Special relationships between agonists and antagonists

Anti-metabolites need not be analogues of metabolites. In fact substances which do not resemble metabolites in any way can be used to combine with or destroy an apoenzyme, a coenzyme, or a substrate (Fildes, 1940). All that is necessary is a high affinity, and the combination is often effected by covalent bonds. These antagonists are not displaceable by the substrate. An example of such an antagonist is the porphyrin, haemin, which splits thiamine into two substances, one containing the pyrimidine ring and the other the thiazole ring. This causes thiamine-deprivation symptoms in animals fed on raw fish, in which this substance is abundant (Woolley, 1952). 

The demonstration of competitive antagonism between two substances does not prove that one of them is a metabolite. Thus the severe contractions of the gut caused by barium chloride are competitively inhibited by sodium sulfate (which is not a normal gut constituent). In this case, there is a chemical reaction 

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