Methyltion of the Lysine Residues of Monellin 1

Summary

The €-amino groups of the lysyl residues of monellin 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. 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 chromatography, gel filtration, fluorescence spectroscopy, polyacryl-amide gel electrophoresis, and amino acid analysis. Although sweetness was maintained after limited methylation, some change in conformation of the protein did occur.