460 THE BIOCHEMISTRY OF B VITAMINS 



before the others. The equations relating the inhibition index to equilib- 

 rium constants and other constants (p. 447) would apply to the particular 

 enzyme system, E 1} and the inhibition index, K 1} would be related to this 

 particular enzyme system. 



Although the analogue may not prevent the formation of P2, it may at 

 higher concentrations prevent the conversion of S to P 3 sufficiently to 

 inhibit the biological system. Consequently, an exogenous supply of Pi 

 would not completely prevent the toxicity of the analogue, since another 

 enzyme system, E s , becomes the limiting process of the biological system, 

 and the particular equilibrium, rate, and other constants of this enzyme 

 determine the inhibition index, K 3 . Thus, in the presence of adequate 

 amounts of Pi, the inhibition index is increased from the K\ value cor- 

 responding to the inhibition of the biosynthesis of Pi, to the value K s , 

 corresponding to the inhibition of the biosynthesis of P 3 . 



Since P x becomes the first limiting product, an external supply of P 3 

 would not be expected to exert any effect on the inhibition in the absence 

 of Pi. However, if both P x and P 3 are supplied to the biological system, 

 the analogue either becomes ineffective as an inhibitor of the system, or 

 at a still higher inhibition index it prevents an additional function of the 

 metabolite. 



Hence, regardless of the metabolite concentration, the addition of the 

 product of an inhibited enzyme system will result either in a complete 

 reversal of the toxicity of the analogue, or in a higher analogue-metabolite 

 ratio corresponding to the inhibition index of another enzyme system 

 utilizing the metabolite. If an analogue prevents the formation of a series 

 of products from the metabolite, there is a definite order in which the 

 products must be added if the effect of each substance is to be demon- 

 strated. In the absence of a product the biosynthesis of which is inhibited 

 at a lower inhibition index, all other products the biosyntheses of which 

 are inhibited at higher inhibition indices do not affect the inhibition index. 



The general shape of the graph obtained by plotting the growth response 

 of the biological system against increasing concentration of an inhibitor 

 at a constant concentration of substrate is related to a specific enzyme 

 system and is dependent upon the dissociation constants, etc., of the 

 particular system (p. 447) . Since two separate enzyme systems involving 

 the same metabolite and inhibitory analogue would not be expected to 

 have similar dissociation constants, etc., the general shape of such a graph 

 or any type of data which depends upon such constants can be employed 

 to show that different enzyme systems are involved. The ratio of the 

 index for maximum inhibition to that for half-maximum inhibition can 

 be useful in such cases. 



