Para-aminobenzoic acid scavenges reactive oxygen species and protects DNA against UV and free radical damage

Abstract

Para-aminobenzoic acid (PABA), once considered a vitamin, is a precursor of folate for certain bacteria and is used as a sunscreen because of its UV-absorbing property. In addition to its mild anti-inflammatory activity, recent studies demonstrate that PABA protects against photocarcinogenesis in hairless mice and nephrotoxicity of cis--diamminedichloroplatinum(II) in rats. However, little is known regarding the mechanism by which PABA exerts these effects. We have hypothesized that PABA may be an effective scavenger of reactive oxygen species. In this report, we showed that PABA reacted with hypochlorite in a ratio of reaction of 1:2, as measured by a spectrofluorometric method developed in this study. PABA reacted with hvdroxyl radicals (·OH) by inhibiting deoxyribose oxidation induced by a Fenton-type reaction system (Fe3+ + EDTA/H2OO2//ascorbic acid). The second-order rate constant for the reaction of PABA with ·OH was approximated 1.07 × 1010 M−1 s−1.. Electron spin resonance studies further demonstrated the reaction of PABA with ·OH. The Fenton-type reaction system also caused damage to calf thymus DNA, and concurrent treatment with UV (254 nm) enhanced the damage by 3 fold. PABA tested at 1.0 mM afforded 58% protection against such damage. Using rose bengal as a singlet oxygen (1OO2)) generator and ESR techniques, we showed that PABA scavenged 1OO2 more effectively than sodium azide, a known quencher of 1OO2.. However, PABA did not scavenge superoxide onions or react with H2OO2.. Thus, in addition to its UV absorbing ability, PABA effectively scavenges certain reactive oxygen species, an effect that may be relevant to its protection against photocarcinogenesis, inflammation, and drug toxicity.