Enzymic and chemical O-methylation of a 4-hydroxyestrone N-acetylcysteine conjugate.

4-Hydroxyestrone N-acetylcysteine conjugate (4-OHE1-2SR) is considered to be an important compound for monitoring the in vivo formation of catechol estrogen quinones, an intermediary in estrogen carcinogenicity. This article describes the selective synthesis of isomeric monomethyl ethers of 4-OHE1-2SR utilizing the formation of a seven-membered ring lactone by dehydration with acetic anhydride. Using these authentic specimens, enzymic and chemical O-methylation were examined. Enzymic O-methylation, using a rat liver cytosolic fraction, of 4-OHE1-2SR gave its 3-methyl ether as the sole product, while preferential O-methylation of 4-hydroxyestrone (4-OHE1) at the C-4 position was confirmed under the same conditions. Methylation of 4-OHE1-2SR with diazomethane gave initially carboxylate methylation, then the corresponding 3-methyl ether almost exclusively, while methylation of 4-OHE1 also gave its 3-methyl ether preferentially. However, much more rapid formation of the 3-methyl ether was observed with 4-OHE1-2SR than with 4-OHE1 itself. These results show that the hydroxy group at the C-3 position of 4-OHE1-2SR is more reactive than that at the C-4 position, both chemically and enzymatically.