Ascorbate is a multifunctional micronutrient whose synthesis is lacking in primates

Abstract

Ascorbate (vitamin C) is an essential micronutrient in primates, and exhibits multiple physiological functions. In addition to antioxidative action, ascorbate provides reducing power to α-ketoglutarate-dependent non-heme iron dioxygenases, such as prolyl hydroxylases. Demethylation of histones and DNA with the aid of ascorbate results in the reactivation of epigenetically silenced genes. Ascorbate and its oxidized form, dehydroascorbate, have attracted interest in terms of their roles in cancer therapy. The last step in the biosynthesis of ascorbate is catalyzed by l-gulono-γ-lactone oxidase whose gene Gulo is commonly mutated in all animals that do not synthesize ascorbate. One common explanation for this deficiency is based on the increased availability of ascorbate from foods. In fact, pathways for ascorbate synthesis and the detoxification of xenobiotics by glucuronate conjugation share the metabolic processes up to UDP-glucuronate, which prompts another hypothesis, namely, that ascorbate-incompetent animals might have developed stronger detoxification systems in return for their lack of ability to produce ascorbate, which would allow them to cope with their situation. Here, we overview recent advances in ascorbate research and propose that an enhanced glucuronate conjugation reaction may have applied positive selection pressure on ascorbate-incompetent animals, thus allowing them to dominate the animal kingdom.

Keywords: ascorbate; detoxification; glucuronate conjugation; vitamin C.

Fig. 1.

Interconversion of Asc, ascorbyl radical and DHA and their roles in the vitamin E-mediated elimination of lipid radical. Three major forms of ascorbic acid and their redox-mediated interconversion are depicted on the top. Vitamin E (tocopherol) donates electron to lipid hydroxyl radical (LOO^•) and is converted to tocopherol radical. Asc may donate an electron to a tocopherol radical for recycling and is converted to an ascorbyl radical, which is then oxidized to DHA. Asc collectively can protect lipids from oxidation reactions as well. Schemes show conceptual reactions and are not stoichiometric. R; alkyl side chain.

Fig. 2.

Pathways for Asc synthesis and other related carbohydrate metabolism pathways from glucose. Solid lines indicate carbon flow to Asc synthesis. Dotted lines indicate flows that simultaneously or occasionally procced depending on the physiological conditions of the animals. Gulo in the ER is absent in incompetent animals of the Asc synthesis.

Fig. 3.

Roles of ER in the production of glucose via glucose 6-P and Asc via UDP-glucose from glycogen in rodents. Glucose 1-P from glycogen is the common precursor for plasma glucose and Asc. Glucagon stimulates glycogenolysis but is not involved in UDP-glucose formation. In the meantime, xenobiotics and glutathione consumption stimulate the production of UDP-glucose and the consequent Asc synthesis, but not blood glucose. Dephosphorylation of glucose 6-P by glucose 6-phosphatase (G6Pase) releases glucose in the ER. Gulo oxidizes l-gulono-γ-lactone to Asc by means of the oxidation of molecular oxygen, leading to the production of hydrogen peroxide.

Fig. 4.

Transport system for Asc and DHA and the reductive recycling of DHA. Asc is transported via the SVCT which is driven by a Na⁺ electrochemical gradient. Some types of GLUT are responsible for DHA uptake as well as glucose (Glc) uptake. DHA is reduced to Asc by either enzymatic reactions or an electron donation from glutathione. DHA is partly transported into mitochondria via GLUT and reduced to Asc by electrons from the respiratory chain.

Fig. 5.

Proline hydroxylation and the maturation of collagen in the ER. (A) Proly hydroxylase catalyzes the oxidative conversion of proline to hydroxyproline by employing Asc, molecular oxygen and α-ketoglutarate (αKG) and results in the production of DHA, carbon dioxide (CO₂) and succinate (Suc). (B) Oxidative folding and the prolyl hydroxylation of collagen proceed in the ER lumen. Procollagen and other secretory proteins undergo oxidative folding by means of ERO1 and other thiol oxidases. The resulting procollagen is hydroxylated at proline residues by prolyl hydroxylase coupled with Asc oxidation at multiple positions (~100 proline residues in a molecule) and then secreted out of the cell. DHA may also be involved in thiol oxidation. Both an Asc insufficiency and abundance may cause pathological conditions, such as scurvy and fibrosis, respectively.

Fig. 6.

Epigenetic action of Asc towards gene activation. Electron donation from Asc to TET reaction supports the conversion of 5-methyl cytosine to 5-hydroxyl cytosine and then 5-carboxylcytosine, which is consequently replaced to cytosine by the base excision DNA repair system. Jumonji C (JmjC)-domain-containing histone demethylases (JHDMs) removes methyl groups from lysines in histones. Demethylation of DNA and histones may be able to activate consequent gene expression. Only concept of the reactions are presented schematically.

Fig. 7.

Roles of Asc in controlling HIF-1α action. Under normoxic conditions, prolyl hydroxylase catalyzes the hydroxylation of specific proline residues in HIF-1α, which leads to polyubiquitination by the ubiquitin ligase activity of pVHL, followed by degradation via proteasomes. Under hypoxic conditions, the short oxygen supply suppresses proly hydroxylation, which results in the stabilization of HIF-1α. After translocation to the nucleus, HIF-1α dimerizes with HIF-1β, which is constitutively present largely in the nucleus, and together with other transcriptional factors, activates transcription of corresponding genes, such as erythropoietin, vascular epidermal growth factor (VEGF), and GLUT.

Fig. 8.

Schematic representation of and crosstalk between two detoxification pathways, glutathione conjugation and glucuronate conjugation in the liver. While glutathione conjugation is catalyzed by glutathione S-transferases (GST), glucuronate conjugation is catalyzed by UGT. UGT may also catalyze the hydrolysis of UDP-glucuronate, resulting in the formation of d-glucuronate. Pathways for glucuronate conjugation and Asc synthesis share the metabolic processes up to UDP-glucuronate. Ablation of enzymes responsible for the downstream reaction causes the accumulation of intermediary compounds that include UDP-glucuronate. As a result, the glucuronate conjugation reaction is enhanced in animals that have a defect in Asc synthesis and preserve glutathione for antioxidation.