The anaerobic photolysis of lens alpha-crystallin: evidence for triplet state mediated photodamage.

Anaerobic solutions of lens alpha-crystallin were subjected to near-UV (greater than 295 nm) irradiation, and the photoproducts were analyzed by fluorescence and room-temperature phosphorescence spectroscopy. The principal photoproduct was excited maximally at 340 nm, fluoresced maximally at 430 nm, and phosphoresced with an emission maximum at 510 nm. The phosphorescence intensity decay of this species was well fit by a sum of two exponentials with lifetimes of 9.2 ms (78%) and 61 ms (22%); this report is the first demonstration of a long-lived triplet state associated with a protein photolysis product. As reported previously, 3trp* is also long-lived in deoxygenated alpha-crystallin solution at room-temperature (Berger and Vanderkooi, 1989, Biochemistry 28, 5501-5508), hence both tryptophan and photoproduct triplet states are good candidates to mediate photodamage. Photolysis experiments in the presence of agents known to alter the tryptophan triplet yield provide evidence for the importance of triplet-state-mediated photodamage of lens crystallins in anaerobic solution. In 30 mM acrylamide where 3trp*, but not 1trp*, is efficiently quenched, anaerobic solutions exhibited marked resistance to protein photodamage, whereas the photoprotection in aerobic solution was minimal. In D2O, where photoionization is suppressed but triplet states are longer-lived, photodamage was accelerated in anaerobic solution but reduced in aerobic solutions. Finally, the anaerobic photodestruction rate was increased in 500 mM Cs+ solution where the triplet yield is increased by a heavy atom effect.