RESUMO
The kinetic mechanism of enzymatic modification of flavonol quercetin with L-cysteine by horseradish peroxidase (HRP) was studied. Reaction of modification of quercetin was followed by recording spectral changes over time at 380 nm. All reactions were performed in 100 mM phosphate buffer pH, 6.0 at 20ºC. Kinetic parameters were determined from graphics of linear Michaelis-Menten equation. The values obtained at specified intervals were: Vmax = 0.17 ÷ 0.91 ΔA380/min, Km = 0.023 ÷ 0.5 mM, kcat = 0.21 ÷ 1.14 ΔA380/min∙nM-1 and Vmax/Km = 0.83 ÷ 26.55 ΔA380/min∙mM-1. It was found that all investigated reactions of the modification of quercetin with L-cysteine by HRP followed an ordered mechanism. We propose that HRP initially reacts with H2O2 than with quercetin and finally with L-cysteine, leading to the introduction of L-cysteine in the structure of quercetin.