The role of nonprotein thiols in enzymatic reduction of 2-nitroimidazoles
Description
The preferential cytotoxicity of 2-nitroimidazoles (2-NIs) towards hypoxic cells arises from the reductive intermediates formed after reduction of the nitro group under a hypoxic environment. The cytotoxicity is enhanced by thiol depleting agents such as N-ethylmaleimide (NEM) and diethyl maleate (DEM) and is reduced by the addition of nonprotein thiols (NPSH) such as glutathione (GSH) and cysteine. The role of thiol depleting agents and NPSH on the reduction of the nitro function has been studied qualitatively and quantitatively by utilizing high pressure liquid chromatographic (HPLC) methods Rat hepatic cytosol and microsomes, buttermilk xanthine oxidase and sonicates of B16 melanoma cells were employed as sources of nitro reductases. Addition of NPSH caused an enhancement in the reduction of the nitro group of the 2-NIs, misonidazole (MISO) and SR2508 under hypoxic conditions. The thiol depleting agents, NEM and DEM decreased the reduction of the nitro function and the inhibition was reversed by the addition of NPSH except in the case of hepatic microsomes. Both NEM and DEM attenuated the enhanced reduction observed after the addition of NPSH A new 2-NI, 3-ethoxy-3-(2-nitroimidazol-1-yl)-1-propene (NBK50) was found to be selectively toxic towards hypoxic cells. NBK50 was designed to release acrolein, a toxic species, upon reductive bioactivation. NBK50 was found to deplete NPSH in B16 cells under oxic and hypoxic conditions but did not react chemically with GSH. The effects of NPSH in modulating the cytotoxicity of NBK50 may represent a balance between an increase in toxicity due to the release of acrolein after nitro reduction stimulated by NPSH and a decrease in toxicity due to the protective effect of NPSH The paradox that the addition of NPSH enhanced the nitro reduction and yet decreased cytotoxicity may be explained by the fact that addition of NPSH facilitates the reduction of the nitro radical anion to other reduced intermediates that may be inactivated by NPSH. However, the depletion of NPSH that inhibits nitro reduction would be expected to allow the accumulation of the nitro radicals anions which may be cytotoxic. (Abstract shortened with permission of author.)