OXIDIZED AMINO-ACIDS IN LENS PROTEIN WITH AGE - MEASUREMENT OF O-TYROSINE AND DITYROSINE IN THE AGING HUMAN LENS

The concentrations of ortho-tyrosine (o-Tyr) and dityrosine (DT) were measured in noncataractous human lenses in order to assess the role of protein oxidation reactions in the aging of lens proteins. The measur ements were conducted by selected ion monitoring-gas chromatography/ma ss spectrometry using deuterium-labeled internal standards, which prov ided both high sensitivity and specificity for the quantitation of o-T yr and DT. Between ages 1 and 78 years, the o-Tyr concentration in len s proteins varied from 0.3 to 0.9 mmol of o-Tyr/mol of Phe (n = 19), w hile DT ranged from 1 to 3 mumol of DT/mol of Tyr (n = 30). There were no significant changes in levels of o-Tyr with lens age. There was a statistically significant, but only slight, increase in DT in lens pro teins with age (approximately 33% increases between ages 1 and 78, r = 0.5, p < 0.01). At the same time, total protein fluorescence, measure d at DT wavelengths (E(x) = 317 nm, E(m) = 407 nm), increased 11-fold between ages 1 and 78 and correlated strongly with age (r = 0.82, p < 0.0001). Although the fluorescence maxima of lens proteins were simila r to those of DT, DT accounted for less than 1% of the DT-like fluores cence in lens protein at all ages. These observations indicate that ox idation of Phe and Tyr plays a limited role in the normal aging of len s proteins in vivo.