90 PHYSIOLOGY 



ALANINE is a-amino-propionic acid : 



CH 3 



CH.NH 2 



COOH 



It is optically active, the alanine derived from proteins being dextro- 

 rotatory. 



Closely allied to alanine is the amino-acid SERINE, which was first 

 obtained by the hydrolysis of silk and has since been found as a con- 

 stituent of a large number of proteins. Its formula is : 



CH 2 OH 



CH.NH 2 



COOH 



i.e. it is amino-oxypropionic acid. Its special interest lies in the fact 

 that it was one of the first of the amino-oxyacids to be isolated, and it 

 is possible in these acids that we must seek the intermediate stages 

 between carbohydrates and proteins. 



AMINO-VALERIANIC ACID has the formula 



CH 2 



CH.NH 2 



COOH 



*. 



It occurs only in small quantities in the protein molecule. 



LEUCINE, one of the oldest known members of the group of amino- 

 acids, is obtained in large quantities from the disintegration of nearly 

 all the animal proteins, of which in some cases it may form as much as 

 20 per cent. It has the formula 



CH 3 CH 3 



CH 



CH 2 



CH.NH 2 



COOH 



i.e. it is amino-isobutyl acetic acid. On evaporating a tryptic digest 



