Biosynthesis simple indole alkaloids

Table I is a compilation of plant species which contain the simple indole alkaloid types of Fig. 1. As mentioned earlier, the main requirement for the inclusion of a certain simple indole alkaloid into Table I is that it contain a tryptamine unit as a readily distinguishable feature in its structure. That tryptamine is a precursor in the biosynthesis of many of the b, c, d, and e type simple indole bases is yet to be shown although it is felt that future work will prove the correctness of such a view. Gramine, the simplest indole alkaloid, has been included in the tryptamine classification a because it is biosynthetically related to tryptophan cryptole-pine has been likewise included therein although its structural relationship to tryptophan appears more obscure (Volume VIII, Chapter 1, pp. 4, 19). The calycanthine type does not possess a tryptamine structure but it is included in the simple indole alkaloid b classification since most of its congeners are tryptamine derivatives and since it exhibits a close biogenetic relationship to this latter (chimonanthine) type (Volume VIII, Chapter 16). Type d is represented by the small number of the so-called canthin-6-one alkaloids (Volume VIII, pp. 260-252, 497-498). The most recent variation of the simple indole alkaloids is found in the Anacardiaceae family. Its indoloquinolizidine nucleus suggests inclusion with type d on the basis of structural and biogenetic similarity. Finally, simple indole alkaloid type e is composed of the well-defined evodiamine (rutaecarpine) structural form (Volume VIII, Chapter 4).

Although the natural occurrence of indole and the biosynthesis of the indole ring system are of importance and relevance to the wider question of the biosynthesis of the complex indole alkaloids, indole will not be discussed in detail here, as it is not an alkaloid. For a comprehensive and critical account of the occurrence of indole and its simple derivatives in plants, the reader is referred to the article by Stowe (8). 

P. viridis grows naturally in Amazonian tropical forests in Central and South America, and the leaves are used as one of the main components in the preparation of the hallucinogenic drink ayahuasca. This plant is a rich source of the psychedelic indole-alkaloid DMT [94] in the mixture that also contains jS-carboline alkaloids provided by Banisteriopsis caapi (Malpighiaceae), mainly harmine, harmaline, and tetrahydroharmine, that besides psychoactive properties, act as reversible MAOIs. MAO acts as a detoxifying enzyme [105], and when inhibited by MAOIs, DMT inactivation is prevented in the gut [106] enabling it to reach the CNS site of action, affording significant psychotropic effect. DMT biosynthesis is relatively simple and is summarized in Scheme 5.2. 

An enzyme does not have to be pure to yield valuable information. In many cases simple cell-free preparations containing a mixture of many enzymes may yield most useful results. These preparations are simply made from bacterial cultures in various ways including ultrasonic disruption. For plants resort is commonly made to cell-free preparations from plant tissue cultures [20]. An impressive example is found in recent work on the biosynthesis of terpenoid indole alkaloids (Section 6.6.2). Also to be noted is the application here of radioimmunoassay [21] to the analysis of radioactive alkaloids. 

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