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It is abundant (3 to 10 mmol/L) in the
cytoplasm, nuclei, and mitochondria, and is the major soluble
antioxidant in these fractions. Thus, GSH can detoxify both
soluble and lipid peroxidases.
The levels of GSH decrease with a suboptimal
nutrient intake, particularly of protein and with exercise.
It also is depleted by oxidative stress, which is usually caused
by an infection, trauma, or major surgery. Since cysteine is
the rate-limiting step for GSH synthesis, both inadequate dietary
protein and proteins with low concentrations of cysteine limit
its availability. Giving cysteine alone in the absence of the
other amino acids does not increase GSH, because dietary cysteine
is rapidly metabolized and furthermore is toxic.18 On the other
hand, dietary administration of g-glutamylcysteine increases
GSH; whey protein contains substantial amounts of this substance.
The glutamylcysteine groups are located primarily in bovine
serum albumin fraction, b-lactoglobulin, and immunoglobulin G1
fraction (Table 1). This substance is rarely found in other
protein sources or plants, except raw egg whites, which are not
eaten.
Selected Diseases
Patients with end stage renal disease often
develop protein calorie malnutrition due to poor appetite, which
leads to a low protein intake. Also, regular hemodialysis removes
protein from the body. These patients are also likely to be
in a state of increased oxidative stress. Whey protein has been
postulated to be an ideal protein supplement for these patients
because it not only is a good source of dietary protein, but
also is rich in cysteine and glutamate for GSH synthesis mitigating
the oxidative stress (Williams ME, on file, Optim Nutrition,
1997).
General depletion of GSH in cancer cells
slows its replication, which is good; but the normal tissue becomes
depleted also, which is not desirable.19 However, whey protein
can decrease the concentration of GSH in in vitro in cancer cells
while not affecting normal cells. This finding may have a direct
implication for cancer patients undergoing chemotherapy, where
whey protein may become the protein source of choice.
Patients with AIDS have subnormal concentrations
of GSH.20 Low concentrations of GSH have been associated with
decreased cell survival, impaired T-cell function, diminished
interleukin (IL)-2 activity, and increased HIV replication.21-24
Survival rates were lower in patients with AIDS and a low GSH
concentration than in patients with higher levels.
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Supplementation with N-acetyl cysteine
(a source of cysteine) improved survival.25 Since whey protein
is such a rich source of cysteine, it is likely to have the same
effect.
In order to obtain the benefit of GSH from
the whey protein, it is important not to allow it to be denatured
during processing. Bounous et al.18 have shown that not all
whey protein concentrates are alike. Animals who received the
most-soluble whey protein (undenatured conformation) had the
best humoral immune response through GSH activity compared with
animals receiving whey that had been denatured during processing
or casein.
Lactoferrin, Alpha-lactalbumin, Beta-lactoglobulin,
and Bovine Serum Albumin
The specially processed whey also contains
other proteins that also enhance immune function and contain
rich sources of cyst(e)ine for GSH synthesis26 (Table 1). These
include lactoferrin, alpha-lactalbumin, beta-lactoglobulin, and
bovine serum albumin.
Lactoferrin controls iron absorption by
binding to it. This keeps the iron from supporting the growth
of intestinal bacteria, in particular those that are pathogenic
like Escherishus coli.27 Lactoferrin may also modulate immune
function.28,29 Normal volunteers who took 40 mg of capsules containing
bovine lactoferrin for 10 days had a 100% increase in the level
of immature cell forms (bands) and significant decreases in eosinophils
and monocytes.28 Serum levels of selected pro-inflammatory cytokines
decreased (TNF-a, IL-6). Cells stimulated with lipopolysaccharide
(LPS) produced less of these two cytokines, again revealing the
blunting of an activated immune response. In another study, mice
consuming the same substance had higher total immunoglobulins
(IgA and IgG) in their intestinal fluid, Peyer's patch, and spleen
cells.30 This finding suggests that lactoferrin acts as an immunostimulating
factor on the intestinal immune system, which depends on its
ability to bind antigens in the intestinal mucosa.
Alpha-lactalbumin is the B subunit of lactose
sythetase, the enzyme that catalyzes the addition of galactose
to glucose to produce lactose.30 This may aid in digestion of
milk by infants. Beta-lactoglobulin is involved with phosphorus
metabolism in the mammary gland, transfer of passive immunity,
and binding of retinol and fatty acids. This binding allows the
efficient uptake of fats by the cells. These two fractions are
also present in milk and are major allergenics in humans; those
with allergies to milk proteins (not the sugar portion, lactose)
should not consume whey. Bovine serum albumin also binds fatty
acids while stimulating pregastric lipases, which aids digestion
in newborns.
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