Derived from ribose, a pentose, pentosidine forms fluorescent cross-links between the arginine and lysine residues in collagen. It is formed in a reaction of the amino acids with the Maillard reaction products of ribose.
Although it is present only in trace concentrations among tissue proteins, it is useful for assessing cumulative damage to 
proteins&mdash;advanced glycation endproducts&mdash;by non-enzymatic browning reactions with carbohydrates.