Electrochemical study of naturally occurring genotoxins ochratoxin A and prodigiosin.

摘要

Ochratoxin A, a ubiquitous mycotoxin produced by a number of Aspergillus species, has been implicated in human kidney carcinogenesis and is believed to be activated by an oxidative process in vivo. To gain a better understanding of the redox chemistry of this important environmental contaminant, electrochemical studies were carried out on OTA, as well as 4-chlorophenol (4-CP). Cyclic voltammetry and UV-visible spectroelectrochemistry indicate that OTA, like 4-CP, undergoes anodic conversion to a quinone species in dry MeCN solution. The implications of this finding could be far-reaching, since many quinones act as electrophiles in vivo, resulting in the alkylation of DNA and certain proteins. Quinones are also known to undergo redox cycling, thereby generating genotoxic reactive oxygen species (ROS). In aqueous solution or basic MeCN, the OTA anion undergoes a 1 e- /1 H+ (n = 1.1, D = 0.5 x 10-5 cm2s-1 in Kphos buffer) oxidation to a radical species, analogous to other 4-substituted phenols. Although the fate of the OTA radical is unknown, cyclic voltammetry studies suggest that it is more stable in MeCN solution than in water. Attempts were made to measure the lifetime of OTA· by fast-scan cyclic voltammetry and double potential step chronoamperometry, but electrode fouling made such a quantitative study difficult. The effect of transition metal binding on the redox chemistry of OTA was also eyed by cyclic voltammetry. Cu(II) coordination lowers the anodic half-peak potential of OTA by ∼0.1 V, and addition of a second equivalent of Cu(II) results in a substantial increase in the peak current of the primary anodic oxidation. This provides important rationale for the enhanced DNA-cleaving ability of the toxin in the presence of Cu(II).; The prodigiosins are oligopyrrolic natural products that have shown great promise as immunosuppressants and chemotherapeutic agents. Although the therapeutic target for these pyrrolic species is unclear, it is known that the prodigiosins facilitate copper-mediated double strand DNA cleavage. This nuclease activity involves the reductive activation of Cu(II), and the oxidation of prodigiosin (Prod) is believed to play a key role. In hopes of establishing a basis for structure---activity relationships in Cu(II)-mediated nuclease activity, Prod and several Prod analogues were examined by electrochemical methods. Cyclic voltammetry suggests that the most effective nucleases are those derivatives that are the most easily oxidized. This is demonstrated by the inactivity of PhenylProd, FuranProd, or AcylProd, in which the electron-rich A pyrrole ring is replaced by the more electron-withdrawing arenes. CV also indicates that Prod is capable of dimerizing/polymerizing upon electrochemical oxidation in MeCN. However, this process does not appear crucial in its DNA cleavage ability, as EEProd (reactive A-alpha position blocked) also exhibits nuclease activity. The electrochemical characterization of Cu(II)• Prod was attempted, but the Prod ligand proved susceptible to oxidation by the metal to form insoluble products.