Background: Protein cross-linking and fluorescence are widely recognized markers of oxidative aging in human proteins. Oxidative protein aging is a combinatorial process in which diversity arises from the heterogeneity of the targets and is amplified by the nonselective nature of the reactants. The cross-links themselves defy analysis because they are generally embedded in a covalent matrix. Arthropods rely upon oxidative cross-linking in the hardening of the cuticle - a process known as sclerotization. Among arthropods, scorpions are noteworthy in that the process of sclerotization is accompanied by the buildup of strong visible fluorescence. To date, the nature of the fluorescent species has remained a mystery.
Results: We have identified one of the soluble fluorescent components of the scorpions Centuroides vittatus and Pandinus imperator as beta-carboline - a tryptophan derivative that has previously been identified by hydrolysis and oxidation of lens protein. We have also shown that beta-carboline-3-carboxylic acid is released from both scorpion exuvia (the shed cuticle) and human cataracts upon hydrolysis, suggesting that the protein-bound beta-carboline and free beta-carboline have common chemical origins.
Conclusions: Cataractogenesis and cuticular sclerotization are disparate oxidative processes - the former is collateral and the latter is constitutive. The common formation of beta-carbolines shows that similar patterns of reactivity are operative. These fundamental mechanisms provide predictive insight into the consequences of human protein aging.