Cellular Injury by Oxidants

Abstract

Oxidants, generated by stimulated leukocytes, induce a variety of distinct biochemical changes in target cells. HOCl, produced by the action of peroxidase on H₂O₂ in the presence of chloride ion, acts at low molar concentration (10–20 μM) to damage proteins on cell membranes and destroy their function. H₂O₂ rapidly permeates cells and causes inhibition of ATP synthesis via both glycolytic and oxidative phosphorylation (mitochondrial) pathways. In the glycolytic pathway, damage is limited to the step involving glyceraldehyde 3-PO₄ dehydrogenase (GAPDH). This results from both an attack of H₂O₂ on GAPDH and, indirectly, by a reduction in concentration of the GAPDH cofactor, nicotinamide adenine dinucleotide (NAD). This latter effect was found to result from activation of the enzyme, poly-ADP ribose polymerase, an enzyme involved in DNA repair. Indeed, DNA damage in target cells was found at low concentrations of H₂O₂ (20–80 μM) in many cell types. Both strand breaks and base hydroxylation were observed, and resulted from the generation of hydroxyl radical from the H₂O₂ in the presence of transition metal. The consequence of the damage of DNA was found to be either cell injury and death or mutations of the base sequence and amino acid residues. These latter effects led to malignant transformations in cultured cells in both tissue cultures of the cells and in vivo in athymic mice. Exposure of a protooncogene, K-ras 4B, also led to the development of a malignant transformation by virtue of mutations in codon positions 12 and 61. Thus, oxidant effects on target cells can lead to damage to multiple functional pathways inside the cells, on the one hand, and malignant transformation via DNA damage, on the other.