Oxidation of ochratoxin A by an Fe-porphyrin system: Model for enzymatic activation and DNA cleavage

Ochratoxin A (OTA, 1) is a fungal toxin that facilitates single-strand DNA cleavage, DNA adduction, and lipid peroxidation when metabolically activate d. To model the enzymatic activation of OTA, we have employed the water-sol uble iron(III) meso-tetrakis(4-sulfonatophenyl)porphyrin (FeTPPS) oxidation system. In its presence, OTA has been found to facilitate single-strand cl eavage of supercoiled plasmid DNA through production of reactive oxygen spe cies (ROS) (i.e., the hydroxyl radical, HO.). The reaction of OTA with the FeTPPS oxidation system also generated three hydroxylated products (chlorin e atom still attached), which was taken as evidence for production of the k nown 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 o f excess hydrogen peroxide (H2O2) and sodium ascorbate, a hydroquinone spec ies (OTHQ, 5) was detected in which an OH group has replaced the chlorine a tom of OTA. The production of OTHQ (5) was dependent on the presence of the reducing agent, sodium ascorbate, which suggested that the oxidation catal yst furnished the quinone derivative OTQ (6) that was subsequently reduced to OTHQ (5) by ascorbate. Utilizing a synthetic sample of OTHQ (5), the hyd roquinone 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 t he carbonyl groups. Our findings imply that the hydroquinone (OTHQ) and qui none (OTQ) metabolites of OTA have the ability to cause alkylation/redox da mage and have allowed us to propose a viable pathway for oxidative damage b y OTA.