Figure 7. Proposed mechanisms for glutathione (GSH) interaction with photooxoA2E and methylglyoxal, an A2E photodegradation product. A: GSH would transfer two hydrogens from two GSH molecules to an endoperoxide on A2E, resulting in the m/z+2 pattern associated with photooxidized forms of A2E in the m/z 640–736 region of Figure 3B-D [33]. B: GSH adduct formation with an endoperoxide on A2E would occur via nucleophilic attack and ring opening [34,35]. Simple addition of GSH at the site of an endoperoxide would involve the formation of an unstable hydroperoxide (OOH) moiety (B, i) and for example would account for m/z 947 in Figure 4B, insert. Alternatively, GSH conjugation could involve attack of the endoperoxide bridge (O-O) by the GSH thiolate followed by carbonyl formation and GS insertion (B, ii); this mechanism would account for m/z 931 in Figure 4B, insert [33,36,37]. C: Adduct formation with an epoxide would be expected to occur [35]; however, the appropriate product (m/z 915) was not detected. D: GSH can react with methylglyoxal (MG) released upon A2E photodegradation to form an MG-GSH hemi-thioacetal [38]; this adduct accounts for m/z 380 and 402 in Figure 6F.