Simplified Colorimetry of α-Amino Nitrogen in Plasma, Serum, or Urine

We describe a simple procedure for estimating amino acids, based on the formation of charge-transfer complexes with p-benzoquinone. The reaction is insensitive to ammonia, urea, purines, creatinine, and other nitrogenous compounds present in serum or urine, except amines, peptides, and proteins. The reagent is cheap, stable, and does not require prior treatment of urine before determination. Specifity, precision, and normal values of this method are reported and compared with those obtained by a colorimetric ninhydrin technique.