Lupanine biosynthesis

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 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... 

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

Quinolizidine Alkaloids.—Important new information (cf. Vol. 11, p. 4) has been obtained on the biosynthesis of quinolizidine alkaloids such as lupanine (27) in experiments with enzyme preparations from Lupinus polyphyllus cell suspension cultures26 and with chloroplasts.27 These alkaloids are formed from three molecules of lysine by way of cadaverine (25),1,2 and the enzymic evidence26,27 is that conversion of cadaverine into these alkaloids occurs without release of intermediates until 17-oxosparteine (26) is generated the enzyme is a transaminase and not a diamine oxidase. 

Essentially only lupanine (27) is accumulated in cell suspension cultures of Lupinus polyphyllus,29,30 Sarothamnus scoparius,29 and Baptista australis,29,31 whereas the intact plants accumulate other alkaloids. It is reasonable to assume that the cultures will accumulate alkaloids early rather than late in a biosynthetic pathway. Thus lupanine (27) is identified as a likely intermediate in the biosynthesis of the other alkaloids of these plants. In the case of B. australis, these alkaloids are of the pyridone type, e.g. anagyrine (28) and cytisine (29).31 Earlier results with... 

C02 had indicated that lupanine (27) is an intermediate in the biosynthesis of... 

Biosynthesis of piperidine alkaloids from lysine/cadaverine commonly occurs via A piperideine (31). Three molecules are utilized for the construction of lupanine (27), and an attractive biosynthetic route involving the all-trans-isomer of... 

The model scheme developed for the biosynthesis of lupanine from A piperideine and isotripiperideine33 has been adapted for the biosynthesis of matrine (32).35 At the moment, the two hypothetical pathways26,33,35 for the biosynthesis of quinolizidine alkaloids are manifestly different (cf. Vol. 11, p. 4 and Vol. 8, p. 3) one uses A piperideine (31) as an intermediate the other does not. Where the points of fundamental agreement between the two models lie, and which model is a more accurate picture of what is really happening, are questions that remain to be answered. 

In accord with a general body of evidence on the biosynthesis of alkaloids as against that of pipecolic acid (see above), L-lysine has been shown to be the preferred precursor for lupanine (27) and D-lysine the preferred precursor for l-pipecolic acid (24) in Lupinus angustifolia,36 A high retention of tritium, present at C-4 and C-5 in the lysine, on formation of (27) is to be noted. 

Alkaloid metabolism in lupine was proved by Wink and Hartmann to be associated with chloroplasts (34). A series of enzymes involved in the biosynthesis of lupine alkaloids were localized in chloroplasts isolated from leaves of Lupinus polyphylls and seedlings of L. albus by differential centrifugation. They proposed a pathway for the biosynthesis of lupanine via conversion of exogenous 17-oxosparteine to lupanine with intact chloroplasts. The biosynthetic pathway of lupinine was also studied by Wink and Hartmann (35). Two enzymes involved in the biosynthesis of alkaloids, namely, lysine decarboxylase and 17-oxosparteine synthetase, were found in the chloroplast stoma. The activities of the two enzymes were as low as one-thousandth that of diaminopimelate decarboxylase, an enzyme involved in the biosynthetic pathway from lysine to diaminopimelate. It was suggested that these differences are not caused by substrate availability (e,g., lysine concentration) as a critical factor in the synthesis of alkaloids. Feedback inhibition would play a major role in the regulation of amino acid biosynthesis but not in the control of alkaloid formation. 

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 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. 

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,... 

The biosynthesis of lupanine (131), 5,6-dehydrolupanine, anagyrine (132), rhombifoline (134), thermopsine (133), cytisine (135), and iV-methylcytisine has been examined in Thermopsis rhombifolia and T. caroliniana with C02-Lupanine was the first alkaloid to be labelled and it was followed by 5,6-dehydro-lupanine and anagyrine (132). Then the first of the tricyclic bases, rhombifoline... 

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. 

Quinolizidine Alkaloids.—Cadaverine is known to be a precursor for quinoliz-idine alkaloids. (For discussion of the biosynthesis of these alkaloids see also previous Reports). Recent experiments have shown that cadaverine is a precursor for alkaloids (anagyrine, pachycarpine, ammodendrine, N-methylcytisine, and cytisine) in Ammodendron karelinu too. Metabolism of lupanine, anagyrine, ammodendrine, and pachycarpine in the plant was also studied. ... 

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. 

Lupine alkaloids are formed in the green, aerial parts of Lupinus polyphyllus that incorporate labeled cadaverine into the lupanine skeleton, consistent with the fact that the enzymes of alkaloid biosynthesis, in this case, are located in the chloroplast stroma (Hartmann, 1985). Roots of the intact plants or in vitro cultured roots do not. A similar situation obtains for coniine in Conium maculatum, where the en-Z5nnes occur in both the chloroplasts and mitochondria. However, alkaloids are rarely formed in plastids (Hartmann, 1985), but are usually formed in the cytoplasm. Chloroplasts are not only the site of photosynthesis, but also of lipid, amino acid, and terpenoid biosynthesis (Schultz et al., 1985 Wink, 1987). 

Quinolizidine alkaloid biosynthesis Cadaverine, (+)-P- coumaioylepilupinine/lupinine, (—)-13a-tigloyloxymuttflOTme/lupanine... 

Little secure information exists on the detail of biosynthesis between cadaverine and the various alkaloids although the incorporation, in appropriate fashion, of three molecules of radioactive A -piperideine 6.18) into lupanine 6.54) has been recorded label from C-6 appeared at C-2, C-15, and, by inference, C-10, whereas C-2 label appeared at C-17, C-11, and, by inference, C-6 [38]. (Much less definitive results were obtained for matrine [36].) This was taken, together with a careful consideration of the chirality at the asymmetric centres of all the quinolizidine alkaloids, as evidence for rearrangement within the A -piperideine trimer 6.59) [38], formed from 6.18), leading to the quinolizidine alkaloids [38]. 

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... 

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