UVA PHOTOLYSIS USING THE PROTEIN-BOUND SENSITIZERS PRESENT IN HUMAN LENS

This research was undertaken to demonstrate that the protein-bound chr omophores in aged human lens can act as sensitizers for protein damage by UVA light. The water-insoluble (WI) proteins from pooled human and bovine lenses were solubilized by sonication in water and illuminated with UV light similar in output to that transmitted by the cornea. An alysis of the irradiated proteins showed a linear decrease in sulfhydr yl groups with a 30% loss after 2 h. No loss was seen when native alph a-crystallin was irradiated under the same conditions. A 25% loss of h istidine residues was also observed with the human lens WI fraction, a nd sodium dodecyl sulfate polyacrylamide gels indicated considerable p rotein cross-linking. Similar photodamage was seen with a WI fraction from old bovine lenses. While the data show the presence of UVA sensit izers, some histidine destruction and protein cross-linking were also obtained with alpha-crystallin and with lysozyme, which argue that par t of the histidine loss in the human WISS was likely due to tryptophan acting as a sensitizer. A preparation of human WI proteins was irradi ated with a total of 200 J/cm(2) of absorbed light at 10 nm intervals from 290 to 400 nm. Photodamage of cysteine SH groups (35%) and methio nine (28%) was maximum at 330 nm and diminished linearly at longer wav elengths. The major loss of tryptophan (80%) occurred at 290 nm, but d estruction was observed throughout the UVA range. Tyrosine was 35% des troyed at 290 nm but decreased sharply to only 5% at 330 nm. A constan t loss of histidine (20%) was seen at all wavelengths from 290 to 360 nm, with some loss (7-8%) even at 400 nm. These action spectra show th at the human lens WI fraction contains a collection of protein-bound U VA sensitizers that can cause protein photodamage similar to that seen in cataractous lenses.