Biochemistry and Cell Biology of Secondary Metabolites

Many secondary metabolites in plants promote human health not only as pharmaceuticals but also as dietary supplements and functional foods (Askin et al., 2002; Briskin, 2000). Plant-produced recombinant proteins are also important as botanical therapeutics (Askin et al., 2002). As suggested by their diverse functions, secondary metabolites are structurally divergent (see Croteau et al., 2000 or other chapters) and are usually classified into several major groups. Terpenoids and steroids, which are formed from isoprenoid moieties, include the antineoplastic diterpene paclitaxel, from T. brevifolia Nutt; the antimalarial sesquiterpene artemisinin, isolated from Artemisia annua L.; diosgenin, a precursor for oral contraceptives and hormonal drugs, from Dioscorea spp.; and the cardiotonic steroidal glycoside digoxin, from Digitalis spp. Phenylpropanoids, derived from the shikimate pathway, include the antineoplastic lignan, podophyllotoxin, from Podophyllum peltatum L. Alkaloids, which are a diverse family of alkaline nitrogen-containing compounds, include many biologically active chemicals such as caffeine, coniine, morphine, nicotine, and strychnine. We focus here on the alkaloids, justified by their relatively high beneficial biological activities, and especially the isoquinoline alkaloids, the terpenoid indole alkaloids, and the tropane alkaloids, which are currently the subjects of intensive study (De Luca and Laflamme, 2001; Facchini, 2001; Facchini and St-Pierre, 2005; Hashimoto and Yamada, 2003; Kutchan, 2005a,b; Verpoorte and Memelink, 2002; Zhao et al., 2005). Excellent reviews of the potential of lignans in cell cultures (Petersen and Alfermann, 2001), as well as monoterpenes (Lange and Croteau, 1999) and taxol (Jennewein and Croteau, 2001), have already been published.