THE PROTEINS 87 



Cystine itself is compounded of two cysteine molecules joined togethei 

 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 

 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 con- 

 stituents 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 substance 

 and to isolate an osazone resembling in its characters those derived from the 

 sugars. Since then various observers have shown that this reducing sub- 

 stance 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 convinced that it forms 

 an integral part of these proteins. Both egg-white and serum contain 

 proteins belonging to the class of mucins, ovomucoid and serum mucoid, each 

 of which yields on acid hydrolysis from 16 to 30 per cent, glucosamine. 

 various observers have obtained results varying from 1 to 16 per cent, 

 samine for crystallised egg albumin, it seems possible that in ever) 

 the crystals carried down with them some of the carbohydrate-rich 

 and that the varying results were due to the different amounts 

 present in the crystals. By our ordinary methods it is impossible to prepai 

 a specimen of either egg albumin or serum albumin which is entn 

 this amino-derivative of carbohydrate. 



Connected with this group of proteins may be reckoned i 



