6 THE SIMPLER NATURAL BASES 



practical definition is to describe a base as a substance which is pre- 

 cipitated by phosphotungstic acid. Adopting this criterion we consider 

 creatinine to be a base but creatine not. 



Alkaloid. ^Some writers have used this term to include all natural 

 bases, but the objections to this are evident from what has been said 

 above, and the word is best restricted to complex vegetable heterocyclic 

 bases derived from pyridine, quinoline, etc. 1 There is no doubt as to 

 what is generally meant by an alkaloid, but nevertheless a rigid definition 

 is almost impossible. On the one hand narceine, for instance, is a 

 typical alkaloid from opium, but the nitrogen atom does not form part 

 of a ring ; narceine is an amine. On the other hand histidine and its 

 derivatives are not classed as alkaloids, although they contain the 

 heterocyclic glyoxaline ring, which is also present in pilocarpine. The 

 latter substance is an undoubted alkaloid. In a few cases the inclusion 

 of bases in this monograph is arbitrary ; thus hordenine, which is usually 

 called an alkaloid, has been included on account of its relationship to 

 tyrosine ; ephedrine, which is isomeric with hordenine, has been ex- 

 cluded. All betaines have been included, for no typical alkaloid shows 

 a betaine structure. One further point should be noted. The typical 

 alkaloids are generally found only in one or a few closely related 

 species, but the simpler natural bases, in accordance with their close 

 connection with proteins and phosphatides, have generally a much 

 wider distribution. 



Ptomaine was originally applied by Selmi to bases from corpses 

 and afterwards became identical with putrefaction base (Brieger). 

 Some writers have restricted the term to poisonous bases. Lately it 

 has fallen into disuse. 



Leucomaine was a term used by Gautier for animal bases such as 

 creatinine, which are not formed by putrefaction ; this term is now 

 quite obsolete. 



Toxins are poisonous bacterial products which when injected cause 

 the production of anti-bodies, neutralising their poisonous properties ; 

 an example is diphtheria toxin. Gautier has applied the word, in a 

 different sense, to simple poisonous putrefaction bases. 



1 Winterstein and Trier define plant alkaloids as nitrogenous substances which can no 

 longer be utilised for building up protein. Thus they would call betaine an alkaloid. 



