PURINES 109 



acids ends here. On the other hand, there is considerable 

 evidence to show that for the more complex amino-acids 

 animals depend upon plants. We shall consider this more 

 fully in connection with nutrition, merely noting at this 

 stage that the capacity of the animal body for synthesising 

 amino-acids is hmited to the very simplest of these. The 

 possible conversion of amino-acids into compounds other 

 than protein is discussed later (see pp. 190 and 201). 



3.— PURINES 



The purines form a group of closely related substances 

 found extensively in hving tissues. They may be regarded 

 as composed of two urea groups united together through 

 a central chain of three carbon atoms so as to form a 

 double ring. 



Purine, though itself only of theoretical importance, may 

 be taken as a starting-point. It has the formula C5H4N4, or 



N=CH 



I I 



CH C— NH. 



II II >H 

 N C W 



Its principal derivatives may be thus classified :— 

 1. Amino derivatives (with or without oxygen) : — 



