IO2 PHYSIOLOGICAL CHEMISTRY. 



(a) p. 101. It is still soluble. Why has it not been coagulated? 

 Repeat the above experiments with powdered serum albumin and 

 see how the results compare with those just obtained. 



SULPHUR IN PROTEIN. 



Sulphur is believed to be present in two different forms in the 

 protein molecule. The first form, which is present in greatest 

 amount, is that loosely combined with carbon and hydrogen. Sul- 

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

 mercaptan and lead-blackening sulphur. The second form is com- 

 bined 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. Test for Loosely Combined Sulphur. To equal volumes 

 of KOH and egg albumin solutions in a test-tube add 1-2 drops of 

 plumbic acetate solution and boil the mixture. Loosely combined 

 sulphur is indicated by a darkening of the solution, the color deep- 

 ening into a black if sufficient sulphur is present. Add hydrochloric 

 acid and note the characteristic odor evolved from the solution. 

 Write the reactions for this test. 



2. Test for Total Sulphur (Loosely Combined and Oxidized). 

 Place the substance to be examined ( powdered ' egg albumin) 

 in a small porcelain crucible, add a suitable amount of solid fusion 

 mixture (potassium hydroxide and potassium nitrate mixed in the 

 proportion 5:1) 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 hydrochloric acid, heat it to the boiling- 

 point and add a small amount of barium chloride solution. A 

 white precipitate forms if sulphur is present. What is this pre- 

 cipitate? 



GLOBULINS. 



Globulins are- simple proteins especially predominant in the vege- 

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

 common 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 



