Incorporation of chirally deuterated putrescines into pyrrolizidine alkaloids: a reinvestigation
1998
Graser, G. | Witte, L. | Robins, D.J. | Hartmann, T.
Based on previous tracer work and recent enzymatic studies it can be predicted that incorporation of (S)-[1-2H]putrescine via the symmetrical intermediate homospermidine into the necine base moiety of pyrrolizidine alkaloids (PAs) should proceed with 50% retention of deuterium. However, values of only 34 to 34.5% retention had been found independently in two laboratories in the past. These results were confirmed in this study. Deuterium isotope effects during homospermidine formation as a reason for the low retention could be excluded by GC mass spectral studies. Doubly-labelled [2H-14C]putrescine was fed to Senecio vulgaris root cultures and by means of quantitative GC mass spectrometry the specific 2H-retention was established for various intermediates of PA-biosynthesis such as putrescine, spermidine and homospermidine. The results clearly indicate that 2H is stereoselectively lost from (S)-[1-2H]-labelled putrescine during its reversible inter-conversion with spermidine. This loss corresponds precisely to the above mentioned difference between measured and predicted 2H-retention. Since (S)-[1-2H]-labelled putrescine is incorporated into spermidine with deuterium retention, it is most likely the 2H is lost during the conversion of spermidine into putrescine. The mechanism of this unusual reaction which is insensitive to beta-hydroxyethylhydrazine (a potent diamine oxidase inhibitor) needs to be elucidated.
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