Tocopherols and Tocotrienols
metabolism of tocotrienols is different from that of ¦Á-tocopherol.
¦Á-Tocopherol is almost exclusively secreted into
chylomicrons by the enterocytes in the intestine,
whereas tocotrienols are secreted into both chylomicrons
and small high-density lipoproteins (HDL).
The ratio of tocotrienols secreted into
chylomicrons and HDL varies greatly amongst individual
of tocotrienols in newly formed chylomicrons at
peripheral tissues does not occur because the truncated
apoprotein B-48 in the chylomicrons does not have the
required domains for the uptake receptors.
Newly formed chylomicrons deliver lipids to
muscles and adipose tissue via lipoprotein lipase
actions and the chylomicron remnants are transported to
the liver. Vitamin
E is subjected to discrimination by the ¦Á-tocopherol
transfer protein (¦Á-TTP) in the liver before they are
re-secreted into nascent very low-density lipoproteins (VLDLs).
The extent of discrimination depends on the
relative binding affinity of individual forms of vitamin
E with ¦Á-TTP. ¦Á-Tocotrienol
has 12.4% binding affinity of ¦Á-tocopherol.
In layman term, for every 100 molecules of ¦Á-tocopherol
sequestered by ¦Á-TTP, the corresponding amount for ¦Á-tocotrienol
is 12.4 molecules, whereas the balance 87.6 molecules
are unprotected, and subjected to catabolism.
The relative binding affinity for ¦Ã- and ¦Ä-tocotrienols
has not been reported.
From literature data, these binding affinities
may be very low. In
other words, most of the tocotrienols secreted into
chylomicrons are not re-secreted into nascent VLDLs and
they undergo catabolism leading to secretion of
metabolites in the urine.
VLDL has apo B-100 with docking domains for the
also delivers lipids to muscles and adipose tissue via
lipoprotein lipase actions and become
intermediate-density lipoproteins (IDL).
Further lipoprotein lipase action on IDL forms
low-density lipoprotein (LDL).
IDL and LDL deliver vitamin E to peripheral
tissues via LDL-receptors mediated endocytosis.
secreted into small HDL are independent of ¦Á-TTP action
and can be delivered directly for uptake into selective
peripheral tissues via scavenger receptor-mediated
selective vitamin E uptake.
Unlike LDL-receptors, scavenger receptors are not
evenly distributed in the peripheral tissues, they are
more abundant in the fatty tissues.
Therefore tocotrienols uptake via direct vitamin
E uptake is concentrated in selective tissues whereas
vitamin E uptake via LDL-receptors is more evenly
distributed in the peripheral tissues.
maximum tocotrienols absorption, the ¦Á-tocopherol
content should be minimized in order to enable
tocotrienols to have a chance for ¦Á-TTP sequestering.
Hepatic re-circulation of ¦Á-tocopherol and also
¦Á-tocopherol from dietary intake will jeopardize the
absorption of tocotrienols via chylomicrons absorption
¦Á-tocopherol is more harmful than the synthetic
counterpart due to its high binding affinity with ¦Á-TTP
and therefore suppresses the absorption of tocotrienols
further details, kindly refer to "Unleashing the
untold and misunderstood observations on vitamin E"
doi: 10.1007/s12263-010-0180-z. Genes &
Nutrition, 2010 published online first.