The Antioxidant Activities of Vitamin C and Lipoic Acid in the Body
| My primary research
focus under the direction of Drs. Balz Frei and Tory M. Hagen is to determine
the biochemical mechanisms by which vitamin C and lipoic acid attenuate
oxidative stress caused by transition metals like iron and copper. We are
particularly interested in learning whether vitamin C and lipoic acid are
effective in ameliorating the metal-dependent oxidative stress in humans
with hemochromatosis (iron-overload disease), Wilson's disease (abnormal
copper accumulation), and during aging. Understanding the physiological
interactions that may occur between these antioxidants and metals is important
because of suggestions that excess amounts of certain reactive metals may
play a role in the pathogenesis of a number of chronic disorders, such as
cancer and atherosclerosis, and neurological diseases like Alzheimer's and
Parkinson's. Both vitamin C and lipoic acid are naturally occurring compounds
that serve important roles in a number of biological processes. Vitamin
C and lipoic acid are also potent antioxidants that can scavenge various
reactive oxygen and nitrogen radicals. Structurally, vitamin C and lipoic
acid are quite distinct from each other. While the structure of vitamin
C resembles that of other basic sugar compounds, lipoic acid contains two
sulfur groups that are required for its metabolic and antioxidant functions.
Because of these differences, these compounds have quite different reactivity
with transition metal ions like iron and copper. For instance, lipoic acid
has been shown to bind to these metals while vitamin C does not, or at least
to a much lesser extent. However, it is not clear how these differences
in their structure affect their antioxidant activity in the body.
The critical role of vitamin C in ameliorating the adverse effects of reactive oxygen and nitrogen radicals has been well established. In addition, numerous epidemiological studies strongly support the protective role of vitamin C in decreasing the incidence of chronic diseases like atherosclerosis where oxidative stress caused by excessive oxygen or nitrogen radicals may play a causal role. Despite these reports, there are concerns that vitamin C could potentially promote reactive oxygen radical generation, especially in persons suffering from iron-overload. The basis for this concern is that in test tube experiments, vitamin C serves as an excellent catalyst for metal-mediated hydroxyl radical generation, thereby acting as a pro-oxidant. However, it has been unknown if these pro-oxidant reactions occur in the body. To address this issue, we have recently completed a study using human plasma to test the antioxidant or potential pro-oxidant effects of vitamin C.
We added iron to freshly isolated human plasma and then examined the effects of different amounts of vitamin C added to the plasma on metal-dependent oxidation of plasma lipids and proteins. In the body, most or all of the iron and copper is bound to transport or storage proteins and, therefore, unable to react with vitamin C. In the presence of excess iron added to the plasma, exceeding the binding capacity of the transport and storage proteins, vitamin C was oxidized rapidly. However, we failed to detect any significant formation of lipid hydroperoxides (sensitive markers of lipid oxidation) if vitamin C was initially present. Interestingly, lipid hydroperoxide formation was only observed in plasma samples that had been depleted of endogenous vitamin C. Similar results were also obtained when copper was used instead of iron. In all of these experiments, the rate of vitamin E oxidation was unaffected by either the addition of vitamin C and/or metals, suggesting that protection by vitamin C is not dependent on the regeneration of vitamin E from its oxidized form. Thus, in human plasma, vitamin C protects against, rather than promotes, the metal-dependent oxidation of lipids.
To examine the effect of vitamin C and metals on proteins, we measured protein carbonylsmarkers of oxidative protein damagein plasma. We found that the addition of iron or copper caused a significant increase in the levels of protein carbonyls, indicating damage. Although vitamin C added to plasma failed to protect against protein carbonyl formation, it did not further enhance metal-dependent protein oxidation. Thus, these results demonstrate that vitamin C does not promote protein damage in human plasma, even in the presence of excess metal ions.
Excess iron accumulation can also occur naturally during the aging process even in individuals without hereditary hemochromatosis. Although the mechanisms and physiological consequences of aberrant iron accumulation during aging are not well understood, the age-associated increase in iron load may contribute significantly to increased oxidative stress. Several animal studies, including our own using rats, showed that there is a marked increase in total iron content in the brain associated with old age. Although the exact contribution of the increased iron burden to oxidative stress or neurodegeneration is not clear, we observed a significant inverse correlation between total iron burden and vitamin C content, indirectly showing that iron may further enhance oxidant generation in the aging brain.
A dietary intervention with antioxidants that can effectively lower or inhibit the reactivity of free iron in the brain may be important in attenuating the age-related increase in oxidative stress. (R)-alpha-lipoic acid is a thiol compound found naturally in plants and animals. Certain enzymes found only in the mitochondria in animal cells reduce lipoic acid to dihydrolipoic acid, which is a potent antioxidant. Lipoic acid increases or maintains levels of other antioxidants, particularly vitamin C and glutathione (an important intracellular antioxidant). Aside from their potent antioxidant function, lipoic acid and dihydrolipoic acid can chelate free metal ions, such as iron and copper, in vitro, resulting in the loss of their potentially harmful reactivity, probably through subsequent excretion.
To investigate the effects of aging on iron accumulation and antioxidant status, we examined iron, vitamin C, and glutathione levels in the frontal cortex of brains from young and old rats. Consistent with other published data, we observed a 50% increase in the total iron content in the cortex in the old rats compared to young rats. In addition to the age-related iron accumulation, we observed a 27% decline in the total vitamin C levels in the old rats. Interestingly, we did not observe any statistically significant differences in the total glutathione concentrations in the cortex, although there was an age-dependent increase in the amount of oxidized glutathione. Thus, increased iron burden in conjunction with the decline in tissue antioxidant status may contribute to a significant age-dependent increase in oxidative stress.
We decided to examine whether supplementation with lipoic acid can reverse the age-dependent increase in the iron content of the brain and also decrease oxidative stress. We fed young (3-4 months) and old (28 months) rats a diet with or without 0.2% (w/w) lipoic acid for two weeks. Based on the food consumption, rats of all age groups ingested about 50 mg of lipoic acid per kilogram of body weight. We found a significant decrease in the total iron content in the brains of old animals fed lipoic acid compared to their age-matched controls not given lipoic acid. In fact, the iron content in old rat brains was no longer significantly different from the iron content in the young unsupplemented animals. Lipoic acid did not lower the iron content in young rat brains, suggesting that lipoic acid mainly removes the excess iron accumulated with age. We also found a 30% increase in vitamin C levels in the brains from old rats treated with lipoic acid. Consistent with this observation, lipoic acid also increased the amount of glutathione in its reduced form.
Although the mechanisms responsible for the observed decrease in iron by lipoic acid in the aging brain are not known, our results provide a basis for studies in humans on the use of lipoic acid as a therapeutic agent in either slowing or preventing neurodegenerative disorders. Lipoic acid may be a safe and effective means to lower the age-related increase in iron content in the brain and also bolster protection against free radical damage, but much more work needs to be done before any recommendations for humans can be confidently made.
updated May, 2002
Scientific Giant with Nutritional Research
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