Lysine, alkaloid precursor

The precursors of true alkaloids and protoalkaloids are aminoacids (both their precursors and postcursors), while transamination reactions precede pseudoalkaloids (Tables 1 and 10). It is not difficult to see that from all aminoacids only a small part is known as alkaloid precursors (Table 19). Both true and proto alkaloids are synthesized mainly from the aromatic amino acids, phenylalanine, tyrosine (isoquinoline alkaloids) and tryptophan (indole alkaloids). Lysine is the... 

L-lysine furnishes alkaloids with at least four different nuclei. It is a protein amino acid, one of the most important alkaloid precursors. L-lysine-derived... 

The Early Stages of Alkaloid Biosynthesis. — It is well established that L-lysine (13) is incorporated into some piperidine alkaloids by way of a symmetrical intermediate it is accepted that this symmetrical intermediate is cadaverine (17), which is also an alkaloid precursor. Lysine, however, is incorporated into other alkaloids, e.g. anabasine (12) (Section 1.5) and sedamine (18), without the intervention of any symmetrical intermediate. Cadaverine (17), although able to act as an alkaloid precursor. 

The results of extensive experiments with variously labelled samples of lysine closely define the way in which this amino acid is assimilated into the alkaloids. Thus C-2 and C-6 of the precursor become C-2 and C-6, respectively, in 6.19) [9], 6.21) [10, 11], and 6.20) [12]. Although cadaverine 6.26) is also an alkaloid precursor it cannot be an intermediate following lysine because any label at C-2 or C-6 of the amino acid would become spread over both C-1 and C-5 of the symmetrical diamine 6.26) a single lysine label would thus be spread over both C-2 and C-6 of the alkaloids. 

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). ... 

True alkaloids derive from amino acid and they share a heterocyclic ring with nitrogen. These alkaloids are highly reactive substances with biological activity even in low doses. All true alkaloids have a bitter taste and appear as a white solid, with the exception of nicotine which has a brown liquid. True alkaloids form water-soluble salts. Moreover, most of them are well-defined crystalline substances which unite with acids to form salts. True alkaloids may occur in plants (1) in the free state, (2) as salts and (3) as N-oxides. These alkaloids occur in a limited number of species and families, and are those compounds in which decarboxylated amino acids are condensed with a non-nitrogenous structural moiety. The primary precursors of true alkaloids are such amino acids as L-ornithine, L-lysine, L-phenylalanine/L-tyrosine, L-tryptophan and L-histidine . Examples of true alkaloids include such biologically active alkaloids as cocaine, quinine, dopamine, morphine and usambarensine (Figure 4). A fuller list of examples appears in Table 1. 

Figure 16. L-lysine is a precursor of piperidine, quinolizidine and indolizidine alkaloids.

Lythraceae Alkaloids.—Results showing that lysine (15) is a precursor for decodine (22) and decinine (23) in Decodon verticillatus, which were published in preliminary form (cf. Vol. 1, p. 6), are now available in a full paper.8 Label from either C-2 or C-6 of the amino-acid was found to be spread equally over C-5 and C-9 of the alkaloids, indicating that ring A derived from this amino-acid and that incorporation was via a symmetrical intermediate. Cadaverine (16),... 

Most of the work on the biosynthesis of Lythraceae alkaloids has been done by Spenser et al. (10, 84-87). First, the validity of the pelletierine hypothesis (c) of Ferris et al. (62) has been tested. The pelletierine (126) nucleus is generated from L-lysine (181) via cadaverine (182), and presumably A -piperideine (132) and its side chain originate from the acetate. Incorporation of radioactivity from 14C-labeled samples of these precursors to decodine (6) and decinine (2) in Decodon verticilatus has been investigated (85, 87). 

The chirality of a precursor product relationship was determined by the use of doubly labeled lysine, in which one enantiomer was labeled only with tritium and the other with tritium and 14C (55). Comparison of the 3H/14C ratios of substrate and products demonstrated that decodine and decinine were derived from L-lysine, whereas pipecolic acid (186) was derived from D-lysine. Thus, pipecolic acid does not serve as a precursor of Lythraceae alkaloids (57). 

Lysine.—Lysine is a common precursor of piperidine alkaloids. Of the two enantiomers of this amino-acid, the L-isomer is the more direct precursor, in plants, for piperidine alkaloids, e.g. anabasine, whereas D-lysine is more directly implicated in the biosynthesis of pipecolic acid (24)1,2,23 (cf Vol. 7, p. 7). It has now been shown that a pathway exists in the plant Nicotiana glauca,24 and also in the micro-organism Neurospora crassa2S which transforms D-lysine into L-lysine by way of L-pipecolic acid (24). 

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. 

Alkaloids thus represent one of the largest groups of natural products, with over 10,000 known compounds at present, and they display an enormous variety of structures, which is due to the fact that several different precursors find their way into alkaloid skeletons, such as ornithine, lysine, phenylalanine, tyrosine, and tryptophan (38-40). In addition, part of the alkaloid molecule can be derived from other pathways, such as the terpenoid pathway, or from carbohydrates (38-40). Whereas the structure elucidation of alkaloids and the exploration of alkaloid biosynthetic pathways have always commanded much attention, there are relatively few experimental data on the ecological function of alkaloids. This is the more surprising since alkaloids are known for their toxic and pharmacological properties and many are potent pharmaceuticals. 

The majority of alkaloids have been found to be derived from amino acids, such as tyrosine, phenylalanine, anthranilic acid, tryptophan/tryptamine, ornithine/arginine, lysine, histidine and nicotinic acid (Fig. 2.1). However, alkaloids maybe derived from other precursors such as purines in case of caffeine, terpenoids, which become aminated after the main skeleton has been synthesized i.e. aconitine or the steroidal alkaloids, are found in the Solanaceae and Liliaceae. Alkaloids may also be formed from acetate-derived polyketides, where the amino nitrogen is introduced as in the hemlock alkaloid, coniine. 

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