Further experiments on the inhibition of aconitase by enzymically prepared fluorocitric acid

17-OXO STEROIDS AND ARYL SULPHAMATE SYNTHESIS 261greater than for 2 mM-2-naphthylamine.Its apparent affinity for 3,B-methoxyandrost-5-en-17one is high, Ki being 1'5 zM.5. The corresponding values for guinea-pigliver arylamine sulphokinase are: Ki, 05 mM-2- naphthylamine; Ki, less than 0-2,uM-oxo steroid.6.An explanation of these effects is suggested.It is based on a mechanism formally similar to the partial competitive inhibition of Dixon & Webb (1958), in which a ternary enzyme-steroidsubstrate complex (dissociation constant K ) can be formed.The rate of breakdown of this complex to give the reaction products is identical with that of the breakdown of the normal enzyme-substrate complex (dissociation constant K, ).If K, is greater than K, in such a system then the steroid will be an inhibitor of the enzyme; if K, is less than K, the steroid will be an activator.