Lupine alkaloids biosynthesis

Saito K, Koike Y, Suzuki H, Murakoshi I (1993) Biogenetic implication of lupin alkaloid biosynthesis in bitter and sweet frams of Lupinus luteus and Lupinus albus. Phytochemistry 34 1041-1044... 

Golebiewski, W. M. and Spenser, I. D. 1976. The biosynthesis of lupine alkaloids. A reexamination. Journal of American Chemical Society, 21 6726-6728. 

Biosynthesis, transport and storage of quinolizidine alkaloids in lupins. Alkaloid concentrations are given as % dry weight. 

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. 

Simple bicyclic compounds form a rather small subset of the lupine (or lupin) alkaloids, the overwhelming majority of which have tricyclic or tetracyclic structures based on the quinolizidine motif. These alkaloids are characteristic metabolites of the Papilionoideae, a sub-family of the Leguminosae (Fabaceae), although representative examples have also been isolated from several other plant families. The simple lupine quinolizidines were surveyed in Volume 28 of this treatise (7), while later reviews in Volumes 31 (5) and 47 (9) comprehensively covered all classes of lupine alkaloids, including those containing indolizidine components. Much relevant material is also to be found in the review on the biosynthesis of pyrrolizidine and... 

Possible pathways of biosynthesis of the necic acids have been outlined by Adams and Gianturco (74, 134), based upon multiple condensations of acetate units. The possible derivation of the necines has been considered by Robinson to be closely analogous to the biosynthetic scheme for some of the lupin alkaloids (222-224). 

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]. 

Scheme 2.5. Mannich Reaction in the Biosynthesis of Lupine Alkaloids... 

Robert Robinson in 1917. More recently, modern biosynthetic work has provided a great deal of information about alkaloid biosynthesis and many alkaloids have been efficiently synthesized by routes that generally parallel those followed in nature. The case of the lupine alkaloids can be cited as one example of the role of the Mannich reaction in biosynthesis. Scheme 2.5 provides a rough outline of the biosynthesis of this alkaloid system from the amino acid lysine. 

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). 

Figure 1.18a. Proposed metabolic grid for the biosynthesis and interconversions of quinolizi-dine alkaloids and related products in Fabaceae (Papilionaceae). For definition of symbols, see Table 1.2. Construction of the metabolic grid of lupine alkaloids The data on the various conversions of the lupine alkaloids were used to construct short metabolic pathways. Data concerning the distribution of lupine alkaloids in Leguminosae were drawn from Boit (1961) Cranmer and Turner (1967) Mears and Mabry (1971) Aslanov et al. (1972) Wicky and Steinegger (1965) Faugeras and Paris (1968) Bratek and Wiewiorowski (1959) Balcar-...

Another aspect is also frequently applied, which is the alkaloid origin. Nicotine, for example, is included among the tobacco alkaloids and quinolizidine alkaloids occurring in different types of lupine are lupine alkaloids. For practical reasons, classification based on these principles has also been used in the following sections. Today, classification according to the precursors of alkaloid biosynthesis is often promoted. This, however, requires a detailed knowledge of the biogenesis mechanisms (Table 10.2). 

Fig. 121. The most important lupin alkaloids and their biosynthesis.

The biochemistry and molecular biology of quinolizidine alkaloid biosynthesis have not been fully characterized. Quinolizidine alkaloids are formed from lysine via lysine decarboxylase (LDC) whereby cadaverine is the first detectable intermediate (Figure 4.16). Biosynthesis of the quinolizidine ring is thought to arise from the cychzation of cadaverine units via an enzyme-bound intermediate (Suzuki et al. 1996). Lysine decarboxylase and the quinolizidine skeleton-forming enzyme have been detected in chloroplasts of Lupinus polyphyllus (common lupin) (Wink and Hartmann 1982). Once the quinolizidine skeleton has been formed it is modified by dehydrogenation, hydroxylation or esterification to generate the diverse array of alkaloid products. 

Site of alkaloid formation, transport, and accumulation. QA are formed in the aerial green parts of legumes, especially in the leaves (.9) In lupin leaves we succeeded in localizing the key enzymes of QA biosynthesis in the chloroplast (10, 11), where the formation of the precursor lysine also takes place. Like most of the processes that are located in the chloroplast, QA biosynthesis is regulated by light (.8) and QA formation fol lows a light-dependent diurnal rhythm (, 13). The alkaloids formed in the leaves are translocated via the phloem (13, 14) all over a lupin plant, so that all plant parts contain alkaloids. QA are accumulated and stored preferentially in epidermal and subepidermal tissues of stems and leaves (15, 16). Especially rich in alkaloids are the seeds, which may contain up to 5% (dry weight) alkaloid (equivalent to 200 mmol/ kg). ... 

Nowacki, E. 1963. Inheritance and biosynthesis of alkaloids in lupin. Genetica Polonica, 4 161-202. 

Hartmann, T. 1988. Secondary metabolism of lupins Biosynthesis, translocation and accumulation of the quinolizidine alkaloids. In Proceedings of the 5th International Lupin Conference. (Twardowski, T., ed.) pp. 64-78. Poznari, Polish Academy of Sciences, Institute of Bioorganic Chemistry. 

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]. 

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