Tolerance and toxicity assessment of a novel methionine source, dipeptide Met-Met on common carp Cyprinus carpio

Abstract

Methionine is the first-limiting essential amino acid in plant-based diet, and also is the most toxic amino acid. The study aimed to evaluate the tolerance and toxicity of a dipeptide DL-methionyl-DL-methionine (Met-Met) in the diet of common carp Cyprinus carpio. Five iso‑nitrogenous (∼ 380 g kg−1) and isoenergetic (∼185 MJ kg−1) diets were formulated including a positive control diet (100 g kg−1 fish meal, FM) supplemented with 3 g kg−1DL-Met and four low fish meal diets (40 g kg−1 fish meal), with 0 (M0), 3 (M3), 6 (M6), and 15 (M15) g kg−1 Met-Met being supplemented. The Met-Met supplementation levels of 6 and 15 g kg−1 were designed as the 2- and 5-fold of the optimal level (3 g kg−1) for the tolerance and toxicity assessment, respectively. After 10 weeks of feeding, the lowest growth performance was observed in the M0 group, as evidenced by the lowest weight gain rate and specific growth rate. Contents of protein and amino acid in whole-body, and retentions of nitrogen and amino acid were highest in the M6 group, while these values decreased in M15 group. Fish fed diet M15 showed lower levels of albumin, high density lipoprotein cholesterol (HDL), total cholesterol, alkaline phosphatase, and higher alanine aminotransferase, total bile acid, total antioxidant capacity (T-AOC) compared with M3. Fish fed diet M15 had lower levels of glucose, HDL, and higher T-AOC and malonaldehyde compared to M6. Genes related to bile acid metabolism (cyp8b1, rxrα, fxr), anti-inflammatory (tgfβ1, tgfβ2, il10), proinflammatory (tnfα, il1β, il17) were significantly down-regulated in the M15 group compared to M3. However, caspase-3 expression significantly up-regulated in the M15 group compared with M6. Increased signals of hepatic vacuolation and fibrosis were observed in the M0 and M15 groups. Besides M15 group showed higher expression of apoptosis signal marker, cleaved caspase-3 in hepatopancreas. This study clarified the safety margin of Met-Met was 2× dosage as 6 g kg−1, while 15 g kg−1 Met-Met caused deleterious consequences, manifested as the highly occurred of cholestatic liver disease.