Enzymatic browning, Inhibition, Nitroanilines, Tyrosinase

Tyrosinase also known as polyphenol oxidase (PPO) is a copper-containing mono-oxygenase, which is responsible for melanization in animals and the enzymatic browning of fruits. It displays two distinct enzymatic activities: hydroxylation of monophenols to o-diphenols (monophenolase activity) and oxidation of the latter to o-quinones (diphenolase activity). The inhibition of tyrosinase is very important and encourages us to takes special attempts to search for new inhibitors. For the first time in the present study, the effects of 2-nitroaniline (a), 3-nitroaniline (b) and 4-nitroaniline (c) as well as their newly synthesized vanillin derivatives (2-nitrobenzenaminium 4-formil-2-metoxyphenolate (d), 3-nitrobenzenaminium 4-formil-2- metoxyphenolate (e) and 4-nitrobenzen aminium 4-formil-2-metoxyphenolate (f)) were studied on the oxidation of dopamine hydrochloride by mushroom tyrosinase. Among them, 4-nitroaniline (c) exhibited the strongest inhibitory effect, while acted as an activator. For these compounds, the IC50 follows the order of b> f= a> e> c. Compounds a, b and f were competitive while c and e were un-competitive inhibitors. The results indicated that the relative positioning of amino and nitro groups is important in the inhibition of the enzyme