α-Tocopherol Inhibits the Respiratory Burst in Human Monocytes

Abstract

Vitamin E (-tocopherol), one of the most important natural antioxidants, is assumed to be beneficial in the prevention of cardiovascular diseases.-Tocopherol exhibits acyl-peroxyl-radical scavenger properties and exerts cell-mediated actions in the hemovascular compartment , such as inhibition of superoxide anion (O 2 .) production by leukocytes. The aim of this study was to examine the mechanism underlying the inhibitory effect of-tocopherol on O 2. production by human monocytes. In activated monocytes O 2. is produced by the NADPH-oxidase enzyme complex. The oxidase activation elicited by phorbol myristate acetate (PMA) requires membrane translocation of several cytosolic factors. We found that in human PMA-stimulated adherent monocytes,-to-copherol (but not-tocopherol) inhibited O 2. production in intact cells but had no effect on a membrane preparation containing activated NADPH-oxidase, suggesting that-tocopherol impairs the assembly process of the enzyme complex. We showed that translocation and phosphorylation of the cytosolic factor p47 phox were reduced in monocytes preincubated with-tocopherol. We verified that the tryptic phosphopeptide map of mono-cyte p47 phox was similar to that of neutrophil p47 phox , indicating that several serine residues were phospho-rylated. Peptides whose phosphorylation is dependent on protein kinase C (PKC) were phosphorylated to a lesser degree when p47 phox was immunoprecipitated from-tocopherol-treated monocytes. In vitro, the activity of PKC from monocytes was inhibited by-tocoph-erol in a specific manner compared with that of-to-copherol or Trolox®. Membrane translocation of PKC was not affected. These results show that-tocopherol inhibits O 2. production by human adherent monocytes by impairing the assembly of the NADPH-oxidase and suggest that the inhibition of phosphorylation and translocation of the cytosolic factor p47 phox results from a decrease in PKC activity.