The Functional Characterization of a Site-Specific Apigenin 4′-O-methyltransferase Synthesized by the Liverwort Species Plagiochasma appendiculatum

Apigenin, a widely distributed flavone, exhibits excellent antioxidant, anti-inflammatory, and antitumor properties. In addition, the methylation of apigenin is generally considered to result in better absorption and greatly increased bioavailability. Here, four putative Class II methyltransferase genes were identified from the transcriptome sequences generated from the liverwort species Plagiochasma appendiculatum. Each was heterologously expressed as a His-fusion protein in Escherichia coli and their methylation activity against apigenin was tested. One of the four Class II OMT enzymes named 4′-O-methyltransferase (Pa4′OMT) was shown to react effectively with apigenin, catalyzing its conversion to acacetin. Besides the favorite substrate apigenin, the recombinant PaF4′OMT was shown to catalyze luteolin, naringenin, kaempferol, quercetin, genistein, scutellarein, and genkwanin to the corresponding 4′-methylation products. In vivo feeding experiments indicated that PaF4′OMT could convert apigenin to acacetin efficiently in E. coli and approximately 88.8 µM (25.2 mg/L) of product was synthesized when 100 µM of apigenin was supplemented. This is the first time that a Class II plant O-methyltransferase has been characterized in liverworts