Inhibition of NADPH oxidase activation in endothelial cells by ortho-methoxy-substituted catechols

NADPH oxidase is a major enzymatic source of oxygen free radicals in stimulated endothelial cells (ECs). The ortho -methoxy-substituted catechol, apocynin (4-hydroxy-3-methoxyacetophenone), isolated from the traditional medicinal plant Picrorhiza kurroa , inhibits the release of superoxide anion (O 2 · m ) by this enzyme. The compound acts by blocking the assembly of a functional NADPH oxidase complex. The underlying chemistry of this inhibitory activity, and its physiological significance to EC proliferation, have been investigated. A critical event is the reaction of ortho -methoxy-substituted catechols with reactive oxygen species (ROS) and peroxidase. Analysis of this reaction reveals that apocynin is converted to a symmetrical dimer through the formation of a 5,5' carbon-carbon bond. Both reduced glutathione and L -cysteine inhibit this dimerization process. Catechols without the ortho -methoxy-substituted group do not undergo this chemical reaction. Superoxide production by an endothelial cell-f...