PREVENTION OF AMINO-ACID RACEMIZATION DUR ING GUANIDINATION - A PREREQUISITE FOR MEASUREMENT OF PROTEIN DIGESTIBILITY BY HOMOARGININE LABELING

Homoarginine labeling (guanidination) is used to calculate true precec al protein digestibility. A particular worry is that guanidination of proteins at alkaline pH might cause formation of D-amino acids. If D-a mino acids show decreased protein digestibility in vivo, as seen in vi tro, then the homoarginine method would underestimate protein digestib ility. Therefore, the degree of protein racemization was measured duri ng guanidination of casein at pH values between pH 9 and 11 and temper atures between 4 degrees and 65 degrees C. Optimal conditions for the guanidination reaction were 4 degrees C and pH 10.5-11 or 22 degrees C and pH 10. A higher pH value at 22 degrees C or temperatures above 22 degrees C at each pH leads to the formation of appreciable amounts of D-amino acids.