Real-time oxygen sensing as a powerful tool to investigate tyrosinase kinetics allows revising mechanism and activity of inhibition by glabridin.

A new method for studying tyrosinase kinetics and inhibition by oxygen sensing is described and matched to the conventional spectrophotometric approach. The stoichiometric ratio of O uptake to dopachrome formation was 1.5 ± 0.2 for substrate l-tyrosine and 1.0 ± 0.1 for l-DOPA. With both methods, we reinvestigated mushroom tyrosinase inhibition by glabridin from Glycyrrhiza glabra. The two methods agreed showing mixed-type inhibition for monophenolase and diphenolase activities, at variance with previous literature. Average K (K) values for glabridin were 13.6 ± 3.5 (281 ± 89) nM and 57 ± 8 (1312 ± 550) nM, for monophenolase and diphenolase inhibition, respectively, with IC of 80 ± 8 nM and 294 ± 25 nM, respectively, at 1 mM substrate. For reference kojic acid K (K) were 10.9 ± 8 (217 ± 55) µM and 9.9 ± 1.4 (21.0 ± 5.2) µM, for monophenolase and diphenolase, respectively, with respective IC of 33 ± 8 μM and 17 ± 3 μM. Glabridin's activity is among the highest in nature, being about three orders of magnitude higher than previously reported.