Effects of dietary cystine and tyrosine on melanogenesis pathways involved in skin color differentiation of Malaysian red tilapia

A 68-day experiment was conducted to investigate the effects of dietary cystine (Cys) and tyrosine (Tyr) on skin melanogenesis pathways in Malaysian red tilapia (initial weight: 15.04 ± 0.03 g). Six practical diets containing extra 0.1%, 0.5%, 1% cystine (Cys) and 0.5%, 1%, 2% tyrosine (Tyr) based on the control diet were prepared, accordingly named as Control, Cys-0.1%, Cys-0.5%, Cys-1%, Tyr-0.5%, Tyr-1% and Tyr-2% respectively. Each diet was fed to triplicate groups of fish during the trial. At the end of the experiment, the final weight for each group was not significantly different (P > 0.05). However, the dietary Cys and Tyr had significant effects on the melanin level and tyrosinase activity in the skin of fish. The melanin content in the skin of fish was lowest in Cys-0.5% group, with a higher level observed at Cys-0.1% and Cys-1% group. The final melanin of fish skin in Cys-1% was higher than that in Cys-0.5% and Cys-0.1% (P < 0.05). At the end of the experiment, the melanin in ventral skin of fish in Tyr-2% was higher than that in Tyr-0.5% and Tyr-1% (P < 0.05), and the melanin in dorsal skin of fish in Tyr-0.5% was lower than that in Tyr-1% and Tyr-2% (P < 0.05). The final tyrosinase activity of fish skin in Cys-1% was highest, and the final tyrosinase activity in Tyr-1% and Tyr-2% were significantly higher than that in Tyr-0.5% (P < 0.05). In serum, the tyrosinase activity was highest in Cys-1%, and the tyrosinase activity in Tyr-2% was significantly higher than that in Tyr-1% and Tyr-0.5% (P < 0.05). Moreover, the ventral skin melanin zone in Cys-1% treatment is largest by slice microstructure observation. The expression of tyrosinase mRNA in Tyr-2% group was highest in ventral skin of fish. At the end of the experiment, tyrosinase mRNA in the dorsal skin of fish was lowest in Cys-0.5%, with a higher level observed at Cys-0.1% and Cys-1% group. The final expression of tyrosinase mRNA in Tyr-0.5% was lower than that in Tyr-1% and Tyr-2% in dorsal skin of fish (P < 0.05). The final expressions of slc7a11 mRNA in Tyr-1% was higher than that in Tyr-0.5% in dorsal skin of fish (P < 0.05). Present results demonstrated that dietary Cys and Tyr could affect the melanin synthesis pathway in the red tilapia.