Dietary sufficiency of sulfur amino acid compounds influences plasma ascorbic acid concentrations and liver peroxidation of juvenile hybrid striped bass ( Morone chrysops × M. saxatilis)

Abstract

The influence of sulfur amino acid (SAA) sufficiency in aquafeeds on tissue antioxidant status including liver ascorbic acid and lipid peroxidation has not been investigated with any fish to date. In this study, a basal diet consisting of lyophilized fish muscle and a crystalline amino acid premix without supplemental SAAs and containing 0.51% total SAAs (methionine + cystine) served as the negative control to which three graded levels of dl-methionine ( dl-met) or dl-2-hydroxy-4-methylthiobutanoic acid (MHA) were added. Quadruplicate groups of juvenile hybrid striped bass were fed each diet in a recirculating system for 6 weeks. Increases in dl-met or MHA in the diets linearly increased weight gain, survival, feed efficiency, protein efficiency ratio, protein conversion efficiency, whole-body protein and some amino acids in whole-body tissues. Because high inclusion levels of MHA (4.5–5.4 g/kg) increased intraperitoneal fat and also decreased whole-body protein and amino acids, protein conversion efficiency was used to determine relative bioavailability of MHA, which was 74.6% as effective as dl-met on an equal-weight basis. Fish fed 5.4 g MHA/kg diet had significantly higher ascorbic acid concentration in circulation than fish fed the basal diet or diets including 1.5, 3.0 g dl-met/kg or 4.5 g MHA/kg. The thiobarbituric acid reactive substances (TBARS) in liver also were negatively related to sufficiency of dietary SAA compounds. The present study is the first to provide evidence that methionine deficiency in aquafeeds may reduce/exhaust reservoirs of antioxidants such as ascorbic acid in fish tissue, which may result in irreversible oxidative stress and potentially bilateral cataracts as well as aggravate growth retardation, depressed feed efficiency and mortality.