Tryptophan metabolites from young human lenses and the photooxidation of ascorbic acid by UVA light.
OBJECTIVE
To determine whether there are UVA light-responsive sensitizers in young human lenses capable of initiating the oxidation of ascorbic acid in the absence of oxygen.


METHODS
Lens homogenates were fractionated, and low-molecular-weight (LMW) components were separated from the proteins by filtration through a 3000-MWt cutoff filter. Aliquots of each fraction were assayed for sensitizer activity by UVA irradiation (337-nm cutoff filter) with 0.1 mM ascorbic acid, measuring ascorbate oxidation by loss of absorbance at 265 nm. Two major peaks were isolated from a human lens water-soluble (WS)-LMW fraction on a reversed-phase column and were identified by mass spectrometry.


RESULTS
All human lens fractions oxidized ascorbate when irradiated by UVA light. Most of the sensitizer activity in young human lenses was in the LMW fractions. An action spectrum for the WS-LMW fraction from human lens showed activity throughout the UVA region. Assays with and without oxygen showed little or no difference in ascorbate oxidized, arguing for a direct transfer of an electron in a so-called type 1 reaction. A human lens WS-LMW fraction contained two major peaks of activity. The greater peak was identified as 3-hydroxykynurenine glucoside (3OHKG) by mass spectrometry and its absorption spectrum, whereas the lesser peak was identified as 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid glucoside (AHBG). The activities were 1.1 and 2.8 nmol of ascorbate oxidized in 30 minutes/nmol 3OHKG and AHBG, respectively.


CONCLUSIONS
The filter compounds present in human lenses can absorb UVA light and cause the oxidation of ascorbic acid in the presence and absence of oxygen, possibly initiating the glycation of lens proteins.
Ortwerth
Beryl J
BJ
Mason Eye Institute, University of Missouri, Columbia, Missouri 65201, USA. ortwerthb@health.missouri.edu
Bhattacharyya
Jaya
J
Shipova
Ekaterina
E
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology; D001205:Ascorbic Acid / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003459:Crystallins / Q000378:metabolism / Q000528:radiation effects*; D005960:Glucosides / Q000378:metabolism*; D005978:Glutathione / Q000494:pharmacology; D006801:Humans; D007908:Lens, Crystalline / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects*; D013058:Mass Spectrometry; D008875:Middle Aged; D010084:Oxidation-Reduction; D010654:Phenylbutyrates / Q000378:metabolism*; D011838:Radiation-Sensitizing Agents / Q000378:metabolism*; D013347:Subcellular Fractions; D014364:Tryptophan / Q000378:metabolism*; D014466:Ultraviolet Rays
C402223:3-hydroxykynurenine glucoside; D003459:Crystallins; D005960:Glucosides; D010654:Phenylbutyrates; D011838:Radiation-Sensitizing Agents; C088377:4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-glucoside; D014364:Tryptophan; D005978:Glutathione; D001205:Ascorbic Acid