The GULO gene for vitamin C synthesis is found exclusively in mammals
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REFUTED
the evidence says no
refutedsupported
the weight of evidence
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Scientific evidence shows that the GULO gene for vitamin C synthesis is not found exclusively in mammals, as it is present in various non-mammalian species including non-bilaterians, protostomes, deuterostomes, fishes, and birds.
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 Fe<sup>2+</sup>. 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 Fe<sup>2+</sup> 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 H<sub>2</sub>O<sub>2</sub> 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.
<h4>Background</h4>Vitamin C (VC) is an indispensable antioxidant and co-factor for optimal function and development of eukaryotic cells. In animals, VC can be synthesized by the organism, acquired through the diet, or both. In the single VC synthesis pathway described in animals, the penultimate step is catalysed by Regucalcin, and the last step by L-gulonolactone oxidase (GULO). The GULO gene has been implicated in VC synthesis only, while Regucalcin has been shown to have multiple functions in mammals.<h4>Results</h4>Both GULO and Regucalcin can be found in non-bilaterian, protostome and deuterostome species. Regucalcin, as here shown, is involved in multiple functions such as VC synthesis, calcium homeostasis, and the oxidative stress response in both Deuterostomes and Protostomes, and in insects in receptor-mediated uptake of hexamerin storage proteins from haemolymph. In Insecta and Nematoda, however, there is no GULO gene, and in the latter no Regucalcin gene, but species from these lineages are still able to synthesize VC, implying at least one novel synthesis pathway. In vertebrates, SVCT1, a gene that belongs to a family with up to five members, as here shown, is the only gene involved in the uptake of VC in the gut. This specificity is likely the result of a subfunctionalization event that happened at the base of the Craniata subphylum. SVCT-like genes present in non-Vertebrate animals are likely involved in both VC and nucleobase transport. It is also shown that in lineages where GULO has been lost, SVCT1 is now an essential gene, while in lineages where SVCT1 gene has been lost, GULO is now an essential gene.<h4>Conclusions</h4>The simultaneous study, for the first time, of GULO, Regucalcin and SVCTs evolution provides a clear picture of VC synthesis/acquisition and reveals very different selective pressures in different animal taxonomic groups.
Vitamin C is an organic compound biosynthesized in plants and most vertebrates. Since its discovery, the benefits of vitamin C use in the cure and prevention of various pathologies have been frequently reported, including its anti-oxidant, anti-inflammatory, anticoagulant, and immune modulatory properties. Vitamin C plays an important role in collagen synthesis and subsequent scurvy prevention. It is also required in vivo as a cofactor for enzymes involved in carnitine and catecholamine norepinephrine biosynthesis, peptide amidation, and tyrosine catabolism. Moreover, as an enzymatic cofactor, vitamin C is involved in processes of gene transcription and epigenetic regulation. The absence of the synthesis of L-gulono-1,4-lactone oxidase, a key enzyme in the pathway of vitamin C synthesis, is an inborn metabolism error in some fishes and several bird and mammalian species, including humans and non-human primates; it is caused by various changes in the structure of the original <i>GULO</i> gene, making these affected species dependent on external sources of vitamin C. The evolutionary cause of <i>GULO</i> gene pseudogenization remains controversial, as either dietary supplementation or neutral selection is evoked. An evolutionary improvement in the control of redox homeostasis was also considered, as potentially toxic H<sub>2</sub>O<sub>2</sub> is generated as a byproduct in the vitamin C biosynthesis pathway. The inactivation of the <i>GULO</i> gene and the subsequent reliance on dietary vitamin C may have broader implications for aging and age-related diseases, as one of the most important actions of vitamin C is as an anti-oxidant. Therefore, an important aim for medical professionals regarding human and animal health should be establishing vitamin C homeostasis in species that are unable to synthesize it themselves, preventing pathologies such as cardiovascular diseases, cognitive decline, and even cancer.
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