ENZYMES 



minimal amounts necessary of substances in the second category. 

 The concentrations of the first category are in milHgrams per cubic 

 centimeter, whereas those of the second are in fractions of a micro- 

 gram per cubic centimeter. 



The enzyme-trace substance theory in the form stated above is 

 appHcable only to naturally occurring substances in the diet or growth 

 medium. There is an alternative form of the theory which applies 

 to all substances regardless of their origin, and it may be stated in the 

 following terms: Any substance which in trace amounts induces pro- 

 found biological effects does so either by participating in or by spe- 

 cifically afifecting some enzyme system. If valid, this theory must be 

 regarded as one of the cornerstones of the science of pharmacology. An 

 extensive list can now be compiled of substances, part if not all of whose 

 pharmacological actions can be explained in terms of enzyme effects 

 {cj. Table I). 



Table I 



One cannot but be impressed by the vindication of the enzyme 

 hypothesis in at least fourteen instances, most of them reported within 

 the last few years, in contrast to the absence of a single authenticated 

 case in which any other principle of mechanism has been shown to 

 operate. At present, the enzyme theory is really not much more than 

 a good working hypothesis. But while it may be premature to regard 

 all the biological eflfects of trace substances exclusively in terms of 



