Chemistry of collagen cross-links: glucose-mediated covalent cross-linking of type-IV collagen in lens capsules.

The incubation of lens capsules with glucose in vitro resulted in changes in the mechanical and thermal properties of type-IV collagen consistent with increased cross-linking. Differential scanning calorimetry (d.s.c.) of fresh lens capsules showed two major peaks at melting temperatures Tm 1 and Tm 2 at approx. 54 degrees C and 90 degrees C, which can be attributed to the denaturation of the triple helix and 7S domains respectively. Glycosylation of lens capsules in vitro for 24 weeks caused an increase in Tm 1 from 54 degrees C to 61 degrees C, while non-glycosylated, control incubated capsules increased to a Tm 1 of 57 degrees C. The higher temperature required to denature the type-IV collagen after incubation in vitro suggested increased intermolecular cross-linking. Glycosylated lens capsules were more brittle than fresh samples, breaking at a maximum strain of 36.8 +/- 1.8% compared with 75.6 +/- 6.3% for the fresh samples. The stress at maximum strain (or 'strength') was dramatically reduced from 12.0 to 4.7 N.mm.mg-1 after glycosylation in vitro. The increased constraints within the system leading to loss of strength and increased brittleness suggested not only the presence of more cross-links but a difference in the location of these cross-links compared with the natural lysyl-aldehyde-derived cross-links. The chemical nature of the fluorescent glucose-derived cross-link following glycosylation was determined as pentosidine, at a concentration of 1 pentosidine molecule per 600 collagen molecules after 24 weeks incubation. Pentosidine was also determined in the lens capsules obtained from uncontrolled diabetics at a level of about 1 per 100 collagen molecules. The concentration of these pentosidine cross-links is far too small to account for the observed changes in the thermal and mechanical properties following incubation in vitro, clearly indicating that another as yet undefined, but apparently more important cross-linking mechanism mediated by glucose is taking place.