Summary

V. Summary

Benzylisoquinoline represent a group of about 2500 structures containing many physiologically active members. Among the benzylisoquinoline alkaloids, is one of the pharmaceutically important members that are still derived from the poppy , which produces over 80 alkaloids derived from L-. Through the biosynthetic pathway that leads from L-tyrosine to morphine, salutaridinol is acetylated by salutaridinol 7 -O-acetyltransferase (SalAT) and, subsequently, the group is removed to form the pentacyclic morphinan ring system of . SalAT was initially identified and isolated as a cDNA clone that corresponds to the internal amino acid sequences of the native enzyme purified from a cell suspension culture of Papaver somniferum (Grothe et al., 2001). SalAT catalyzes the conversion of the phenanthrene salutaridinol to salutaridinol acetate the immediate precursor of thebaine along morphine biosynthetic pathway. Because salutaridinol acetate non-enzymatically rapidly decomposes to thebaine, it was not clarified whether in vivo thebaine formation is an enzymatic or a non-enzymatic reaction. There were some indications which suggest that this elimination is an enzymatic - dependent process. Because of the instability of the acetate, this “thebaine synthase” was assumed to be physically correlated with salutaridinol 7 -O-acetyltransferase. Based upon this hypothesis, a yeast two-hybrid system was used in an attempt to isolate a cDNA that encodes a putative “thebaine synthase”. The Cyto Trap yeast two-hybrid system is a new way to identify protein- protein interactions by searching the cytoplasm instead of the nucleus of the yeast cells. The project and its results could be summarized as the following.

1. Bait plasmid was constructed by cloning the cDNA of Salutaridinol 7-O- acetyltransferase into the pSos vector, though fusing the bait to human gene (hSos) product. The generated SalAT- hSos; fusion was transformed into yeast cdc25H strain. 2. A cDNA library from Papaver somniferum stems was prepared for protein- protein interaction screening, and cloned into pMyr vector which fuses the target gene to myristylation factor that anchors the protein to the yeast cell membrane.

3. Both fusion proteins (1 and 2) were co-transformed and co-expressed in the cdc25H yeast strain that harbors a temperature-sensitive mutation of the cdc25 gene and the yeast cells were incubated at the restrictive temperature of 37 °C on galactose

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dependent medium for cDNA library screening and identification of putative positive interactors. Approximately 400 putative positive clones could be isolated from which 345 were sequenced and analyzed.

4. cDNA encoding the SalAT neighboring reductase (SalR) and the distant Norclaurine 6-O- methyl transferase (6OMT) were generated and unidirectional cloned into vector MCS as target proteins. SalAT show the ability to interact with SalR, but did not interact with 6OMT.

5. Gene expression of 400 interactors were analyzed along with alkaloid profiling by LC- MS in cell culture suspension, and developing poppy seedlings to correlate the expression of interactor cDNAs with the time of accumulation of morphinan alkaloids. LC-MS analysis of 4-months plants of Papaver somniferum shows morphine as main alkaloid followed by and thebaine, respectively. Codeine was the first alkaloid to be detected in the greenhouse seedlings (12 days) meanwhile morphine was detected after 20 days, other alkaloids were in very low concentrations during the first month. Morphine contents continue to increase in much higher rates than codeine during the second month. One of the interactors showing high expression during alkaloid accumulation, the major latex protein 146, was selected for further characterization.

6. Salutaridinol 7-O-acetyltransferase (SalAT) which catalyzes the conversion of salutaridinol to salutaridinol-7-O- acetate, and the “thebaine synthase” candidate protein recombinant major latex protein (MLP 146) were heterologously over- expressed in E.coli and purified for a potential role in enzymatic thebaine formation. Enzyme assays were performed by incubations of salutaridinol and SalAT in the presence or absence of MLP146.

7. The product analysis was performed by LC-MS/MS and ESI selected reaction monitoring (SRM). The mass-spectrometric data exhibit signals for salutaridinol, salutaridinol acetate and thebaine in all incubations. Recombinant salutaridinol 7-O- acetyltransferase performance in converting salutaridinol into salutaridinol acetate was

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very active in a wide pH range values (6.5-9) as observed by thebaine formation without any participation of MLP 146 protein.

8. Biochemical data from enzymatic activity assays suggests that salutaridinol acetate undergoes a subsequent spontaneous and non enzymatic elimination at pH 6.5 as well as in neutral and alkaline environment leading to the formation of thebaine suggesting that thebaine formation is not a pH dependent process. The presence of by-products at acidic and neutral conditions as described by Lenz and Zenk (1995a) were not detectable. Full scan data show no signals for neodihydrothebaine, nudaurine, , and [8, 9-dihydro 5H-2, 12-dimethoxy-1-hydroxy-7-methyl-dibenz [d,f ]azonium]acetate. The MS spectra shows an unknown compound with protonated molecular ion at m/z 332, where the structure is not elucidated.

9. Recombinant major latex protein (MLP 146) appeared as a regulatory factor in thebaine accumulation depending on pH value; e.g. the rate of thebaine accumulation was dramatically declined regardless of the amount of MLP 146 at acidic conditions

Conclusion

Yeast two-hybrid assays for isolation of SalAT interaction proteins pulled out a relatively high number of cDNA as candidates for “thebaine synthase”. One of those candidates was the MLP 146 which could not be identified biochemically as the putative “thebaine synthase”. Rather, thebaine was detected rapidly and directly after incubations of salutaridinol and the recombinant salutaridinol 7-O-acetyltransferase only, no other proteins were needed. Based on the previous observations we suggest that in vivo thebaine formation in opium poppy is a non enzymatic rather than enzymatic step, most likely influenced by major latex protein.

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