Space-time distribution of γ-glutamyl transferase activity in Agaricus bisporus

Abstract

γ-Glutamyl transferase is involved in the biosynthesis of two characteristic γ-glutamyl compounds occurring in Agaricus bisporus: agaritine and γ-glutaminyl-4-hydroxybenzene. Agaritine was shown to be a precursor of potential toxic aryl diazonium ions and γ-glutaminyl-4-hydroxybenzene was demonstrated to be one of the main substrates implicated in mushroom browning. γ-G;utamyl transferase activity was measured in various tissues of A. bisporus fruitbodies at different developmental stages and in mycelium grown on synthetic and compost media. Gills and skin, which exhibit the highest levels of γ-glutamyl amino acids, also present the highest levels of γ-glutamyl transferase activity. Stipe base tissue, which is characterised by a lack of agaritine and the presence of its hydrolysis product hydroxymethylphenylhydrazine, also exhibits high levels of γ-glutamyl transferase activity. Thus, in the gills and in the skin, γ-glutamyl transferase could be mainly involved in the synthesis of γ-glutamyl derivatives and, in the stipe base, in the hydrolysis of agaritine. γ-Glutamyl transferase activity measured in mycelia was rather low but significantly greater in mycelium grown on compost medium than on synthetic medium. These results are in agreement with the lack of γ-glutaminyl-4-hydroxybenzene in mycelium grown on artificial medium. Cap flesh and stipe tissues show the lowest γ-glutamyl transferase activity. The elucidation of the role of γ-glutamyl transferase in the synthesis of one of the main substrates for mushroom browning opens new perspectives in attempts to optimise post-harvest quality.