ELEMENTARY CHEMICAL PHYSIOLOGY 201 



hsematin and the histone globin. In certain of the lower forms of 

 life, molluscs for example, there is believed to exist a conjugated 

 protein hsemocyanin in which the pigment substance contains 

 copper in place of iron. 



Gluco-proteins. This class of protein has as a prosthetic group 

 some form of carbohydrate, very frequently glucosamine. Gluco- 

 samine is a sugar which contains an amino group in its molecule. 



They are often separated into three groups, mucins, pseudomucins and 

 nrucoids. The mucins are soluble in water and dilute alkali, but are pre- 

 cipitated with excess acetic acid. They give viscid solutions. The pseudo- 

 mucins are equally soluble but are not precipitated by acetic acid. The 

 mucoids are soluble in water and in dilute acids and alkalis. Their solutions 

 are not viscid. Mucins are found in the saliva, sputum and synovial fluid. 

 Pseudomucin is present in fluid from ovarian cysts. Mucoids are found in 

 the cornea, the lens, in cartilage, tendon and bones. Mucoid-like substances 

 can be also obtained from serum. The so-called nubecolae of urine is also 

 a mucoid. The different gluco -proteins yield varying amounts of reducing 

 substance on hydrolysis. 



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

 add to it a few drops of 1 per cent, 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. If enough mucin can be collected the presence 

 of carbohydrate can be shown by boiling with a mineral acid, then 

 doing an ordinary reduction test. 



EXPERIMENT. Mucin can be prepared from connective tissue where it is 

 very abundant, by extracting the latter with a weak alkali (lime water). 

 The mucin is precipitated by a weak acid, and the resulting precipitate then 

 boiled for about ten minutes with hydrochloric acid (1 part concentrated 

 acid -f- 3 parts water). The resulting solution is cooled and neutralised, 

 and examined for protein and carbohydrate. 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 carbohydrate 

 moiety. 



Nucleo-proteins. These are derived for the most part from the 

 nuclein of the cells and are, therefore, widely distributed. The 

 prosthetic group combined with protein is nucleic acid. They may, 

 indeed, be regarded as protein salts of nucleic acid. The two com- 

 ponents are readily separated, especially by the action of alkali. 

 Two varieties a and /? nucleo-proteins are recognised. It is question- 

 able if the /? form is actually a constituent of the cell nucleus. 



Nucleo-proteins on digestion with pepsin are not completely broken down. 

 A certain amount of the protein is left in combination with the nucleic acid ; 

 this material is called nuclein, i.e. nuclein is the residue left after peptic 

 digestion of a nucleo-protein. Nuclein in its turn may be split into protein 

 and nucleic acid. Nucleic acid, on complete hydrolysis, yields various sub- 

 stances, a carbohydrate, phosphoric acid, two purine and two pyrimidine bases. 



