A Study on the Effect of Copper Methionine in Alleviating Yellowing in Channel Catfish

An 8-week trial evaluated dietary copper methionine (CM) on body and flesh color in yellowed channel catfish. Basic diets were supplemented with CM at 0 (control, Cu1), 7 (Cu2), 14 (Cu3), 21 (Cu4), and 27 (Cu5) mg/kg, with corresponding measured copper levels of 10.82, 12.63, 14.35, 15.58, and 17.40 mg/kg. In skin, GSH levels in Cu2-Cu5 groups and GSH-Px activities in Cu2-Cu4 groups were increased, and in muscle, CAT activities in Cu3-Cu5 groups and GSH levels in Cu4-Cu5 groups were increased. Gene expression analysis showed downregulated keap1 (Cu2–Cu4 in skin; Cu3–Cu5 in muscle) and nfκb (Cu2–Cu5 in skin), upregulated nrf2 (Cu3–Cu5 in skin; Cu4 in muscle) and il-10 (Cu4 in skin). Colorimetric analysis showed reduced the values of b* in skin (Cu3–Cu5) and muscle (Cu2–Cu5). Uranidin content decreased in skin (Cu3–Cu5) and muscle (Cu4–Cu5), while melanin and tyrosinase increased in skin (melanin: Cu2–Cu3; tyrosinase: Cu3–Cu4) and muscle (Cu3–Cu5). The expression of melanogenesis-related genes (mitf, tyr, etb, camk2, slc24a5) was increased in skin and muscle of Cu3-Cu5 groups, while the uranidin synthesis gene xdh was downregulated. Overall, optimal effects were observed at 14–21 mg/kg CM (measured copper: 14.35–15.58 mg/kg).