The free amino acids of human blood plasma.

The presence of amino acid nitrogen in the non-protein nitrogen fraction of blood plasma was first established in 1912 by Van Slyke and Meyer (1). Chemical, microbiological, and chromatographic techniques have subsequently been employed to determine many of the individual amino acids which contribute to the total or-amino nitrogen as currently measured by the ninhydrin-CO2 method of Hamilton and Van Slyke (2). In the studies reported in this communication, chromatographic methods in which columns of Dowex 50-X4 are employed (3) have been applied to this problem (Fig. 1). It has been possible to identify with a high degree of probability twenty-eight ninhydrin-positive substances in protein-free plasma and to determine most of them quantitatively. The results, taken in conjunction with the values for glutamine obtained by Archibald (4) and Hamilton (5), account for nearly 100 per cent of the total a-amino N of postabsorptive plasma.