The nature and properties of squirrel lens yellow pigment.

The low molecular weight yellow pigment in the gray squirrel lens is confirmed to be n-acetyl-3-OH-kynurenine (NAK). This conclusion is based upon the results of studies of the compound's optical, chromatographic, and mass spectroscopic properties. The original tentative identification of this compound was reported by Van Heyningen (1971, 1973). The pigment is distributed equivalently throughout the lens. Its absorbance increases with age, but the increase is related to the growth of the lens and not to an increase in pigment concentration. The function of NAK in the squirrel lens is to enhance vision by eliminating short-wavelength light that causes scattering and chromatic aberration. The pigmented lens also protects the retina from near-UV radiation-associated damage to the photoreceptors (Collier and Zigman, 1987).