Reduction in dietary lysine increases muscle free amino acids through changes in protein metabolism in chickens.

Taste is crucial to meat quality, and free Glu is an important taste-active component in meat. Our recent study showed that the short-term feeding of a low-Lys diet increases the concentration of free Glu and other free amino acids in chicken muscle and improves its taste. Here, we investigated the mechanisms by which the feeding of a low-Lys diet increases free Glu in chicken muscle. Two groups (n = 10 per group) of 28-day-old female Ross strain broiler chickens were fed diets with a graded Lys content of 90% or 100% of the recommended Lys requirement (according to National Research Council [1994] guidelines) for 10 D. Free amino acid concentrations and the mRNA abundance of protein metabolism-related genes were measured in breast muscle, and breast muscle metabolome analysis was conducted. Free Glu in muscle was increased by 51.8% in the Lys 90% group compared with the Lys 100% group (P < 0.01). Free threonine, glutamine, glycine, valine, isoleucine, leucine, tyrosine, phenylalanine, histidine, and 3-methyl-histidine concentrations in breast muscle were also increased in the Lys 90% group (P < 0.05). Metabolome analysis also showed that free amino acids were increased in the Lys 90% group. The mRNA abundance of μ-calpain, caspase-3, and 20S proteasome C2 subunit were increased in the Lys 90% group (P < 0.05). Moreover, the free Glu concentration in muscle was correlated with mRNA abundance of μ-calpain (r = 0.74, P < 0.01), caspase 3 (r = 0.69, P < 0.01), 20S proteasome C2 subunit (r = 0.65, P < 0.01), and cathepsin B (r = 0.52, P < 0.05). Our study suggests that the feeding of a low-Lys diet to chickens increased the free Glu content of breast muscle by promoting protein degradation.