The Maillard Reaction: Protein Modification by Ascorbic Acid

Age-related nuclear cataract is widely thought to affect all individuals of the human population, provided everyone lives long enough for lens opacities to develop. The accumulation of age-related chemical modifications in lens crystallins fits the definition of an aging process. Whether these are due to protein deamidation and racemization, truncation, deamination, oxidation, or glycation, most of these are by and large, with rare exceptions, not repaired and irreversible. This chapter focuses on chemical damage by the Maillard reaction in ascorbic acidrich tissues, such as lens and brain, i.e. glycation and cross-linking reactions primarily mediated by ascorbic acid. Ascorbic acid, like oxygen, glucose, and other small molecules is essential for life, but paradoxically deleterious by forming H₂O₂, hydroxyl radicals, photosensitizers, singlet oxygen radicals, and advanced glycation end products.