Detection of nonenzymatic browning products in the human lens.

The nonenzymatic browning or Maillard reaction is an aging process in stored foods. The initial stage of this reaction, nonenzymatic glycosylation, has been shown to occur in the human lens. The possible occurrence of further steps of the Maillard reaction involving lysine residues and glucose has been investigated. A lipid-free protein extract from a pool of human cataractous lenses was reduced, alkylated, and digested with pronase. The digest was reduced with [3H]borohydride, acid hydrolyzed and fractionated by Sephadex G-15 chromatography. The fractions eluting ahead of epsilon-1-deoxyglucitolyllysine were pooled and separated with an amino acid analyzer. Four fluorescent, yellow, and radioactive peaks were obtained. One of these, which co-eluted with tyrosine, was isolated, acetylated, and further analyzed by reverse phase chromatography using HPLC. Two new peaks were separated which co-chromatographed with lysine derivatives isolated from the nonenzymatic browning reaction of alpha-tert-butyloxycarbonyllysine with glucose. Control experiments showed that they were not artifacts due to acid hydrolysis of epsilon-glucitolyllysine. These results suggest that dehydration and rearrangement of the Amadori product, epsilon-fructosyllysine, has occurred in vivo, thus leading to the formation of at least two nonenzymatic browning products.