Biosynthesis, Characterization, and Bioactivity of L-Alanyl-L-tyrosine in Promoting Melanin Synthesis

L-Alanyl-L-tyrosine (L-Ala-Tyr) is a dipeptide formed by the condensation of L-alanine methyl ester and L-tyrosine. After entering the body, it can be rapidly broken down to release tyrosine. In this study, L-Ala-Tyr was successfully prepared by using α-ester acyltransferase as biocatalyst and alanine methyl ester (L-Ala-OMe) and tyrosine (L-Tyr) as acyl donor and nucleophile, respectively. The dipeptide yield was increased from 15 to 50% by optimizing the conditions: boric acid-borax (0.2 mol/L), 30°C, pH 9.5, 2:1 acyl donor to nucleophile ratio, DES (ChCl/urea), and 15%(v/v) water content. The catalytic product is then isolated and purified. The structure of the product was identified by high-performance liquid chromatography, mass spectrometry, proton nuclear magnetic resonance, and carbon spectroscopy. Its biological activity was preliminarily determined by the B16-F10 mouse melanoma cell model. The results showed that the purity of L-Ala-Tyr prepared by the separation and purification method of this study was 96.8%, and the mass spectrometry and nuclear magnetic resonance spectroscopy showed that the structure of the peptide was consistent with the expected structure. In addition, the preliminary physiological activity identification results show that L-Ala-Tyr has no toxic effect on cells in the concentration range of 100-800 μmol·L^-1, and at the optimal concentration, compared with the positive control 8-methoxypsoralen, it can promote the production of melanin.