Loss of free amino groups in the water-insoluble fraction of nuclear senile cataracts.

Free amino groups were analyzed in the water-insoluble fraction (WIF) of cataractous human lenses of the nuclear sclerosis, pigmented type. Two modified versions of the trinitrobenzene sulfonic acid (TNBS) procedure for the determination of free amino groups in proteins (AFSA Habeeb, Anal Biochem 14:328-336, 1966) were used for this purpose. The concentration of free amino groups in WIF was found to be inversely related to the dry weight of this fraction. Taking WIF weight as a measure of the severity of nuclear cataract, it can be thus said that a relative loss of free amino groups apparently occurs during the insolubilization process associated to this type of senile cataract. The disappearance rate has been estimated as about 0.9%--NH2 per 1 mg of material added to WIF. After considering other possible alternatives (loss of accessibility of amino groups to TNBS, dilution of amino groups by materials with a low primary amine content) we have interpreted this finding as most likely due to the postsynthetic blockade of lysine epsilon-amino functions in WIF proteins. The potential involvement of lysine residues in Schiff base formation is discussed within the context of current views of cataractogenesis. The reaction of lysine epsilon-NH2 with unidentified carbonyl compounds could represent a partial but quantitatively important mechanism of yellowing in nuclear cataracts.