Aromatization of a Bicyclic Steroid Analog, 3-Oxodecalin-4-ene-10-carboxaldehyde, by Liver Microsomal Cytochrome P450 2B4

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Abstract
Several purified isoforms of microsomal cytochrome P450 were previously shown in this laboratory to catalyze the oxidative deformylation of a variety of alpha- or beta-branched aldehydes with the production of olefins and formic acid. In the present study, 3-oxodecalin-4-ene-10-carboxaldehyde (ODEC, numbered according to the convention for steroids) was synthesized as a bicyclic analog of the aldehyde that is known to be the terminal intermediate in the enzymatic conversion of androgens to estrogens. ODEC undergoes aromatization in a reconstituted enzyme system containing liver microsomal cytochrome P450 2B4 and NADPH-cytochrome P450 reductase, along with NADPH and phosphatidylcholine, under aerobic conditions. The products, 3-hydroxy-6,7,8,9-tetrahydronaphthalene (HTN) and formic acid, were identified by mass spectrometry. The corresponding 10-carbinol does not undergo oxidative aromatization with P450 2B4, and with ODEC as substrate, other microsomal P450 cytochromes are either weakly active (isoforms 2C3 and 3A6) or inactive (isoforms 2E1, 1A2, and 2G1). Cytochrome b5 stimulates the P450 2B4-catalyzed reaction with ODEC about 2.6-fold but has no effect with the other P450s. In two respects the conversion of the bicyclic model compound to HTN with P450 2B4 was shown to be similar to that of the steroid aromatase reaction. Deuterium in the formyl group of ODEC was retained in the formic acid that was produced and isolated as the 4-nitrobenzyl derivative, and with preparations of ODEC containing deuterium in the 1 alpha position or the 1 alpha and 2 alpha positions, it was shown that the desaturation reaction is specific for removal of the 1 beta-hydrogen, thus involving a stereospecific cis elimination of formate. Cytochrome b5 has no effect on the stereospecificity of the reaction.

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