Kinetic Properties of a Soluble Catechol O‐Methyltransferase of Human Liver

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Abstract
When 2‐hydroxyoestradiol‐17β was incubated in the presence of S‐adenosylmethionine with a 380‐fold purified catechol O‐methyltransferase from human liver, 2‐methoxyoestradiol‐17β and 2‐hydroxyoestradiol‐17β 3‐methyl ether were identified as the only metabolites. No dimethylation, transmethylation or demethylation was observed.The purified enzyme was active only in the presence of cysteine and divalent cations; magnesium was found to be the most effective cation. The temperature optimum for the methylation of 2‐hydroxyoestradiol‐17β was 42°C; the activation energy amounted to 20.9 kcal/mol. The enzyme activity had two pH optima; the pH optimum between 6.8 and 8.4 was due to maximal formation of 2‐methoxyoestradiol‐17β, whereas the pH optimum at 9.2 was due to the formation of both the 2‐ and 3‐monomethyl ether.The formation of the two isomeric monomethyl ethers increased to a maximum at a substrate concentration of 50 μM (Km value 14 μM) and then decreased with an inflection point between 75 and 100 μM (K1 value under standard assay conditions 95 μM). Whereas both monomethyl ethers were formed at the same rate up to 50 μM substrate concentration, the decrease of formation of 2‐hydroxyoestradiol‐17β 3‐methyl ether was more pronounced than that of 2‐methoxyoestradiol‐17β. Both substrate inhibition and the ratio of methylation depended on the concentrations of S‐adenosylmethionine and MgCl2. Product inhibition was also demonstrated with K1 values of 24 μM for 2‐methoxyoestradiol‐17β, 80 μM for 2‐hydroxyoestradiol‐17β 3‐methyl ether and 39 μM for S‐adenosylhomocysteine. In contrast, increasing amounts of S‐adenosylmethionine did not produce inhibition or changes of the ratio of methylation (Km value 8.5 μM).