Methyltion of the Lysine Residues of Monellin

Abstract

The .epsilon.-amino groups of the lysyl residues of monellin [from Dioscoreophyllum cumminsii] were reductively methylated with formaldehyde and sodium borohydride; 20-40% of the lysines could be methylated with essentially complete retention of the sweetness of the protein [tested in human subjects]. The methylated protein yielded dimethyllysine and monomethyllysine upon acid hydrolysis. 3H-Labeled methylated monellin was also prepared with [3H]formaldehyde as the methyl donor; this derivative could be useful in binding studies to taste receptors. The methylated monellin was studied by ion-exchange chromatogrpahy, gel filtration, fluorescence spectroscopy, polyacrylamide gel electrophoresis and amino acid analysis. Although sweetness was maintained after limited methylation, some change in conformation of the protein did occur.