Carotenoid Cleavage Products: An Introduction - ACS Symposium

Chapter 1

Carotenoid Cleavage Products: An Introduction

Downloaded by UMEA UNIV on September 25, 2013 | http://pubs.acs.org Publication Date (Web): July 8, 2013 | doi: 10.1021/bk-2013-1134.ch001

Peter Winterhalter*,1 and Susan E. Ebeler2 1Institut

für Lebensmittelchemie, Technische Universität Braunschweig, Schleinitzstraße 20, 38106 Braunschweig, Germany 2Department of Viticulture and Enology, University of California, Davis, One Shields Avenue, Davis, California 95616, United States *E-mail: [email protected].

During the last decade there has been tremendous progress in the area of bio-oxidative carotenoid cleavage and the resulting cleavage products. This introductory chapter will briefly describe the major classes of carotenoid metabolites as well as the major achievements in this field, i.e. the discovery of specific carotenoid cleavage enzymes and novel plant hormones, as well as several unexpected new functions of carotenoid metabolites.

The last ACS symposium on carotenoid metabolites in the year 2001 has had a strong focus on volatile carotenoid cleavage products. These compounds are widespread in the plant kingdom and constitute important ingredients for the flavor and perfume industry (1). Although the evidence for a formation via bio-oxidative carotenoid cleavage was very strong at that time, the responsible cleavage enzymes were still not known. Only in recent years the so-called carotenoid cleavage dioxygenases (CCDs) have been identified and a new insight into carotenoid metabolism has been obtained. The discovery of CCDs together with the detection of a novel group of plant hormones (strigolactones) lead to an enormous interest in carotenoid metabolism and the number of publications covering carotenoid cleavage grew almost exponentially. In view of these developments this symposium proceedings will try to briefly highlight recent discoveries in this rapidly growing research field.

© 2013 American Chemical Society In Carotenoid Cleavage Products; Winterhalter, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Carotenoid Cleavage Enzymes Two different classes of carotenoid cleavage enzymes are known to be present in plants. The first class represents the so-called 9-cis-epoxycarotenoid dioxygenases (NCEDs) responsible for abscisic acid formation from neoxanthin and violaxanthin through cleavage of the carotenoid chain at the 11,12-position (2). The second class of cleavage enzymes are the so-called carotenoid cleavage dioxygenases (CCDs). The first CCD characterized on a molecular level was from Arabidopsis thaliana and labeled as AtCCD1 (3). When expressed in E. coli, AtCCD1 was found to cleave several carotenoids in the 9,10 and 9′,10′-position giving rise to a C14-dialdehyde and two C13-carotenoid endgroups. CCD1 is a non-heme enzyme, requiring only Fe2+ as a cofactor. Convincing evidence for a dioxygenase mechanism has been obtained from labeling experiments (4). In total, the family of carotenoid cleavage enzymes in A. thaliana consists of 9 members (CCD 1, 4, 7, 8 and NCED 2, 3, 5, 6, and 9) (5, 6). For an overview cf. refs. (7–9).

Figure 1. Examples for carotenoid-derived metabolites.

4 In Carotenoid Cleavage Products; Winterhalter, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Carotenoid Cleavage Products Plant carotenoids are not only essential for photosynthesis and photoprotection (10) they also give rise to the formation of numerous biologically active cleavage products which inter alia include aroma compounds, vitamins, phytohormones, and apocarotenoid pigments (Figure 1).


Aroma Compounds A comprehensive overview on the occurrence and formation of carotenoidderived aroma compounds from non-volatile precursors can be found in the ACS Symposium Volume 802 (1). Some structures of important volatiles are outlined in Figure 1. They include inter alia the C10-compound safranal and the C13apocarotenoids ß-ionone and ß-damascenone which are obtained by cleavage of the carotenoid chain in the 7,8/7′,8′- and 9,10/9′,10′-position, respectively.

Figure 2. Formation of key flavor compounds from the respective parent carotenoid. 5 In Carotenoid Cleavage Products; Winterhalter, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


In many cases, a three-step mechanism is required for the formation of the aroma compounds in food: (i) CCD cleavage of the parent carotenoid giving rise to a primary cleavage product, (ii) enzymatic transformation (e.g., reduction and glycosylation) in plant tissues, and (iii) acid-catalyzed transformation during processing of food. Examples are shown in Figure 2. Whereas most of the aroma active apocarotenoids are derived from the carotenoid endgroup, a limited number is also obtained from the central portion of the carotenoid chain, such as, e.g., the odoriferous marmelo oxides and marmelo lactones (Figure 1). With regard to CCD cleavage in planta a step-wise cleavage of the carotenoid has been suggested. Because of its localization in the cytosol, CCD1 is discussed as being mainly responsible for the cleavage of C27-apocarotenoids. The latter are most likely generated by the action of a plastidial CCD (possible candidates CCD4 or CCD7) which have direct access to the intact carotenoids in the plastids and the so-obtained C27-fragment is then expected to be translocated into the cytosol (8, 9). This hypothesis is still under evaluation.

Figure 3. First steps in the biogeneration of strigolactones. 6 In Carotenoid Cleavage Products; Winterhalter, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 4. Structure of retinoids.

Plant Hormones Apart from the thoroughly studied plant hormone S-(+)-abscisic acid (11) that is formed by NCED-cleavage of 9-cis-violaxanthin and 9-cis-neoxanthin (2), the pathway of formation and function of a novel group of carotenoid-derived plant hormones, i.e. strigolactones, has only recently been elucidated (12, 13). Strigolactones are known since the 1960s as allelochemicals that are secreted from the roots to the rhizosphere. To date, 15 different members have been structurally characterized. Strigolactones were first found to have crucial functions in induction of parasitic plant germination (e.g., witchweed) as well as in establishing the symbiosis with arbuscular mycorrhizal (AM) fungi. In 2008, strigolactones have been identified as plant hormones being responsible for shoot branching. Plants obviously regulate production of strigolactones in response to changes in nutrient supply. Under nutrient starvation, plants produce more strigolactones in order to minimize shoot branching and to promote symbiosis with AM fungi. 7 In Carotenoid Cleavage Products; Winterhalter, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

For the biogeneration of strigolactones the pathway outlined in Figure 3 has been elucidated in the year 2012 (14). After isomerization of all-trans-ß-carotene to the 9-cis-isomer by the isomerase D27, action of CCD7 gives rise to the formation of 9-cis-ß-apo-10′-carotenal, the latter being directly converted into carlactone by CCD8. Conversion of carlactone into the likely precursor of all other strigolactones, i.e., 5-deoxystrigol, is still under active investigation.


Retinoids Retinoids are a family of signaling molecules that are related to vitamin A. The structures of retinoids are outlined in Figure 4. Retinoids are derived from vitamin A which is formed by central cleavage of ß-carotene. Molecular identification of an enzyme cleaving ß-carotene to retinal has first been described in the year 2000 (15). This ß-carotene dioxygenase has been obtained from Drosophila melanogaster by expressing it into an E. coli strain. Whereas vitamin A is vital for vision and immune function, conversion to retinoic acid opens an avenue to even more active compounds which exert multiple effects on embryonic development, cell proliferation, differentiation and apoptosis. Retinoids are thus used in the treatment of human cancers (16).

New Functions of Carotenoid Metabolites Investigation of the biological activity of carotenoid cleavage products is a rapidly growing research field which will continue to produce novel and sometimes also unexpected results. In the case of volatile carotenoid cleavage products a screening of aroma compounds being present in apple fruit revealed a cancer chemopreventive potential of members of the damascone group, such as ß-damascenone, 3-HO-ß-damascone, and related substances (17). Other C13-oxygenated apocarotenoids were identified as allelochemicals (18, 19) and even the plant hormone abscisic acid has recently been found to possess an anti-inflammatory activity in mouse models (20). In view of this, carotenoid metabolites will continue to be a hot research topic in the future.

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Carotenoid Cleavage Products: An Introduction - ACS Symposium

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