Oxidation of Ochratoxin A by an Fe−Porphyrin System:  Model for Enzymatic Activation and DNA Cleavage

Abstract

Ochratoxin A (OTA, 1) is a fungal toxin that facilitates single-strand DNA cleavage, DNA adduction, and lipid peroxidation when metabolically activated. To model the enzymatic activation of OTA, we have employed the water-soluble iron(III) meso-tetrakis(4-sulfonatophenyl)porphyrin (FeTPPS) oxidation system. In its presence, OTA has been found to facilitate single-strand cleavage of supercoiled plasmid DNA through production of reactive oxygen species (ROS) (i.e., the hydroxyl radical, HO(*)). The reaction of OTA with the FeTPPS oxidation system also generated three hydroxylated products (chlorine atom still attached), which was taken as evidence for production of the known hydroxylated metabolites (2-4) of OTA. This result suggested that the FeTPPS system served as a reasonable model for the enzymatic activation of OTA. When the reaction of OTA with FeTPPS was carried out in the presence of excess hydrogen peroxide (H(2)O(2)) and sodium ascorbate, a hydroquinone species (OTHQ, 5) was detected in which an OH group has replaced the chlorine atom of OTA. The production of OTHQ (5) was dependent on the presence of the reducing agent, sodium ascorbate, which suggested that the oxidation catalyst furnished the quinone derivative OTQ (6) that was subsequently reduced to OTHQ (5) by ascorbate. Utilizing a synthetic sample of OTHQ (5), the hydroquinone was found to undergo autoxidation with a t(1/2) of 11.1 h at pH 7.4, and to possess a pK(a) value of 8.03 for the phenolic oxygen ortho to the carbonyl groups. Our findings imply that the hydroquinone (OTHQ) and quinone (OTQ) metabolites of OTA have the ability to cause alkylation/redox damage and have allowed us to propose a viable pathway for oxidative damage by OTA.

Ochratoxin A: Potential epigenetic mechanisms of toxicity and carcinogenicity

“…A role for cytochrome P450 in this reaction was also suggested (Omar et al 1991). Other authors have reported that the OTA hydroquinone/quinone couple was generated from the oxidation of OTA (phenol oxidation) by electrochemical, photochemical and chemical processes (Gillman et al 1999, Dai et al 2002. The quinone is thought to be a likely candidate for covalent binding to DNA.…”

“…It can also undergo reductions to form semiquinone and hydroquinone. Such events are likely to result in redox cycling and in the generation of reactive oxygen species (Gillman et al 1999, Dai et al 2002. The actual relevance of these reactions for the physiological in vivo situation has still to be elucidated.…”

Glutathione Conjugates of Ochratoxin a as Biomarkers of Exposure / Glutationski Konjugati Okratoksina A Kao Biomarkeri Izloženosti

“…OTA possesses a chlorophenolic group, and chlorophenols are well known to undergo oxidative dechlorination reactions to afford quinone/ hydroquinone redox couples (15,16). The hydroquinone metabolite (OTHQ, Figure 1) of OTA has been detected in the urine (17) and kidneys (18) of rats, and in human blood and urine samples (19).…”

“…Figure 3 shows the twodimensional Table 2. The Table also includes 1 H chemical shift data for OTHQ, which was also recorded in DMSO-d 6 (16), and OTA, which was recorded in CDCl 3 by Dais et al (36). The NMR data together with the MS data provided an unambiguous assignment of the OTB-NAC structure, as 1 H signals for the attached NAC moiety could be determined and H-6 of the OTB component was observed at ~ 8 ppm, as expected from the 1 H NMR data for OTHQ (16) and OTA (36).…”

Ochratoxin A kinetics: A review of analytical methods and studies in rat model

“…in rats (Xiao et al, 1996a). The OTAderived quinones (OTQ/OTHQ) (figure 1) and their glutathione conjugate (OTHQ-GS) have been hypothesized as possible reactive metabolites (Dai et al, 2002;Gillman et al, 1999) and related with the genotoxic effects of OTA (Pfohl-Leszkowicz and . The presence of O-labile ester conjugates of OTA (figure 1) was also found in the urine of F344 rats after a single oral dose of OTA .…”