Prevention of amino acid racemization during guanidination--a prerequisite for measurement of protein digestibility by homoarginine labeling.

Homoarginine labeling (guanidination) is used to calculate true prececal protein digestibility. A particular worry is that guanidination of proteins at alkaline pH might cause formation of D-amino acids. If D-amino acids show decreased protein digestibility in vivo, as seen in vitro, then the homoarginine method would underestimate protein digestibility. Therefore, the degree of protein racemization was measured during guanidination of casein at pH values between pH 9 and 11 and temperatures 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.