Degradation of rutin by <i>Aspergillus flavus</i>. Studies on specificity, inhibition, and possible reaction mechanism of quercetinase

Abstract

The dioxygenase produced by Aspergillus flavus that oxidizes quercetin contains 2 moles of copper per mole of enzyme and binds at least 2 moles of substrate. The enzyme is not inhibited by sulfhydryl reagents or affected by H 2 O 2 , but is inhibited by copper-chelating agents and reducing agents. Nitrogen bases with chelating ability are more effective inhibitors than those that do not form metal chelates.The dioxygenase oxidizes flavones that possess a double bond between carbons-2 and -3 and a hydroxyl on carbon-3. Hydroxyls at other positions affect both the relative rates of enzymatic oxidation and the concentration of substrate required to attain half maximal rate (K m ). The evidence indicates that the substrate binds to the copper of the enzyme via the 3-hydroxyl and 4-carbonyl groups of the substrate. A possible mechanism for the reaction is presented.