Determination of the arginine content of proteins by the Sakaguchi procedure

The arginine contents of a variety of protein samples determined by the quantitative Sakaguchi method were in good agreement with the values obtained by quantitative ion-exchange chromatography, after acidic hydrolysis. One protein, α-chymotrypsin, while possessing a linear relationship between the colour obtained in the Sakaguchi test and the protein concentration, gave colour yields which corresponded to the apparent presence of an additional Sakaguchi-positive residue. The appearance of this phenomenon was found to be associated with the process of zymogen activation. Subsequently, it was found that two other serine esterases, trypsin and subtilisin, also exhibited this behaviour. Possible reasons for this have been discussed.