Pyrrolidine1, Tropane, Pyrrozidine, Polyamines2
Pyperidine, Quinolizidine, Indolizidine
Isoquinoline Tetrahydroisoquinoline, Benzyltetrahydroiso-quinoline1, Catecholamines2
Indole1, Carbolines, Quinoline2, Pyrrolindole, Indolamines3
1. Cocaine, Atropine
2. Putrescine, Spermine, Spermidine
Pyperidine, Quinolizidine, Indolizidine
1. Morphine, Curarines, Papaverine
2. Noradrenaline, Adrenaline
2. Quinine, Capthotecin
3. Melatonin, Serotonin
Theobromine, Theophylline, Caffeine
Fig. 11.6 Main classes of alkaloid precursors and derivatives (the example number refers to the corresponding alkaloid class)
of over 1000 isoprene units and derived from polymerisation of geranylgeranyl-PP units (Wendt and Schulz 1998).
A common pathway for alkaloid biosynthesis does not exist. The greater amounts of alkaloid compounds are amino acid derivatives, grouped on the base of their precursor and chemical structure. Therefore, the main groups include alkaloids arising from ornithine, leucine, lysine, tyrosine, tryptophan, histidine and pheny-lalanine, in addition to alkaloids arising from nicotinic (pyridine alkaloids) and anthranilic acid, acetate, isoprenoids and purine (Fig. 11.6). In turn, these classes include some minor divisions. Accordingly, pyrrole alkaloids arise from leucine; pyrrolidine, tropane and pyrrolizidin alkaloids from ornithine; piperidine, quino-lizidine and indolizidine alkaloids derived from lysine; catecholamines, isoquino-line, tetrahydroisoquinoline and benzyltetrahydroisoquinoline alkaloids originated from tyrosine; indolamines, indole, carboline, quinoline, pyrrolindole, ergot alkaloids come from tryptophan and imidazole alkaloids from histidine. The same way, anthranilic acid is the precursor of quinazoline, quinoline and acridine alkaloids, whereas isoprenoid alkaloids include mono- (geraniol), di- (geranylgeranyl-PP) and triterpenes (cholesterol) derivatives (Fig. 11.6) (Cordell et al. 2001; Facchini 2001; Hughes and Shanks 2002).
Alkaloids consist of an enormous number of phytochemicals of toxicological, pharmacological, nutritional and cosmetic interest, and also of ecological importance for plants. For instance, tropane alkaloids include cocaine and atropine, nicotine is a pyridine alkaloid, noradrenaline (or norepinephrine), adrenaline (epinephrine), papaverine, curarines and morphine arise from tyrosine. Melatonin and serotonin are indolamines, vindoline, catharantine (and their derivatives vin-cristine and vinblastine) are indole alkaloids, quinine and capthotecin are quinoline alkaloids and lysergic acid diethylamide (LSD) is an ergot alkaloid, all these arising from tryptophan. Histamine is an imidazole alkaloid, ephedrine and capsaicin derive from phenylalanine, solanin is a steroid glycoalkaloid from cholesterol. Finally, purine alkaloids include theophylline, theobromine and caffeine which are found in tea, cacao and coffee, respectively (Cordell et al. 2001; Facchni 2001; Hughes and Shanks 2002).
In plants, phenylpropanoid metabolism is induced as a general response to stress. Therefore, enhancement of key enzyme activities and accumulation of secondary metabolites are events that occur in order to improve the resistance against pathogen attack and/or tolerance to adverse environmental conditions and pollutants. PAL is an extremely sensitive indicator of stress conditions, and commonly considered as a biochemical marker indicating the activation of plant defences which include the synthesis of both structural and protective compounds (Fig. 11.7). In particular, ozone exposure elevates the level of flux through the phenylpropanoid pathway, thereby supplying carbon skeletons for secondary metabolites (Toumainen etal. 1996).
The enhancement of phenylpropanoid biosynthesis by ozone is well documented. A very early report, dated more than 30 years ago, demonstrated the accumulation of isoflavonoid in soybean following ozone exposure (Keen and Taylor 1975). Ever since, ozone-stimulated induction of transcripts for defence-related genes, the
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