An Evaluation of Lysyl Oxidase–Derived Cross-Linking in Keratoconus by Liquid Chromatography/Mass Spectrometry

PURPOSE Current literature contains scant information regarding the extent of enzymatic collagen cross-linking in the keratoconus (KC) cornea. The aim of the present study was to examine levels of enzymatic lysyl oxidase-derived cross-links in stromal collagen in KC tissue, and to correlate the cross-link levels with collagen fibril stability as determined by thermal denaturation temperature (Tm). METHODS Surgical KC samples (n = 17) and Eye-Bank control (n = 11) corneas of age 18 to 68 years were analyzed. The samples were defatted, reduced (NaBH4), hydrolyzed (6N HCl at 110°C for 18 hours), and cellulose enriched before analysis by C8 high-performance liquid chromatography equipped with parallel fluorescent and mass detectors in selective ion monitoring mode (20 mM heptafluorobutyric acid/methanol 70:30 isocratic at 1 mL/min). Nine different cross-links were measured, and the cross-link density was determined relative to collagen content (determined colorimetrically). The Tm was determined by differential scanning calorimetry. RESULTS Cross-links detected were dihydroxylysinonorleucine (DHLNL), hydroxylysinonorleucine, lysinonorleucine (LNL), and histidinohydroxylysinonorleucine in both control and KC samples. Higher DHLNL levels were detected in KC, whereas the dominant cross-link, LNL, was decreased in KC samples. Decreased LNL levels were observed among KC ≤ 40 corneas. There was no difference in total cross-link density between KC samples and the controls. Pyridinolines, desmosines, and pentosidine were not detected. There was no notable correlation between cross-link levels with fibril instability as determined by Tm. CONCLUSIONS Lower levels of LNL in the KC cornea suggest that there might be a cross-linking defect either in fibrillar collagen or the microfibrillar elastic network composed of fibrillin.