SUBSTRATE INHIBITION 129 



Reaction Constants for Substrates Producing Inhibition " 



" In the nomenclature of Marcus and Talalay, K^ = K^ and K^ 

 OH OH OH 



aK, 



HO 



constants are given in the accompanying tabulation. Four steroids not pro- 

 ducing substrate inhibition are also shown for comparison. The remarkable 

 specificity of interaction is apparent in the marked effects produced by 

 minor structural changes: compare particularly B and G, or D and H, 

 where the difference is only in the a or /5 configuration. The most potent 

 substrate inhibition is given by compounds [a) with the ring system sat- 

 urated, (b) with one =0 and one —OH group at either the 3- or 17-po- 

 sitions, and (c) without substituent groups on the rings (an 11-OH group 

 blocks the inhibition). It is interesting that the constants for G and H are 

 very close and yet the molecules have opposite positions for the oxidized 

 OH group; B and D do not show this similarity however. It may be that 



