Synthesis and properties of the epimeric 6-hydroperoxyandrostenediones, new substrates/inhibitors of human placental aromatase

We have recently reported the formation in bovine adrenals and in rat liver of 6 beta-hydroxy-, and 6-oxoprogesterone via the 6 beta-hydroperoxy intermediate. The presence of steroid hydroperoxides in animal tissues, however transient it may be, is not devoid of physiologic significance in view of their characteristic property as potential radical initiators. Since 6-hydroperoxides of androgens have not previously been described, we have synthesized the 2 epimeric 6 alpha-, and 6 beta-hydroperoxy-4-androstene-3,17-diones by oxygenation of 5-androstene-3,17-dione in an aprotic solvent system in the presence of dibenzoyl-peroxide. Their chemical identity and chirality were established by IR, NMR, GC-MS, and by reduction to the known 6 alpha and 6 beta-alcohols. These hydroperoxide stereoisomers could only be separated without decomposition by HPLC using a non-aqueous mobile phase. In our search for a natural, non-estrogenic inhibitor of human placental aromatase, we have studied the effect on this enzyme complex of 6 alpha- and 6 beta-OOH-androstenedione, as well as of their corresponding 6-hydroxy and 6-oxo metabolites. Aromatase activity was measured by a slightly modified version of Thompson and Siiteri's original assay based on 1 beta,2 beta-tritium exchange to 3H water. The C-6 oxygenated androgens were found to competitively inhibit the aromatase reaction in the following descending order: 6-oxo greater than 6 beta-OH greater than 6 alpha-OOH greater than 6 beta-OOH showing Ki values of resp. 2.5, 5.0, 6.5 and 7.5 microM, suggesting that they are interacting with the same active site. Moreover, both 6 alpha- and 6 beta-hydroperoxyandrostenedione are active substrates for the aromatase, giving KM values of 2.8 and 2.5 microM respectively.