beta-Lactoglobulin A with N-ethylmaleimide-modified sulfhydryl residue, polymerized through intermolecular disulfide bridge on heating in the presence of dithiothreitol.

Roles of sulfhydryl groups on thermal aggregation of beta-lactoglobulin A (betaLG A) at pH 7.5 were investigated. It is known that betaLG A modified at Cys(121) with N-ethylmaleimide (NEM-betaLG A) does not form an aggregate by heating and that dithiothreitol (DTT) reduces cystine residues and induces the intermolecular sulfhydryl/disulfide interchange reaction and/or oxidation. NEM-betaLG A was heated in the presence of DTT. The molecules were linked together with an intermolecular disulfide bridge, and the polymer formed increased with increase in DTT concentration. The largest portion of polymer was formed when DTT was added at around the same molar concentration as that of NEM-betaLG A. Then, polymer formation decreased with further increase in DTT concentration. The results suggest that sulfhydryl/disulfide residues other than Cys(121), generated from cysteine residues, can induce intermolecular sulfhydryl/disulfide interchange reactions to polymer and that thiol compounds, for example, added DTT, are capable of starting such reactions.