INTRODUCTION Flavonoids in Herbs

I. INTRODUCTION

Flavonoids are a large group of polyphenolic compounds that occur commonly in plants. This group contains more than 8000 known compounds, and this number is constantly growing because of the great structural diversity arising from the various hydroxylation, methoxylation, glycosylation, and acylation patterns.

Flavonoids are the pigments responsible for the shades of yellow, orange, and red in flowering plants. They are also important factors for plant growth, development, and defense. Many flavonoids are endowed with biological activities, such as anti-inflammatory, antiallergic, antischemic, antiplatelet, immunomodulatory, and antitumoral activities [1–3]. Flavonoids have also been shown to inhibit several enzymes, including lipoxygenases and cyclooxygenases, mono-oxygenases, xanthine oxidase, mitochondrial succinoxidase, reduced nicotinamide-adenine dinucleotide (NADH) oxidase, phospholipase A2, topoisomerases, and protein kinases [4–6]. The biological activities of flavonoids are thoughy to be due mainly to their antioxidant properties [7–8], which are displayed by limiting the production of reactive oxygen species (ROS) and/or scavenging them.

Flavonoids are components of the diet of numerous herbivores and omnivores, including humans [9]. They are principally found in fruits, vegetables, and popular drinks, such as red wine, tea, beer, and their intake may reach 1 g/day [10]. In addition, flavonoids are present in various herbs* [11].

Approximately 50 species, from Achillea millefolium to Viola tricolor, have been used as herbal remedies for their flavonoid content; some are listed in

Table 1. These preparations have been reported to be effective for the treatment of disorders of peripheral circulation and for the improvement of aquaresis. In addition, flavonoid-based herbal medicines are available in different countries as anti-inflammatory, antispasmodic, antiallergic, and antiviral remedies [12–14].

The pharmacological effects of these phytomedicines are ascribed either to their functions as radical scavengers, reductants, and metal chelators or to alternative nonantioxidant functions, including the interaction with different enzymes, the inhibition of calcium ion influx into the cells, and the regulation of cell signaling [15] and gene expression [16]. However, it should be remembered that the health benefit properties of most medicinal plants high in flavonoids cannot be assigned exclusively to these compounds, since other components present in the phytocomplex may either directly contribute to or display a ‘‘permissive’’ role that enhances the effects of flavonoids. When examining different examples including aquaretic, anti-inflammatory, sedative, and antispasmodic herbs, it is found that the observed pharmacological effect is due to flavonoidic and nonflavonoidic constituents [17].

As natural products, herbs can greatly differ in their composition as a result of genetic factors, climate, soil quality, and other external factors. Therefore, controlled cultivation and selection represent the first steps to ensuring the most consistent concentration of specific ingredients or groups of compounds.

Second, the production of the herbal ingredients by extracting the herbs with solvents must be carefully monitored to select the components that are important to the action and the efficacy of the product. To achieve consistent pharmaceutical quality, the analytical quality control is essential. This is not an easy task, as herbs and related extracts are complex mixtures of constituents with different physicochemical (i.e., analytical) characteristics. With flavonoidcontaining herbs, however, phytochemical data are largely available: i.e., the chemical nature of flavonoids present in these herbs is known. Almost all the flavonoid classes are present in herbs with proven therapeutic activity, including flavonols, flavones, and their dihydroderivatives; isoflavones; catechins; flavanolignans; and anthocyanins. Some of the additional phytochemicals are closely related to flavonoids such as phenolic and hydroxycinnamic acids, whereas others have different chemical natures, including various terpenes (mostly present in volatile oils), coumarin derivatives, phytosterols, and other speciescharacteristic constituents.

The analysis of the flavonoid fraction in the raw herbs and in standardized (i.e., having known potency) extracts may be accomplished by using different approaches [18–19], including high-performance liquid chromatography (HPLC), capillary electrophoresis (CE), and mass spectrometry. HPLC coupled with ‘‘online’’ ultraviolet (UV) detection and/or mass spectrometry (MS) allows data on the chromatographic, UV, and MS behavior of the analytes to be obtained from a single run. This approach remains the method of choice (1) to obtain typical ‘‘fingerprints’’ for the herbal ingredient; (2) to assay single flavonoids; and (3) to detect evidence the presence of adulterants. CE has been proved a valuable alternative to HPLC, because of its high selective power, which allows detection of some flavonoids not separable by HPLC. Unfortunately, CE has not become as popular as HPLC, which remains the technique of choice for routine quality control of flavonoid-containing vegetables [20].

Typical examples of flavonoid herbs examined by HPLC or CE have already been described in a previous contribution of this volume series [11]. This chapter aims to describe three mass techniques, the electrospray ionization MS, (ESI-MS), atmospheric pressure chemical ionization MS (APCI-MS), and ion trap MS (ITMS) techniques, and their application to the analysis of flavonoids in some standardized herbal extracts with proven therapeutic efficacy. In addition, the flavonoid composition of some commonly consumed vegetables with aromatic or savory properties (culinary herbs) is described.