600 METABOLISM 



group is very firmly attached; it remains intact in acids and alkalies and 

 is removable only by a process of oxidation. This can be accomplished by 

 treating the amino acid with such reagents as hydrogen peroxide or per- 

 manganate, when the amino group is displaced and a so-called ketonic acid 

 formed. The reaction will be evident from the accompanying equation: 



CH 3 CH S 



O + NH,-C-COOH ? O = 0-COOH+ NH 3 



H 



( alanine ) ( pyruvic acid ) 



To illustrate this reaction we have chosen aminopropionic acid or ala- 

 nine, because the substance formed by its oxidation and known as pyruvic 

 acid is of very great importance in intermediary metabolism. It serves 

 as the common substance from which proteins, carbohydrates or fats may 

 be formed, and therefore as the intermediary substance through which 

 one of them may pass on being transformed into another (page 666). The 

 use of two arrows pointing in opposite directions in the above equation 

 indicates that the reaction may proceed readily in either direction. 



The ammonia set free from amino acids may be oxidized to free nitrogen 

 by using nitrous acid according to the general equation: NH 3 -f-HONO= 

 2H 2 0+N 2 . Upon this reaction depends another extremely important 

 quantitative method for measuring the number of amino groups present in 

 protein (Van Slyke). To make the estimation, nitrous acid is allowed 

 to act on the amino acids, and the volume of nitrogen gas set free by the 

 reaction is measured, the principle being similar to that used for the de- 

 termination of urea by the hypobromite method. 



The apparatus employed for decomposing the substance and collecting and measuring 

 the evolved nitrogen consists essentially of a mixing bulb, connected below through stop- 

 cocks with two small burettes, one containing a solution of sodium nitrite and glacial 

 acetic acid, and the other a solution of the substance to be investigated. The upper end 

 of the mixing bulb is connected through a three-way cock with a graduated gas burette 

 and with another bulb containing potassium permanganate solution. By allowing some 

 nitrite and acid solution to run into it and shaking, the mixing bulb is first of all filled 

 to a certain mark with nitrous oxide gas. A measured quantity of the amino solution 

 is then allowed to mix with the nitrite; the apparatus is shaken for five minutes at 15 

 to 20 C., and the evolved nitrogen and nitric oxide are dviven over into the permanganate, 

 which absorbs the nitric oxide, leaving the nitrogen, which is then measured in the burette. 



The apparatus has now been so perfected that numerous analyses may 

 be made with it in a very short time and with a degree of accuracy that 

 is scarcely surpassed in any other biochemical estimation. 



From the point of view of protein chemistry, the most significant reac- 

 tion of the amino acids is their ability to link together to form compounds 



