30 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



Note that the alkali-albumin now requires more acid for its precipitation 

 than in A, the acid which is first added converting the sodium phosphate into 

 acid sodium phosphate. 



Now remove C from the bath. Boil it. Again there is no coagulation, 

 the proteins having been converted into acid-albumin, or syntonin. After 

 cooling, colour with litmus and neutralise with 0*l-per-cent. alkali. At the 

 neutral point a precipitate is formed, soluble in excess of acid or alkali. 

 (Acid-albumin is formed more slowly than alkali-albumin, so it is best to 

 leave this experiment to the last.) 



2. Take some gelatin and dissolve it in hot water. On cooling, the solu- 

 tion sets into a jelly (gelatinisation). 



Take a dilute solution of gelatin, and try all the protein tests with it 

 enumerated on p. 27. Carefully note down your results. 



3. Add a few drops of acetic acid to some saliva. A stringy precipitate 

 of rnucin is formed. 



4. A tendon has been soaked for a few days in lime water. The fibres 

 are not dissolved, but they are loosened from one another owing to the solu- 

 tion of the interstitial or ground substance by the lime water. Take some 

 of the lime-water extract and add acetic acid. A precipitate of mucoid is 

 obtained. The fibres themselves consist of collagen, which yields gelatin on 

 boiling. Vitreous humour or the Whartonian jelly of the umbilical cord is 

 much richer in ground substance than tendon, and, if treated in the same 

 way, a much larger yield of mucoid is obtained. 



The Proteins are the most important substances that occur in 

 animal and vegetable organisms, and protein metabolism is, as already 

 noted, the most characteristic sign of life. 



They are highly complex compounds of carbon, hydrogen, oxygen, 

 nitrogen, and sulphur occurring in a solid viscous condition, or in 

 solution in nearly all parts of the body. The different members of 

 the group present, however, great differences in their chemical and 

 physical properties. 



The proteins in the food form the source of the proteins in the 

 body tissues, but the latter are usually different in composition from 

 the former. The food proteins are in the process of digestion 

 broken up into simpler substances, usually called cleavage products, 

 and it is from these that the body cells reconstruct the proteins 

 peculiar to themselves. As a result of katabolic processes in the 

 body, the proteins are finally again broken down, carbonic acid, 

 water, sulphuric acid (combined as sulphates), urea, and creatinine 

 being the principal final products which are discharged in the urine and 

 other excretions. The intermediate substances between the proteins 

 and such final katabolites as urea will be discussed under Urine. 



The following figures will convince the student how different the 

 proteins are in elementary composition ; Hoppe-Seyler many years 

 ago gave the variations in percentage composition as follows : 



H N 8 



From 51-5 6'9 15'2 0'3 2O9 

 To 54-5 7-3 17-0 2-0 23'5 



