86 PHYSIOLOGY 



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 

 substance is most probably glucosamine : 



CH 2 OH 

 I 

 (CHOH), 



I 

 CH.NH, 



I 

 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. Since 

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

 samine for crystallised egg albumin, it seems possible that in every case 

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

 and that the varying results were due to the different amounts of mucoid 

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

 a specimen of either egg albumin or serum albumin which is entirely free from 

 this ammo-derivative of carbohydrate. 



Connected with this group of proteins may be reckoned the diamino- 

 trioxydodecoic acid already mentioned as occurring among the disintegration 

 products of proteins. 



THE BUILDING UP OF THE PROTEIN MOLECULE 



By simple hydrolysis the protein molecule may be broken down into a 

 large number of amino-acids. Analyses of various proteins show that these 

 amino-acids are present in different proportions in the individual proteins, 

 so that in many cases a large number of identical amino-acid groups must 

 be present in the protein molecule with smaller numbers of other groups. 

 In endeavouring to form an idea of the manner in which the amino-acids can 

 be linked together into one gigantic molecule, Hofmeister first put forward 

 the idea that the linkage follows the general formula : 



CH 2 NH CO 

 or NH CH 9 CO NH- 



