MICROSCOPIC RATE CONSTANTS FOR THE ACETATE ION CATALYZED ISOMERIZATION OF 5-ANDROSTENE-3,17-DIONE TO 4-ANDROSTENE-3,17-DIONE - A MODEL FOR STEROID ISOMERASE

The intermediate dienol 3-hydroxy-3,5-androstadien-17-one (2) in the isomerization of 5-androstene-3,17-dione (1) to 4-androstene-3,17-dione (3) has been generated in acetic acid/acetate buffers. Under these conditions, this dienol partitions almost exclusively to 1 rather than 3. The rate constants for ketonization of 2, acetate-catalyzed deuterium exchange of the C-4 hydrogens of 1 in D2O, and isomerization of 1 to 3 have been used to calculate the microscopic rate constants for the individual steps in the acetate ion catalyzed isomerization. These rate constants provide values for the keto-enol equilibrium constants for 1 (pK(E)1 = 2.7) and 3 (pK(E)3 = 6.0) and the pK(a) of 2 (pK(a)E = 10.0). The relevance of the rate and equilibrium constants to the mechanism of the corresponding reaction catalyzed by steroid isomerase is discussed.