Abstract
The medicinal plant Psychotria ipecacuanha produces ipecac alkaloids, a series of monoterpenoid-isoquinoline alkaloids such as emetine and cephaeline, whose biosynthesis derives from condensation of dopamine and secologanin. Here, we identified three cDNAs, IpeOMT1-IpeOMT3, encoding ipecac alkaloid O-methyltransferases (OMTs) from P. ipecacuanha. They were coordinately transcribed with the recently identified ipecac alkaloid beta-glucosidase Ipeglu1. Their amino acid sequences were closely related to each other and rather to the flavonoid OMTs than to the OMTs involved in benzylisoquinoline alkaloid biosynthesis. Characterization of the recombinant IpeOMT enzymes with integration of the enzymatic properties of the IpeGlu1 revealed that emetine biosynthesis branches off from N-deacetylisoipecoside through its 6-O-methylation by IpeOMT1, with a minor contribution by IpeOMT2, followed by deglucosylation by IpeGlu1. The 7-hydroxy group of the isoquinoline skeleton of the aglycon is methylated by IpeOMT3 prior to the formation of protoemetine that is condensed with a second dopamine molecule, followed by sequential O-methylations by IpeOMT2 and IpeOMT1 to form cephaeline and emetine, respectively. In addition to this central pathway of ipecac alkaloid biosynthesis, formation of all methyl derivatives of ipecac alkaloids in P. ipecacuanha could be explained by the enzymatic activities of IpeOMT1-IpeOMT3, indicating that they are sufficient for all O-methylation reactions of ipecac alkaloid biosynthesis.
Highlights
The nucleotide sequence(s) reported in this paper has been submitted to the DDBJ/GenBankTM/EBI Data Bank with accession number(s) AB527082 to AB527084
Phylogenetic analysis of the IpeOMTs with selected members of plant OMTs, including benzylisoquinoline alkaloid OMTs (Fig. 2), showed that three IpeOMTs are closely related to each other, and the clade exhibited the closest relationship to the flavonoid OMTs and terpenoid-indole alkaloid 16-hydroxytabersonine 16-OMT in C. roseus [29] but not to the OMTs of benzylisoquinoline alkaloids such as 3Ј-hydroxy-N-methylcoclaurine 4Ј-OMT in C. japonica [22], norcoclaurine 6-OMTs in P. somniferum [16], T. tuberosum [18], and C. japonica [22], reticuline 7-OMT [16], norreticuline 7-OMT [19], columbamine 2-OMT [21], and scoulerine 9-OMT [20]
Involvement of IpeOMTs in the Biosynthesis of Ipecac Alkaloids Derived from (1S)-N-Deacetylisoipecoside—Identification of the three ipecac alkaloid O-methyltransferases, IpeOMT1, IpeOMT2, and IpeOMT3, in this study, as well as the ipecac alkaloid -glucosidase, IpeGlu1 [15], enabled us to elucidate a major portion of the biosynthetic pathways to ipecac alkaloids
Summary
The (1S)-diastereomer N-deacetylisoipecoside is deglucosylated and converted to protoemetine, which is condensed with a second molecule of dopamine and further converted to cephaeline and emetine (Fig. 1). We have identified an ipecac alkaloid -glucosidase (IpeGlu1) from P. ipecacuanha root culture that catalyzes the deglucosylation of the (1S)-diastereomer N-deacetylisoipecoside and the (1R)-diastereomer N-deacetylipecoside [15]. Ipecac alkaloids possess one isoquinoline moiety in the carbon skeleton prior to condensation with the second molecule of dopamine to form cephaeline and emetine, which contain two isoquinoline groups (Fig. 1). The substrate specificities of the three recombinant OMTs are sufficient to explain all O-methylation reactions of ipecac alkaloid biosynthesis Based on their functional characterization, we propose the detailed biosynthetic network of ipecac alkaloids in P. ipecacuanha
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
