PROTEINS 157 



spondence between the amount of this substance excreted 

 and the amount of tyrosine ingested shows that these are 

 related. It is therefore beHeved that tyrosine is, under 

 normal circumstances, first changed into homogentisic acid, 

 and that the alcaptonuric cannot oxidise homogentisic 

 acid. 



OH 



/\ H0/\ 



OH 



CH2CHNH2COOH CH2COOH 



Tyrosine Homogentisic acid 



6. Knoop^s Resistant Radical Method. — Substances which 

 are readily oxidised under normal conditions are incom- 

 pletely oxidised when they are Unked to another substance 

 itself resistant to oxidation. By hnking fatty acids to the 

 benzene ring important deductions can be drawn as to the 

 normal metabolism of these acids (see p. 202). 



The location of these changes in a particular organ can 

 be made — 



1. By the perfusion and digestion methods mentioned 

 above. 



2. By studying the effect of removal of the organ under 

 investigation from the circulation. 



3. By a comparative analysis of the blood entering and 

 the blood leaving the organ. 



2. PROTEINS 



The Nature of Proteins 



A protein is a substance containing carbon, hydrogen, 

 oxygen, nitrogen, and sometimes sulphur and phosphorus. 

 Structurally it consists of a large number of amino-acid 

 molecules hnked together by condensation. Into these 



