| Structure Identification: |
| J Biol Chem. 2008 Dec 12;283(50):34650-9. | | The new beta-D-glucosidase in terpenoid-isoquinoline alkaloid biosynthesis in Psychotria ipecacuanha.[Pubmed: 18927081] | Ipecac alkaloids produced in the medicinal plant Psychotria ipecacuanha such as emetine and cephaeline possess a monoterpenoid-tetrahydroisoquinoline skeleton, which is formed by condensation of dopamine and secologanin. Deglucosylation of one of the condensed products N-deacetylisoIpecoside (1 alpha(S)-epimer) is considered to be a part of the reactions for emetine biosynthesis, whereas its 1 beta(R)-epimer N-deacetylIpecoside is converted to Ipecoside in P. ipecacuanha.
METHODS AND RESULTS:
Here, we isolated a cDNA clone Ipeglu1 encoding Ipecac alkaloid beta-D-glucosidase from P. ipecacuanha. The deduced protein showed 54 and 48% identities to raucaffricine beta-glucosidase and strictosidine beta-glucosidase, respectively. Recombinant IpeGlu1 enzyme preferentially hydrolyzed glucosidic Ipecac alkaloids except for their lactams, but showed poor or no activity toward other substrates, including terpenoid-indole alkaloid glucosides. Liquid chromatography-tandem mass spectrometry analysis of deglucosylated products of N-deacetylisoIpecoside revealed spontaneous transitions of the highly reactive aglycons, one of which was supposed to be the intermediate for emetine biosynthesis. IpeGlu1 activity was extremely poor toward 7-O-methyl and 6,7-O,O-dimethyl derivatives. However, 6-O-methyl derivatives were hydrolyzed as efficiently as non-methylated substrates, suggesting the possibility of 6-O-methylation prior to deglucosylation by IpeGlu1. In contrast to the strictosidine beta-glucosidase that stereospecifically hydrolyzes 3 alpha(S)-epimer in terpenoid-indole alkaloid biosynthesis, IpeGlu1 lacked stereospecificity for its substrates where 1 beta(R)-epimers were preferred to 1 alpha(S)-epimers, although Ipecoside (1 beta(R)) is a major alkaloidal glucoside in P. ipecacuanha, suggesting the compartmentalization of IpeGlu1 from Ipecoside.
CONCLUSIONS:
These facts have significant implications for distinct physiological roles of 1 alpha(S)- and 1 beta(R)-epimers and for the involvement of IpeGlu1 in the metabolic fate of both of them. |
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