THE EFFECT OF HYDROGEN ION CONCENTRATION ON ENZYMATIC O-METHYLATION OF CATECHOL ESTROGENS (CLINICAL ANALYSIS ON STEROIDS. XI)

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Incubation of 2-hydroxyestradiol (1) with rat liver homogenate in the presence of Sadenosyl-L-methionine and Mg^<2+> gave two isomeric monomethyl ethers, 2-methoxyestradiol (2) and 2-hydroxyestradiol-3-methyl ether (3). The ratio of the 2-O-methyl ether to the 3-O-methyl ether formed was found to be dependent on hydrogen ion concentration of the medium. At pH 6.0,the ratio was about 3-4,which decreased gradually with increasing pH, and at pH>7.0,the ratio was approaching to unity. PH dependency upon the product ratio was significant when 2,3,17β-trihydroxyestra-1,3,5 (10)-trien-6-one (5) was used as a substrate, although the ratio was completely reversed in contrast with the result obtained with 2-hydroxyestradiol. At pH 6-7,the ratio of the 3-O-methyl ether (7) to 2-O-methyl ether (6) was about 6-8. With increasing pH of the medium, 2-O-methylation increased gradually, and at over pH 8.0 the ratio became constant at about 3. These results suggest that rat liver catechol O-methyltransferase catalyzes the transfer of the methyl group from S-adenosyl-L-methionine to more basic hydroxyl group of the two phenolic hydroxyl groups of estrogen catechols.
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