Different roles for nitrogen monoxide and peroxynitrite in lipid peroxidation induced by activated neutrophils

We studied the roles of nitrogen monoxide (NO√) and peroxynitrite produced by the polymorphonuclear leukocytes (PMNs) isolated from an inflammatory exudate. PMNs were incubated either in a medium with a submicromolar concentration of iron or in a diethylenetriaminepenta-acetic acid (DTPA)-containing medium, and stimulated with phorbol 12-myristate 13-acetate (PMA) to generate free radicals. In both conditions superoxide anion (O2•−), NO√ and peroxynitrite were produced. In the presence of arachidonic acid, malondialdehyde (MDA) was generated. This MDA was generated in one of two way; the peroxynitrite iron-independent mechanism (40%) and the Fenton reaction, caused by free iron (60%). We also observed that the addition of image-arginine was followed by a 42% reduction in MDA, which can be explained by the antioxidant effect of NO√. These results indicate that lipid peroxidation can occur in the absence of iron, through a peroxynitrite-mediated mechanism, and that NO√ may act as an antioxidant when it is produced in large amounts.