Physiologically active secondary products

Perhaps most conspicuous by their absence from the list (cf. Table VIII) are the alkaloids. These substances, generally defined as nitrogen-containing secondary metabolites, are common plant products and usually have distinct physiological activities. Nevertheless, to date no reports seem to have appeared concerning the presence (or absence) of alkaloids as Gossypium metabolites. 

J. Eichhom and M.H. Zenk, in Secondary Products - Physiologically Active Compounds. 43th Annual Congress on Medicinal Plant Research, Halle, Germany, 3-7 September 1995,... 

The main metabolic pathway of C. proceeds through 23- and 26-hydroxylation to 23S,26-dihydroxycal-citriol and further oxidation to calcitriol-26,23-lactone. In total over SO metabolites of calciol and ercalciol have been isolated and identified to date. Most of these secondary products have no or a very reduced antirachitic activity but do possess other interesting physiological properties the relevance of which is being investigated. 

This wais shown in the symposium with some plant hormones which in microorganisms do not possess any comparable physiological activity indicating that it is the existence of an appropriate target mechanism rather than peculiarities in the metabolism which convert a normal secondary product into a hormone. 

During evolution some secondary products have acquired a physiological role not only for intra- or interspecific interactions of individuals but also for the interaction between cells of the producing organisms themselves. Thus, as indicated by their biosynthesis and metabolism, the plant hormones so far known, in spite of their wide distribution, are also secondary products. On the other hand, synthesis and activity of enzsanes of secondary and therefore also of hormone metabolism are regulated not only by substrate-like effectors but also by non-substrate-like ones, for instance by hormones and, in some cases, other metabolites with hormone-like activity. These statements are illustrated by examples. 

Typical examples in this respect are secondary products used as hormones or neurotransmitters (E 3). Table 59 shows that nearly all secondary products with hormone activity in plants or animals are also synthesized in organisms where a comparable physiological activity in absent. Hence it is the development of an appropriate target mechanism rather than any special chemical structure which makes a secondary product act as a hormone. 

Table 63. Secondary products with physiological activity...

In several cases the physiological action of secondary products has become so optimal in the course of evolution that more active substances with similar structures could not as yet be prepared synthetically. Examples in this respect are the cardiac glycosides as well as certain antibiotics, like streptomycin and chloramphenicol, which in spite of great efforts by the pharmaceutical chemists, have found no better substitute. 

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