Food Biochemistry and Food Processing

3 Recent Advances 41

Tocotrienols have more powerful antioxidant proper-
ties than tocopherols but are not absorbed as readily.
The predominant forms of vitamin E in leaves and
seeds are -tocopherol and -tocotrienol, respec-
tively (Munné-Bosch and Alegre 2002). While the
biosynthesis of tocopherols and tocotrienols has
been known for many years, the particular genes
that encode for the different enzymes in the pathway
have only recently been discovered. Researchers are
trying to develop plants with increased vitamin E
levels, and some positive results have already been
achieved.
The first step in the pathway for the biosynthesis
of both tocopherols and tocotrienols is the formation
of homogentisic acid (HGA) from p-hydroxyphenyl-
pyruvate, catalyzed by the enzyme p-hydroxphenyl-
pyruvate dioxygenase (HPPD) (EC 1.13.11.27) (Fig.
3.5) (Grusack and DellaPenna 1999). Tocotrienol

and tocopherol biosynthesis in plants originates

from two different precursors. Tocotrienols are pro-

duced from the condensation of HGA and geranyl-

geranyl diphosphate (GGDP), catalyzed by HGA

geranylgeranyl transferase (HGGT) (EC 2.5.1.32),

and tocopherols are formed from the condensation

of HGA and phytyl diphosphate (PDP), catalyzed by

HGA phytyl transferase (HPT) (EC 2.5.1.62) (Fig.

3.5) (Soll et al. 1980, Schultz et al. 1985, Collakova

and DellaPenna 2001). Researchers from the

Institute of Botany in Germany described the effects

of constitutive expression of HPPD cDNA from bar-

ley (Hordeum vulgare)in tobacco plants. The HPPD

gene was cloned into the pBinAR binary vector, in a

SmaIcloning site located between the 35S CaMV

promoter and the octopine synthase (EC 1.5.1.11)

polyadenylation signal. The construct was then

introduced into AgrobacteriumGV3101, which was

Figure 3.5.Biosynthetic pathway of vitamin E (-tocotrienol and -tocopherol). (Adapted from Cahoon et al. 2003.)