Vitamin C (Ascorbic acid) is a water-soluble vitamin and is highly polar compound. Humans have to get what they need of Vitamin
C from food, including citrus fruits, broccoli, and tomatoes. Human bodies require vitamin C for the growth and repair of tissues in
all parts of the body. Vitamin C is one of many antioxidants that block some of the damage caused by free radicals. There are some
thoughts that vitamin C might help the heart and the blood vessels by preventing clots in veins and arteries, high blood pressure, and
high cholesterol. Essential hypertension is characterized by endothelial dysfunction, arterial stiffness, and increased oxidative stress.
This mini review will focus on the role of vitamin C in these major symptoms.
Ascorbic acid, LDL oxidation, cell adhesion, Endothelium, Oxidative stress.
Vitamin C is sensitive to light, air, and heat, so you’ll get the most
vitamin C if you eat fruits and vegetables raw or lightly cooked.
The best way to get the daily requirement of essential vitamins,
including vitamin C, is to eat a balanced diet that contains a variety
of foods (1).
The presumption that oxidative stress, amongst several other
factors, plays an important role in atherogenesis implies that
the development and progression of atherosclerosis can be
inhibited by antioxidants (2). In this review we discussed several
mechanisms by which the antioxidant ascorbate (vitamin C) affects
atherosclerosis. These mechanisms include inhibition of LDL
oxidation, inhibition of leukocyte adhesion to the endothelium and
vascular endothelial dysfunction.
Ascorbic acid metabolism
Ascorbate donates a single electron to become the ascorbate radical,
which reacts with another ascorbate radical to form a molecule
each of ascorbate and dehydroascorbate (DHA). The latter is
unstable at physiologic pH and if not reduced back to ascorbate
via GSH-dependent mechanisms. It will undergo irreversible ring
opening and loss. In buffers, DHA forms a hemiketal that has amolecular structure resembling that of glucose (3).
Oxidative stress (free radicals generating process) is thought to
play an important role in atherosclerotic vascular disease (4).
Thus, dietary antioxidants such as ascorbate (vitamin C) and
polyphenols can protect against the development and progression
of atherosclerosis in experimental models (4). Numerous
observational studies have shown an inverse association between
antioxidant intake or body status and the risk of cardiovascular
Oxidatively modified LDL has been implicated in the pathogenesis
of atherosclerosis (4 and 5) Although the mode of LDL oxidation
in vivo is incompletely understood, the mechanisms of LDL
oxidation in vitro have been studied extensively (4). Modification
of the protein moiety of LDL (apolipoprotein B-100), either
directly by leukocyte-derived oxidants such as hypochlorous acid or indirectly by lipid hydroperoxide breakdown products such as
4-hydroxynonenal and malondialdehyde, results in a form of LDL
that is internalized by macrophages via the scavenger receptor
pathway leading to foam cell formation. Although redox-active
transition metal ions seem to play a pivotal role in cell-mediated
LDL oxidation (4). Various metal ion-independent mechanisms of
LDL oxidation have been proposed, such as reactive nitrogen and
chlorine species. Furthermore, there is convincing evidence that
in vitro lipid peroxidation in LDL is initiated by α-tocopheroxyl
radicals formed in the lipoprotein on attack by free radicals or
other reactive species. Thus, α-tocopherol can act as a pro-oxidant,
rather than an antioxidant, in LDL incubated in vitro (6 and 7)
Ascorbate and LDL Oxidation
Human plasma and other extracellular fluids contain numerous
water-soluble antioxidants, including ascorbate, urate, bilirubin,
and various thiol compounds (8 and 9). Experimental data on
the effects of vitamin C supplementation of human subjects on
ex vivo LDL oxidation are sparse, mainly because ascorbate is
removed from LDL during isolation from plasma. However, there
is convincing evidence from in vitro studies that physiological
concentrations of ascorbate strongly inhibit LDL oxidation by
vascular cells and neutrophils, as well as in cell-free systems (10).
Ascorbate prevents oxidative modification of LDL primarily by
scavenging free radicals and other reactive species in the aqueous
milieu. Thus, direct and rapid trapping of these aqueous reactive
species by ascorbate prevents them from interacting with and
oxidizing LDL. Ascorbyl radicals formed in this process may be
reduced back to ascorbate by dismutation, chemical reduction
(eg, by glutathione), or enzymatic reduction (eg, by thioredoxin
reductase) (11). Dismutation also produces dehydroascorbic
acid (ADA ), which in turn can be reduced back to ascorbate by
glutathione, thioredoxin reductase, and glutaredoxin. Ascorbate
can also prevent the pro-oxidant activity of α-tocopherol by
reducing the α-tocopheroxyl radical to α-tocopherol, thereby
acting as a “coantioxidant” and inhibiting LDL oxidation.
Adhesion of leukocytes to the endothelium is an important
initiating step in atherogenesis (12-14). Various studies have shown
that monocytes bind selectively to aortic pre lesion areas and
atherosclerotic lesions, which also exhibit increased expression of
adhesion molecules compared with normal tissue(13). Cultured
endothelial cells exposed to inflammatory cytokines or oxidized
LDL exhibit enhanced expression of cell adhesion molecules,
such as intercellular adhesion molecule-1 (ICAM-1), vascular
cell adhesion molecule-1 (VCAM-1), and E-selectin (14). These
adhesion molecules interact with specific ligands expressed on
the surface of leukocytes, such as the β1 and β2 integrins, and
mediate leukocyte rolling, firm attachment to the endothelium, and
subsequent migration into the subendothelial space (14).
Ascorbate and Cell Adhesion :
Two recent human studies have investigated the role of ascorbate in
inhibiting cell-cell adhesion (15 and 16). Smokers have decreased plasma levels of ascorbate, and monocytes isolated from smokers
exhibit increased adhesion to cultured endothelial cells compared
with monocytes isolated from nonsmokers. Supplementation of
smokers with 2 g/day of vitamin C for 10 days elevated plasma
ascorbate levels almost 2-fold and significantly reduced monocyte
adhesion to cultured endothelial cells (15). This finding indicates
that upregulation of ligands on monocytes is inhibited by ascorbate.
In another study, however, supplementation of smokers with 2 g of
vitamin C 2 hours before isolation of monocytes had no effect on
ex vivo monocyte-endothelial cell adhesion or endothelial ICAM-
1 surface expression, despite a >3-fold increase in serum ascorbate
levels. The supplementation period in this study may have been
too short to affect intracellular ascorbate levels.
Several in vivo studies using intravital microscopy in hamsters
have demonstrated an important role of ascorbate in inhibiting
leukocyte–endothelial cell interactions induced by cigarette smoke
or oxidized LDL, likely by antioxidant mechanisms. Lehr et al.,
(1997) showed that the induction of leukocyte adhesion to the
vascular wall elicited by cigarette smoke is due to the formation
of oxidatively modified lipids with platelet-activating factor–like
activity. Administration of ascorbate prevented the accumulation
of these platelet-activating factor–like lipids and the subsequent
leukocyte–endothelial cell interactions (17).
Endothelium Nitric Oxide (NO) synthesis
Endothelium-derived NO (EDNO) is a pivotal molecule in the
regulation of vascular tone and homeostasis (18). In addition
to stimulating vascular smooth muscle cell relaxation and
vasodilation, EDNO exerts a number of potent antiatherogenic
effects, including inhibition of smooth muscle cell proliferation,
platelet aggregation, and leukocyte–endothelial cell interactions
(18). EDNO is synthesized from L-arginine through the action
of constitutive and inducible isoforms of the NADPH-dependent
enzyme NO synthase. The enzyme requires a number of cofactors,
including flavin adenine dinucleotide, flavin mononucleotide,
tetrahydrobiopterin, and possibly thiols (19). Endothelial
vasodilator dysfunction has been observed in patients with
coronary artery disease or subjects with coronary risk factors. Most
of these conditions are associated with increased oxidative stress,
particularly increased production of superoxide radicals, which
can inactivate EDNO (20). In addition, oxidized LDL has been
shown to inhibit the synthesis of EDNO or attenuate its biological
Ascorbate and Endothelium NO synthesis :
Numerous clinical studies have consistently demonstrated
beneficial effects of vitamin C treatment on endotheliumdependent
vasodilation in individuals with coronary artery disease
or coronary risk factors (22).
There are a number of potential mechanisms underlying the
salubrious effects of ascorbate on endothelial function. First,
ascorbate may be decreasing the levels of superoxide radicals
and oxidized LDL, both of which react with and inactivate NO.
Because of the facile reaction between superoxide and NO radicals, relatively high concentrations of ascorbate (≈10 mmol/L) are
required to effectively inhibit the reaction of NO with superoxide.
Such concentrations are potentially achievable in plasma by intraarterial
infusion or in the cytoplasm as a result of cellular uptake of
ascorbate. Second, Ascorbate may indirectly enhance endotheliumdependent
vasodilation by sparing intracellular thiols, which in
turn stabilize EDNO through the formation of biologically active
S-nitrosothiols. Reducing agents such as ascorbate have also been
implicated in the rapid release of NO from S-nitrosothiols. Finally,
Heller et al., 1999 (23) and, more recently, Huang et al., 2000
(24) have shown that physiological concentrations of ascorbate
increase the synthesis and biological activity of NO in cultured
endothelial cells by increasing intracellular tetrahydrobiopterin.
Thus, a very likely mechanism by which intracellular ascorbate
stimulates NOS activity is regeneration of tetrahydrobiopterin
from the trihydrobiopterin radical. Such a mechanism of action
of ascorbate would also prevent NOS from leaking superoxide
From epidemiological studies people who eat foods rich in vitamin
C have a lower risk of high blood pressure than people who have
poorer diets. Eating foods rich in vitamin C is important for overall
health, especially when person is at risk for high blood pressure.
Nutritionists and dieticians’ most frequently recommended
treatment and prevention of high blood pressure is known as the
DASH (Dietary Approaches to Stop Hypertension) diet which
includes lots of fruits and vegetables loaded with vitamin C.
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