Eye lens color: formation and function.

Aromatic amino acids are photooxidized by near-ultraviolet light to colored products that are bound very tightly to protein amino groups. The resulting colored proteins absorb near-ultraviolet light more strongly and are rendered more hydrophobic than the untreated compounds, and they fluoresce at 440 nanometers when excited at 360 nanometers. Coloration in the lenses of diurnally active animals (including man) may be caused by this reaction, and senile cataracts may result. Such changes in many other proteins (as in the skin and retina) could lead to more serious consequences.