A method for the determination of guanidoacetic acid and arginine in biological fluids.

The determination of guanidoacetic acid in biological fluids has assumed greater importance since the demonstration that this compound is an intermediate in the biosynthesis of creatine (l-3). In the reported studies of creatine metabolism (4-6), guanidoacetie acid has been determined exclusively with the Sakaguchi reaction, which is given by monosubstituted guanidines. It has been shown recently that arginine and guanidoacetic acid are the only two monosubstituted guanidines present in significant quantities in man (7) and probably in higher animals. Hence the reliable determination of guanidoacetic acid or arginine by the Sakaguchi reaction requires the complete removal of the other reacting substance. In our investigation of creatine metabolism in tumor-bearing rats, which will be published elsewhere, we became aware of the inadequacy and inconvenience of the available methods for the estimation of guanidoacetic acid. An effort was therefore made to develop a reliable and quantitative assay procedure for the two Sakaguchi-positive compounds in biological materials. The results of such a study are the subject of this communication. The present method is based on the observation that from a mixture of guanidoacetic acid and arginine the weakly cationic resin, Amberlite CG-50, adsorbs the arginine quantitatively. The guanidoacetic acid thus freed from arginine is separated from certain interfering substances by its adsorption on the strongly cationic resin, Nalcite HCR-8. Elution of the two resins gives solutions of arginine and of guanidoacetic acid which can be determined separately and directly by the Sakaguchi reaction.