Recent advances in betalain research

Abstract

Betalains replace the anthocyanins in flowers and fruits of plants of most families of the Caryophyllales. Unexpectedly, they were also found in some higher fungi. Whereas the anthocyanin-analogous functions of betalains in flower and fruit colouration are obvious, their role in fungi remains obscure. The nature of newly identified betalains as well as final structure elucidation of earlier putatively described compounds published within the last decade is compiled in this report. Recent advances in research on betalain biosynthesis is also covered, including description of some ‘early’ reactions, i.e. betalain-specific dopa formation in plants and fungi and extradiolic dopa cleavage in fungi. Work on betalain-specific glucosyltransferases (GTs) has given new insights into the evolution of secondary plant enzymes. It is proposed that these GTs are phylogenetically related to flavonoid GTs. It was found that the decisive steps in betalain biosynthesis, i.e. condensation of the betalain chromophore betalamic acid with cyclo-dopa and amino acids or amines in the respective aldimine formation of the red-violet betacyanins and the yellow betaxanthins, are most likely to be non-enzymatic. Betalains have attracted workers in applied fields because of their use for food colouring and their antioxidant and radical scavenging properties for protection against certain oxidative stress-related disorders.

Introduction

Betalains are water-soluble nitrogen-containing pigments, which comprise the red-violet betacyanins and the yellow betaxanthins. They are immonium conjugates of betalamic acid with cyclo-dopa and amino acids or amines, respectively. The structure scheme shows betalamic acid (1), the chromophore of all betalains, betanidin (2) (Wyler et al., 1963), the aglycone of most of the betacyanins, and indicaxanthin (3) (Piattelli et al., 1964), a proline-containing betaxanthin. These were the first betalains identified by chemical means. Since then, betaxanthins with various other amino acids and amines as well as various betanidin conjugates (glycosides and acylglycosides) were identified by spectroscopic techniques (Strack et al., 1993, Strack and Wray, 1994a).

Within the last decade, a series of publications has described new structures and complete identification of some putative ones as well as elucidation of biosynthetic reactions (Roberts and Strack, 1999). Some early enzymatic reactions in betalain biosynthesis were characterized, polyphenoloxidase (PPO)-type tyrosinase and extradiolic dopa dioxygenase. Furthermore, the decisive steps in the biosynthesis of both betanidin and betaxanthins by aldimine formation were recently identified to proceed non-enzymatically. Finally, betalain-specific glucosyl- and hydroxycinnamoyltransferases were characterized.

Betalains accumulate in flowers, fruits and occasionally in vegetative tissues of plants belonging to most families of the Caryophyllales (Steglich and Strack, 1990). However, there are two exceptions: the Caryophyllaceae and the Molluginaceae accumulate anthocyanins instead, flavonoid-derived pigments occurring in all other families of flowering plants. The Caryophyllales-specific occurrence of betalains is a prominent example of the chemotaxonomic relevance of plant secondary products. Gain and loss of anthocyanins and betalains during plant evolution still remain a mystery (Clement and Mabry, 1996). Molecular studies are needed to elucidate the evolutionary mechanisms leading to the mutual exclusion of the betalain and anthocyanin pathways in flowering plants (Kimler et al., 1970, Stafford, 1994). Betalains were also detected in some higher fungi (Steglich and Strack, 1990), for example in the fly agaric (Amanita muscaria). Whereas the functions of betalains in plant flower and fruit colouration are obvious, their role in fungi is unknown.

There is growing interest in the use of natural pigments for food colouring, since synthetic dyes are becoming more and more critically assessed by the consumer. In food processing, betalains are less commonly used than anthocyanins and carotenoids, although these water-soluble pigments, stable between pH 3 and 7, are well suited for colouring low acid food. The most important source of betanin (4) as colouring agent is the red beet (Beta vulgaris subsp. vulgaris) root. The corresponding cell cultures (Leathers et al., 1992, Trejo-Tapia et al., 1999, Akita et al., 2000) and hairy roots (Mukundan et al., 1998a, Mukundan et al., 1998b, Berzin et al., 1999) cannot compete with the plant's root (50-60 t/ha with ca. 0.5 g betanin/kg) (Stintzing et al., 2000) with respect to betanin accumulation. Besides betanin from red beet, also betacyanins from plants of the Amaranthaceae were tested concerning colour properties and pigment stability in model food systems (Cai et al., 1998, Cai and Corke, 1999, Cai and Corke, 2000).

Betacyanins are a class of compounds with antioxidant and radical scavenging activities (Pedreni ands Escribano, 2000, Escribano et al., 1998, Kanner et al., 2001). It is known that they prevent oxidative processes, which contribute to the onset of several degenerative diseases in human. Since betanin exerts a good bioavailability, red beet products may provide protection against certain oxidative stress-related disorders (Kanner et al., 2001).