Photocatalytic actions of the pesticide metabolite 2-hydroxyquinoxaline: destruction of antioxidant vitamins and biogenic amines – implications of organic redox cycling

Abstract

Toxicity of the pesticide quinalphos may comprise secondary, delayed effects by its main metabolite 2-hydroxyquinoxaline (HQO). We demonstrate that HQO can destroy photocatalytically vitamins C and E, catecholamines, serotonin, melatonin, the melatonin metabolite AMK (N(1)-acetyl-5-methoxykynuramine), and unsubstituted and substituted anthranilic acids when exposed to visible light. In order to avoid HQO-independent ascorbate oxidation by light and to exclude actions by hydroxyl radicals, experiments on this vitamin were carried out in ethanolic solutions. Other substances tested (vitamin E, melatonin, anthranilic acids) were also photocatalytically destroyed by HQO in ethanol. After product analyses had indicated that HQO was not, or only poorly, degraded in the light, despite its catalytic action on other compounds, we followed directly the time course of HQO and ascorbate concentrations in ethanol. While ascorbate was largely destroyed, no change in HQO was demonstrable within 2 h of incubation. Destruction was not prevented by the singlet oxygen quencher DABCO. Obviously, HQO is capable of undergoing a process of organic redox cycling, perhaps via an intermediate quinoxaline-2-oxyl radical. Health problems from HQO intoxication may not only arise from the loss of valuable biomolecules, such as antioxidant vitamins and biogenic amines, but also from the formation of potentially toxic products. Dimerization and oligomerization are involved in several oxidation processes catalyzed by HQO, especially in the indoleamines, in dopamine, and presumably also in vitamin E. Melatonin oxidation by HQO did not only lead to the well-known - and usually protective - metabolite AFMK (N(1)-acetyl-N(2)-formyl-5-methoxykynuramine), but also to a high number of additional products, among them dimers and trimers. DABCO did not prevent melatonin destruction, but changed the spectrum of products. Serotonin was preferentially converted to a dimer, which can further oligomerize. Several indole dimers are known to be highly neurotoxic, as well as oxidation products formed from catecholamines via the adrenochrome/noradrenochrome pathway. Destruction of melatonin may cause deficiencies in circadian physiology, in immune functions and in antioxidative protection.

Toxicity of the quinalphos metabolite 2‐hydroxyquinoxaline: Growth inhibition, induction of oxidative stress, and genotoxicity in test organisms

“…HQO does certainly not induce strong acute effects as the pesticide does, nor can a low LC 50 be expected. A compound like HQO that has a substantial degree of lipophilicitywhich can be easily deduced from the structural formula and what we see in its chromatographic behavior (Behrends et al, 2004)-should rather be suspected to accumulate by time and to induce, by virtue of its redox properties, a steadily rising oxidative load. Therefore, long-lasting exposures are required, which are complicated by photocatalytic actions of the compound, and which can be conducted only at selected concentrations and under selected conditions previously identified in initial experiments.…”

“…Secondary products from melatonin were, however, affected by singlet oxygen also formed under the influence of HQO. Moreover, HQO turned out not to be consumed in the photocatalytic processes, findings leading to the conclusion of organic redox cycling, which regenerates HQO from a radical intermediate (Behrends et al, 2004). Electron donation by the electron-rich HQO to acceptor molecules and formation of an oxygen-centered radical, such as the presumed quinoxaline-2-oxyl radical ( Fig.…”

“…Solutions were exposed for 1 h to light from a slide projector (FA 150, Liesegang, Duesseldorf, Germany), at a photon fluence rate of 1400 mol/(m 2 Â s), using thin-walled vials (Fiolax, Schott, Mainz, Germany), at 258C, and under continuous shaking (other details of equipment: Behrends et al, 2004).…”

“…The assumption of secondary toxicity by a metabolite receives support by various findings. Many effects of quinalphos have been described, which cannot be explained by the molecular properties of the pesticide, including oxidative stress (Gupta et al, 1998;Debnath and Mandal, 2000), compensatory inductions of antioxidant enzymes (Dwivedi et al, 1998;Gupta et al, 1998;Debnath and Mandal, 2000), decrease of the radical scavenger melatonin along with impairment of immunological parameters (Haldar, unpublished data;Behrends et al, 2004), destruction of vitamin E in vivo (Gupta et al, 1998), chromosomal aberrations and micronuclei formation in mice and humans (Pongrac et al, 1989;Rupa et al, 1989Rupa et al, , 1990Rupa et al, , 1991Chauhan et al, 2005), inhibition of mitosis (Chauhan et al, 2005), and estrogenic/antiandrogenic actions Ray et al, 1992;Debnath and Mandal, 2000;Sarkar et al, 2000). In none of these cases, a relationship to the primary action of quinalphos, inhibition of acetylcholinesterase, is obvious.…”

“…When directly applied to the depilated guinea pig skin, quinalphos did not elicit any photosensitization, but phototoxicity became apparent when the pesticide was administered orally to mice, so that the effect should result from a metabolite formed in the organism. In vitro, HQO is capable of photocatalytically destroying vitamins C and E, oxidizing various biogenic amines (serotonin, melatonin, N 1 -acetyl-5-methoxykynuramine, dopamine, norepinephrine, epinephrine), and anthranilic acid derivatives (Behrends et al, 2004). Rates of ascorbate and melatonin destruction were not changed by a singlet oxygen quencher, 1,4-diazabicyclo-(2,2,2)-octane, so that these photoreactions should be based on electron rather than energy transfer.…”