M. Devrese et al., PREVENTION OF AMINO-ACID RACEMIZATION DUR ING GUANIDINATION - A PREREQUISITE FOR MEASUREMENT OF PROTEIN DIGESTIBILITY BY HOMOARGININE LABELING, Zeitschrift fur Ernahrungswissenschaft, 33(4), 1994, pp. 310-312
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.