Inhibition of radical adduct reduction and reoxidation of the corresponding hydroxylamines in in vivo spin trapping of carbon tetrachloride-derived radicals

Abstract

In vivo spin trapping of radical metabolites has become a promising tool in understanding and predicting toxicities caused by different xenobiotics. However, in biological systems radical adducts can be reduced to electron paramagnetic resonance (EPR)-silent hydroxylamines. To overcome this difficulty, different procedures for reoxidation of the reduced radical adducts were systematically investigated and some metabolic inhibitors of nitroxide reduction were tested. As a test system, carbon tetrachloride (CCl 4), a known hepatotoxic substance, was used. CCl 4 is metabolized by liver to .CCl 3 and, in the presence of the spin trap phenyl N- t-butylnitrone (PBN), forms the PBN/ .CCl 3 and PBN/ .CO 2 − radical adducts. These radical adducts were measured in the bile using electron paramagnetic resonance after administration of CCl 4 and PBN to the rat. We have shown that these radical adducts were reduced to the corresponding hydroxylamines in vivo, since immediately after the collection of bile only traces of the radical adducts could be detected, but after oxidation by different procedures such as bubbling with oxygen, addition of mild oxidant potassium ferricyanide or autoxidation the EPR spectra intensity increases, indicating that the hydroxylamines had been re-oxidized back to nitroxides. The collection of bile into plastic Eppendorf tubes containing the sulfhydryl reagent N-ethylmaleimide (NEM) or the enzyme ascorbate oxidase did not increase the intensity of the spectra significantly, demonstrating that neither reduction by reduced glutathione (GSH) nor ascorbic acid occurred ex vivo. However in the presence of NEM faster re-oxidation was observed. A new radical adduct that was not observed previoulsy in any in vivo experiment and which exhibited 13C hyperfine coupling was detected when the rats were injected with 13CCl 4. We have proven that this is the same adduct detected previously in vitro in microsomal incubations of CCl 4, PBN, GSH, and reduced nicotinamide adenine dinucleotide phosphate (NADPH). As a general rule, we have shown that a variety of oxidation procedures should be tried to detect the different radical adducts which are otherwise not observable due to the in vivo reduction of radical adducts.