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UV Filter Compounds in Human Lenses: the Origin of 4-(2-Amino-3-hydroxyphenyl)-4-oxobutanoic Acid O-ß-D-Glucoside

From the Australian Cataract Research Foundation, Department of Chemistry, University of Wollongong, Wollongong, New South Wales, Australia.

PURPOSE. To investigate UV filter synthesis in the human lens, in particular the biosynthetic origin of the second most abundant UV filter compound, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-ß-D-glucoside.

METHODS. Human lenses were analyzed by high-performance liquid chromatography (HPLC) after separate incubation with ³H-tryptophan (³H-Trp), ß-benzoylacrylic acid, D,L--amino-ß-benzoylpropionic acid, or D,L-3-hydroxykynurenine O-ß-D-glucoside. The effect of pH on the model compound D,L--amino-ß-benzoylpropionic acid and D,L-3-hydroxykynurenine O-ß-D-glucoside was also investigated.

RESULTS. UV filters were not detected in fetal lenses, despite a 5-month postnatal lens displaying measurable levels of UV filters. In adults no radiolabel was incorporated into 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-ß-D-glucoside after ³H-Trp incubations. ß-Benzoylacrylic acid was readily reduced in lenses. D,L--Amino-ß-benzoylpropionic acid and D,L-3-hydroxykynurenine O-ß-D-glucoside slowly deaminated at physiological pH and were converted to ß-benzoylpropionic acid and 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-ß-D-glucoside, respectively, after lens incubations.

CONCLUSIONS. UV filter biosynthesis appears to be activated at or near birth. Compounds containing the kynurenine side chain slowly deaminate, and in the lens, the newly formed double bond is rapidly reduced. These findings suggest that 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-ß-D-glucoside is derived from L-3-hydroxykynurenine O-ß-D-glucoside through this deamination-reduction process. The slowness of the deamination presumably accounts for the absence of incorporation of radiolabel from ³H-Trp into 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-ß-D-glucoside.

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