Determination of the amino-acid nitrogen in the urine

Abstract

Since the abandonment of the Pfaundler method for determination of the amino-acid nitrogen of the urine, the only direct methods proposed which have met with any favor, are the gasometric method of D. D. Van Slyke and the titration method of Henriques and Sörensen. In our hands the Van Slyke method as originally described 1 has not proved entirely satisfactory, (1) because of the difficulty of removing all the ammonia after conversion of urea, and (2) because poly-peptids and other condensation products of amino-acids, e. g., hippuric acid, are estimated as well as free amino-acid nitrogen. The method, therefore, probably gives results which are too high.

The Henriques and Sörensen method as improved by the authors themselves 2 likewise presents some difficulties. For example, as objected by de Jager and since admitted by Henriques and Sörensen, in the presence of large quantities of ammonia the total titration is less than the sum of the ammonia N and amino-acid N done separately. It is therefore necessary first to remove the ammonia. Henriques and Sörensen recommend for this the method of Krieger and Reich slightly modified. This is essentially the method well known in this country by Shaffer's name. Since the method is based on distillation under diminished pressure, it is not adapted to rapid determinations in a series of urines simultaneously. Besides, as used by Henriques and Sörensen, we have not been able to obtain as high ammonia figures as by the Folin method. Use of the latter method for removal of ammonia, while perfectly satisfactory for small samples of urine (10 or 20 c.c.) is not satisfactory for a sample large enough to give a titration for amino-acid nitrogen (40-50 c.c.).