in the diets increased, although not statistically significant. A trend of decreasing superoxide dismutase, catalase, and selenium-independent glutathione peroxidase levels in tryptophan-rich diets was also observed.