Reversible modification of lysine in beta-lactoglobulin using citraconic anhydride. Effects on the sulfhydryl groups

Acylation of lysine in beta-lactoglobulin-B with citraconic anhydride resulted in a loss of free sulfhydryl groups. These were not regenerated under the conditions used to remove the modifying groups from lysine. Gel filtration and polyacrylamide gel electrophoresis of the citraconylated and decitraconylated beta-lactoglobulin showed the presence of high molecular weight components. Modification of sulfhydryl groups with N-ethylmaleimide prior to citraconylation prevented the formation of these high molecular weight components. The heterogeneity of the decitraconylated protein was attributed to a combination of intermolecular disulfide bonding of subunits caused by structural changes occurring during lysine modification and to alkylation of free sulfhydryl groups via the citraconyl double bond.