Heat-Induced Formation of Intermolecular Disulfide Linkages between Thaumatin Molecules That Do Not Contain Cysteine Residues

Abstract

Thaumatin, a sweet protein that contains no cysteine residues and eight intramolecular disulfide bonds, aggregates upon heating at pH 7.0 above 70 degrees C, and its sweetness thereby disappears. The aggregate can be solubilized by heating in the presence of both thiol reducing reagent and SDS. This molecular aggregation depended on the protein concentration during heating and was suppressed by the addition of N-ethylmaleimide or iodoacetamide, indicating a thiol-catalyzed disulfide interchange reaction between heat-denatured molecules. An amino acid analysis of the aggregates suggested that the cysteine and lysine residues were reduced, and the formation of a cysteine residue and a lysinoalanine residue was confirmed. The reduction and formation of these residues stoichiometrically satisfied the beta-elimination of a cystine residue. The disulfide interchange reaction was catalyzed by cysteine; that is, a free sulfhydryl residue was formed via beta-elimination of a disulfide bond. Intermolecular disulfide bonds were probably formed between thaumatin molecules upon heating at pH 7.0, which led to the aggregation of thaumatin molecules.