636 METABOLISM 



To illustrate this reaction we have chosen aminopropionie acid or alanine, because 

 the substance formed by its oxidation and known as pyruvic acid is of very great im- 

 portance in intermediary metabolism. It serves as the common substance from which 

 proteins, carbohydrates or fats may be formed, and therefore as the intermediary sub- 

 stance through which one of them may pass on being transformed into another (page 

 698). The use of two arrows pointing in opposite directions in the above equation in- 

 dicates 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 + HONO=2H 2 O + N 2 . Upon this 

 reaction depends another extremely important quantitative metttod for measuring the 

 number of amino groups prese'nt 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 driven 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. 



Protein Synthesis. From the point of view of protein chemistry, the 

 most significant reaction of the amino acids is their ability to link to- 

 gether to form compounds called peptides. This linking occurs between 

 the amino group of one amino acid and the carboxyl group of the next. 

 When alanine and glycocoll, with which we are familiar, are thus linked 

 together, the reaction takes place according to the equation: 



H CH 3 H 



CH, | | 



| /IH + HO; OC-C-NH 2 = HOOC-C-NH-CO-C-NH 2 



HOOC-C-N I ! : I 



\H H H H 



(alanine) (glycocoll) (alanyl - glycocoll) 



In this manner, then, a so-called dipeptide is formed, in which it will 

 be observed there still remain free carboxyl and amino groups, thus per- 

 mitting the linking on of other amino-acid groups to form tripeptides or 

 tetrapeptides or other polypeptides. Indeed, this process of condensa- 

 tion may go on practically indefinitely, a polypeptide containing eighteen 

 amino-acid groups namely, three leucine and fifteen glycocoll groups hav- 



