Kinetic study of the oxidation of gamma-L-glutaminyl-4-hydroxybenzene catalyzed by mushroom (Agaricus bisporus) tyrosinase

Despite the importance of the substrate gamma-L-glutaminyl-4-hydroxybenzene (GHB) in the melanin biosynthesis pathway in mushrooms Agaricus bisporus, the kinetics of its oxidation catalyzed by tyrosinase has never been proper ly characterized. For this purpose GHB and its corresponding o-diphenol (GD HB) were isolated and purified from A. bisporus mushrooms. The kinetic cons tants that characterize the action of tyrosinase on GHB and GDHB are V-max( GHB) = 2.10 +/- 0.10 mu M/min, K-m(GHB) = 0.30 +/- 0.03 mM, V-max(GDHB) = 2 10.0 +/- 7.3 mu M/min, and K-m(GDHB) = 7.80 +/- 0.41 mM. The oxygen kinetic constants for tyrosinase in the presence of these compounds are V-max(O2(G HB)) = 3.20 +/- 0.21 mu M/min, K-m(O2(GHB)) = 1.50 +/- 0.12 mu M, V-max(O2( GDHB)) = 200.2 +/- 8.1 mu M/min, and K-m(O2(GDHB)) = 100.2 +/- 8.2 mu M. Th ese values were compared to those obtained for the pair L-tyrosine/L-DOPA. The kinetic and structural reaction mechanisms of tyrosinase were corrobora ted for these physiological phenolic compounds.