Vitamin C: The state of the art in disease prevention sixty years after the Nobel Prize

133 Pages·1998·2.3 MB·English

by  H. Sies

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R. Paoletti • H. Sies
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Vitamin (
The state of the art in disease
prevention sixty years after
the Nobel Prize
Springer
RODOLFO PAOLETTI
Institute of Pharmacological Sciences
Faculty of Pharmacy
University of Milan (Italy)
HELMUT SIES
Institute of Physiological Chemistry
University of Dusseldorf (Germany)
JOACHIM BUG
Merck Pharma, Self-Medication International
Merck KGaA
Darmstadt (Germany)
ENZO GROSSI
Ethical Drugs Medical Department
Bracco Spa
Milan (Italy)
ANDREA POLl
Nutrition Foundation of Italy
Milano
© Springer-Verlag Italia, Milano 1998
ISBN-13: 978-88-470-0027-8 e-ISBN-13: 978-88-470-2244-7
DOl: 10.1007/978-88-470-2244-7
Library of Congress Cataloging-in-Publication Data: Vitamin C : the state of the art in disease
prevention sixty years after the Nobel Prize 1 [edited by] R. Paoletti ... let al.]. p. cm. Includes
bibliographical references and index. ISBN 8847000270 1. Vitamin C--Physiological effect. 2.
Vitamin C--Therapeutic use. 3. Chemoprevention.1. Paoletti, Rodolfo. [DNLM: 1. Ascorbic Acid
-pharmacology. 2. Ascorbic Acid--therapeutic use. 3. Primary Prevention. QU 210 V8372 1998]
QP772.A8V574 1998 615'.328--DC21 DNLM/DLC for Library of Congress 98-24805 CIP
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Preface
Vitamin C, or ascorbic acid, has a long and multifaceted scientific history.
In 1937, the Nobel Prize for Physiology and Medicine was awarded to Albert
Szent-Gyorgyi'in recognition of his discoveries concerning the biological oxida
tion processes with special reference to vitamin C', and the Nobel Prize for
Chemistry was shared by Sir Norman W. Haworth, who was the first to synthesize
the vitamin. Vitamin C is a potent antioxidant, and this action represented the
theoretical basis for various lines of investigation on this molecule in which the
potential role of ascorbic acid in the prevention and treatment of a series of dis
eases, whose pathogenesis is linked to an excess of free radicals such as athero
sclerosis and cancer, have been examined.
These data have been analyzed in detail by experts in biochemistry, epidemi
ology, and preventive and clinical medicine in the International Symposium
Vitamin C, the state of the art in disease prevention sixty years after the Nobel
Prize, held in Monte Carlo from October 31 to November 1, 1997, under the aus
pices and the scientific endorsement of the Nutrition Foundation of Italy and
with the financial support of Bracco SpA and Merck.
The basic mechanisms of action of ascorbic acid, its interaction with other
antioxidants, and its preclinical and clinical effects on a wide range of human
diseases have been discussed. Special attention has been devoted to the problem
of the optimal intake of ascorbic acid and to the safety issues connected with its
use. The positive effects of vitamin C in pathological conditions such as the com
mon cold and a few respiratory diseases have been considered as well.
This volume presents the point of view of the speakers invited to the meeting.
We hope it will be of help to the increasing number of scientists working on the
biological properties of this important and fascinating natural substance.
Rodolfo Paoletti
Helmut Sies
Joachim Bug
Enzo Grossi
June 1998 Andrea Poli
Table of Contents
Antioxidants and Human Health ................. .
H. SIES, W. STAHL
Modulation of Cell Death by Oxidants and Antioxidants . 13
M.B. HAMPTON, S. ORREN IUS
Metabolism and Metabolic Interactions of Vitamin C. 21
A.M. BODE, J.G.G. VETHANAYAGAN
Antioxidants and Cancer Prevention in 1997 29
T. BYERS, J. MOUCHAWAR
Vitamin Ca nd Gastric Cancer Prevention 41
C.J. SCHORAH
Vitamin Ca nd Cardiovascular Risk Factors 51
G. BLOCK
Vitamin Ca nd Cardiovascular Disease: Mechanisms of Action ..... 59
B.FREI
Vitamin Ca nd Infectious Diseases . 73
H.HEMILA
Vitamin Ci n Respiratory Diseases ........... . 87
P.C.BRAGA
Recommended Vitamin CI ntake:
From Molecular Mechanisms to Clinical Application ....... 107
M. LEVINE, S.c. RUMSEY, Y. WANG, J. PARK, R. DARUWALA, N. AMANO
The Safety of High Doses of Vitamin C . ........... 125
A.T. DIPLOCK
Antioxidants and Human Health
H. SIES, W. STAHL
Introduction
Human health should be maintained from fertilization throughout life, and
long-term exposure to potentially deleterious agents is obviously a factor in
determining the function of the organism as well as the development of age
related degenerative diseases. Oxidants are potentially deleterious agents and,
consequently, antioxidants playa role in the prevention of damage due to oxi
dants (for reviews on various aspects, see [1,2]). Compounds known as antioxi
dants recently have been recognized as modulators of gene expression and of
signal transduction pathways; these novel functions may not necessarily be
based on the antioxidant properties, but rather on the function as ligands to
regulatory proteins. The present contribution focuses first on oxidants, then on
antioxidants; a previous review may be consulted [3].
Oxidants
Aerobic life is associated with the generation of reactive oxygen species, even
under physiological conditions [4,5]. Reactive oxygen species are responsible
for the oxidative damage of biological target molecules such as DNA, lipids,
carbohydrates, or proteins; the reactive oxygen species include peroxyl radicals
(ROOO), the nitric oxide radical (NOO), the superoxide anion radical (0/-), the
hydroxyl radical (OHO), singlet oxygen ('02), peroxynitrite (ONOO-), and
hydrogen peroxide (H202). There are various pathways for the generation of
reactive oxygen species in the human organism. The superoxide radical anion
appears to playa central role, since other reactive intermediates are formed in
reaction sequences initiated by 02°-. Superoxide is generated by enzymatic
one-electron reduction of oxygen from xanthine oxidase, NADPH oxidase, by
leakage of the respiratory chain, or by redox-cycling mechanisms [6]. It has
Institut fUr Physiologische Chemie lund Biologisch-Medizinisches Forschungszentrum,
Heinrich-Heine-Universitat DUsseldorf, Postfach 101007, D-40001-DUsseldorf, Germany
2 H. Sies, W. Stahl
been estimated that about I %-3% of the oxygen we utilize is converted to 02·
(4).
Hydrogen peroxide is a nonradical reactive species which is reduced to
water by catalase or by glutathione (GSH) peroxidases. Recent evidence sug
gests that hydrogen peroxide is involved in signal transduction, regulating the
expression of genes through the NFKB and AP-l pathways [7,8).
The most reactive species is the hydroxyl radical, with an estimated half-life
of about 10-9 s, and formed in vivo upon high energy irradiation (e.g., x-rays)
by homolytic cleavage of water or from endogenous hydrogen peroxide in
metal catalyzed processes (Fenton reaction; iron -catalyzed Haber-Weiss reac
tion). Ultraviolet light is not capable energetically of splitting water, but it can
cleave hydrogen peroxide to yield two molecules of the hydroxyl radical. The
high reactivity of the hydroxyl radical implies immediate reaction at the site
where it is generated. Consequently, there is no practicable defense in terms of
interception of the hydroxyl radical, because too high a concentration of a trap
ping molecule would be required (in the lOO-mM range), which would not be
feasible for osmotic reasons.
The peroxyl radical (ROO·) is a relatively long-lived species (seconds), with
a considerable diffusion path length in biological systems. Peroxyl radicals can
be generated in the process of lipid peroxidation, which is initiated by the
abstraction of an hydrogen atom from polyunsaturated fatty acids (PUFA).
Further products generated in lipid peroxidation are alkoxyl radicals (RO·) and
organic hydroperoxides (ROOH). The latter can rearrange to endoperoxide
intermediates which decompose to yield aldehydes. The reaction of aldehydes
with amine groups has been suggested to be a mechanism involved in the mod
ification of the protein part of lipoproteins.
Singlet molecular oxygen (102) is another nonradical reactive oxygen
species which is suggested to be formed in vivo. Its half-life has been estimated
to be 10-6 s, depending on the nature of the solvent. 102 can interact with other
molecules either by transferring its excitation energy or by combining chemi
cally. Preferential targets for chemical reactions are double bonds, e.g., in
PUFAs or in DNA bases such as guanine (for reviews see [9-11)).
The nitric oxide radical (NO·) has attracted attention in the past few years,
being a signaling compound formed enzymatically from arginine. It relaxes
smooth muscle cells in blood vessel walls, resulting in lowered blood pressure.
Nitric oxide is also produced by activated macrophages, contributing to prima
ry immune defense. An excess of nitric oxide is cytotoxic. Nitric oxide reacts
directly with 02"- to yield peroxynitrite (ONOO-) which, in turn, is capable of
oxidizing guanines in DNA, of generating strand breaks in DNA, or of inducing
lipid peroxidation in lipoproteins. It also interferes with cellular signaling path
ways by nitrating tyrosine residues in proteins (for a recent overview, see (12)).
These reactive oxygen species are produced by endogenous reactions in the
organism, but we are also exposed to reactive oxygen species generated from
external sources. Numerous compounds of prooxidant nature are delivered to
the organism via the diet. Cigarette smoke contains an array of radicals and
Antioxidants and Human Health 3
ozone is also a reactive oxygen species [13].
Phagocytic cells such as neutrophils, monocytes, or macrophages defend the
system against foreign organisms by synthesizing large amounts of 02°- or
nitric oxide as a part of the killing mechanism. Several diseases are accompa
nied by excessive phagocyte activation, resulting in tissue damage which is at
least in part due to the activity of reactive oxygen species.
Antioxidants
Enzymatic and Nonenzymatic Defense Systems In Vivo
To counteract the prooxidant load, a diversity of antioxidant defense systems
are operative in biological systems, including enzymatic and nonenzymatic
antioxidants [3]. An antioxidant has been defined as "any substance that, when
present in low concentrations compared to that of an oxidizable substrate, sig
nificantly delays or inhibits the oxidation of that substrate" [2, 14].
The major enzymes directly involved in the detoxification of reactive oxygen
species are superoxide dis mutase, scavenging 02°-, as well as catalase and GSH
peroxidases which reduce hydrogen peroxide and organic hydroperoxides,
respectively. Subtypes of GSH peroxidases are selenium-dependent. An elevat
ed intake of selenium has been associated with protective effects against cancer
in animal studies. The preventive effects of selenium in humans are under
investigation [15]. Indirect antioxidant functions are mediated by enzymes that
restore endogenous antioxidant levels; for example, GSH levels are replenished
upon reduction of the GSH disulfide (GSSG) by GSH reductase. Further, reac
tive intermediates produced in reactions of pro oxidants and biological mole
cules (e.g., epoxides) are conjugated by phase-II detoxification enzymes such as
GSH-S-transferases to favor their excretion. Another strategy of preventing the
formation of reactive oxygen species is the control of the levels of free transi
tion metals such as iron or copper ions. Metal binding proteins responsible for
the transport of metal ions prevent the initiation of lipid peroxidation or DNA
damage. Major metal binding proteins are ferritin, transferrin, and ceruloplas
min.
A variety of endogenous, low-molecular-weight compounds are also
involved in antioxidant defense. GSH, the major cytosolic thiol, serves as a
cofactor of several detoxifying enzymes (GSH peroxidases, GSH-S-transferases)
and is involved in the reduction of protein disulfides [16]. Other endogenous
compounds such as ubiquinol-l0, urate, or bilirubin also exhibit antioxidant
activities.
Dietary Antioxidants
The human diet contains an array of different compounds exhibiting antioxi
dant activities. The most prominent representatives of dietary antioxidants are
4 H. Sies, W. Stahl
ascorbate (vitamin C), tocopherols (vitamin E), carotenoids (Fig. 1), and
flavonoids. Apart from vitamin C, each of these groups of antioxidants consists
of a number of structurally different compounds. Regarding vitamin E, the term
denotes several different tocopherols and tocotrienols and their isomers: the
naturally occurring major form is RRR-a-tocopherol. As to carotenoids, more
than 600 different carotenes and oxocarotenoids have been identified up to
now, and about 50 different carotenoids may occur in the human diet.
Vitamin C is one of the most powerful natural antioxidants; its structure
was identified by Szent-Gyorgyi [17]. The life of the eminent scientist Albert
Szent-Gyorgyi has been described vividly [18], and several monographs on vit
amin C are available [19-21]. Ascorbate is water-soluble and is found in high
Ascorbic Acid (Vitamin C)
a-Tocopherol (Vitamin E)
B-Carotene
Lycopene
Fig. 1. Structures of some antioxidant nutrients
Antioxidants and Human Health 5
concentrations in many tissues; human plasma contains about 60 flmol ascor
bate/I. Upon reaction with reactive oxygen species it is oxidized in two one-elec
tron steps to dehydroascorbate via the ascorbyl free radical. Dehydroascorbate
is recycled back to ascorbate by the dehydroascorbate reductases. Thus, dehy
droascorbate is found in low levels as compared to vitamin C. As a scavenger of
reactive oxygen species, ascorbate has been shown to be effective against super
oxide radical anion, hydrogen peroxide, the hydroxyl radical, peroxynitrite, and
singlet oxygen. Important sources of ascorbate in the diet are fruits, broccoli,
cauliflower, Brussels sprouts, and cabbage; its content may exceed 100 mg as cor
bate/lOO g fresh weight. The efficacy of absorption from the lumen of the gas
trointestinal tract decreases with increasing dose levels [22].
There is evidence that vitamin C is capable of regenerating tocopherol from
the tocopheroxyl radical which is formed upon inhibition of lipid peroxidation
by vitamin E [23-25]. This process would allow for the transfer of a radical load
from a lipophilic compartment to an aqueous compartment where it is taken
care of by further enzymatic defense systems. In the presence of free transition
metal ions (iron and copper) and ascorbate, the hydroxyl radical can be gener
ated and initiation of lipid peroxidation may occur. Vitamin C has additional,
well-established biological functions, including cofactor activity for several
important enzymes [22]. Further, vitamin C suppresses the formation of
nitrosamines from nitrite, which is of importance with regard to beneficial
effects against cancer.
The term vitamin E is a generic description for all tocols and tocotrienol
derivatives which exhibit the biological activity of a-tocopherol. This group of
compounds is highly lipophilic and operative in membranes or lipoproteins.
Their most important antioxidant function appears to be the inhibition of lipid
peroxidation scavenging lipid peroxyl radicals to yield lipid hydroperoxides and
a tocopheroxyl radical (see [26] for review). The latter is less reactive than per
oxyl radicals towards neighboring PUFAs and acts as a chain-breaking antioxi
dant. The tocopheroxyl radical might be either reduced by ascorbate and GSH
or further oxidized to the respective quinone. Since only small amounts of toco
pheryl quinone are detectable in human blood and tissues, the regenerative
pathway in vivo appears to be favored. In addition to the peroxyl radical scav
enging properties, further interactions with reactive oxygen species have been
described, including quenching of singlet oxygen [27] and interaction with per
oxynitrite. The richest sources of vitamin E in the diet are vegetable oils (soy
bean, corn, cotton seed, and safflower) and products made from these oils such
as margarine and mayonnaise. Further, wheat germ, nuts, and some leafy green
vegetables contribute to the dietary vitamin E supply. Vitamin E plasma levels
in the human are about 22 flmol!l; the compound is also found in tissues such
as liver, kidney, fat, or adrenals. In the liver the RRR-isomer of a-tocopherol is
preferentially incorporated into very low density lipoproteins (VLDL), which
are further catabolized in the circulation. Thus, RRR-a-tocopherol is the major
form of vitamin E in LDL.