N-acetyl-p-benzoquinone imine (NAPQI), a reactive metabolite of acetaminophen (APAP), can arylate and oxidize protein and nonprotein thiols in the pathogenesis of APAP-induced hepatotoxicity. We report the first direct evidence for the formation of a labile ipso adduct between glutathione (GSH) and NAPQI using a combination of techniques including liquid chromatography/tandem mass spectrometry and liquid chromatography/NMR spectroscopy. Decomposition kinetics of the GSH-NAPQI ipso adduct and product ratios suggested that the ipso adduct was readily reversible back to NAPQI under neutral and basic conditions. The significance of the ipso adduct is that it may migrate from its site of formation to other cell compartments where it can either oxidize protein thiols or covalently modify them. Ipso adduct formation with protein thiols was demonstrated with a cysteine protease, papain, whose catalytic activity relies on the presence of an active site cysteinyl thiol. The formation and reactions of cysteinyl thiol ipso adducts of NAPQI provides significant new insights into possible reactions of quinone imines with cellular peptides and proteins.

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After-cooking discoloration of potatoes has been a serious problem in several growing areas east of the Mississippi River for more than 20 years.This problem was acute with the 1953 crop but occurred in only rare lots of the 1954 crop. Several hundred samples of potatoes from the 1954 Maine crop were screened by staff members of the Maine Agricultural Experiment Station but little incidence of this type of discoloration was found. Blackening of cooked potatoes was again troublesome with the 1955 crop.The extensive literature on after-cooking discoloration of potatoes was recently reviewed by E. Yanovsky (22), and will not be considered in detail here. This type of discoloration usually occurs as greying or blackening, in greatest intensity at the stem end just beneath the skin. Flesh of affected tubers typically appears normal in the raw state.After-cooking discoloration is not only encountered in home and restaurant cooked products but in all types of processed potato products. This discoloration can cause difficulties to processors, particularly in canned potatoes, frozen products, and dehydrated potato products. Caldwell ef al.(3) reviewed thoroughly the various types of discoloration encountered in the processing and use of dehydrated potatoes.Water blanching and acid dipping treatments of the raw, peeled stock are of limited value in inhibiting after-cooking discoloration. Sulfiting is of some help but cannot be used with potatoes to he processed with meat, since foodst.uffs containing meat must be sulfite-free for shipment in interstate commerce.In the past 10 years, at least two investigations have been made of a colorless, fluorescent substance extracted from potatoes \vith acidified alcohol. Lewis and Doty (12) believed this substance to be the precursor of the pigment causing potato blackening, since it \zas readily converted to a black pigment by heating above 35" C. However, Bowman and Hanning (2) concluded that there was no clear relationship bet\veen the extent of fluorescence obtained with this substance and the tendency for potatoes to blacken. Many possible causes have been suggested for after-cooking discoloration, but few facts have been established and much of the published data are contradictory when compared. Some of the factors proposed as being related to after-cooking discoloration are : potato variety ; soil pH and composition ; growing conditions ; storage conditions ; ratio of protein nitrogen to nonprotein nitrogen ; and the amounts of tyrosinase, iron, phosphorus, and manganese in the tubers.