Vitamin A and Carotenoids

Chemistry, Analysis, Function and Effects

By Victor R. Preedy

The Royal Society of Chemistry

Copyright © 2012 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-368-7

Contents

Vitamin A and Carotenoids in Context,
Chapter 1 Retinol, Retinoic Acid, Carotenes and Carotenoids: Vitamin A Structure and Terminology Gerald Wollard, 3,
Chapter 2 Vitamin A in the Context of Other Vitamins and Minerals Jennifer H. Lin and Kuang-Yu Liu, 23,
Chapter 3 The Importance of β-Carotene in the Context of Vitamin A Hans K. Biesalski and Donatus Nohr, 39,
Chapter 4 Vitamin A in the Context of Supplementation Frank T. Wieringa, Marjoleine A. Dijkhuizen and Jacques Berger, 55,
Chemistry and Biochemistry,
Chapter 5 The Chemistry of Vitamin A Alessandra Gentili, 73,
Chapter 6 Nomenclature of Vitamin A and Related Metabolites Niketa A. Patel, 90,
Analysis,
Chapter 7 Structural Analysis of Vitamin A Complexes with DNA and RNA H. A. Tajmir-Riahi and P. Bourassa, 97,
Chapter 8 Encapsulation of Vitamin A: A Current Review on Technologies and Applications Beatrice Albertini, Marcello Di Sabatino and Nadia Passerini, 113,
Chapter 9 Thermal Degradation of β-Carotene in Food Oils Alam Zeb, 129,
Chapter 10 Provitamin A Carotenoids: Occurrence, Intake and Bioavailability Torsten Bohn, 142,
Chapter 11 Vitamin A – Serum Vitamin A Analysis Ronda F. Greaves, 162,
Chapter 12 Liquid Chromatography-based Assay for Carotenoids in Human Blood Taiki Miyazawa, Kiyotaka Nakagawa and Teruo Miyazawa, 184,
Chapter 13 Capillary Liquid Chromatographic Analysis of Fat-soluble Vitamins and β-Carotene Sheng Zhang and Li Jia, 204,
Chapter 14 Assay of Carotenoid Composition and Retinol Equivalents in Plants Sangeetha Ravi Kumar and V. Baskaran, 221,
Chapter 15 LC-NMR for the Analysis of Carotenoids in Foods Chisato Tode and Makiko Sugiura, 250,
Chapter 16 LC-DAD-tandem MS Analysis of Retinoids and Carotenoids: Applications to Bovine Milk Alessandra Gentili and Fulvia Caretti, 261,
Chapter 17 HPLC-DAD-MS (ESI+) Determination of Carotenoids in Fruit Pasquale Crupi, Victor R. Preedy and Donato Antonacci, 282,
Chapter 18 Thin-layer Chromatographic Analysis of Pro-vitamin A Carotenoids Alam Zeb, 303,
Chapter 19 Extraction of Carotenoids from Plants: a Focus on Carotenoids with Vitamin A Activity Anita Oberholster, 316,
Chapter 20 Quantification of Carotenoids, Retinol, and Tocopherols in Milk and Dairy Products Beatrice Duriot and Benoit Graulet, 332,
Chapter 21 Simultaneous Ultra-high-performance Liquid Chromatography for the Determination of Vitamin A and Other Fat-soluble Vitamins to Assess Nutritional Status Fernando Granado-Lorencio, Inmaculada Blanco-Navarro and Belén Pérez-Sacristán,
Function and Effects,
Chapter 22 Distribution and Concentrations of Vitamin A and their Metabolites in Human Tissue Ewa Czeczuga-Semeniuk, Janusz W. Semeniuk and Adrianna Semeniuk, 381,
Chapter 23 Vitamin A Deficiency: An Overview Teresa Barber, Guillermo Esteban-Pretel, Maria Pilar Marín and Joaquín Timoneda, 396,
Chapter 24 Retinoic Acid Receptors and their Modulators: Structural and Functional Insights Albane le Maire, William Bourguet, Hinrich Gronemeyer and Angel R. de Lera, 417,
Chapter 25 Retinoic Acid in Development Don Cameron, Tracie Pennimpede and Martin Petkovich, 438,
Chapter 26 Retinol/Vitamin A Signaling and Self-renewal of Embryonic Stem Cells Jaspal S. Khillan, Himanshu Bhatia and Liguo Chen, 457,
Chapter 27 Retinoic Acids and their Biological Functions Joseph L. Napoli, 470,
Chapter 28 Vitamin A and Cancer Risk Siddhartha Kumar Mishra and Mi Kyung Kim, 485,
Chapter 29 Vitamin A and Immune Function Charles B. Stephensen, 501,
Chapter 30 Vitamin A and Brain Function Christopher R. Olson and Claudio. V. Mello, 516,
Chapter 31 The Importance of Vitamin A during Prgenancy and Childhood: Impact on Lung Function Hans K. Biesalski and Donatus Nohr, 532,
Subject Index, 555,

CHAPTER 1

Retinol, Retinoic Acid, Carotenes and Carotenoids: Vitamin A Structure and Terminology

GERALD WOOLLARD

Department of Chemical Pathology, Lab Plus, Auckland City Hospital, Auckland, New Zealand

E-mail: geraldw@adhb.govt.nz

1.1 Introductory Remarks

The fact that the terminology vitamin A is used colloquially in everyday conversations and in commercial products within the cosmetic industry tends to belie the fascinating nature of this compound and to understate the importance of retinol (and the carotenoids) in the biological world. There can hardly be a more intriguing set of compounds which are intrinsically related to so many fundamental biological processes. Any discussions concerning the structure of vitamin A are never complete without due regard to the carotenoids themselves.

To discuss the chemical and biochemical behavior of vitamin A and the carotenoids takes the reader on a journey from fundamental photosynthetic processes in plants and into the realm of human nutrition and pathology. Vitamin A is born out of these plant-derived products and transposed into a set of animal compounds which have their own specific carrier proteins [retinol-binding protein (RBP)] and nuclear receptors [retinoic acid receptor (RAR)]. This notion in itself is remarkable and models the idea of the interdependence of the natural world. The evolutionary process leading to retinoid/carotenoid biological complicity can be bewildering to consider in that parallel chemistries can be used by unrelated species for unrelated purposes.

1.2 Structure and Function of Carotenoids

It is instructive to consider briefly what carotenoids are and how their intended function dictates their structure. It gives an appreciation of their general chemical structure and what characteristics are essential. An example of a very familiar carotenoid, β-carotene is shown in Figure 1.1. A cursory glance at its basic structure shows the obvious feature of a long conjugated central chain with two rings (identical in β-carotene) at each end.

1.2.1 Central Carotenoid Chain

β-carotene represents a convenient prototype carotenoid to assist with the appreciation of carotenoid structure. It is ubiquitous in the natural plant and animal world, physiologically and nutritionally important in itself and as a precursor carotenoid for the production of other compounds. The general properties of carotenoids can be discussed by consideration of β-carotene.

1. The conjugated polyene structure is paramount to carotenoid function because the electrons in the double bonds are delocalized and have a lower ground energy state. This allows visible light to be absorbed.

2. Carotenoids act as chromophores with high extinction coefficients. They confer colour to fruit or flowers to attract birds and insects (for seed propagation) or by the birds themselves to enhance dichromatic behavior between the genders. Animals may modify this basic structure to extend the chromophore to make other carotenoids such as astaxanthin, the intense red pigment evident in salmon.

3. The ability to absorb light is at the very heart of the photosynthetic apparatus. The process is enormously complicated and will be discussed in detail later in this book. Basically, β-carotene itself or the other two important photosynthetic carotenoids lutein and zeaxanthin play multiple roles:

(a) Capture incoming photons and passing on this energy for use in photosynthesis (carotenoids contribute 20–30% of the absorbed light energy).

(b) Broaden the absorption spectrum of the photosystem because carotenoids have a wider spectrum than chlorophyll and the hydroxylated carotenoids (lutein and zeaxanthin) specifically have a bathochromic red shift in their absorbance characteristics.

(c) Absorb excess light energy (the intensity of sunlight obviously varies greatly) and remove it by dissipation as heat (i.e. by increased vibrational energy of the carotenoid chain).

(d) Quench the high energy of other excited molecules such as singlet-state oxygen and triplet-state chlorophyll which protects the photosynthetic molecules from damage.

(e) Confer chemical protection by capture of excited singlet oxygen with chemical attachment to the carotenoid (the polyene structure is able to dissipate the free radical). This is a sacrificial action in which the carotenoid is chemically altered.

(f) Carotenoids are involved with membrane stabilization and may also conduct electrons (molecular wires) between other molecules such as cytochromes and chlorophylls.

4. In order that carotenoids can perform these photosynthetic tasks, they must maintain an unsubstituted polyene chain and all-trans geometry. Bending the central polyene chain has a profound effect on the geometry of β-carotene and also changes the ground state energy of the delocalized electrons. Hence, although there are potentially a large number of cis-isomers, they are less common in nature.

5. The central chain in most common carotenoids is unaltered. Any alteration causes a change of function or chain cleavage.

1.2.2 The End Ring Systems

The unsubstituted end ring systems have a single double bond. In β-carotene it extends the conjugation of the chain at both identical ends. A complete set of compounds have the double bond shifted along one position (see biochemical pathway in Figure 1.3) to produce a set of geometric isomers based on a carotene. The double bond can also occur on the methyl group. These ring systems are occasionally referred to as ionones. The three variations are α-ionone, β-ionone and γ-ionone depending on the position of the double bond. The names are derived from the volatile fragrant compounds produced by cleavage of the carotene central chain near the end ring (see Figure 1.2). They contribute to the aroma of rose petals. The principle reason for pointing out the ionone structures is to emphasis that in order for a carotenoid to be a precursor to vitamin A it must have at least one β-ionone end ring.

In contrast to the central chain system, substitution of the end rings is common. Most reactions of carotenoids are oxidations, with reductions being fairly rare. The end chains are usually altered by the additions of a hydroxy, keto or epoxy group. This can occur at either end and these transformations can be different forming non-symmetrical products.

1.3 Biosynthesis

Perhaps the best way to understand the structure and the relationship between the various carotenoids is to let nature be the teacher. The biosynthetic assembly of the carotenoids is from an intermediary metabolic precursor isoprene (2-methyl-1,3-butadiene). Isoprene is a fundamental plant building block of many plant-derived compounds which includes sterols, tocopherols (vitamin E), phylloquinone (vitamin K) and countless other terpenoids with characteristic odours and flavours. Isoprene is a branched five carbon molecule and therefore carotenoids can be expected to have multiples of five carbon atoms. Although C30 carotenoids exist, by far the largest group is the C40 carotenoids, i.e. they are synthesized from eight isoprenoid precursor molecules.

1.3.1 Biosynthetic Pathway

The basic biosynthetic pathway is shown in Figure 1.3. This occurs only in photosynthetic plants and microorganisms. Animals are incapable of de novo synthesis of carotenoids but are capable of modification of dietary carotenoids to a range of compounds of importance to animal physiology.

The first committed step of carotenoid biosynthesis is the C20 compound geranylgeranylphosphate in which the isoprenes are linked in a head to tail arrangement. The subsequent transitions are catalyzed by diiron proteins or cytochrome p450 enzymes.

1.3.2 Key Observations for Carotenoid Biosynthesis

The details are unimportant for this discussion, except to make some observations concerning carotenoid structure. Here are several specific key features of this pathway:

1. The coupling of the two C20 precursors takes place in a ‘tail to tail’ arrangement so that the central position reverses the chains to produce the colourless compound phytoene. Hence the central two methyl groups are in 1:6 positions, whereas all the others methyl groups have 1:5 relationships. This is common to all carotenoids as well as squalene (precursor to cholesterol) but not to all higher terpenoids, e.g. phytol.

2. The synthetic pathway divides at lycopene into an alpha and a beta pathway. These two series are geometric isomers with the solitary shift of a single unsaturated position on the ring system. This is not an insignificant phenomenon because each series leads to a distinct set of downstream products of separate importance which are not necessarily interconvertible by isomerization.

3. The trivial names (see below for discussions on trivial names) for isomers of carotene can at times be confusing. Consider α-carotene, β-carotene, δ-carotene and [zi]-carotene: it may be construed from their common names that they are intimately related in structure, biosynthetic origin and/or function but this is not really the case.

4. The biosynthetic pathway initially produces unsubstituted hydrocarbon carotenes, principally α-carotene and β-carotene. These are both very important compounds in themselves but this pathway proceeds to metabolize these carotenes to hydroxylated, keto or epoxy carotenoids. Arbitrarily, carotenoids can be classified in two classes:

(a) Carotenes which are strictly hydrocarbons.

(b) Xanthophylls which are carotenes that have one or more oxy-substitutions anywhere on their structure (usually end rings). The xanthophylls are more polar than the carotenes.

5. Most reactions involving carotenoids are oxidations and are irreversible. However, the ‘xanthophyll cycle’ on the lower right of Figure 1.3 is reversible. It is oxidative recycling of zeaxanthin after conversion into two epoxy carotenoids antheraxanthin and violaxanthin. This is an important plant process.

Both plants (including microorganisms) and animals (including marine species) can convert the basic carotenoids shown in Figure 1.3 into a bewildering array of more complex carotenoids. Over 700 are currently known and they serve a range of essential metabolic functions in a range of environmental habitats. Together with the porphyrins, carotenoids are one of the great colour chemistries in the biological world.

1.4 Trivial Names of Carotenoids

1.4.1 Origins of Trivial Names

Not surprisingly carotenoids were named by the original discoverer which reflects the plant from which they were originally extracted. The most obvious of these is carotene from carrots, zeaxanthin from wheat (the botanical name is Zea mays) and lutein the principle yellow pigment in the macula lutea in the retina. The terminology xanthophyll refers to the more yellow colour of the polar pigments from autumn leaves (from the Greek words for yellow leaf). Lutein and zeaxanthin are both xanthophylls. These non-systematic names are still in common use but are inexact because they do not reveal the stereochemistry and often cannot differentiate the multitude of isomeric forms of substituted carotenoids.

1.4.2 Major Nutritional Carotenoids

It is worthwhile retaining the trivial names for the most important nutritional carotenoids. Indeed there is nothing to be gained by deviating from them because their simplicity outweighs any confusion that is created in assigning structure. In plasma of higher animal (including humans) the profile of carotenoids reflects their respective diets. Quantitatively, the most common carotenoids observed in plasma are the major plant carotenoids β-carotene, α-carotene, lycopene, β-cryptoxanthin, β-canthaxanthin, lutein and zeaxanthin. There may be up to 40 measurable carotenoids including a range of cis-isomers but many are minor dietary components. Carotenoids in humans have been studied extensively for their relationship to various diseases.

1.4.3 Provitamin A Carotenoids

The trivial names of the five most common pro-vitamin A carotenoids are β-carotene, α-carotene, β-cryptoxanthin, β-canthaxanthin and β-echinenone. These trivial names are retained in nutritional literature because of their widespread acceptance. β-Cryptoxanthin, β-canthaxanthin and β-echinenone are pro-vitamin A xanthophylls. This is because these three xanthophylls have hydroxyl (and keto) groups attached at one end ring only. The most common xanthophylls, lutein and zeaxanthin, are substituted at both ends and are not pro-retinol precursors. The α-carotenoids can be pro-vitamin A because they can still possess a β-ionone end ring.

1.4.4 Ambiguities in Trivial Names

Trivial nomenclature can be ambiguous or slightly deceiving. For instance, with β-carotene there is a β-ionone ring at both ends. However, α-carotene does not have two α-ionone rings, it has one α- and one β-ring. Moreover, in the semi-systematic naming system α-carotene is not named α- because it is called the end ring and has a ε-descriptor not α- (see Table 1.1). It is advisable to be aware of this. Even more unusual is that lutein and zeaxanthin are geometric isomers with exactly the same relationship as α-carotene and β-carotene but they have completely disparate names.

1.5 IUPAC Definitions

International Union of Pure and Applied Chemistry (IUPAC) systematic naming of all organic compounds is very rigorous and is based solely on exact assignment of structure and not biological rationale or relationships (IUPAC 1978).

(Continues…)Excerpted from Vitamin A and Carotenoids by Victor R. Preedy. Copyright © 2012 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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