THE INDIVIDUAL GROUPS OF PROTEINS 4 6c> 



Saliva. 



Presence of Mucin. 



About 10-15 c.c. of saliva are collected in a beaker. Its re- 

 action is faintly alkaline, but may be neutral or slightly acid at first 

 from bacterial decomposition in the mouth. 



On adding acetic acid, the mucin is precipitated and is insoluble 

 in excess of acetic acid ; on stirring the liquid, the mucin collects to- 

 gether on the rod and may thus be removed. The remaining liquid 

 contains only traces of protein as shown by e.g. Millon's reagent. 



The precipitate on solution in dilute alkali carbonate will be 

 found to give most of the general reactions for proteins. It is also- 

 soluble in 'I per cent, hydrochloric acid. 



On boiling the solution with dilute hydrochloric acid for about 

 5 minutes, neutralising and testing with Fehling's solution, a small 

 quantity of cuprous oxide will settle out on standing. 



Presence of Thiocyanic Acid. 



Saliva generally contains small quantities of thiocyanic acid". 

 This is shown by treating it with a drop of ferric chloride solution. 

 A red colour is produced. The red colour is discharged by a drop of 

 mercuric chloride solution. (Smoker's saliva generally contains more 

 thiocyanic acid than that of non-smokers.) 



Presence of Diastase. 



This has been shown on p. 399. 



Mucin of the Mucous Membrane of the Respiratory Passages. 



This mucin is prepared from sputum. The sputum, free from cells and 

 food particles, of a clear gelatinous appearance, is put into alcohol and 

 shaken. It becomes fibrous and is separated from particles of protein, etc., by 

 straining through cloth. It is shaken with (i) 0*5 per cent. HCl ; (2) o'i 

 per cent. Na 2 CO 3 ; (3) 0-5 per cent. HCl to remove protein and nucleo- 

 protein. It is dissolved in dilute caustic soda solution ; the solution is filtered 

 and centrifuged, acidified with acetic acid and precipitated with alcohol. 



The mucin is a fibrous solid. It gives an opalescent solution with water 

 which becomes clear on adding alkali. It is slightly soluble in 'i per 

 cent, hydrochloric acid. Its solution is precipitated by saturation with am- 

 monium sulphate. 30-35 per cent, of reducing carbohydrate is formed by 

 boiling it for 3 hours with 3 per cent, hydrochloric acid. 



Mucin of Serosa. 



This mucin is present in synovial fluid and sometimes in ascitic fluid to 

 which it gives the slimy consistency. The liquid, 'diluted if necessary with water, 

 is precipitated by adding acetic acid so that the content of acid is -i per cent. 

 The precipitate is dissolved in water containing alkali and reprecipitated with 

 acetic acid. The process is repeated and the precipitate washed with 

 alcohol and ether. 



The mucin is soluble in water containing alkali ; the solution does not 

 coagulate on heating and has a slimy appearance. The substance is not 

 soluble in excess of acetic acid, but is soluble in very dilute hydrochloric 

 acid. It is precipitated by half-saturation with ammonium sulphate. A re- 

 ducing carbohydrate is formed on hydrolysis. 



