Structure-activity studies in E. coli strains on ochratoxin A (OTA) and its analogues implicate a genotoxic free radical and a cytotoxic thiol derivative as reactive metabolites.

Ochratoxin A (OTA), its major metabolite in rodents, ochratoxin alpha, and seven structurally related substances were assayed for SOS DNA repair inducing activity in Escherichia coli strain PQ37. At concentrations of 0.1-4 mM, OTA, chloroxine, 5-chloro-8-quinolinol, 4-chloro-meta-cresol and chloroxylenol induced SOS DNA repair in the absence of an exogenous metabolic activation system. Ochratoxin B, ochratoxin alpha, 5-chlorosalicylic acid and citrinin were inactive, but all except ochratoxin alpha were cytotoxic. Thus, the presence of chlorine at C-5 appears to be one determinant of genotoxicity in these substances. Amino oxyacetic acid, an inhibitor of the cysteine conjugate beta-lyase, decreased the cytotoxicity of OTA but did not alter its genotoxic activity, suggesting the formation of a cytotoxic thiol-containing derivative. The mechanisms by which OTA and some of its active analogues induce SOS DNA repair activity was further investigated in E. coli PQ37 and in three derived strains (PQ300, OG100 and OG400), containing deletions within the oxy R regulon. The response in strain PQ37 was measured in the absence and presence of Trolox C, a water-soluble form of vitamin E. Trolox C completely quenched the genotoxicity of OTA, and the effect was similar in the mutant and wild-type strains. These results implicate an OTA-derived free radical rather than reduced oxygen species as genotoxic intermediate(s) in bacteria.