308 PRACTICAL PHYSIOLOGY 



decomposition product. They also yield the hexone bases, since 

 protamin forms part of their molecule (see table, p. 299). 



EXPERIMENT IX. With pieces of finger-nail show that keratin gives 

 xanthoproteic, Millon's, and lead sulphide test. 



THE COMPOUND PROTEINS. 



In this group we have proteins to which groups other than protein 

 are united to form a complex molecule. The chief groups are : 

 (i) The chromo-proteins. 

 (ii) The gluco-proteins. 

 (Hi) The nucleo-proteins. 



The Chromo-proteins. As the name signifies these are " coloured " 

 proteins, and its members are pigments, of which the chief is haemo- 

 globin. (See chapter on Blood.) 



The Gluco-proteins are compounds of protein with a large carbo- 

 hydrate group. Many proteins not contained in this class, such as egg 

 albumin and nucleo-proteins, contain carbohydrate, but not in such 

 large amounts. The chief members of the class are the mucins. 



EXPERIMENT X. Collect some saliva in a test tube, note its 

 viscidity; add to it a few drops of 1 % acetic acid; a stringy precipitate 

 of mucin results. It is insoluble in excess of acetic acid. Filter. To 

 residue add a few drops of weak sodium carbonate solution, when the 

 precipitate will dissolve. Test this with protein colour tests, including 

 Molisch. 



EXPERIMENT XI. Mucin has been prepared from connective tissue 

 where it is very abundant, by extracting the latter with a weak alkali 

 (lime water). The mucin has been precipitated by a weak acid. The 

 resulting precipitate has then been boiled for about ten minutes with 

 hydrochloric acid (1 part concentrated acid + 3 parts water), and the 

 resulting solution cooled and neutralised. Examine portions of the 

 resulting solutions. Divide the solution into portions, a and b. 



To (a) apply the Biuret reaction a violet or pink colour is produced, 

 showing the presence of the protein moiety. 



To (b) add a drop of copper sulphate solution, and, if necessary, some 

 caustic alkali till a blue solution is obtained. Now boil, when reduction 

 to cuprous oxide will occur, demonstrating the presence of the carbo- 

 hydrate moiety. 



Besides forming the ground substance of the connective tissues, 

 mucin is also secreted on to the surface of all mucous membranes, 

 where it acts as a lubricant. 



Besides the mucins, this group also contains the mucinoids, bodies distinguished 

 from mucin in not being viscous in nature and not being so easily precipitated by 



