Vitamins C and E: beneficial effects from a mechanistic perspective

Abstract

The mechanistic properties of two dietary antioxidants that are required by humans, vitamins C and E, are discussed relative to their biological effects. Vitamin C (ascorbic acid) is an essential cofactor for α-ketoglutarate-dependent dioxygenases. Examples are prolyl hydroxylases, which play a role in the biosynthesis of collagen and in down-regulation of hypoxia-inducible factor (HIF)-1, a transcription factor that regulates many genes responsible for tumor growth, energy metabolism, and neutrophil function and apoptosis. Vitamin C-dependent inhibition of the HIF pathway may provide alternative or additional approaches for controlling tumor progression, infections, and inflammation. Vitamin E (α-tocopherol) functions as an essential lipid-soluble antioxidant, scavenging hydroperoxyl radicals in a lipid milieu. Human symptoms of vitamin E deficiency suggest that its antioxidant properties play a major role in protecting erythrocyte membranes and nervous tissues. As an antioxidant, vitamin C provides protection against oxidative stress-induced cellular damage by scavenging of reactive oxygen species, by vitamin E-dependent neutralization of lipid hydroperoxyl radicals, and by protecting proteins from alkylation by electrophilic lipid peroxidation products. These bioactivities bear relevance to inflammatory disorders. Vitamin C also plays a role in the function of endothelial nitric oxide synthase (eNOS) by recycling the eNOS cofactor, tetrahydrobiopterin, which is relevant to arterial elasticity and blood pressure regulation. Evidence from plants supports a role for vitamin C in the formation of covalent adducts with electrophilic secondary metabolites. Mechanism-based effects of vitamin C and E supplementation on biomarkers and on clinical outcomes from randomized, placebo-controlled trials are emphasized in this review.

Copyright © 2011 Elsevier Inc. All rights reserved.

Figures

Fig. 1

Vitamin C acts as a cofactor for prolyl 4-hydroxylase. The starting point for the catalytic cycle is where the co-substrate, α-ketoglutaric acid, coordinates with the enzyme-bound FeII (step 1). Activation of molecular oxygen (2) and subsequent decarboxylation of α-ketoglutaric acid (3) leads to the formation of the highly energetic FeIV=O reagent that hydroxylates proline residues in procollagen (steps 4 and 5, inner catalytic cycle) [217-219]. If decarboxylation of α-ketoglutaric acid and subsequent formation of the FeIV=O species takes place in the absence of a substrate molecule (proline residue), the FeIV=O species will oxidize a molecule of ascorbic acid in order to regain activity (steps 6 and 7, outer cycle). Thus, ascorbic acid is consumed stoichiometrically in the uncoupled reaction but is not consumed when substrate is available for oxidation. The iron is bound to prolyl 4-hydroxylase through interactions with amino acid residues, His 412, Asp414, and His 483 [10]. The co-substrate, α-ketoglutaric acid, binds to the enzyme-bound iron through two coordination sites as shown. These coordination sites can be occupied by ascorbate upon departure of succinate from the FeIV=O species in the uncoupled reaction (outer cycle).

Fig. 2

Antioxidant effects of vitamins C and E on lipid peroxidation (LPO). The LPO chain reaction can be initiated by many radical species (indicated by R•) and converts LH into LOO•, which attacks another LH generating L• (paths 1 and 2, dotted oval). Ascorbic acid may scavenge the initiating radical species R• and reduce the tocopheroxyl radical, generating the ascorbyl radical, which can be reduced by glutathione dependent enzymes. Key to reaction steps: 1, initiating event; 2, radical propagation reaction; 3, termination of the radical reaction by tocopherol (TocH); 4, dismutation of ascorbyl radicals (Asc•–); 5, reduction of dehydroascorbate (DHAsc) by GSH-dependent dehydroascorbate reductase; 6, GSH peroxidase (GPx); 7, further oxygenation and non-enzymatic cleavage of carbon-carbon bonds yields 4-hydroperoxy-2(E)-nonenal (HPNE); 8, reduction yields 4-hydroxy-2(E)-nonenal (HNE).

Fig. 3

Proposed mechanism of NO biosynthesis by NOS, a heme-containing flavo-enzyme (adapted from [109]). In the first reaction, arginine is hydroxylated to form Nω-hydroxyarginine, which is oxidatively converted in the second reaction into NO and citrulline. The oxygen in both reactions originates from a heme FeIV=O species, which is formed by oxygenation of heme-FeII, acceptance of an electron from tetrahydrobiopterin (BH4), protonation and loss of a water molecule (steps 1-4; in step 5, the FeIV-oxo complex arises from the FeV-oxo complex by electron transfer from a ligand nitrogen atom to iron. NO is released in the second reaction via an intermediate FeIII-NO complex. The resulting heme-FeIII species is reduced back to heme-FeII by the flavoprotein domain (step 6). In endothelial NOS, the heme-iron is bound to Cys184 of the enzyme [220].

Fig. 4

Michael adduction of acrolein with ascorbate and subsequent metabolism of the acrolein– ascorbate adduct in cultured human THP-1 monocytes.

Fig. 5

Antioxidant effect of vitamin E. α-Tocopherol reacts with a lipid hydroperoxyl (LOO•) radical. The resultant tocopheryl radical is resonance stabilized, does not react with oxygen (unlike L• radicals) and it can be converted back to α-tocopherol by ascorbate.