Effects of denaturants on the sweet-tasting protein monellin.

Effects of the denaturants urea and guanidine-HCl on the sweet-tasting protein monellin have been studied. The pH at which monellin is initially treated with denaturant is an important factor in retention of sweetness, but the pH maintained during subsequent removal of denaturant by dialysis has no effect on activity. Recovery of sweetness of denaturant-treated monellin is favored when denaturation occurs at acid pH. Monellin treated with either 6 M guanidine-HCl or 8 M urea at acid pH retains all of its sweetness following removal of denaturant, but urea treatment at neutral pH leads to some irreversible loss of sweetness. Monellin precipitates from solution under some conditions during removal of denaturant by dialysis, and the precipitated protein is no longer sweet. Precipitation is least under acid conditions. Aggregated protein was demonstrated by gel filtration chromatography. The single sulfhydryl group of monellin was not demonstrable in the precipitated protein, having apparently become oxidized during denaturation and formation of the aggregated protein. The data support the hypothesis that the tertiary structure is important in the ability of monellin to elicit a sweet sensation.