Synthesis and purification of 3-hydroxykynurenine-O-beta-glucoside, a primate lens ultraviolet filter, and its application in a two-step assay for beta-glucosidase activity

Synthesis of l-Kynurenine and Homo-l-Kynurenine via an Aza-Fries Rearrangement

l-Kynurenine, a non-proteinogenic amino acid, is the primary metabolite of tryptophan via the kynurenine pathway. Kynurenine is involved in a variety of biological processes occurring in the human body, notably in the central nervous system. Thus, the study of this molecule offers multiple opportunities for drug discovery; however, an essential prelude for biological studies is to secure the supply of kynurenine and analogues thereof. A simple synthetic procedure for the efficient preparation of enantiomerically pure l-kynurenine from l-aspartic acid and its implementation to prepare homo-l-kynurenine from l-glutamic acid is presented. The approach relies on a photochemical aza-Fries rearrangement of the corresponding acyl-aniline as the fundamental transformation.

Optical density of the aging human ocular media in the visible and the UV

We analyzed the literature on the absorption in the young and aging human eye media. Five templates were derived to provide an adequate description of the spectra from 300 to 700 nm for the lens, cornea, aqueous, and vitreous. Two templates were found in all media. They stand for Rayleigh scatter and the absorbance of tryptophan. Three additional templates for the lens represent absorbance in kynurenine derivatives, such as 3-hydroxykynurenine glucoside (3HKG), and absorbance in two substances found at older age. Except for Rayleigh scatter, all templates have a Gaussian shape. Aging-trend functions were derived that show a linear slope on an age-squared scale. The result can be used to correct for media losses in visual perception tasks, in fundus reflectometry, and in studies on light damage.

Absorption of the eye lens and macular pigment derived from the reflectance of cone photoreceptors

We measured the amplitude of the directional component of the bleached fundus reflectance, the so-called optical Stiles-Crawford effect, as a function of wavelength. The directional reflectance originates from within the outer segments of the photoreceptors. Thus only two anterior absorbers are of importance: macular pigment and the crystalline lens. Analysis of spectra obtained in pseudophakes established that the cone photoreceptors act as spectrally neutral reflectors. The reflectance spectra, expressed in density units, resembled the macular pigment density spectrum. Studying age effects in the lens of normal subjects resulted in a description of the optical density of the lens in terms of a "young" and an "aged" template. The young template represents the pigment O-beta-glucoside of 3-hydroxykynurenine, which dominates the light absorption in young eyes and decreases with age. The aged template represents the pigments accumulating in the lens with age. The total optical density increased with age, but it was lower in the wavelength region 500-650 nm than was previously assumed on the basis of psychophysical studies. Analysis of the spectra also provided precise individual estimates of the optical density of macular pigment. Finally, we observed a decrease in the photoreceptor reflectivity with age, possibly reflecting a degradation of the photoreceptors.