Ascorbic acid metabolism and functions: A comparison of plants and mammals

Abstract

Ascorbic acid is synthesised by eukaryotes, the known exceptions being primates and some other animal groups which have lost functional gulonolactone oxidase. Prokaryotes do not synthesise ascorbate and do not need an ascorbate supply, so the functions that are essential for mammals and plants are not required or are substituted by other compounds. The ability of ascorbate to donate electrons enables it to act as a free radical scavenger and to reduce higher oxidation states of iron to Fe2+. These reactions are the basis of its biological activity along with the relative stability of the resulting resonance stabilised monodehydroascorbate radical. The importance of these properties is emphasised by the evolution of at least three biosynthetic pathways and production of an ascorbate analogue, erythroascorbate, by fungi. The iron reducing activity of ascorbate maintains the reactive centre Fe2+ of 2-oxoglutarate-dependent dioxygenases (2-ODDs) thus preventing inactivation. These enzymes have diverse functions and, recently, the possibility that ascorbate status in mammals could influence 2-ODDs involved in histone and DNA demethylation thereby influencing stem cell differentiation and cancer has been uncovered. Ascorbate is involved in iron uptake and transport in plants and animals. While the above biochemical functions are shared between mammals and plants, ascorbate peroxidase (APX) is an enzyme family limited to plants and photosynthetic protists. It provides these organisms with increased capacity to remove H2O2 produced by photosynthetic electron transport and photorespiration. The Fe reducing activity of ascorbate enables hydroxyl radical production (pro-oxidant effect) and the reactivity of dehydroascorbate (DHA) and reaction of its degradation products with proteins (dehydroascorbylation and glycation) is potentially damaging. Ascorbate status influences gene expression in plants and mammals but at present there is little evidence that it acts as a specific signalling molecule. It most likely acts indirectly by influencing the redox state of thiols and 2-ODD activity. However, the possibility that dehydroascorbylation is a regulatory post-translational protein modification could be explored.

The Chemistry of Protein Oxidation in Food

“…Ascorbate ( 1 , vitamin C) plays a dual role in protein redox chemistry. It can reduce Fe 3+ to Fe 2+ through the generation of a monodehydroascorbate radical 2 (Scheme ) . This regeneration of Fe 2+ is also the function of ascorbate as a cofactor in enzymatic oxidation reactions, e.g., proline hydroxylation during collagen synthesis .…”

“…This regeneration of Fe 2+ is also the function of ascorbate as a cofactor in enzymatic oxidation reactions, e.g., proline hydroxylation during collagen synthesis . Dismutation of radical 2 or further oxidation leads to dehydroascorbic acid ( 3 ) . On the other hand, ascorbate can repair protein radicals such as tryptophanyl and tyrosyl radicals and act as an antioxidant .…”

“…Ascorbate (1,vitamin C) plays adual role in protein redox chemistry.I tc an reduce Fe 3+ to Fe 2+ through the generation of am onodehydroascorbate radical 2 (Scheme 1). [71] This regeneration of Fe 2+ is also the function of ascorbate as [a] Only samples taken between 1960 and 2000 were considered from Fanetal.,2008. [59] [b] Data given on adry-matter basis for soy beans [61] were coverted to fresh-weightu sing ad ry mass of 88 g/100 g. Scheme 1.…”

Die Chemie der Proteinoxidation in Lebensmitteln

“…B. der Hydroxylierung von Prolin bei der Collagenreifung . Die Disproportionierung des Radikals 2 oder weitere Oxidation führt zur Dehydroascorbinsäure ( 3 ) . Andererseits kann Ascorbat Proteinradikale, z.…”

“…[72] Die Disproportionierung des Radikals 2 oder weitere Oxidation führt zur Dehydroascorbinsäure (3). [71] Andererseits kann Ascorbat Proteinradikale, z. B. lokalisiert an Tyrosin-oder Tryptophanresten, reparieren und so als Antioxidans fungieren.…”