The Role of Nonenzymatic Glycation and Carbonyls in Collagen Cross-Linking for the Treatment of Keratoconus

Abstract

Data presented here suggest that CXL is carbonyl dependent and involves the formation of AGE cross-links. Six possible cross-linking mechanisms are discussed.

Photochemical Kinetics of Corneal Cross-Linking with Riboflavin

“…4,5 The underlying molecular photochemical mechanisms of corneal cross-linking with Rf have been investigated in several studies, [4][5][6] but the role of oxygen in this process, especially in modulating Type-I and Type-II photochemical kinetic mechanisms, is not well understood and remains speculative. Analysis of chemical kinetics, in addition to observed tissue oxygen concentrations, suggests that UV-Aexcited molecules of Rf act as the predominant agent in corneal cross-linking.…”

Determination of the Excimer Laser Ablation Rate in Previously Cross-linked Corneas

“…23,33 Another explanation might be the creation of additional crosslinks between proteoglycans and collagen in the anterior stroma during CXL. 34,35 To break these additional bonds, additional energy is needed.…”

Resistance of Corneal RFUVA–Cross-Linked Collagens and Small Leucine-Rich Proteoglycans to Degradation by Matrix Metalloproteinases

“…Compared to native collagens, RFUVA causes distinct decreases in molar percentages of Met, Tyr, His, Hlys, and Lys in cross-linked collagens, suggesting that these amino acids are modified by RFUVA, just as they are as free amino acids in solution, with the latter being very susceptible to photodegradation in the presence of RFUVA. 49 During corneal RFUVA cross-linking treatment, RF is a key component as it is excited by UVA into its triplet state, generating singlet oxygen that can react further with various molecules, 41 inducing chemical bonds to form that covalently link amino groups of collagen fibrils 50 with those of other proteins in the corneal stroma extracellular matrix. 42 These reactions may involve tyrosine residues, 51 advanced glycation end products, 50,52 or changes in secondary or tertiary structure.…”

“…49 During corneal RFUVA cross-linking treatment, RF is a key component as it is excited by UVA into its triplet state, generating singlet oxygen that can react further with various molecules, 41 inducing chemical bonds to form that covalently link amino groups of collagen fibrils 50 with those of other proteins in the corneal stroma extracellular matrix. 42 These reactions may involve tyrosine residues, 51 advanced glycation end products, 50,52 or changes in secondary or tertiary structure. 53,54 Tyrosine residues in the terminal, telopeptide domains of collagen alpha chains can form pi-pi complexes, 55 leading to dityrosine cross-links.…”

“…42 In addition, irradiation of RFUVA can induce lysine to form glycosyl-lysine cross-links (Maillard reaction) and so-called advanced glycation end products (AGEs), 40,52 suggesting that corneal cross-linking is carbonyl-dependent and involves the formation of AGE cross-links. 41,50 RFUVA thus causes apparent disappearance of tyrosine, histidine, methionine, and lysine/ Mouse myeloma cell line NS0-derived Ala20-Phe298, with a C-terminal 6-His tag R&D Systems Decorin 38 Spodoptera frugiperda Sf21(baculovirus)-derived Gly17-Lys359, with a C-terminal 6-His tag R&D Systems Biglycan 37.5 Mouse myeloma cell line NS0-derived Asp38-Lys368, with a C-terminal 6-His tag R&D Systems hydroxylysine while concomitantly converting their structures while still in peptide linkage to participate in covalent crosslinks (which are not detected during normal amino acid analyses) ( Table 1). Preventing protein degradation by MMPs is an essential goal in the clinical efficacy of the cross-linking treatment for keratoconus because already demonstrated increased MMPs activity is one of the most important factors causing corneal thinning.…”