Secondary metabolites flavonoids

Plant metabolism can be separated into primary pathways that are found in all cells and deal with manipulating a uniform group of basic compounds, and secondary pathways that occur in specialized cells and produce a wide variety of unique compounds. The primary pathways deal with the metabolism of carbohydrates, lipids, proteins, and nucleic acids and act through the many-step reactions of glycolysis, the tricarboxylic acid cycle, the pentose phosphate shunt, and lipid, protein, and nucleic acid biosynthesis. In contrast, the secondary metabolites (e.g., terpenes, alkaloids, phenylpropanoids, lignin, flavonoids, coumarins, and related compounds) are produced by the shikimic, malonic, and mevalonic acid pathways, and the methylerythritol phosphate pathway (Fig. 3.1). This chapter concentrates on the synthesis and metabolism of phenolic compounds and on how the activities of these pathways and the compounds produced affect product quality. 

Flavonoids are secondary metabolites generally occurring in various plants as glycosides. The chemical structure of flavonoids shows high variety. The basic structure of flavons and flavonols is the 2-phenylbenzo-gamma-pyrone. Flavonoids generally contain two phenol rings linked with a linear three-carbon chain (chalcones) or with three carbon... 

Because of its importance, the application of planar chromatography for the analysis of various secondary metabolites in plants such as heterocyclic oxygen compounds (coumarins, flavonoids, anthocyanins, etc.) has been reviewed many times [143,144],... 

About 120 chemical constituents have been identified in chamomile as secondary metabolites, including 28 terpenoids, 36 flavonoids and 52 additional compounds [4]. A substantial part of drag effects are determined by the essential oil content. Oil is collected from flower heads, either by steam distillation or solvent extraction, for yields of 0.24-1.90% of fresh or dry plant tissue. Among the essential oil constituents the most active are /-/-a-bisabolol and chamazulene. /-/-a-bisabolol has demonstrated anti-inflammatory, antispasmodic, antimicrobial, antiulcer, sedative and CNS activity. Chamazulene is also anti-inflammatory. Topical applications of chamomile preparation have shown benefit in the treatment of eczema, dermatitis and ulceration [5]. 

The 4-coumarate CoA ligase (4CL EC 6.2.1.12) enzyme activates 4-coumaric acid, caffeic acid, ferrulic acid, and (in some cases) sinapic acid by the formation of CoA esters that serve as branch-point metabolites between the phenylpropanoid pathway and the synthesis of secondary metabolites [46, 47]. The reaction has an absolute requirement for Mg " and ATP as cofactors. Multiple isozymes are present in all plants where it has been studied, some of which have variable substrate specificities consistent with a potential role in controlling accumulation of secondary metabolite end-products. Examination of a navel orange EST database (CitEST) for flavonoid biosynthetic genes resulted in the identification of 10 tentative consensus sequences that potentially represent a multi-enzyme family [29]. Eurther biochemical characterization will be necessary to establish whether these genes have 4CL activity and, if so, whether preferential substrate usage is observed. 

Bennett, R.N. et al., Profiling glucosinolates, flavonoids, alkaloids, and other secondary metabolites in tissues of Azima tetracantha L. (Salvadoraceae), J. Agric. Food Chem., 52, 5856, 2004. 

Ormrod, D.P., Landry, L.G., and Conklin, P.L., Short-term UV-B radiation and ozone exposure effects on aromatic secondary metabolite accumulation and shoot growth of flavonoid-deficient Arabidopsis mutants, Physiol Plant., 93, 602, 1995. 

Aromatic Amino Acid Biosynthesis. The shikimate pathway is the biosynthetic route to the aromatic amino acids tryptophan, tyrosine and phenylalanine as well as a large number of secondary metabolites such as flavonoids, anthocyanins, auxins and alkaloids. One enzyme in this pathway is 5-enolpyruvyl shikimate-3-phosphate synthase (EPSP synthase) (Figure 2.9). 

The bioactive secondary metabolites reported from Broussonetia kazinoki can be classified into major two groups, alkaloids and flavonoids (Table 1), Fig. (1). The Kusano group at Osaka University of Pharmaceutical Sciences in Japan reported over 20 pyrrolidine alkaloids, broussonetines A-H, K-M, O-T, V-X, and Mi, and broussonetinines A and B, four pyrrolidinyl piperidine alkaloids, broussonetines I, J, Ji, and J2, two pyrroline alkaloids, broussonetines U and U, and one pyrrolizidine alkaloid, broussonetine N, from hot water extracts of B. kazinoki [16-24]. As shown in Table 1, some of these alkaloids exhibited strong... 

Phytochemical studies of these species led to the isolation and structural characterization, by NMR and MS analysis, of many secondary metabolites, mainly flavonoids, and expecially flavonol glycosides, clerodane diterpenes, and triterpenes having the lupane, ursane, and oleanane skeletons. Particularly flavonoids and their glycosides have a chemotaxonomic interest in the genus and in general in the Chrysobalanaceae family. 

In the course of our phytochemical work we studied seven Licania species all belonging to Venezuelan flora, collected in Puerto Ayacucho, Estado Amazonas and in the Parque Nacional Henry Pittieri, Maracay, Estado Aragua. A number of new and known secondary metabolites, mainly flavonoids, especially flavonol glycosides, sterols, and triterpenes having the lupane, ursane, and oleanane skeleton were isolated and characterized. The last part of this chapter deals also with the isolation of clerodane diterpenes from the methanol extract of L. intrapetiolaris by Oberlies et al., 2001 [4]. All the Licania species investigated up to now are reported in Table 1. 

In addition to the molecular techniques, technical advances both in chromatographic techniques and in identification tools, particularly the diverse forms of mass spectrometry, has allowed successful challenges to the separation and characterization of compounds of increasing complexity, poor stability, and low abundance [Whiting, 2001]. Information generated utilizing these techniques has resulted in characterization of a plethora of complex secondary metabolites that, in conjunction with the characterization of the enzymatic steps, has permitted the complete or partial elucidation of the flavonoid and the phenolic pathways present in many plants (Figs. 1.35 and 1.36). 

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