Elsevier

Phytochemistry

Volume 56, Issue 1, January 2001, Pages 5-51
Phytochemistry

Review
The chemical diversity and distribution of glucosinolates and isothiocyanates among plants

https://doi.org/10.1016/S0031-9422(00)00316-2Get rights and content

Abstract

Glucosinolates (β-thioglucoside-N-hydroxysulfates), the precursors of isothiocyanates, are present in sixteen families of dicotyledonous angiosperms including a large number of edible species. At least 120 different glucosinolates have been identified in these plants, although closely related taxonomic groups typically contain only a small number of such compounds. Glucosinolates and/or their breakdown products have long been known for their fungicidal, bacteriocidal, nematocidal and allelopathic properties and have recently attracted intense research interest because of their cancer chemoprotective attributes. Numerous reviews have addressed the occurrence of glucosinolates in vegetables, primarily the family Brassicaceae (syn. Cruciferae; including Brassica spp and Raphanus spp). The major focus of much previous research has been on the negative aspects of these compounds because of the prevalence of certain “antinutritional” or goitrogenic glucosinolates in the protein-rich defatted meal from widely grown oilseed crops and in some domesticated vegetable crops. There is, however, an opposite and positive side of this picture represented by the therapeutic and prophylactic properties of other “nutritional” or “functional” glucosinolates. This review addresses the complex array of these biologically active and chemically diverse compounds many of which have been identified during the past three decades in other families. In addition to the Brassica vegetables, these glucosinolates have been found in hundreds of species, many of which are edible or could provide substantial quantities of glucosinolates for isolation, for biological evaluation, and potential application as chemoprotective or other dietary or pharmacological agents.

Introduction

The first observations on the unique properties of glucosinolates and isothiocyanates or mustard oils, as they are commonly known, were recorded at the beginning of the 17th century as a result of efforts to understand the chemical origin of the sharp taste of mustard seeds. The discovery and early history of glucosinolates and the participation of the enzyme myrosinase (a β-thioglucosidase) in their conversion to isothiocyanates, are the subjects of an interesting and scholarly review by Challenger (1959). The glucosinolates known by the trivial names sinigrin (2-propenyl or allyl glucosinolate; 1, Fig. 1) and sinalbin (4-hydroxybenzyl glucosinolate; 1, Fig. 1) were isolated early in the 1830s from black (Brassica nigra) and white (Sinapis alba) mustard seeds, respectively.2 (See Table 1, Table 2 for glucosinolate numbers used throughout this review.) The first general, although incorrect, structure for these compounds was proposed at the end of the nineteenth century by Gadamer (1897), who concluded that the side chain was linked to the nitrogen rather than the carbon atom of the “NCS” group. Despite certain difficulties the structure was generally assumed to be correct until 1956, when Ettlinger and Lundeen (1956a) pointed out the inadequacies of the Gadamer structure to explain certain properties of these compounds, proposed the now correct structure, and described the first chemical synthesis of a glucosinolate (Ettlinger and Lundeen, 1957). The remaining structural issue of the geometrical isomerism at the Cdouble bondN bond was established to be Z (or anti-) by X-ray crystallographic analysis of sinigrin (see Fig. 1; Marsh and Waser, 1970).

Glucosinolates are β-thioglucoside N-hydroxysulfates [also known as (Z)-(or cis)-N-hydroximinosulfate esters or S-glucopyranosyl thiohydroximates], with a side chain (R) and a sulfur-linked β-d-glucopyranose moiety.

In the last 40 years, a succession of reviews have addressed the biology and chemistry of glucosinolates (e.g. Kjær, 1961, Kjær, 1974; Ettlinger and Kjær, 1968, Kjær and Olesen Larsen, 1973, Kjær and Olesen Larsen, 1976; Underhill et al., 1973; Underhill, 1980; Fenwick et al., 1983; Chew, 1988; Duncan and Milne, 1989; Brown and Morra, 1997; Halkier, 1999; Mithen et al., 2000), and their distribution among plants (Rodman, 1981). More narrowly focused reviews have examined the indole glucosinolates (McDanell et al., 1988), or specifically glucosinolates in the family Brassicaceae (Kjær, 1976). Similar coverage (i.e. of glucosinolates of crop plants, primarily the Brassica vegetables) has been provided by Stoewsand (1995) and Rosa et al. (1997). Many other even more narrowly focused reviews have concentrated on specific plant families or on specific aspects of glucosinolate biology and they are referenced herein, as appropriate.

The present review provides a comprehensive survey of the chemical structures of all known glucosinolates and the plant families from which they have been isolated. It provides a single source of their chemical structure, their trivial names, and groups these compounds into families according to their structural similarities. We discuss, mostly by reference, the state of scientific understanding of the synthesis, biosynthesis and ecological importance of glucosinolates and their conversion to isothiocyanates and other products by myrosinase. To our knowledge, there has been no recent effort to provide a comprehensive compilation and cataloging of isolated glucosinolates, their structures, systematic and trivial (common) names, and their distribution among plant species. Although we have attempted to do so herein, undoubtedly there are omissions. Since many of these compounds were identified before modern spectroscopic techniques were available, some of the structural assignments of glucosinolates to plant taxa may require revision.

Section snippets

Glucosinolate distribution among plants

There is now a voluminous literature on the glucosinolates of the plant family, Brassicaceae, which alone contains more than 350 genera and 3000 species. Of the many hundreds of cruciferous species investigated, all are able to synthesize glucosinolates (Kjær, 1976). Among the Brassicaceae, the genus Brassica contains a large number of the commonly consumed species. Brassica sp. glucosinolates have been the subject of scholarly reviews by Kjær, 1974, Kjær, 1976), Fenwick et al. (1983), Chew

Types of glucosinolates

We have grouped glucosinolates into a number of chemical classes on the basis of structural similarities. The most extensively studied glucosinolates are the aliphatic, ω-methylthioalkyl, aromatic and heterocyclic (e.g. indole) glucosinolates, typified by those found in the Brassica vegetables (e.g. compounds 1, Fig. 1 in Table 1 and Fig. 1). Glucosinolate side chains, however, are characterized by a wide variety of chemical structures (Fig. 1). The most numerous glucosinolates are those

Hydrolysis of glucosinolates by plant and microbial myrosinases

Glucosinolates are very stable water-soluble precursors of isothiocyanates, and are typically present in fresh plants at much higher levels than their cognate isothiocyanates. Under carefully controlled conditions designed to extract glucosinolates and isothiocyanates completely, while preventing myrosinase activity, some fresh plants have been shown to contain almost exclusively glucosinolates (Fahey et al., 1997). The relatively nonreactive glucosinolates are converted to isothiocyanates upon

Glucosinolate content of plants

Glucosinolate content in plants is about 1% of dry weight in some tissues of the Brassica vegetables (Rosa et al., 1997), although the content is highly variable (Kushad et al., 1999, Farnham et al., 2000), and can approach 10% in the seeds of some plants, where glucosinolates may represent one-half of the sulfur content of the seeds (Josefsson, 1970). Most species contain a limited number of glucosinolates (generally less than one dozen) although as many as 23 different glucosinolates have

Biotic interactions of glucosinolates and isothiocyanates

The antibacterial activities of glucosinolates/isothiocyanates (Kjær and Conti, 1954; Procházka and Komersová, 1959; Virtanen, 1962; Wagner et al., 1965; Dornberger et al., 1975; Johns et al., 1982; Uda et al, 1993; Brabban and Edwards, 1995; Delaquis and Mazza, 1995; Hashem and Saleh, 1999; Lin et al., 2000) and their antifungal activity (Drobinca et al., 1967; Kojima and Ogawa, 1971; Mari et al, 1993; Delaquis and Mazza, 1995; Mayton et al., 1996; Manici et al., 1997; Hashem and Saleh, 1999)

Analytical methods

Since the work of Ettlinger and Lundeen, 1956a, Ettlinger and Lundeen, 1956b, Ettlinger and Lundeen, 1957), much effort has been devoted to developing methods for the efficient isolation and identification of glucosinolates (Betz and Fox, 1994). Most early identifications relied on paper or thin-layer chromatography of the glucosinolates or of their presumptive hydrolysis products (e.g. an investigation of the glucosinolates from the seeds of 151 different crucifers by Danielak and Borkowski,

Glucosinolates/isothiocyanates and cancer chemoprotection

Over the past 20 years, compelling evidence has been obtained linking increased consumption of fruits and vegetables, especially cruciferous vegetables, to reduced incidence of many types of cancer (Steinmetz and Potter, 1991, Steinmetz and Potter, 1996; Block et al., 1992; Doll, 1992; Verhoeven et al., 1996; Michaud et al., 1999; Talalay, 1999). Ingestion of about two servings per day of cruciferous vegetables may result in as much as a 50% reduction in the relative risk for cancer at certain

Concluding remarks

The genus Brassica, represents only 1 of over 350 genera in the Brassicaceae family which, in turn, is only 1 of 16 families of glucosinolate-containing higher plants (Table 3). Many glucosinolate-containing genera contain plants that have been used for food or medicinal purposes by various cultures for many centuries (e.g. capers, Capparis spinosa; wasabi, Wasabia japonica; Arugula, Eruca sativa; Radish, Raphanus sativus) and are currently being investigated for their fungicidal,

Acknowledgements

The assistance of Pamela Talalay, Kristina L. Wade and Katherine K. Stephenson in critical reading of the manuscript and in final manuscript preparation is gratefully acknowledged. Work in the authors’ laboratories was supported by generous gifts from the Lewis and Dorothy Cullman Foundation, Charles B. Benenson and other Friends of the Brassica Chemoprotection Laboratory and by a Program Project grant (PO1 CA 44530) from the National Cancer Institute, Department of Health and Human Services,

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    This paper is dedicated to Professor Anders Kjær, who, with his collaborators in Lyngby, Denmark, has contributed immeasurably to the scientific community’s understanding of glucosinolates and to knowledge of their chemistry, biosynthesis, metabolism, and their relationship to the plants from which they were isolated; more glucosinolates have been isolated and characterized in Professor Kjær’s laboratory than anywhere else in the world.

    1

    Present address: Jefferson Medical College, Thomas Jefferson University, 1020 Locust Street, Philadelphia, PA 19107, USA.

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