Oxidation versus addition reactions of glutathione during the interactions with quinoid thioethers of 4-(dimethylamino)phenol

Article abstract of DOI:10.1021/tx00044a016

4-(Dimethylamino)phenol (DMAP) is a potent cyanide antidote which forms many equivalents of ferrihemoglobin in vivo and in vitro. During this process formation of phenoxyl radicals was observed which are reduced by ferrohemoglobin, thereby sustaining a catalytic cycle of ferrihemoglobin formation, or which disproportionate to give the quinone imine of DMAP. In the presence of thiols, e.g., glutathione (GSH), formation of 4- (dimethylamino)-2-(glutathion-S-yl)phenol (2-GS-DMAP), 4-(dimethylamino)- 2,6-bis(glutathion-S-yl)phenol (2,6-bis-GS-DMAP), and 4-(dimethylamino)- 2,3,6-tris(glutathion-S-yl)phenol (2,3,6-tris-GS-DMAP) was observed. While the trisubstituted glutathione conjugate is a stable end product, 2-GS-DMAP and 2,6-bis-GS-DMAP were still reactive and produced ferrihemoglobin. It is concluded that formation of polysubstituted DMAP thioethers is a result of sequential oxidation/addition reactions with quinoid intermediates. Formation of glutathione disulfide (GSSG) was minimal during the interaction of oxidized DMAP or 2-GS-DMAP with glutathione but became significant when oxidized 2,6-bis-GS-DMAP reacted with GSH. Thus it is conceivable that the bulky glutathione substituents in 2,6-bis-GS-DMAP render the addition of a third GSH molecule to the quinone imine derivative more difficult, and other reactions may get a chance. The reaction mechanism of GSSG formation has not been fully resolved, but a radical pathway mechanism involving thiyl radicals is proposed. Oxidation and addition reactions were also observed in the absence of oxygen when ferrihemoglobin served as oxidant. In the presence of oxygen, however, GSSG formation was increased, partly due to hydrogen peroxide formation, partly due to an additional trapping reaction of the glutathione disulfide radical anion. It appears that product orientation during the sequential oxidation/addition reactions is largely directed by steric factors of the bulky glutathione residues.