On the role of copper and iron in DNA cleavage by ochratoxin A. Structure-activity relationships in metal binding and copper-mediated DNA cleavage

Ochratoxin A (OTA, 1: X = Cl) is a fungal carcinogen that facilitates single-strand DNA cleavage and DNA adduction when metabolically activated. To determine if redox-active transition metals induce OTA-mediated DNA damage, we have examined the toxin's ability to bind Cu(II) and Fe(III) in aqueous media and facilitate DNA cleavage in their presence using agarose gel electrophoresis and supercoiled plasmid DNA. Using fluorescence spectroscopy, 1 was found to bind Cu(II) readily at physiological pH, while acidic conditions (pH 2.6) were employed to study Fe(III) binding due to the formation of Fe-oxide precipitates at higher pH values. Structure-activity relationships employing synthetic derivatives of 1 implied that 1 binds both Cu(II) and Fe(III) by its phenolic oxygen, while the carboxylic acid of its phenylalanine moiety binds Cu(II), but does not appear to play a role in Fe(III) coordination at pH 2.6. In terms of metal-mediated DNA cleavage, no role for 1 could be detected in Fe-induced DNA strand scission. With Cu(II), DNA cleavage by the 1:1 copper-bound complex of 1 could only be initiated by addition of a suitable reducing agent (sodium ascorbate). However, 1 was found to facilitate DNA cleavage by the Cu(II) complex of 1,10-phenanthroline (Cu(OP)(2)); a prototypical Cu-mediated nuclease system that cleaves DNA upon activation by an external reducing agent. Structure-activity relationships employing analogs lacking the chlorine atom, ochratoxin B (2: X = H), and the lactone (12), indicated that the chlorine atom is essential for activity of the OTA in potentiating DNA cleavage by Cu(OP)(2). The implications of our findings to the genotoxic properties of 1 are discussed.