Modification of the amino and hydroxyl groups of lysozyme with carboxylic acid anhydrides: a comparative study

Abstract

Modification of hydroxyl groups by carboxylic acid anhydrides is an unwanted reaction when these reagents are used to modify protein amino groups. It is important to know the specificity of different anhydrides, and the stability of the modified hydroxyl groups. Lysozyme was treated with acetic, succinic, maleic, monomethylmaleic and dimethylmaleic anhydrides, and the extent of modification of the amino and hydroxyl groups was evaluated. Amino groups were found to be much more reactive than the hydroxyl groups. Of the reagents used, dimethylmaleic anhydride is the most specific for amino groups, with practically no modification of hydroxyamino acid residues. The modified hydroxyl groups are preferentially deacylated at alkaline pH. For acetic, succinic and maleic anhydrides, the half-lives of the modified serine (25–45 min) and threonine (14–30 h) residues were determined at pH 10.0 and 37°C. Under these conditions, the modified hydroxyl groups can be selectively deacylated without practically affecting the modified amino groups.