Electrochemical oxidation of ochratoxin A: correlation with 4-chlorophenol

Ochratoxin A (OTA, 1A) is a mycotoxin implicated in human kidney carcinogenesis, in which oxidative activation is believed to play a key role. To gain an understanding of the oxidative behavior of the toxin, we have carried out an electrochemical study and have observed a direct correlation between the electrochemistry of OTA and 4-chlorophenol (4-ClPhOH). Cyclic voltammetry (CV) of OTA in acetonitrile (MeCN) showed that the toxin exhibits an irreversible oxidative half-peak potential (E(p/2)) of 1.81 V vs saturated calomel electrode (SCE); the corresponding value for 4-ClPhOH is 1.59 V. For both compounds, subsequent scans revealed the appearance of the respective hydroquinone/benzoquinone couple, which formed from the oxidation of the parent para-chlorophenol moiety. The hydroquinone of OTA (OTHQ, 2) exhibited E(p/2) = 1.21 V in MeCN. Deprotonation of OTA to form the phenolate (OTA(-)) lowered the potential to E(p/2) = 1.0 V in MeCN. However, from the oxidation of OTA(-), no evidence for the OTHQ(2)/OTQ(3) redox couple was found. In aqueous phosphate buffer (pH 6-8), the electrochemical behavior of OTA mimicked that observed for OTA(-) in MeCN; E(p/2) was approximately 0.8 V vs SCE and subsequent scans did not generate the OTHQ/OTQ redox couple. From capillary electrophoresis (CE), a diffusion coefficient (D) of 0.48 x 10(-5) cm(2) s(-1) was determined for OTA in phosphate buffer, pH 7.0. Combining this value with electrochemical data suggested that OTA undergoes a 1H(+)/1e oxidation in aqueous media. The biological implications of these findings with respect to the oxidative metabolism of OTA, and other chlorinated phenols, are discussed.