Chemistry Reference
In-Depth Information
tocopherols (
- and
-) are reassembled to chylomicrons by the Golgi of the
mucosal cells (Brigelius-Flohe and Traber, 1999). The chylomicrons are
stored as secretory granula and eventually excreted by exocytosis to the
lymphatic compartment from which they reach the bloodstream via the
ductus thoracicus.
The transfer of vitamin E from chylomicrons is regulated by complex
mechanisms that control lipid and lipoprotein metabolism (Traber and Sies,
1996; Traber and Arai, 1999). Chylomicrons are degraded to remnants by
lipoprotein lipase (LPL) and some
- and
-tocopherols are transferred to
peripheral tissues such as muscle, adipose tissue, skin and brain by this
enzyme-mediated mechanism (Traber and Sies, 1996; Jiang et al., 2001;
Morrissey and Kiely, 2005). The resulting chylomicron remnants are then
taken up by the liver, where most of the remaining
- and only small amounts
of
-tocopherols are reincorporated into nascent very low density lipopro-
teins (VLDLs) by a specific 32-kDa
-tocopherol transfer protein (
-TTP),
which enables further distribution of
-tocopherol throughout the body.
Approximately 50% of the dietary intake of
-tocopherol appears to be
degraded in the liver by a cytochrome P450-dependent process (Parker et al.,
2000) and is then excreted, primarily in the urine (Swanson et al., 1999).
Catabolism of
-tocopherol by this route occurs only when the daily intake
of
-tocopherol exceeds 150 mg (Schultz et al., 1997) or the plasma concen-
tration of
-tocopherol is above a threshold of 30-40 mmol/l (Schuelke et al.,
2000). Plasma or serum concentrations of
-tocopherol are typically around
20-35 mmol/l.
-Tocopherol concentrations are approximately 5-15% of
those of
-tocopherol and generally remain around 1 mmol/l even after
supplementation (Hayes et al., 1993; O'Byrne et al., 2000). The highest
concentrations of
-tocopherol in the body are in the adipose tissues and
adrenal glands (Drevon, 1991). Adipose tissues are also the major stores of
the vitamin, followed by the liver and skeletal muscle. The rate of uptake and
turnover of
-tocopherol by different tissues varies greatly (Burton et al.,
1990). Uptake is most rapid into lungs, liver, spleen, kidney and red blood
cells (in rats t
½
<
15 days) and slowest in brain, adipose tissues and spinal cord
(t
½
<
30 days). Likewise, depletion of
-tocopherol from plasma and liver
during times of dietary deficiency is rapid, whereas adipose tissues, brain,
spinal cord and neural tissues are much more difficult to deplete (Bourne and
Clement, 1991).
A considerable, but variable, proportion (typically 30-70%) of ingested
vitamin E is unabsorbed and therefore excreted in the faeces, making this the
main route of elimination (Kayden and Traber, 1993), and is influenced by
experimental conditions and a variety of luminal and physiological factors
(Cohn, 1997). When large doses of vitamin E are administered, much of it is
secreted in the bile, which may account for its relative safety compared to
