RING ADDITION OF THE ALPHA-AMINO GROUP OF GLUTATHIONE INCREASES THE REACTIVITY OF BENZOQUINONE THIOETHERS

2-(Glutathion-S-yl)-1,4-benzoquinone was found to be remarkably unstab le in phosphate buffer (pH 7.4) even in the absence of oxygen. Intramo lecular addition of the alpha-amino group of the glutamate residue to the quinone ring yielded ultimately 2,3-(glutathion-N,S-yl)-1,4-benzoq uinone and 2,6-(glutathion-N,S-yl)-1,4-benzoquinone in a 3:1 ratio alo ng with 2-(glutathion-S-yl)-1,4-hydroquinone. Kinetic studies indicate d that the cyclization reactions proceeded at a rate k(1) of 0.093 min (-1), while intermolecular reactions followed a second-order kinetics with a k(2) of 94 M-1 min(-1) (pH 7.4, 37 degrees C), resulting in mul tiple polymerization products. Both intramolecular amino adducts of 2- (glutathion-S-yl)-1,4-benzoquinone are prone to hydrolysis, leading to the insertion of an additional OH group in the ring. These S-substitu ted trihydroxybenzene derivatives are particularly susceptible to auto xidation. The model compound -(N-acetylcystein-S-yl)-2-hydroxy-1,4-hyd roquinone was shown to form readily two atropoisomeric biphenyls upon autoxidation: tylcystein-S-yl)-2',3,3',4,6,6'-hexadroxybiphenyl, indic ating C-C coupling, presumably via semiquinone radical intermediates. Thus, the sequence of glutathione S-addition, followed by oxidation, N -addition, oxidation, and hydrolysis, constitutes a novel and very eff ective activation pathway of quinones for eliciting oxidative stress. These data underline the fact that glutathione conjugates of autoxidiz able aromatics are no obligatory stable end products of a detoxication reaction. The possible toxicological impacts of intra- and intermolec ular addition reactions of quinoid thiol conjugates are discussed.