Racemization of tyrosine in the insoluble protein fraction of brunescent aging human lenses.

With increase in age, proteins of the eye lens undergo covalent modifications associated with interprotein cross-linking, loss of protein solubility, generation of protein-associated pigments, and racemization of some optically active residues. Precipitated protein aggregates localized in the nucleus of brunescent lenses are modified to such an extent that they become resistant to the commonly used methods of extraction by reducing agents or detergents. Thus, little information is available on the types of protein modifications that occur in the nuclear region of the lens. Here we report the simultaneous solubilization and digestion of the entire nuclear pellet of brunescent human cataract lenses into constituent amino acids and their derivatives through protease action at neutral pH. Fractionation of the resulting hydrolyzate by reverse phase chromatography, followed by spectroscopic and biochemical analysis of one of the purified fractions, showed the presence of racemic tyrosine in the amino acid mixture. Racemization of L-amino acid residues in protein chains is associated with the aging process. This is the first report of racemization of tyrosine in any system studied so far.