96 PHYSIOLOGY 



tive of the three-carbon propionic acid. On reduction it gives a 

 body known as cysteine, which is a-amino-thiopropionic acid. 



CH 2 SH 

 CH.NH 2 



COOH 



Cystine itself is compounded of two cysteine molecules joined together 

 by their sulphur atoms and has the formula 



CH 2 S S CH 2 

 CH.NH 2 CH.NH 2 

 COOH COOH 



E. OTHER CONSTITUENTS OF THE PROTEIN MOLECULE 

 When we* add together the total amino- acids obtainable by the 

 acid disintegration of any given protein, a considerable proportion of 

 the original protein remains unaccounted for. This remainder must 

 have a greater content in hydrogen and oxygen than the amino- acids 

 enumerated above, and it has been suggested that among the missing 

 unascertained constituents of proteins may be oxyamino-acids, of 

 which serine would form one of the lowest members. The isolation of 

 such substances would present considerable interest, in that it would 

 supply the intermediate stages between the constituent groups of the 

 protein molecule and the carbohydrates, the first product of assimilation 

 by living organisms. Only one such intermediate body has so far 

 been isolated, namely, glucosamine. an amino- derivative of glucose. 

 It was first shown by Pavy that from the products of disintegration of 

 a protein such as egg-white it was possible to obtain a reducing sub- 

 stance 'and to isolate an osazone resembling in its characters those 

 derived from the sugars. Since then various observers have shown 

 that this reducing substance is most probably glucosamine : 



CH 2 OH 

 (CHOH) 3 

 CH.NH 2 



CHO 



Although this substance may be obtained from crystallised egg 

 albumin or crystallised serum albumin, authorities are not yet 



