Modification of proteins by 3-hydroxyanthranilic acid: the role of lysine residues

The mechanism of reaction of proteins with 3-hydroxyanthranilic acid (3OHA) under oxidizing conditions has been examined. A range of proteins were found to tan when exposed to oxidized 3OHA. One exception was lysozyme which tanned only after being denatured by reduction and carboxymethylation. Chemical modification experiments using bovine serum albumin (BSA) suggested that lysine was the primary site of reaction in 3OHA-mediated protein tanning. This reactivity of 3OHA toward lysine was confirmed by autoxidizing 3OHA in the presence of amino acid homopolymers. The rate of modification of both BSA and polylysine was pH dependent. At neutral pH, a component of the coloration of the protein was found to be due to the formation of a lysyl-p-quinone adduct. Other products appear to arise through addition to the 3OHA quinone imine. Poly-(Glu,Lys) was tanned by 3OHA at a greatly reduced rate, suggesting that electrostatic interactions may influence the reaction with lysine residues and may provide an explanation for the lack of tanning of lysozyme. Despite the reaction between 3OHA and lysine, amino acid analysis revealed little quantitative change in the lysine content of proteins even after exposure to 3OHA for a period of 24 h. These results support the proposal that reaction with lysine residues is the major route of protein tanning by 3-hydroxyanthranilic acid.