Collagen in the ageing human intervertebral disc: an increase in covalently bound fluorophores and chromophores.

Human disc tissue gradually changes in colour from white in the young to yellow brown in the elderly. It was investigated to what degree this colouration and an associated fluorescence (which are characteristic of the non-enzymic reaction products of sugars or oxidized lipids with proteins), were the result of covalent derivatives on the collagen and other extracellular matrix proteins. Annulus fibrosus was obtained from four subjects aged 19 to 92 years. Papain-solubilized samples of tissue showed increasing yellow colour and the glycation-related fluorescence with age. On chromatography of CNBr-digests of tissue, the yellow colour and fluorescence remained bound to the CNBr-peptide fragments of the collagen and other matrix proteins. One peptide, alpha 1(II)CB12 (from type II collagen), was selected for purification and shown to contain increasing amounts of the characteristic fluorescence with age. Sequencing by Edman degradation over 24 cycles confirmed the identity of the peptide, and by analysis of a portion of the PTH-derivatives showed fluorescence at cycle 11, a lysine residue. The results imply that much of the yellow colour and characteristic fluorescence that accumulate in ageing human discs are contributed by covalent adducts (possibly derived from non-enzymic reactions with carbohydrates or lipids) linked to the collagen and probably to other long-lived matrix proteins. The disc is perhaps particularly susceptible to such protein modifications because, being large and avascular, of its tendency to a low oxygen tension. Such modifications to structural proteins may contribute to the commonly observed degeneration and impaired material function of ageing disc tissue.