80 PHYSIOLOGY 



stances act as feeble conductors of the electric current, i.e. as electrolytes. 

 The charge carried by an amino-acid and its ionisation depends upon the 

 conditions in which it is placed. Since it may act either as the cation or the 

 anion, it is spoken of as an ampJioteric electrolyte. 



One reaction of the amino-acids is of special interest in connection with 

 the respiratory functions of the body, namely, the formation of carbamino- 

 acids. If a stream of carbon dioxide be passed into a mixture of an amino- 

 acid, e.g. glycine, with lime, the carbon dioxide is taken up. On filtering 

 the mixture a clear liquid passes through which gradually in course of time 

 deposits a precipitate of calcium carbonate. The filtrate first obtained 

 contains a compound of calcium, calcium glycine carbonate. The formula is 

 as follows : 



CH 2 .NH 



\o.co 



COO Ca 



METHODS OF SEPARATING AMINO-ACIDS. By the hydrolysis of 

 protein by means of acid or of trypsin, we obtain a complex mixture of 

 amino-acids. From this mixture certain amino-acids are separated with 

 ease. Thus, tyrosine, which is extremely insoluble, crystallises out on 

 concentrating the fluid, and further concentration leads to the separation of 

 leucine. The other acids, which keep each other mutually in solution, are 

 however very difficult to isolate. We owe to Fischer the first general 

 method for their separation. We may take one experiment as an example. 



Five hundred grammes of casein are heated for some hours under a reflux con- 

 denser with 1| litres of strong hydrochloric acid. The liquid is then saturated with 

 gaseous hydrochloric acid and allowed to stand for three days in the ice-chest. Crystals 

 of hydrochlorate of glutamic acid separate out. The nitrate from these crystals is 

 evaporated at 40 C. under diminished pressure to a syrupy consistence, and is then 

 dissolved in 1| litres of absolute alcohol. Hydrochloric acid is then passed into the 

 solution to complete saturation, the mixture being warmed for a short time on the 

 water bath, and the mixture is once more evaporated to a syrupy consistence. By this 

 treatment all the aminoracids have been converted into the hydrochlorates of their 

 esters, e.g. : 



CH 2 NH 2 HC1 C 2 H 4 NH 2 HC1 



I ' I 



COOC 2 H 5 COOC 2 H 5 &c. 



From the hydrochlorates the esters are set free by the addition of potassium carbonate, 

 the mixture being cooled in a freezing mixture. By this means the esters of aspartic 

 and glutamic acids are separated and are extracted by shaking with ether. The 

 remaining esters are then liberated by the addition of 33 per cent, caustic soda together 

 with potassium carbonate, and are again extracted by ether. The combined ethereal 

 solutions are dried by standing over fused sulphate of soda and then evaporated, when 

 a residue containing the free esters is obtained. These esters are then separated by 

 fractional distillation under a very low pressure obtained by means of the Fleuss 

 pump, the second receiver of the apparatus being cooled in liquid air. The various 

 fractions of aminoesters obtained in this way are hydrolysed the lower fractions by 

 boiling for some hours with water, the higher fractions by boiling with baryta. The 



