Photochemical and photophysical studies on human lens constituents

The young human lens contains species (3-hydroxy kynurenine; 3-HK and its glucoside; 3-HKG) which absorb most light between 300 and 400 nm. Photochemical studies have indicated that these compounds are relatively inefficient sensitizers of lens proteins. An investigation of the fluorescent properties of 3-HKG indicate that it contains a fast deactivation pathway (ps) which would be expected to have minimal photochemical effect on the integrity of the lens. Further phot physical studies on 3-HK indicates that it has an even faster fluorescent lifetime (less than 10 ps) with a much lower quantum yield of fluorescence (0.001 vs 0.03 for 3-HKG). With aging, the human lens proteins undergo numerous changes including a generalized yellowing. These chromophores exhibit a higher quantum yield of fluorescence, an increase in the fluorescent lifetime by 2 orders of magnitude and the formation of two long lived transient species (microsecond). These species might be expected to drastically increase the susceptibility of the human lens to ambient radiation. Based upon quantitative experimental comparisons with 3-HK this does not seem to be the case. Further time resolved studies on old lens proteins indicate that the two transient species are interconnected in that the first transient species is the precursor to the second. The implications of this mechanism on the integrity of the lens and origin of those chromophores is discussed.