Purification and Properties of a S‐Adenosylmethionine: Isoflavone 4′‐O‐Methyltransferase from Cell Suspension Cultures of Cicer arietinum L.

Abstract

An O‐methyltransferase catalyzing the transfer of the S‐methyl group of S‐adenosyl‐l‐methionine to the 4′‐hydroxyl group of isoflavones has been detected in seedlings of Cicer arietinum L. and in cell suspension cultures of this plant. After transfer of the cell cultures to fresh medium the activity of the methyltransferase rose after a lag period of about 2 h and reached a maximum after about 20 h and then declined. Illumination of the cell cultures had no effect on the enzyme activity. The enzyme has been purified 360‐fold by (NH4)2SO4 fractionation, chromatography on DEAE‐cellulose and hydroxyapatite and Sephadex G‐200 gel filtration. The S‐adenosylmethionine:isoflavone 4′‐O‐methyltransferase was isolated as a soluble enzyme with a pH optimum of 9 in glycine‐NaOH buffer. It requires–SH‐protecting agents for activity. The molecular weight of the enzyme was estimated by the elution volume from a Sephadex G‐200 column to be about 110000. The enzyme is specific for the methylation of 4′‐hydroxyisoflavones. Intermediates in the bio‐synthetic pathway to isoflavones, e.g. p‐hydroxycinnamic acid and 5, 7, 4′‐trihydroxyflavone, were not methylated. The Km values for daidzein (7, 4′‐dihydroxyisoflavone) and S‐adenosylmethionine were determined to be 80 μM and 0.16 mM, respectively. Because of the high specificity of the methyltransferase for isoflavones it can be concluded that methylation is the last step in the biosynthesis of 4′‐methoxyisoflavones. The enzyme was inhibited by S‐adenosylhomocysteine. The Ki values for S‐adenosylhomocysteine versus daidzein and S‐adenosylmethionine were determined to be 0.3 mM and 30 μM, respectively. From the inhibition kinetics it can be concluded that the reaction follows an Ordered BiBi mechanism with S‐adenosylmethionine and S‐adenosylhomocysteine as leading reaction partners. Copyright © 1974, Wiley Blackwell. All rights reserved