K2P--a novel cross-link from human lens protein.

We report here the isolation of a novel acid-labile yellow chromophore from the enzymatic digest of human lens proteins and the identification of its chemical structure by LC-MS and NMR. This new chromophore exhibited a UV absorbance maximum at 343 nm and a molecular mass of 370 Da. One- and two-dimensional NMR analyses elucidated the structure as being 1-(5-amino-5-carboxypentyl)-4-(5-amino-5-carboxypentyl-amino)-3-hydroxy-2, 3-dihydropyridinium, a cross-link between the epsilon-amino groups of two lysine residues and a five-carbon atom ring. We assigned it the trivial name of K2P. Quantitative determinations of K2P in individual normal human lens or cataract lens water-soluble and water-insoluble protein digests revealed a significant enhancement of K2P in the early stage of brunescent cataract lens proteins (type I/II, 613 +/- 362 pmol/mg of water-insoluble sonicate supernatant (WISS) protein or 85 +/- 51 pmol/mg of water-soluble [WS] protein) when compared with aged normal human lens proteins (261 +/- 93 pmol/mg of WISS protein or 23 +/- 15 pmol/mg of WS protein). Furthermore, a gradual decrease of K2P in the late stages of brunescent cataract lenses with the development of the browning color in the lens argues different coloration mechanisms during the processes of normal aging and cataract development. This new cross-link may serve as a quantitatively significant biomarker for assessing the role of lens protein modifications during aging and in the pathogenesis of cataract.