The Steric Mechanisms of Enzymatic Aromatization of 19-Norsteroids

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The stereochemistry of hydrogen removal from C-2 during the placental and microbial aromatization of 19-norsteroids has been investigated. The substrates for this purpose, epimeric [2-3H] estr-4-ene-3, 17-diones (IV, VIII), were prepared along the route which had previously been developed. The tritiated compounds were admixed with an appropriate amount of [4-14C] estr-4-ene-3, 17-dione and recrystallized until constant isotope ratio was achieved. Each substrate was incubated with the human placental preparation according to the procedure of Ryan. The transformation products, estrone and 1β-hydroxyestr-4-ene-3, 17-dione, and recovered substrate were separated by preparative thin-layer chromatography, diluted with the carrier and then recrystallized to constant isotope ratio, respectively. Estrone formed from the substrate with the label at 2β exhibited a 79% loss, while that from the 2α-labeled substrate lost only 20% tritium. The double-isotope labeled substrate was also incubated with respiring cultures of Bacillus sphaericus. The aromatized product from the 2β-tritiated substrate lost 80% of the label, whereas the 2α-epimer showed only 11% loss. Elimination of hydrogen from C-2 in the aromatization process with human placenta and microorganism is thus stereoselectively β.

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