Mechanism of Coloration of Human Lenses Induced by Near-Ultraviolet-Photo-Oxidized 3-Hydroxykynurenine (With 1 color plate)

Abstract

Tha absorption spectra of human cataractous lenses, classified into four groups on the basis of coloration, were measured by a spectrophotometer equipped with an integrating sphere that is able to detect diffuse reflectance of solid materials. The results revealed that pale-yellow cataractous lenses showed a characteristic absorption spectrum with a peak near 365 nm, coincident with that of 3-hydroxykynurenine, and that brunescent cataractous lenses with high pigmentation showed an absorption spectrum with a peak over 400 nm, coincident with that of xanthommatin, a dimerized compound of 3-hydroxykynurenine. Depending on the coloration of the lens, the position of maximum absorption shifted from 365 to 410 nm and the optical density increased at visible regions. The changes in absorption spectra of pale-yellow lens homogenates were studied in the presence or absence of 3-hydroxykynurenine and by near-ultraviolet (UV) irradiation. Brown pigments were formed in the insoluble proteins of lens homogenates by 48 h of exposition of the samples with 3-hydroxykynurenine to UV irradiation. The absorption spectra of the brown pellets detected by an integrating sphere corresponded well to those of pellets which were obtained from the homogenates of intact human brunescent cataractous lenses. These results strongly suggest that 3-hydroxykynurenine and UV irradiation are involved in the brown pigmentation of human cataractous lenses by formation of xanthommatin.