Ultraviolet filter compounds in human lenses: 3-hydroxykynurenine glucoside formation

Experiments have been conducted on various aspects of the biosynthetic pathway leading to the formation of 3-hydroxykynurenine glucoside (3OHKG), the major tryptophan-derived human lens UV filter compound. Measurable levels of the reactive metabolite, 3-hydroxykynurenine (3OHKyn) were found in normal human lenses, including lens nuclei. 3OHKyn was able to enter organ-cultured lenses and to serve as a substrate for production of 3OHKG. Thus lenticular 3OKHyn can potentially be derived either by metabolism of Trp in the lens or from transport into the lens. Lens incubation followed by dissection showed that the equatorial region was probably the major site of synthesis of 3OHKG. The concentration of 3OHKG was relatively constant along the visual axis but was significantly lower in the equatorial region: a pattern also found for Kyn and 3OHKyn. No evidence, however could be obtained for hydrolysis of 3OHKG by lens homogenates.