Chemistry Reference
In-Depth Information
vitamins A and D (Bramley et al., 2000). Both bile acids and pancreatic juices
are important for the absorption of vitamin E (Traber and Sies, 1996). This
has been established clearly in patients where secretion of either, or both, is
severely diminished as in patients with cystic fibrosis or pancreatitis. The
simultaneous intake of fat is necessary to stimulate bile flow and the secretion
of pancreatic enzymes to allow micelle formation. However, the amount of
fat necessary to ensure absorption may be small (Roodenburg et al., 2000).
12.4.3.
Tocopherols and Prevention of Lipid Oxidation
Vitamin E (as
-tocopherol,
-TOH) is an indispensable component of
biological membranes with membrane-stabilizing properties and high
antioxidant activity. The overall mechanisms of lipid peroxidation and anti-
oxidant protection in biological and food systems have been reviewed exten-
sively (Buettner, 1993; Frankel, 1998; Morrissey et al., 1998; Morrissey and
Kerry, 2004). The antioxidant activity of chain-breaking antioxidants is deter-
mined by how rapidly they scavenge free radicals, the ease of hydrogen transfer
from an antioxidant to a free radical and the difference in the standard one-
electron reduction potentials (Buettner, 1993). A compound, the reduction
potential of which is lower than that of the reduction potential of a free radical
or oxidized species, can donate a hydrogen to that free radical if the reaction is
kinetically feasible. The peroxyl radical (LOO
.
)(E
o'
¼
+1000 mV) has a
reduction potential greater than
-TOH (E
o'
¼
+500 mV), meaning that
hydrogen transfer between the antioxidant and LOO
.
is energetically favour-
able (Buettner, 1993; Morrissey et al., 1994). When the chromanol phenolic
group of
-TOH encounters an LOO
.
, a hydroperoxide (LOOH) is formed and
in the process an
-tocopheroxyl radical (
-TO
.
) is generated:
TOH
þ
LOO
:
E
o
;
¼þ
500mv
!
LOOH
þ
TO
:
The rate constant (k
1
) for this chain-inhibition reaction is 8
10
4
l/mol/s (Buettner,1993) or 2.35
10
6
l/mol/s (Azzi and Stocker, 2000),
which is considerably higher than that for the other tocopherols and related
phenols. Because the rate constant (k
2
) for the chain propagation reaction
between LOO
.
and an unsaturated fatty acid (LH) (LOO
.
þ
LH
!
LOOH)
is much lower than k
1
, at approximately 10
2
l/mol/s,
-tocopherol out-
competes the propagation reaction and scavenges the LOO
.
10
4
times
faster than LH reacts with LOO
.
. Thus, the kinetics of antioxidants, and
particularly
-TOH, require that only a relatively small amount be present
for them to act as effective antioxidants (Buettner, 1993; Morrissey et al.,
