Inhibition effects of Maillard reaction products derived from l-cysteine and glucose on enzymatic browning catalyzed by mushroom tyrosinase and characterization of active compounds by partial least squares regression analysis

Abstract

Inhibition of tyrosinase activity by Maillard reaction products derived from cysteine and glucose (Cys-MRPs) was studied. Pre-incubation of mushroom tyrosinase with Cys-MRPs decreased enzyme activity with increasing reaction time. We show that Cys-MRPs irreversibly block the active site of mushroom tyrosinase and that the competitive inhibitors dithiothreitol and kojic acid protect the enzyme from Cys-MRPs inactivation. Correlation of tyrosinase inhibition ability, volatile compounds, non-volatile compounds (HMF, DDMP and maltol), and Maillard reaction conditions of Cys-MRPs was analyzed by partial least squares regression (PLSR). 3-Ethyl-2-formylthiophene, α-dimethylformylthiophene, 2,6-dimethylpyrazine, ethylpyrazine, 2-ethyl-6-methylpyrazine, 2-methyl-3-(2-thienyldithio) thiophene, and furfural showed a significant and positive contribution to inhibition ability, while 2-propionylfuran and α-dimethyl-2-formylfuran showed a significant but negative correlation with inhibition ability. Of the three non-volatile compounds analyzed, only 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one (DDMP) showed a significant and positive correlation with inhibition ability, while HMF and maltol showed a weak negative correlation. The reaction temperature and time showed a significant and positive correlation with inhibition rate, whereas the ratio of sugar to amino acid showed a negative effect within the experimental range.