Sparteine biosynthesis

Wink, M. 1987. Site of lupanine and sparteine biosynthesis in intact plants and in vitro organ cultures. Z. Naturforsch. 42, 868-872... 

The evidence that lupanine is not an intermediate in sparteine biosynthesis came from experiments in which Lupinus arboreus plants were exposed to radioactive carbon dioxide for varying periods the sparteine became radioactive in every experiment but the lupanine remained consistently non-radioactive. 

The crystal structures of lupanine (15) and its derivatives were investigated as free bases (54-56) as well as protonated forms (57-60). In all structures examined ring A was a half-chair. The conformation of ring C is a boat, and rings B and D have chair conformations. In other cases rings B, C, and D had chair conformations (54-57,59). Lupanine derivatives mamanine (16) and pohakuline (17), possible metabolites in the biosynthesis of sparteine, were studied by... 

Quinolizidine Alkaloids.—Biosynthesis of quinolizidine alkaloids, e.g. sparteine (28), is from lysine (15) by way of cadaverine (16), as shown in Scheme 4. Three cadaverine units (as indicated by the thickened bonds) are required for the construction of alkaloids such as sparteine (28).10 Although something has been discerned about the biosynthetic relationships of various quinolizidine alkaloids, the nature of early intermediates beyond cadaverine has remained quite elusive.10 Exciting new results obtained with crude enzyme preparations from cell suspension cultures of Lupinus polyphyllus indicate why this is so. 

The most common group of alkaloids possessing a quinolizidine nucleus is that of the lupine alkaloids which can simply be classified as bicyclic (lupinine/epilupinine type), tricyclic (cytisine type) or tetracyclic, (sparteine/lupanine or matrine type). Fig. (23). This grouping is made according to structure complexity and without considering biosynthesis, as the detailed biosynthetic pathways are still not completely understood. 

The evidence against sparteine being an intermediate in lupanine biosynthesis is less convincing. When L. polyphyllus plants were exposed to radioactive carbon dioxide the lupanine became radioactive. Attempts to detect sparteine in the extracts by chemical and radiochemical analysis gave negative results. This result suggests that sparteine is not present in this plant and is not, therefore, an intermediate in lupanine biosynthesis. However, it is possible that sparteine is involved but that the pool size is too small to be detected. 

The alkaloid sparteine was isolated only from the plant Chelidonium majus. It differs in its constitution from the already mentioned groups of alkaloids which were derived from 1-benzylisoquinoline precursors. Schiitte (443) studied the biosynthesis of sparteine in Chelidonium majus by means of radioactive cadaverine. He arrived at the conclusion that in this plant the biosynthesis takes the same pathway as in Lupinus luteus L. 

Quinolizidine alkaloids, such as sparteine, lupanine and cytisine are relatively weak inhibitors at this target (they strongly affect ACh receptors and Na+ channels see Tables 3-15). The stages which are inhibited are the loading of aminoacyl-tRNA with amino acids and the elongation step. The inhibitory activity was visible in heterologous systems, but protein biosynthesis in the producing plants (here lupins) was not affected [23]. 

Quinolizidine Alkaloids.—Previous results demonstrate that the quinolizidine skeleton in its entirety derives from lysine.Further research has indicated that lysine is a precursor of all the alkaloids of this type in five species of Leguminosae. From the levels of activity observed in the individual alkaloids it was concluded that saturated alkaloids are precursors for those with a pyridone ring. This was supported by the observation that label from radioactive sparteine (24) and lupanine (25) appeared in more highly oxidized alkaloids. (This compares with a similar situation in the biosynthesis of matrine-type alkaloids. ) A metabolic grid for the biosynthesis of quinolizidine alkaloids from lysine was proposed, based on these results,... 

Thermopsine (133) appeared after rhombifoline (134). As in the biosynthesis of the alkaloids of Lupinus angustifolius, no evidence could be found for the formation of sparteine. Finally, two unknown alkaloids, it was noted, were labelled early in both Thermopsis species. 

In vitro tissue and cell cultures of lupin plants are not appropriate systems for the study of biosynthesis of lupin alkaloids, because the production ability by in vitro culture is rather low, i.e., 10 2 to lO times compared with that of differentiated plants. The production of the alkaloids of lupinine- and sparteine-groups by cell culture have been reported by us [59] and by Wink s group [60]. We have also successfully produced matrine in green callus culture and in multiple shoots of Sophora flavescens [61]. The producibility of matrine was positively correlated with the chloroplast formation. This indicates that the formation of carbon skeleton of matrine-type alkaloids also likely takes place in chloroplasts in plant cells as postulated in that of sparteine-type alkaloids [62]. 

Some interesting relations are observed in the absolute configuration and biosynthesis of the alkaloids. Firstly, in nature, there are enantiomeric series of sparteine/lupanine-type alkaloids (Table II). Both antipodal alkaloids, 7S 9S and 7R 9R alkaloids, exist in the group of saturated-ring A alkaloids, e.g., sparteine and lupanine whereas the alkaloids of a-pyridone-ring A, e.g., anagyrine, cytisine and their derivatives, have only 7R.9R configuration. 

C14H22N2O, Mr 234.34, mp. 80.5-81 °C, [oId -7.5° (+5.2°) (C2H5OH). A tricyclic quinolizidine alkaloid of the sparteine type from 4 genera of the Fabaceae Cytisus, Diplotropis, Lupinus, Ormosia). A. exists as 2 optically active isomers The biosynthesis of A. in plants is assumed to involve ring opening and side chain degradation of lupanine as a precursor Ut. Waterman 8, 197-239. Planta Med. 59, 289 (1993). Pelletier 2, 105-148. 

Tracer studies using C, N-doubly labeled cadaverine and NMR labeling experiments have shown that three units of cadaverine are incorporated into sparteine and two of the C—bonds from two of the cadaverine units remain intact [62, 63]. In accordance with the labeling patterns observed, two hypothetical models for the biosynthesis of the tetracyclic alkaloids have been formulated. 

Fig. 235. Proposed scheme for the biosynthesis of sparteine 1 17-Oxosparteine synthase...

Sparteine has been isolated also from Chelidonium majus (Papaver-aceae) with the help of TLC Schutte [210] has studied the biogenesis of sparteine from radioactive cadaverine in this plant and concluded that the biosynthesis proceeds as in Lupinus luteus. Other work on the TLC of quinolizidine alkaloids has been published [lb, 26, 27]. 

Enzymes of Quinolizidine Alkaloid Biosynthesis.- In the past four years Hartmann, Wink, and their coworkers have obtained enzymes from Lupinus (especially L.polyphyllus) species which catalyse the formation of the tetracyclic quinolizidine alkaloids such as sparteine and lupanine from lysine. A novel biogenetic scheme has been proposed which accommodates these new results, and is consistent with previous biosynthetic studies on these alkaloids in intact plants . This new hypothesis (with some minor modifications by this reporter) is illustrated in Scheme 4. The first step in this sequence is the decarboxylation of lysine to yield cadaverine (38). This lysine decarboxylase was isolated from the chloroplasts of L. polyphyHub lea.ves, It is also present in... 

---