Plant alkaloids, nitrogen

Alkaloids are compounds that contain nitrogen in a heterocyclic ring and are commonly found in about 15-20% of all vascular plants. Alkaloids are subclassified on the basis of the chemical type of their nitrogen-containing ring. They are formed as secondary metabolites from amino acids and usually present a bitter taste accompanied by toxicity that should help to repel insects and herbivores. Alkaloids are found in seeds, leaves, and roots of plants such as coffee beans, guarana seeds, cocoa beans, mate tea leaves, peppermint leaves, coca leaves, and many other plant sources. The most common alkaloids are caffeine, theophylline, nicotine, codeine, and indole... 

Alkaloids, nitrogen-containing compounds generally found as secondary metabolites in plants, are also classical examples of renewables. In contrast to terpenes, they show a great variety in molecular structure, and the different classes of alkaloids are usually based on their basic ring systems. Many pharmaceutically active... 

Herbert, R. B. 2001. The biosynthesis of plant alkaloids and nitrogenous microbial metabolites. Nature Product Reports, 18 50-65. 

Table 1 Approximate percentages of nitrogen In some common plant alkaloids...

The structures of some of the better known plant alkaloids are shown in Figure 23-1. You will recognize some of them by name even if you have never seen their structures before. Many of the alkaloids are polycyclic structures and have other functional groups in addition to basic nitrogen. You will see that the nitrogens of alkaloids frequently are tertiary amine functions. 

Basic nitrogen compounds similar to the plant alkaloids also occur in animals, although the description animal alkaloid seldom is used. Certain amines and ammonium compounds play key roles in the function of the central nervous system (Figure 23-3) and the balance of amines in the brain is critical for normal brain functioning. Also, many essential vitamins and hormones are basic nitrogen compounds. Nitrogen bases also are vital constituents of nucleic acid polymers (DNA and RNA) and of proteins (Chapter 25). 

Alkaloids are nitrogen-based compounds frequently found in plants. Plant alkaloids that are used in treating cancer in humans include traditional agents, such as vincristine and vinblastine, and newer agents, such as vinorelbine, paclitaxel, and docetaxel (Table 36-4). These agents are also known as antimitotic drugs be-... 

Cocaine is the most potent naturally occurring stimulant. It is found as an alkaloid (nitrogen-containing organic base) in the leaves of the Erythroxylon coca trees in the Andes Mountains. Coca leaves contain 0.5-1.8% cocaine (benzoylmethylecgonine or BZ) that can be refined to nearly 100% purity. Research indicates that the plant produces cocaine to kill insects that prey on it. In humans, it is a central nervous system (CNS) stimulant. 

As was the case with terpenes, the function of alkaloids in plants is not known. It has been proposed that they are merely nitrogen-containing waste products of plants, like urea in animals. However, most plants reutilize nitrogen, rather than wasting it. Furthermore, it is difficult to imagine why such complex structures would be needed to store waste nitrogen. Like terpenes, alkaloids have been proposed to serve as protection from herbivores and insects. However, only a few examples of such protection can be demonstrated. Whatever the role of alkaloids is, some 70% to 80% of plants manage to do quite nicely without them. 

The first antagonists were natural products and in particular alkaloids (nitrogen-containing compounds derived from plants). 

B. Naturally Occurring Heterocyclic Amines Alkaloids Alkaloids are defined as plant-produced nitrogenous bases that have... 

The plant alkaloid castanospermine 155 and the related piperidine alkaloid 1-deox-ynojirimicin 160, like several other polyhydroxylated octahydroindolizidines, piperidines and pyrrolidines, are potent glycosidase inhibitors. These nitrogen bases are of considerable interest for the study of biosynthetic processes and, in addition, castanospermine and some of its derivatives may be of clinical value as antineo-plastic agents and as drugs in the treatment of AIDS. 

Most plant alkaloids are derivatives of tertiary amines, while others contain primary, secondary or quarternary nitrogen. The basicity of individual alkaloids varies considerably, depending on which of the four types is represented. The pK values (dissociation constants) lie in the range of 10-12 for very weak bases (e.g. purines), of 7-10 for weak bases (e.g. Cinchona alkaloids) anti of 3-7 for medium-strength bases (e.g. Opium alkaloidvSh... 

Protein and free amino acids found in tobacco leaf contribute significantly through pyrodegradation and pyrosynthesis to the formation of many nitrogenous compounds found in tobacco smoke. The nonvolatility of these compounds either as free acids, proteins, or members of tobacco pigment, for example, porphyrins, make them particularly liable to pyrolytic destruction because they, unlike nicotine and the other plant alkaloids, are not readily volatilized and swept away as the more intense heat of the cigarette coal approaches (3724). 

Previous practice, of listing earlier Reports in this series as the first references, "" is continued. One of the most recent reviews on alkaloid biosynthesis is again cited for background information another, more extensive, review has appeared which includes a discussion of the biosynthesis of nitrogeneous microbial metabolites, as well as that of plant alkaloids."... 

The biosynthetic route to most plant alkaloids and nitrogenous microbial metabolites is quite closely defined by the results of straightforward tracer feeding experiments. For these, work with the enzymes involved provides extra detail, with helpful confirmation for an already deduced sequence (see, e.g., nicotine"... 

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