Microbial Mufanfs 49 



the isotopic evidence for incorporation already mentioned, 

 show that the compound is an essential intermediate. But 

 if the enzyme that forms or utilizes the compound can be 

 shown to be present in extracts of the wild type and absent 

 from extracts of a mutant that is blocked in the biosyn- 

 thetic chain in question — and if the enzyme assay is sensi- 

 tive enough so that the observed difference between the 

 two extracts is large — then one can begin to feel confident 

 that he is dealing with an essential intermediate. 



One more possibility, however, must be considered: that 

 the true intermediates might be unstable or conjugated 

 compounds, never present in more than trace amounts. 

 The compounds that are detected as growth factors or as 

 accumulation products would then be merely cast-off de- 

 rivatives, pallid reflections of the living reality, just as, in 

 Plato's parable of the cave, objects sensed in the world 

 around us were considered to be mere shadows of an 

 ideal world of true reality. Such a scheme might be repre- 

 sented as follows: 



A and B would represent the true intermediates, and A' 

 and B' the parallel side products. An auxotrophic mutant 

 blocked between A and B would be unable to convert not 

 only A to B, but also A' to B'. A distinction can be made, 

 however, as Adelberg has pointed out (2), on the basis 

 of the fact that a single enzyme could convert A to B, 

 whereas three enzymes would be required to convert A' 

 to B'.^ We are led, therefore, to the following conclusion: 

 that essential intermediates can be recognized at present 

 with maximal certainty only in pairs, by showing (a) that 

 the wild type (either cell or extract) can convert these 



3 We are referring here to substances (A and A') that are not in rapid 

 spontaneous equilibrium with each other. 



