Betalains of Celosia argentea

Abstract

The betalains of yellow, orange and red inflorescences of common cockscomb (Celosia argentea var. cristata) were compared and proved to be qualitatively identical to those of feathered amaranth (Celosia argentea var. plumosa). In addition to the known compounds amaranthin and betalamic acid, the structures of three yellow pigments were elucidated to be immonium conjugates of betalamic acid with dopamine, 3-methoxytyramine and (S)-tryptophan by various spectroscopic techniques and comparison to synthesized reference compounds; the latter two are new to plants. Among the betacyanins occurring in yellow inflorescences in trace amounts, the presence of 2-descarboxy-betanidin, a dopamine-derived betacyanin, has been ascertained. The detection of high dopamine concentration may be of toxicological relevance in use of yellow inflorescences as a vegetable and in traditional Chinese medicine, common uses for the red inflorescences of common cockscomb.

Introduction

The plant genus Celosia consisting of about 60 species in the family Amaranthaceae (Caryophyllales) is native in subtropical and temperate zones of Africa, South America and South East Asia. Celosia argentea var. cristata (L.) Kuntze, (Celosia cristata L.) (common cockscomb), and Celosia argentea var. plumosa (Burvenich) Voss, (Celosia plumosa Burvenich) (feathered amaranth), are widespread ornamental plants. Seedlings, young leaves and inflorescences are used as a vegetable in China and other countries (Palada and Crossman, 1999), and also dried leaves, inflorescences and seeds are used in traditional Chinese medicine (Wong, 1994, Xu, 1996). Among the garden varieties, inflorescence colour variation from yellow to various shades of red and violet occurs. At an early date (Wehmer, 1929) it was claimed that red inflorescences of common cockscomb contain a nitrogen-containing pigment. Later, amaranthin and betanin were found in red inflorescences, whereas violet ones contain additional hydroxycinnamoyl-amaranthins (celosianins) (Piattelli and Minale, 1964, Minale et al., 1966). The structures of the latter were presumed to be identical with compounds from cell suspension cultures of Chenopodium rubrum L. identified as 2″-O-E-(4-coumaroyl)-amaranthin (celosianin I) and 2″-O-E-feruloyl-amaranthin (celosianin II) (Bokern et al., 1988, Strack et al., 1988). Protein extracts from inflorescences of C. plumosa catalysed the transfer of ferulic acid from 1-O-feruloyl-β-glucose to amaranthin forming celosianin II (Bokern et al., 1992). The knowledge of betaxanthins in the Amaranthaceae family is very limited (Reznik, 1975), although betalamic acid has been electrophoretically detected in Celosia species (Reznik, 1978). Yellow betaxanthin-synthesizing seedlings of C. plumosa reacted upon illumination with a 3–4-fold increase in betaxanthin content (Giudici de Nicola et al., 1973, Giudici de Nicola et al., 1974). Dopa feeding to inflorescences of Amaranthus caudatus and Celosia argentea did not induce betaxanthin formation (Rink and Böhm, 1991), whereas the administration of this central intermediate to seedlings of A. caudatus led to the occurrence of a betaxanthin mixture (French et al., 1974, Colomas, 1977, Bianco-Colomas, 1980) from which vulgaxanthin II [(S)-Glu-BX] and miraxanthin II [(S)-Asp-BX] were identified by comparison to authentic samples, but dopaxanthin [(S)-Dopa-BX] was entirely absent (Giudici de Nicola et al., 1975). Cell cultures of A. caudatus and Celosia argentea showed only pigmentation by betalains in auxin-free, but GA₃-containing nutrient solutions (Constabel and Nassif-Makki, 1971, Bohm and Rink, 1988). As no betaxanthins from Celosia have so far been identified, structural elucidation of the betaxanthins of two Celosia varieties was performed, which led to two new betaxanthins. In addition, the levels of pharmacologically active precursors were determined.