Effects of dietary lysine level on growth, feed utilization, serum biochemical indexes, lysine metabolizing enzyme activity and related gene expression of Leuciscus brandti

This experiment was aimed to investigate the effects of lysine on growth, feed utilization, serum biochemical indexes, lysine metabolism enzyme activity and its related gene expression of Leuciscus brandti. Six isoenergetic (17 MJ/kg) and isonitrogenous (37% CP) experimental diets were formulated with different levels of lysine (1.82%, 2.27%, 2.72%, 3.17%, 3.62% and 4.07%). Total 900 L.brandti with initial weight (13.44 ± 1.10) g were randomly divided into 6 groups (3 replicates per group and 50 L.brandti per replicate) and were reared with 6 experimental diets for 8 weeks. The results indicated that when the dietary lysine level reached 3.17%, the weight gain (WG) and specific growth rate (SGR) of L.brandti in this group not significantly different from that in the 3.62% group, but significantly higher than that other groups (P < 0.05). The feed efficiency (FE) and protein efficiency (PER) of the 2.27%, 2.72%, 3.17%, 3.62% and 4.07% groups were significantly higher than that of the 1.82% group (P < 0.05). When the lysine level is 3.17%, the content of insulin-like growth factors-1 (IGF-1), total cholesterol (TC), total amino acid (TAA), total protein (TP), globulin (GLB) and albumin (ALB) were significantly higher than those of the 1.82% group in the serum (P < 0.05); at the same time, the content of blood urea nitrogen (BUN), the alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were significantly lower than those of the 1.82% group (P < 0.05). In the hepatopancreas, the lysine-ketoglutarate reductase (LKR) activity among all groups had significant differences (P < 0.05), and it increased significantly with the increase of lysine level; the aminoadipate-semialdehyde synthase (AASS) activity among the 2.72% group, 3.17% group, 3.62% group and 4.07% group had no significant difference, but they were significantly higher than those of the 1.82% group and 2.27% group (P < 0.05). In the serum, the LKR activity among the 2.72% group, 3.17% group and 3.62% group had no significant difference, but they were significantly higher than the 1.82% group and 4.07% group (P < 0.05); the AASS activity of the 2.72% and 3.17% groups was significantly higher than that in 1.82% and 2.27% groups, and significantly lower than that in 3.62% and 4.07% groups (P < 0.05). In the hepatopancreas, the AASS gene expression among the 2.72% group, 3.17% group, 3.62% group and 4.07% group was no significant difference, but they were significantly higher than the 1.82% group (P < 0.05); the cationic amino acid transporter-1(CAT1) gene among the 3.17% group and 3.62% group was no significant difference, but they were significantly higher than those of the 1.82% group, 2.27% group, 2.72% group and 4.07% group (P < 0.05). In conclusion, when the lysine level was 3.17% of the diets or the lysine level was 8.5% of the protein, it could promote the growth, feed utilization, protein metabolism, lysine metabolism and transport ability of L.brandti.