Abstract
Summary Two distinct O-methyltransferases (OMTs), specific for phenylpropanoids (COMT) and for quercetin (QOMT), were partially purified from apple fruit cell suspension culture by fractional precipitation with ammonium sulphate and gel filtration chromatography. Both enzymes had different molecular weights which amounted to 87 and 47 Kds for the COMT and QOMT, respectively. The latter exhibited absolute requirement for Mg 2+ and was inhibited by SH-group reagents. Whereas COMT activity had similar properties as those reported previously, QOMT was distinct from other flavonoid OMTs in: (a) being specific for flavonols with 3′, 4′-dihydroxy-groups; (b) it attacks mainly position 3 of quercetin and not 3′; (c) it accepts partially O-methylated substrates, such as 3-methyl- or 7-methylquercetin with the formation of 3,7-dimethyl- and 3,7,4′-trimethyl derivatives, indicating several OMT activities; (d) the differences in molecular weight and p H optimum from other reported lavonoid OMTs. The instability of QOMT preparation precluded further purification for the separation of individual OMT activities.
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