28 CHEMICAL CONSTITUENTS OF BODY AND FOOD. 



FIG. 7. Leucine crystals. After Kiihne. 



usual method which a chemist follows in attempting to unravel the 

 constitution of any substance, is first to discover the way in which it decom- 

 poses (analysis), and then to build up the original material once more from 

 the simple compounds so obtained (synthesis). In the case of the proteids 

 there have been many observations on the analytical side, but synthesis 

 has not yet been successful. We will first consider the results of 

 analysis, next the attempts at synthesis, and finally state some of the 

 theories founded on these observations. 



The decomposition products of proteids. The experiments which 



have been performed fall into two 

 main groups: the first, designed with 

 a view to determine the series of 

 changes a proteid undergoes in its 

 passage through the body ; the second, 

 with the object of investigating the 

 chemical substances obtained as 

 cleavage products by artificial means 

 in the laboratory. In the first group 

 the progress which has been made 

 is slight, great and obvious difficulties 

 being encountered at nearly every 

 step ; the end products, carbonic 

 anhydride, water, urea, uric acid, 

 ammonia, etc., are known, but the 

 intermediate substances, resulting 

 from metabolic changes within the 

 cells and tissues, are still in the 

 region of conjecture. 



In the alimentary canal itself there are, however, changes which are 



within the grasp of the investi- 

 gator, and the proteoses, al- 

 buminates, and peptones there 

 formed will be treated under 

 the head of " Digestion." Here, 

 too, under the prolonged action 

 of the pancreatic juice, simpler 

 nitrogenous substances, such 

 as leucine, tyrosine, aspartic 

 acid, and ammonia, are formed 

 in small quantities. Leucine 

 (C H 13 N 2 0) is empirically 

 amido-caproic acid, but of the 

 numerous possible isomerides 

 which could be included under 

 that name, leucine has 

 been shown to be a-amido- 

 isobutylacetic acid, (CH 3 ) 9 CH. 

 CH 2 CH 2 (NH 2 ) COOH. ^The 

 leucine obtained on pancreatic 

 digestion is dextrorotatory. 

 Levorotatory and optically in- 



FIG, 8. -Tyrosine crystals. After Frey. active varieties of leucine exist, 



and some of them have been 



