Tyrosinase Inhibition and Kinetic Details of Puerol A Having But-2-Enolide Structure from Amorpha fruticosa

Puerol A (1) from Amorpha fruticosa showed highly potent inhibition against both monophenolase (IC₅₀ = 2.2 μM) and diphenolase (IC₅₀ = 3.8 μM) of tyrosinase. We tried to obtain a full story of enzyme inhibitory behavior for inhibitor 1 because the butenolide skeleton has never been reported as a tyrosinase inhibitor. Puerol A was proved as a reversible, competitive, simple slow-binding inhibitor, according to the respective parameters; k₃ = 0.0279 μM^-1 min^-1 and k₄ = 0.003 min^-1. A longer lag-phase and a reduced static-state activity of the enzyme explained that puerol A had a tight formation of the complex with E_met. Dose-dependent inhibition was also confirmed by high-performance liquid chromatography (HPLC) analysis using N-acetyl-l-tyrosine as a substrate, which was completely inhibited at 20 μM. A high binding affinity of 1 to tyrosinase was confirmed by fluorescence quenching analysis. Moreover, puerol A decreased melanin content in the B16 melanoma cell dose-dependently with an IC₅₀ of 11.4 μM.

Keywords: Amorpha fruticosa; anti-pigmentation; binding affinity; puerol A; tyrosinase.