Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 135

must be freed from the chloroplasts and
enter a step of micelle formation before
being adsorbed. The main forms retained
in blood and tissues are the D - and, to a
lower extent, the J -form of tocopherol. In
addition to its antioxidant activity,
tocopherol is involved in the modulation
of specifi c signalling pathways (reviewed
by Galli and Azzi, 2010). In addition,
minor forms of vitamin E such as
tocotrienol may share with tocopherol
several benefi cial effects for human health
such as the prevention of cancer. However,
the prevention of cardiovascular disease
and the anti-tumoural effect of tocopherol
have been questioned recently and require
further investigation (Galli and Azzi, 2010).
In plants, tocochromanols have an
antioxidant function in photosynthetic
membranes, by controlling lipid peroxid-
ation. The main tocochromanol found in
plants, tocopherol, has been shown to play
a crucial role in the protection of plants
against photo-oxidative stress (Falk and
Munné-Bosch, 2010). In addition, toco-
pherol defi ciency affects other plant
functions, such as germination and photo-
assimilate transport, suggesting additional
but still ill-defi ned roles in plants. The
function of tocotrienols is less clear,
although they have been shown potentially
to serve as antioxidants in photosynthetic
membranes (Matringe et al. , 2008).

and raspberry having the highest vitamin E
content (Table 8.1) (Chun et al. , 2006).

8.3.3 Vitamin E biosynthetic pathway

Whilst the biosynthetic pathway of vitamin
E was proposed in the 1970s, the fi rst
enzyme of that pathway was cloned in
1998 (reviewed by Dellapenna, 2005;
Mène-Saffrané and DellaPenna, 2009).
Since then, vitamin E biosynthetic genes
have been identifi ed using a combination
of genetic and genomic tools in the model
plant species Arabidopsis and sub-
sequently in many other plant species.
More recently, vitamin E biosynthetic
genes have also been identifi ed in several
fruit species including tomato (Almeida et
al. , 2011), apple (Seo et al. , 2011), mango
(Singh et al. , 2011) and palm fruit
(Tranbarger et al. , 2011).
Tocochromanols are composed of a
polar chromanol head group derived from
a cytosolic aromatic amino acid pathway,
the shikimate pathway, and by a lipophilic
polyprenyl side chain, derived from the
plastidial MEP pathway, which synthesizes
the isoprenoid precursor IPP (Fig. 8.2).
Tocopherols have a saturated side chain
derived from phytyl diphosphate (PDP),
produced from geranylgeranyl diphosphate
(GGDP) or from chlorophyll a , whilst
tocotrienols have an unsaturated side chain
derived from GGDP. The committed step in
head-group synthesis is the formation of
homogentisate from hydroxyphenylpyruv-
ate catalysed by 4-hydroxyphenylpyruvate
dioxygenase (HPPD). Prenylation of
homogentisate with either PDP or GGDP
yields 2-methyl-6-phytylquinol (MPBQ)
and 2-methyl-6-geranylgeranylbenzoquinol
(MGGBQ), the fi rst intermediates in
tocopherol and tocotrienol synthesis,
respectively. Subsequent steps of methyl-
ation of MPBQ and MGGBQ by 2-methyl-6-
phytyl-1,4-benzoquinone methyltransferase
(MPBQMT/VTE3), tocopherol cyclase (TC/
VTE1) and J -tocopherol methyltransferase
( J -TMT/VTE4) yield the differently methyl-
ated D -, E -, J - and G -forms of tocopherol
and tocotrienols.

8.3.2 Vitamin E in fl eshy fruits

The main plant sources of vitamin E in the
human diet are by far the plant-derived
oils, derived mostly from seeds. Oil
extracted from the mesocarp of palm fruit,
the major oil crop in the world and also
rich in provitamin A, is very rich in both
D -tocopherol (25.6 mg per 100 g) and
J -tocopherol (31.6 mg per 100 g), and in
D -tocotrienol (14.3 mg per 100 g) (FAO/
WHO, 2004). By comparison, sunfl ower
oil content is 48.7, 5.1 and 0 mg per 100 g
for D -tocopherol, J -tocopherol and
D -tocotrienol, respectively. The vitamin E
content of other fl eshy fruits is much
lower, with avocado, mango, blackberry