Lignin as a Novel Tyrosinase Inhibitor: Effects of Sources and Isolation Processes

Lignin is the most abundant aromatic biopolymer in nature and its value-added application has attracted great attention. In this work, the inhibitory effect and mechanism of lignin on tyrosinase activity were investigated to develop lignin as a novel tyrosinase inhibitor. Six lignin samples isolated by alkali and ethanol organosolv processes from three typical lignocellulosic feedstocks were used to evaluate the effects of the lignin sources and isolation processes on the antityrosinase activity. The lignin heterogeneity including purity, molecular weight, and chemical structure was characterized detailedly by component determination, GPC, FTIR, 2D NMR, and Py-GCMS analyses. The enzyme studies showed that inhibitory activities of ethanol organosolv lignins were obviously stronger than those of alkali lignins. For lignins from different sources, corn stalk lignin (gramineae organosolv lignin, GOL) presented highest inhibitory effect with an IC50 value of 0.276 mg/mL, which was comparable to that of positive control p-hydroxy benzaldehyde (0.233 mg/mL). The inhibitory kinetics suggested that the ethanol organosolv lignin from corn stalk was a reversible mixed-type inhibitor. The fluorescence quenching studies demonstrated that the interaction of GOL with the enzyme was a significant molecular mechanism to inhibit the enzymatic activity. Consequently, these results suggest that lignin possesses antityrosinase activity and can be potentially used as an enzyme inhibitor in overtyrosinase activity control fields.