Inhibition of mushroom polyphenol oxidase by agaritine

Agaritine [(beta-N-(gamma-L(+)-glutamyl)-4-hydroxymethylphenylhydrazine] was isolated and purified from mushrooms (Agaricus bisporus). This paper reports for the first time the inhibition of the monophenolase and diphenolase activities of mushroom polyphenol oxidase by this abundant and characteristic compound from the Agaricus genus. Agaritine showed uncompetitive inhibition (K-I = 2.3 mM) and partial competitive inhibition (K-I = 0.13 mM; alpha = 2.5) when L-DOPA and L-tyrosine were assayed, respectively. In addition to the ability of agaritine to inhibit the enzyme per se, this compound was capable of removing the enzymatically generated o-quinones in the assay. To solve this problem, the inhibition was characterized by using 3-methyl-2-benzothiazolinone hydrazone as nucleophile reagent to form a stable nucleophile-quinone adduct that could not be attacked by agaritine. The inhibition of mushroom polyphenol oxidase by other inhibitors is compared to the inhibition by agaritine. Moreover, a possible role for agaritine in the browning of mushrooms is discussed.