title: Kinetics of paraquat and copper reactions with nitroxides: the effects of nitroxides on the aerobic and anoxic toxicity of paraquat.
abstract: The toxicity of paraquat (PQ2+) is attributed to intracellularly formed PQ*+, O(2)*-, H(2)O(2), and secondary.OH radicals generated through Fenton-like reactions. Yet, no antidote for PQ2+ toxicity in human has been found also due to poor cell permeability of many common antioxidants that remove toxic species predominantly extracellularly. Cell-permeable nitroxides, which scavenge xenobiotic-derived deleterious radicals and detoxify redox-active metal ions, would be expected to ameliorate PQ2+ toxicity. We have studied using pulse radiolysis the kinetics of the reactions of 2,2,6,6-tetramethyl-piperidinoxyl (TPO) and 4-OH-TPO with PQ*+ and CuIIL(2) (L = 1,10-phenanthroline, 2,2'-bipyridyl) in the absence and presence of DNA. We found that the rate constant for the reaction of PQ*+ with the nitroxides is about 4 orders of magnitude lower than that with O(2). In addition, the rate of the reaction of the nitroxides with CuIL(2) decreases as [DNA] increases, which suggests that nitroxides react significantly slower with bound metal ions. These results explain the failure of 4-OH-TPO to protect bacterial and mammalian cells from PQ2+ toxicity under air. In contrast, the rate of the reaction of PQ*+ with CuIIL(2) was unaffected by DNA. Furthermore, copper toxicity has been attributed mainly to CuI and was observed predominantly for cells subjected to anoxic conditions. It implied that nitroxides would be effective protectants if PQ2+ induces toxicity also under anoxia. Surprisingly, we found that PQ2+ toxicity under anoxia was even greater than that under air, and under these conditions 4-OH-TPO protected the cells from PQ. These results indicate that the mechanism underlying the anoxic toxicity of PQ2+ differs from that operating in the presence of oxygen, and that reduced transition metal ions are most probably the species responsible for PQ2+ anoxic toxicity.
authors.affiliation: Department of Physical Chemistry and Molecular Biology, School of Medicine, The Hebrew University of Jerusalem, 91940, Israel.
authors.forename: Sara
authors.initials: S
authors.lastname: Goldstein
authors.forename: Amram
authors.initials: A
authors.lastname: Samuni
authors.forename: Yaacov
authors.initials: Y
authors.lastname: Aronovitch
authors.forename: Dina
authors.initials: D
authors.lastname: Godinger
authors.forename: Angelo
authors.initials: A
authors.lastname: Russo
authors.forename: James B
authors.initials: JB
authors.lastname: Mitchell
mesh_terms: D000818:Animals; D002478:Cells, Cultured / Q000187:drug effects; D003300:Copper / Q000378:metabolism*; D006224:Cricetinae; D003412:Cricetulus; D004247:DNA / Q000378:metabolism; D004347:Drug Interactions; D004926:Escherichia coli / Q000187:drug effects*; D006540:Herbicides / Q000378:metabolism / Q000633:toxicity*; D000860:Hypoxia / Q000378:metabolism*; D007700:Kinetics; D009589:Nitrogen Oxides / Q000378:metabolism*; D010269:Paraquat / Q000378:metabolism / Q000633:toxicity*; D011675:Pulse Radiolysis
chemical_list: D006540:Herbicides; D009589:Nitrogen Oxides; D003300:Copper; D004247:DNA; D010269:Paraquat