Jasminka Mizdrak

Tryptophan-derived ultraviolet filter compounds covalently bound to lens proteins are photosensitizers of oxidative damage

The human eye is chronically exposed to light of wavelengths >300 nm. In the young human lens, light of wavelength 300-400 nm is predominantly absorbed by the free Trp derivatives kynurenine (Kyn), 3-hydroxykynurenine (3OHKyn), and 3-hydroxykynurenine-O-beta-D-glucoside (3OHKynG). These ultraviolet (UV) filter compounds are poor photosensitizers. With age, the levels of the free UV filters in the lens decreases and those of protein-bound UV filters increases. The photochemical behavior of these protein-bound UV filters and their role in UV damage are poorly elucidated and are examined here. UVA illumination of protein-bound UV filters generated peroxides (principally H2O2) in a metabolite-, photolysis-time-, and wavelength-dependent manner. Unmodified proteins, free Trp metabolites, and Trp metabolites that do not bind to lens proteins gave low peroxide yields. Protein-bound 3OHKyn (principally at Cys residues) yielded more peroxide than comparable Kyn and 3OHKynG adducts. Studies using D2O and sodium azide implicated 1O2 as a key intermediate. Illumination of the protein-bound adducts also yielded protein-bound Tyr oxidation products (DOPA, di-tyrosine) and protein cross-links via alternative mechanisms. These data indicate that the covalent modification of lens proteins by Kyn derivatives yields photosensitizers that may enhance oxidation in older lenses and contribute to age-related nuclear cataract.

Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine?

Since crystallins in the human lens do not turnover, they are susceptible to modification by reactive molecules over time. Methylation is a major post-translational lens modification, however the source of the methyl group is not known and the extent of modification across all crystallins has yet to be determined. Sites of methylation in human lens proteins were determined using HPLC/mass spectrometry following digestion with trypsin. The overall extent of protein methylation increased with age, and there was little difference in the extent of modification between soluble and insoluble crystallins. Several different cysteine and histidine residues in crystallins from adult lenses were found to be methylated with one cysteine (Cys 110 in γD crystallin) at a level approaching 70%, however, methylation of crystallins was not detected in fetal or newborn lenses. S-adenosylmethionine (SAM) was quantified at significant (10-50 μM) levels in lenses, and in model experiments SAM reacted readily with N-α-tBoc-cysteine and N-α-tBoc-histidine, as well as βA3-crystallin. The pattern of lens protein methylation seen in the human lens was consistent with non-enzymatic alkylation. The in vitro data shows that SAM can act directly to methylate lens proteins and SAM was present in significant concentrations in human lens. Thus, non-enzymatic methylation of crystallins by SAM offers a possible explanation for this major human lens modification.

Identification of the new UV filter compound cysteine-L-3-hydroxykynurenine O-β-D-glucoside in human lenses

UV filters protect the human lens and retina from UV light‐induced damage. Here, we report the identification of a new UV filter, cysteine‐l‐3‐hydroxykynurenine O‐β‐d‐glucoside, which is present in older normal human lenses. Its structure was confirmed by independent synthesis. It is likely this novel UV filter is formed in the lens by nucleophilic attack of cysteine on the unsaturated ketone derived from deamination of 3‐hydroxykynurenine O‐β‐d‐glucoside. Quantitation studies revealed considerable variation in normal lens levels that may be traced to the marked instability of the cysteine adduct. The novel UV filter was not detected in advanced nuclear cataract lenses.

Detection, quantification, and total synthesis of novel 3-hydroxykynurenine glucoside-derived metabolites present in human lenses.

PURPOSE 3-Hydroxykynurenine O-β-D-glucoside (3OHKG) protects the lens from UV damage, and novel related species may act analogously. The aim of this study was to detect, quantify, and elucidate the structures of novel 3-hydroxykynurenine glucoside-derived metabolites present in the human lens. METHODS Compounds were detected and quantified by liquid chromatography with tandem mass spectrometry (LC-MS/MS) in 24 human lenses of different ages, of which 22 were normal and two had cataract. Structures of these were confirmed through total synthesis. RESULTS 3OHKG concentrations decreased with age in the lens nuclei, whereas the levels of three novel species, 4-(2-amino-3-hydroxyphenyl)-2-hydroxy-4-oxobutanoic acid O-β-D-glucoside (3OHKG-W), 3-hydroxykynurenine O-β-D-glucoside yellow (3OHKG-Y), and 2-amino-3-hydroxyacetophenone O-β-D-glucoside (AHAG), increased, though to different extents. In contrast, the concentrations present in the cortex of the lens remained constant with age. CONCLUSIONS Three novel 3OHKG-derived metabolites have been detected in extracts from human lenses.