INCORPORATION OF CHIRALLY DEUTERATED PUTRESCINES INTO PYRROLIZIDINE ALKALOIDS - A REINVESTIGATION

Based on previous tracer work and recent enzymatic studies it can be p redicted that incorporation of (S)-1-H-2]putrescine via the symmetrica l intermediate homospermidine into the necine base moiety of pyrrolizi dine alkaloids (PAs) should proceed with 50% retention of deuterium. H owever, values of only 34 to 34.5% retention had been found independen tly in two laboratories in the past. These results were confirmed in t his study. Deuterium isotope effects during homospermidine formation a s a reason for the low retention could be excluded by GC mass spectral studies. Doubly-labelled [H-2-C-14]putrescine was fed to Senecio vulg aris root cultures and by means of quantitative GC mass spectrometry t he specific H-2-retention was established for various intermediates of PA-biosynthesis such as putrescine, spermidine and homospermidine. Th e results clearly indicate that H-2 is stereoselectively lost from (S) -[1-H-2]-labelled putrescine during its reversible inter-conversion wi th spermidine. This loss corresponds precisely to the above mentioned difference between measured and predicted H-2-retention. Since (S)-[1- H-2]-labelled putrescine is incorporated into spermidine with deuteriu m retention, it is most likely the H-2 is lost during the conversion o f spermidine into putrescine. The mechanism of this unusual reaction w hich is insensitive to beta-hydroxyethylhydrazine (a potent diamine ox idase inhibitor) needs to be elucidated. (C) 1998 Elsevier Science Ltd . All rights reserved.