PROTEINS 107 



is that loosely combined with carbon and hydrogen. An example of 

 this combination is shown in cystine, 



CH 2 -S S-CH 2 



I I 



CH-NH 2 CH-NH 2 



COOH COOH 



Sulphur in this form is variously termed unoxidized, loosely combined,, 

 mercaptan, and lead-blackening sulphur. The second form is combined 

 in a more stable manner with carbon and oxygen and is known as 

 oxidized or acid sulphur. The protamines are the only class of sulphur- 

 free proteins. 



TESTS FOR SULPHUR, 



1. Tests for Unoxidized Sulphur. (a) To equal volumes of KOH and egg 

 albumin solutions in a test-tube add 1-2 drops of lead acetate solution and boil the 

 mixture. Unoxidized sulphur is indicated by a darkening of the solution, the 

 color deepening into a black if sufficient sulphur is present. Add hydrochloric 

 acid and note the characteristic odor evolved from the solution. Write the reac- 

 tions for this test, (b) Place equal volumes of KOH and egg albumin solutions 

 in a test-tube and boil the mixture vigorously. Cool, make acid with glacial 

 acetic acid and add 1-2 drops of lead acetate. A darkening indicates the pres- 

 ence of unoxidized sulphur. 



2. Test for Total Sulphur (Unoxidized and Oxidized). Place the substance 

 to be examined (powdered egg albumin) in a small porcelain crucible, add a suit- 

 able amount of solid fusion mixture (sodium carbonate and potassium nitrate 

 mixed hi the proportion 2:1) and heat carefully until a colorless mixture results. 

 (Sodium peroxide may be used in place of this fusion mixture if desired.) Cool, 

 dissolve the cake in a little warm water and filter. Acidify the filtrate with hydro- 

 chloric acid, heat it to the boiling-point and add a small amount of barium chlo- 

 ride solution. A white precipitate forms if sulphur is present. What is this 

 precipitate? 



GLOBULINS 



Globulins are simple proteins especially predominant in the vege- 

 table kingdom. They are closely related to the albumins and in com- 

 mon with them give all the ordinary protein tests. Globulins differ 

 from the albumins in being insoluble in pure (salt-free) water. They 

 are, however, soluble in neutral solutions of salts of strong bases with 

 strong acids. Most globulins are precipitated from their solutions by 

 saturation with solid sodium chloride or magnesium sulphate. As a 

 class they are much less stable than the albumins, a fact shown by the 

 increasing difficulty with which a globulin dissolves during the course of 

 successive reprecipitations. 



We have used an albumin of animal origin (egg albumin), for all 



