Carotenoids in Higher Plants

Abstract

Of nearly 450 naturally occurring carotenoids, only one quarter have been found in higher plants (for list and structures, see Davies, [24]). Higher plant carotenoids were prominent among the carotenoids whose structures were elucidated by the classical chemical approaches of Karrer and of Kuhn [50] and, by 1952, as many as 60 of the 95 carotenoids then known had been isolated from higher plants [38]. This situation resulted not so much from an early intrinsic interest in these particular pigments as from the fact that higher plant tissues were available on a sufficient scale to provide the large amounts of pure carotenoid necessary for structural determinations by chemical methods. Since the advent of small-scale preparative (e.g., thin-layer chromatography) and physico-organic methods (e.g., mass spectrometry) and with the ready availability , through the culture collections, of a wealth of pure strains and mutants of microorganisms, the interests of carotenoid chemists have broadened to include algae, bacteria, and fungi. It is from smaller-scale studies of the pigments of such organisms that many unusual carotenoid structures have emerged in recent years [57]. Nevertheless, the higher plants, because of their fundamental importance to man, are of continuing interest. This is reflected in the many recent studies that have contributed to our current understanding of the distribution , metabolism, and function of higher plant carotenoids. A. Nomenclature Because many of the established trivial names of higher plant (and other) carot-enoids are related more to their original source (e.g., chrysanthemaxanthin, eschscholtzxanthin) than to their chemical structure, the introduction of a standardized semi-systematic nomenclature [22, 23] is a welcome rationalization. Although nearly 450 carotenoid structures exist in nature, all are based on only 7 different end groups and, of these, only 4 are found in higher plant carotenoids W, £, K and tjJ; see Fig. 1). The end groups at either extremity of the (fully unsaturated all-trans) polyene chain are used to form the C40 carotene (hydrocarbon) stem names (e.g., f3,/3-carotene, /3,iO-carotene, tjJ,tjJ-carotene). These names are modified by appropriate prefixes and suffixes (with locant designation; see Fig. 1) to denote changes in the carbon skeleton (seco-, apo-), hydrogenation level (hydro-, dehydro-, retro-), oxygenation (ofxanthophylls; epoxy- ,-01,-one,-oic acid), and configuration (cis, trans, R, S).