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      In Poppy/In Mammals                           Emetine                                  Plumbagin

The analgesic isoquinoline alkaloids morphine and codeine are isolated for pharmaceutical use from the opium poppy Papaver somniferum. The morphine biosynthetic pathway is an attractive target for molecular genetics because of the potential to tailor the alkaloid profile of the opium poppy by genetic engineering. We have succeeded in isolating a number of the genes of morphine biosynthesis and, more recently, in using these genes to genetically engineer this plant to have altered alkaloid patterns. Transgenic opium poppy that can accumulate more morphine or selected alkaloid biosynthetic intermediates have been achieved to date. We are also interested in the structure and in the protein-protein interactions of the morphine biosynthetic enzymes. These studies should provide information on the regulation and substrate/product specificities of the enzymes in the morphine biosynthetic pathway.


Syrup of ipecac is derived from the roots of the Central American plant Psychotria ipecacuanha. The active component is the isoquinoline-terpenoid alkaloid emetine, a chemical that induces vomiting in cases of accidental poisoning. In lower concentrations, emetine is an effective anti-amoebic compound. We are investigating the biosynthesis of emetine at the enzyme and gene levels in emetine-producing ipecac root cultures. Although it has been proposed that these tetrahydroisoquinoline-monoterpene alkaloids are biosynthesized from dopamine and secologanin, the complete biosynthetic pathway still remains to be elucidated. A long-term goal of this project could be the production of emetine in a microbial system such as yeast.


Acetate-derived naphthoquinones occur only in a few plant families of the taxonomic order Caryophyllales. These secondary metabolites serve as defense compounds due to their antimicrobial properties and as antifeedants against insects. Plumbagin (2-methyl-5-hydroxy-1,4-naphthoquinone), the prototype of this class of compounds, has allelopathic effects and possesses several useful pharmacological activities, for example antiinflammatory and anticancer effects.
         The biosynthetic origin of plumbagin and related metabolites was proven by feeding labeled acetate units to plants. From these experiments it was proposed that a polyketide synthase provides the backbone of the naphthoquinones by condensing six activated acetate units in the form of acetyl-CoA and malonyl-CoA. The postulated intermediate hexaketide of this reaction undergoes two aldol cyclizations and is probably modified by a ketoreductase. The resulting naphthalene intermediate is then oxidized to a naphthoquinone, which can be further modified, e.g. by hydroxylation, dimerization or reduction.
         In order to identify the enzyme that catalyzes the first step in the biosynthesis of acetate-derived naphthoquinones, two polyketide synthase (PKS) cDNAs were isolated from the naphthoquinone producing plants Drosophyllum lusitanicum (Portuguese sundew) and Plumbago indica (rose-colored leadwort). The purified recombinant PKSs showed activity in enzyme assays. However, instead of naphthoquinones alpha-pyrones derived from three to six acetate units were formed, the main product being a hexaketide phenylpyrone. Since the acetate-derived naphthoquinones in D. lusitanicum and P. indica are also composed of six acetate units, we propose that the isolated PKSs are indeed involved in naphthoquinone formation in planta, but additional co-factors are required for the correct cyclization of the linear hexaketide intermediate. Ongoing research focuses on the characterization of such co-factors and other enzymes catalyzing downstream reactions of naphthoquinone biosynthesis.


Morphine, one of the strongest analgesic compounds known in human physiology is administered by ingestion or injection. It is a major alkaloid in the latex of the plant Papaver somniferum. Morphine has been found to be present in trace amounts in human cells and in ca. 10 nM concentration in mammals and the question was - is it of dietary origin or does it occur endogenously? Studies done in our lab have shown for the first time that morphine is biosynthesized in mammalian tissues such as human neuroblastoma- and pancreas carcinoma cells. Incorporation experiments with ¹⁸O₂ and separate feeding experiments involving heavy isotope-labeled precursors like DOPA have shown unequivocally that morphine found in mammals is of endogenous and not of dietary origin. With these results we developed a putative pathway for the biosynthesis of endogenous morphine in mammals and compared this pathway with that in the poppy plant. Enzymatic studies revealed first clues as to the enzymatic mechanisms involved in the formation of endogenous morphine in mammals and confirmed the proposed biosynthetic pathway. Our main interest focuses presently on the discovery of enzymes in mammals being involved in the biosynthesis of endogenous morphine and on the detection of endogenous morphine in mammalian tissue, especially brain.