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(Hypertension. 2008;51:211.)
© 2008 American Heart Association, Inc.

Original Articles

Apocynin Is Not an Inhibitor of Vascular NADPH Oxidases but an Antioxidant

Sabine Heumüller; Sven Wind; Eduardo Barbosa-Sicard; Harald H.H.W. Schmidt; Rudi Busse^*; Katrin Schröder; Ralf P. Brandes

From the Institut für Kardiovaskuläre Physiologie (S.H., E.B-S., R.B., K.S., R.P.B.), Johann Wolfgang Goethe-Universität, Frankfurt am Main, Germany; Rudolf-Buchheim-Institut für Pharmakologie (S.W.), Justus-Liebig-Universität, Gießen, Germany; and the Department of Pharmacology and Centre for Vascular Health (H.H.H.W.S.), Monash-University, School of Biomedical Sciences, Victoria, Australia.

Correspondence to Ralf P. Brandes, Institut für Kardiovaskuläre Physiologie, Johann Wolfgang Goethe-Universität, Theodor-Stern-Kai 7, D-60596 Frankfurt am Main, Germany. E-mail r.brandes{at}em.uni-frankfurt.de

A large body of literature suggest that vascular reduced nicotinamide-adenine dinucleotide phosphate (NADPH) oxidases are important sources of reactive oxygen species. Many studies, however, relied on data obtained with the inhibitor apocynin (4'-hydroxy-3'methoxyacetophenone). Because the mode of action of apocynin, however, is elusive, we determined its mechanism of inhibition on vascular NADPH oxidases. In HEK293 cells overexpressing NADPH oxidase isoforms (Nox1, Nox2, or Nox4), apocynin failed to inhibit superoxide anion generation detected by lucigenin chemiluminescence. In contrast, apocynin interfered with the detection of reactive oxygen species in assay systems selective for hydrogen peroxide or hydroxyl radicals. Importantly, apocynin interfered directly with the detection of peroxides but not superoxide, if generated by xanthine/xanthine oxidase or nonenzymatic systems. In leukocytes, apocynin is a prodrug that is activated by myeloperoxidase, a process that results in the formation of apocynin dimers. Endothelial cells and smooth muscle cells failed to form these dimers and, therefore, are not able to activate apocynin. Dimer formation was, however, observed in Nox-overexpressing HEK293 cells when myeloperoxidase was supplemented. As a consequence, apocynin should only inhibit NADPH oxidase in leukocytes, whereas in vascular cells, the compound could act as an antioxidant. Indeed, in vascular smooth muscle cells, the activation of the redox-sensitive kinases p38-mitogen-activate protein kinase, Akt, and extracellular signal–regulated kinase 1/2 by hydrogen peroxide and by the intracellular radical generator menadione was prevented in the presence of apocynin. These observations indicate that apocynin predominantly acts as an antioxidant in endothelial cells and vascular smooth muscle cells and should not be used as an NADPH oxidase inhibitor in vascular systems.

Key Words: apocynin • NADPH oxidase • Nox1 • Nox4 • leukocytes • reactive oxygen species

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Apocynin, NADPH Oxidase, and Vascular Cells: A Complex Matter

Rhian M. Touyz
Hypertension 2008 51: 172-174. [Full Text] [PDF]

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