The Alkaline Decomposition of Serine

A method for the quantitative estimation of arginine is included by Van Slyke (1911-12) in his procedure for the determination of the nitrogen distribution numbers of proteins. The amino acids present in the protein hydrolysate are divided into two groups by means of phosphotungstic acid. The portion precipitated by t,his reagent is referred to as the basic or diamino fraction and the portion not so precipitated is called the monoamino fraction. Potassium hydroxide, sufficient in amount to give a concentration of approximately 33 per cent, is added to an aliquot part of a solution of the amino acids of the basic fraction and the mixture is boiled for 6 hours under a reflux condenser. The arginine present is thereby decomposed into ornithine, ammonia, and carbon dioxide, and the cystine present is partially decomposed with the evolution of ammonia. At the end of the allotted time, the apparatus is disconnected, water is added, and the ammonia is removed by distillation. The amount of arginine in the original hydrolysate is calculated from the quantity of ammonia evolved, corrections being made for the solubility of argmine phosphotungstate and for the partial decomposition of the cystine. Plimmer and Rosedale (1925) suggest that for the determination of arginine the division of the amino acids into two groups be omitted. They report that the monoamino fractions from various proteins evolve considerable quantities of ammonia when boiled with alkali. In explanation of this fact, they present the view that the solubility of arginine phosphotungstate is significantly greater than is allowed for by Van Slyke. They conclude: “The arginine value of a protein is the sum of the figures obtained from the di-amino and mono-amino fractions, or the figure obtained