Nitro-Tyrosine as Promoter of Free Radical Damage in a DNA Model System

Abstract

Nitro-tyrosine considerably promotes the degradation of DNA, when incubated with Cu²⁺ and ascorbate in oxygenated aqueous solution. This deleterious process requires oxygen and can be inhibited with catalase, indicating that H₂O₂ is involved, via the reduction of oxygen. Menadione and 2,4,6-trinitro-benzenesulfonate, known to catalyze particularly fast such reduction of oxygen, were only slightly more active than nitro-tyrosine. Degradation of DNA can be explained by a site-specific Fenton type reaction of H₂O₂ with the DNA-Cu⁺ complex.

DNA-Cu⁺ + H₂O₂ → DNA' ' 'OH + Cu²⁺ + OH⁻

Copper-chelating agents (EDTA and penicillamine) prevent DNA degradation, whereas OH-scavengers (t-butanol) are ineffective. The deleterious activity of nitro-tyrosine (and of other nitroaromatics) in the DNA model system may indicate important toxicological implications, since aromatic nitration is a significant mode of action of nitrogen dioxide.

Key Words:

• Copper-catalyzed reactions
• DNA
• oxidative damage
• hydrogen peroxide generation
• menadione
• nitro-tyrosine