This research was undertaken to demonstrate that the protein-bound chromophores 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. Analysis of the irradiated proteins showed a linear decrease in sulfhydryl groups with a 30% loss after 2 h. No loss was seen when native alpha-crystallin was irradiated under the same conditions. A 25% loss of histidine residues was also observed with the human lens WI fraction, and sodium dodecyl sulfate polyacrylamide gels indicated considerable protein cross-linking. Similar photodamage was seen with a WI fraction from old bovine lenses. While the data show the presence of UVA sensitizers, some histidine destruction and protein cross-linking were also obtained with alpha-crystallin and with lysozyme, which argue that part of the histidine loss in the human WISS was likely due to tryptophan acting as a sensitizer. A preparation of human WI proteins was irradiated with a total of 200 J/cm2 of absorbed light at 10 nm intervals from 290 to 400 nm. Photodamage of cysteine SH groups (35%) and methionine (28%) was maximum at 330 nm and diminished linearly at longer wavelengths. The major loss of tryptophan (80%) occurred at 290 nm, but destruction was observed throughout the UVA range. Tyrosine was 35% destroyed at 290 nm but decreased sharply to only 5% at 330 nm.(ABSTRACT TRUNCATED AT 250 WORDS)