The kinetics of the oxidation of 17β-hydroxy-Δ 1,3,5(10)-estratrienes which differ in D ring size, in C 3, substituent and in the character of fusion of the B and C rings by CrO 3 in AcOH has been studied. An increase in size of the D ring in the substrate diminishes reactivity of the 17β-hydroxy group in the oxidation. It has been found that the reactivities of the 17β and 17β-hydroxy groups of the Δ 1,3,5(10)-estratriene derivatives depend upon the electronic nature of the substituent at C 3, explained by a long range effect operating from the A to D ring. The pK'a's of phenolic steroids have been measured and it was shown that introduction of the Δ 8(9) double bond increases the degree of dissociation of the phenolic hydroxyl. Expansion of the D ring and also reduction of the 17(17a)-keto group to 17β (17aβ) hydroxy group decreases the degree of dissociation of the phenolic hydroxyl as a result of the long range effect from the D to A ring. A linear correlation has been established between the oxidation and dissociation rate constants, confirming the existence of an interaction between C 3 and C 17(17a) at the expense of the long range effects from the A to D ring and vice versa. The long range effect apparently arises from a combination of inductive and conformational effects of the substituent. The kinetics of enzymic oxidation of 17β-hydroxy-Δ 1,3,5(10)-estratrienes by soluble 17β-estradiol dehydrogenase from human placenta has been investigated. In the case of 17β-hydroxy-Δ 1,3,5(10)-estratrienes of natural configuration, a linear correlation has been established between the values of the maximum rate constant and the oxidation rate constant. An assumption can thus be made about the major role of the contribution of the specific reactivity of the substrate to that of the enzyme-substrate complex, involving the size of the D ring and the long range effect of the substituent at C 3 upon the reaction center at C 17(17a). For 17β-hydroxy-Δ 1,3,5,(10)-estratrienes of the 8-isoconfiguration it is the specific reactivity of the substrate that makes the major contribution to the maximum rate values; this specific activity is a result of the substrate's configuration, which is responsible for the stability of the enzyme-substrate complex.
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