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Elimination of Lactose Interference in the Determination of Available Lysine Using Fluorine-19 Nuclear Magnetic Resonance Spectroscopy¹

Eastern Regional Research Center, Philadelphia, PA 19118

ABSTRACT

When the ¹⁹F nuclear magnetic resonance method for determining free -amino groups of lysine residues in intact proteins is used on mixtures containing lactose, the amount of available lysine determined decreases with increasing lactose concentration. The reaction of lactose with lysine residues appears to be much more strongly promoted in dimethyl-sulfoxide than in aqueous environments as shown by comparable experiments in water. If the method is modified to remove lactose by dialysis prior to the reaction with S-ethyl trifluorothioacetate, the number of available -amino groups determined equals that of the pure proteins. The modified method as tested with nonfat dry milk gives excellent agreement with results obtained from a Kakade-Liener method.

¹ Reference to brand or firm name does not constitute endorsement by the US Department of Agriculture over others of a similar nature not mentioned.