DESTRUCTIVE AND NON-DESTRUCTIVE METHODS 185 



not contain this enzyme. In this way, by association of destructive and non- 

 destructive methods, both of which are indispensable, it is possible to 

 demonstrate with certainty that the metabolic sequence is A-B-X. 



Nevertheless, it can be objected that a product accumulated by one 

 mutant and utilized by another mutant could very possibly be the product 

 of a side-reaction. 



For example, consider the reaction scheme 



t 

 A' 



It is possible to imagine, if the block is between A and B, that an extract 

 of the wild strain of the micro-organism would covert A' into B whilst an 

 extract of the blocked mutant would not. Is this a reason for placing A' in 

 the direct metabolic sequence to X, that is for writing A ^^ A' -^ B ^' X} 

 No, because the isolation of enzymes enables us to demonstrate that a single 

 enzyme is capable of converting A into B and that, for the conversion of A' 

 into B, two enzymes are necessary; in addition to the first, v/e require an 

 enzyme catalysing the reaction: A' -^ A. This is an illustration of the 

 importance of studies on purified enzymes, alongside experiments using 

 non-destructive methods. 



REFERENCES 



Harris, H. (1953). An Introduction to Human Biochemical Genetics. Univ. Press, 



Cambridge. 

 Wagner, R. P. and Mitchell, H. K. (1955). Genetics and Metabolism. Wiley, 



New York. 



