184 UNITY AND DIVERSITY IN BIOCHEMISTRY 



of a compound. In this way one can measure the turnover of a substance, 

 that is the rate at which its molecules are replaced (although the concen- 

 tration remains the same) when it is in a steady state resulting from an 

 equilibrium between the rate of synthesis on the one hand, and the rate of 

 breakdown or incorporation, on the other. 



III. THE USE OF MUTANT STRAINS OF 

 MICRO-ORGANISMS 



The name auxotrophes is given to those mutant forms of a micro- 

 organism which are dependent on the provision of a growth factor not 

 required by the natural form. At the present time, a very large number of 

 mutants are known which are characterized by the loss of a given enzyme, 

 their metabolism is blocked at the stage of a definite chemical reaction. 

 To define the particular reaction which is blocked, two sets of information 

 are required : a knowledge of the substances which the mutant can use as 

 growth factors, and knowledge of the substances which accumulate in the 

 cell. Let us suppose that A and B are two different precursors of X. If 

 we have a mutant which is an auxotrophe for X, which accumulates A, 

 and which responds by growing when B is supplied, we can deduce that 

 the block is situated after A and before B in the series of metabolic 

 reactions. 



Distinction between a Possible Precursor and an 

 Obligatory Intermediate 



To show that a given substance can serve as a precursor of a second 

 substance is one thing; to show that it is in fact the normal intermediate 

 in the organism is quite another. The study of mutants of micro-organisms 

 has revealed the existence of auxotrophes for each of the naturall)'^ occurring 

 amino acids. This illustrates very well the idea of an obligatory metabolic 

 pathway, at least in these organisms. 



Let us once more consider the case of A and B, precursors of X in a 

 micro-organism. The wild strain of this micro-organism is able, as can be 

 demonstrated by the use of isotopes or by means of the purified enzymes, 

 to convert A and B into X. A single enzyme, extracted from the micro- 

 organism and purified, converts A into B. A mutant auxotrophic for X does 



REFERENCES 



Calvin, M., HEmELBERCER, Ch., REm, J. C, Tolbert, B. M., Yanknich, P. F. 



(1949). Isotopic Carbon. Techniques in its measurement and chemical manipulation. 



Wiley, New York. 

 Kamen, M. D. (1948). Radioactive Tracers in Biology. Academic Press, New York. 



