Historical overview - Retinoids and Carotenoids

In 1912, McCollum and, independently, Osborne and
Mendel, separated in almost pure form the proteins,
fats, carbohydrates, mineral matter and water from a
variety of foods. Mixtures of the isolated nutrients
were then fed to animals which soon sickened and
died. It was concluded that the food from which the
nutrients were derived contained some additional
factor that is necessary to sustain life. In 1915 McCollum
and Davis isolated from animal fats and fi sh oils
a ‘fat-soluble A’ that was essential to rats for growth
and also cured eye disorders. In 1921 Bloch reported
that a diet containing full milk and cod-liver oil cured
xerophthalmia in infants and concluded that the eye
affl iction was due to the absence of the fat-soluble A
in the diet. In the meantime it was discovered that
green vegetables also possess fat-soluble A activity
and in 1930 Moore provided evidence that carotene
was converted to vitamin A in the body. The provitamin
role of β-carotene became obvious after Karrer
elucidated the structures of β-carotene and retinol.
The biochemical function of vitamin A in vision was
established by Wald in 1935.
In 1987, two independent research groups led by
Pierre Chambon in Paris and Ronald Evans in California
made the important discovery that retinoic
acid acts in the manner of a steroid hormone through
binding to a specifi c nuclear receptor.
7.2 Chemistry and biological functions
Vitamin A-active compounds, defi ned as compounds
having qualitatively the biological activity of retinol,
are represented by retinoids and provitamin A carotenoids.
The retinoids comprise retinol, retinaldehyde
and retinoic acid, together with their naturally occurring
and synthetic analogues. The naturally occurring
retinoids are sometimes referred to as preformed
vitamin A because they do not require metabolic
conversion, as do carotenoids, in order to become
biologically active. Carotenoids are represented by
β-carotene and chemically related pigments that are
Key discussion topics
• Ingested provitamin A carotenoids, after absorption,
are converted fi rst to retinaldehyde and then to retinol
in the enterocytes.
• Retinoic acid, the metabolite responsible for most
of the nonvisual functions of vitamin A, is produced
intracellularly by oxidation of diet-derived retinol via
retinaldehyde and also from β-carotene.
• Various types of retinoid-binding proteins transport
the hydrophobic retinoids within aqueous extracellular,
cytosolic and nuclear compartments.
• Vitamin A is stored in the liver as retinyl esters and
mobilized for use in a highly regulated process.
• Circulating retinol concentrations are homeostatically
regulated to remain constant.
• The function of vitamin A in vision is based upon
the binding of 11-cis retinaldehyde with the protein
opsin to form the visual pigment, rhodopsin.
• All-trans retinoic acid and 9-cis retinoic acid are
hormonal metabolites of vitamin A that mediate
tissue-specifi c expression of target genes through
their binding to two types of nuclear retinoid receptors.
• When bound to DNA, retinoic acid receptor (RAR)
functions as a transcriptional activator in the presence
of its hormonal ligand, and as a repressor in the
absence of ligand.
• Heterodimer formation between the retinoid X receptor
(RXR) and the nuclear receptors for retinoic
acid, thyroid hormone and vitamin D3 allow RXR to
function as a master controller for signals from various
converging hormonal pathways.
• The COUP-TF orphan receptors act as negative regulators
of retinoid hormone response pathways.
• Retinoid receptors antagonize the AP-1 activator
thereby preventing excessive cell proliferation.
• Retinoic acid induces the transcription of many genes
encoding proteins that are involved in cell differentiation
and a variety of biochemical processes.
• Disrupted expression patterns of retinoid-responsive
cytokine genes account for impaired antibody-mediated
immunity in vitamin A defi ciency.
• Retinoic acid exerts profound effects on pattern formation
during embryogenesis.
• Carotenoids act as biological antioxidants by trapping
peroxyl free radicals and deactivating singlet
oxygen.
• The cancer-preventing action of retinoids correlates
with enhanced gap junctional communication of
growth controlling signals.
Vitamin A: retinoids and carotenoids 135
responsible for the colour of many vegetables and
fruits. From a nutritional viewpoint, these pigments
are classifi ed as provitamin A carotenoids and inactive
carotenoids. In nature, carotenoids are synthesized
exclusively by higher plants and photosynthetic
microorganisms, in which they function as accessory
light-harvesting pigments to chlorophyll. Although
animals are unable to synthesize carotenoids, they
can assimilate them through their diet.
Vitamin A is required for several essential life processes,
including metabolism, haematopoiesis, bone
development, pattern formation during embryogenesis,
the maintenance of differentiated epithelia,
and immunocompetence. These processes can be
supported by all forms of vitamin A, including the
provitamin A carotenoids. The other vitamin A-dependent
processes, namely vision and reproduction,
specifi cally require either retinol or retinaldehyde.
Retinoic acid cannot support these functions because
it cannot be reduced back metabolically to retinaldehyde.
Hence animals maintained on retinoic acid as
the only source of vitamin A become both blind and
sterile, but are otherwise in good general health.
Retinoids promote the differentiation of a variety
of cell lines in culture, including epithelial cells and
chondrocytes. That this function occurs in vivo is
demonstrated by the replacement of secretory cells in
epithelia by keratin-producing cells when an animal
is deprived of vitamin A. The effects of vitamin A on
cellular differentiation are due to the control of gene
expression by retinoic acid in selected tissues, the protein
products being responsible for the effects. Not
all vitamin A-responsive genes are up-regulated by
retinoic acid; some are down-regulated.
In this text the precise terms ‘retinoids’ and ‘carotenoids’
are used where the specifi city is known. The
non-specifi c term ‘vitamin A’ is used in the sense of
vitamin A activity and therefore accounts for provitamin
A carotenoids as well as retinoids.
7.2.1 Preformed vitamin A
The structures of three physiologically important
retinoids are shown in Fig. 7.1. The parent vitamin
A compound, retinol, comprises a cyclohexenyl (β-
ionone) ring attached at the carbon-6 position to a
polyene side chain. The four double bonds in the side
chain give rise to cis/trans isomerization. The predominant
isomer, all-trans retinol, possesses maximal
(100%) vitamin A activity and is frequently accompanied
in foodstuffs by smaller amounts of 13-cis
retinol, which has 75% relative activity. An aldehyde
form, 11-cis retinaldehyde, is the chromophore in the
retina of the eye, while all-trans and 9-cis retinoic acid
are active metabolites of retinol found in most if not
all tissues. Changes in the molecular state of oxidation
and cis/trans isomerization are of physiological
importance in modifying the biological activity of
retinoids.
7.2.2 Provitamin A carotenoids
Carotenoids are classifi ed chemically as carotenes,
which are hydrocarbons, and xanthophylls, which
have one or more oxygen-containing groups (e.g.
hydroxyl) either on the ring or in the chain. Most
naturally occurring carotenoids contain 40 carbon
atoms. In some instances, C40-carotenoids undergo
partial oxidative cleavage in the plant tissues to give
shortened molecules known as apocarotenoids. The
majority of xanthophylls in plant tissues occur as
mono or bis esters of saturated long-chain fatty acids
(e.g. palmitic acid). For a carotenoid to have vitamin A activity, its
structure must incorporate a molecule of retinol, i.e.
an unsubstituted β-ionone ring with an 11-carbon
polyene chain. The most ubiquitous and nutritionally
most important carotenoid, β-carotene (Fig. 7.2), is
composed of two molecules of retinol joined tail to
tail, thus β-carotene possesses maximal provitamin A
activity. The structures of all other provitamin carotenoids
incorporate only one molecule of retinol, hence
theoretically contribute 50% of the biological value of
β-carotene. Over 500 naturally occurring carotenoids
have been isolated and characterized; of these, about
50 possess provitamin A activity in varying degrees.
In plant and animal tissues the carotenoids are usually
found associated with lipid fractions in noncovalent
association with membranes and lipoproteins,
and they accumulate in the chloroplasts of green
leaves. In nature, carotenoids exist mainly as their alltrans
forms. Food processing and preservation methods,
especially canning, induce cis–trans isomerization,
leading to a reduced vitamin A potency.