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Review
. 2023 Feb 3;11(2):397.
doi: 10.3390/microorganisms11020397.

Pleiotropic Functions of Nitric Oxide Produced by Ascorbate for the Prevention and Mitigation of COVID-19: A Revaluation of Pauling's Vitamin C Therapy

Hideo Yamasaki  1 , Hideyuki Imai  1 , Atsuko Tanaka  1 , Joji M Otaki  1
Affiliations
Review

Pleiotropic Functions of Nitric Oxide Produced by Ascorbate for the Prevention and Mitigation of COVID-19: A Revaluation of Pauling's Vitamin C Therapy

Hideo Yamasaki et al. Microorganisms. .

Abstract

Linus Pauling, who was awarded the Nobel Prize in Chemistry, suggested that a high dose of vitamin C (l-ascorbic acid) might work as a prevention or treatment for the common cold. Vitamin C therapy was tested in clinical trials, but clear evidence was not found at that time. Although Pauling's proposal has been strongly criticized for a long time, vitamin C therapy has continued to be tested as a treatment for a variety of diseases, including coronavirus infectious disease 2019 (COVID-19). The pathogen of COVID-19, SARS-CoV-2, belongs to the β-coronavirus lineage, which includes human coronavirus, severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS). This review intends to shed new light on vitamin C antiviral activity that may prevent SARS-CoV-2 infection through the chemical production of nitric oxide (NO). NO is a gaseous free radical that is largely produced by the enzyme NO synthase (NOS) in cells. NO produced by upper epidermal cells contributes to the inactivation of viruses and bacteria contained in air or aerosols. In addition to enzymatic production, NO can be generated by the chemical reduction of inorganic nitrite (NO2-), an alternative mechanism for NO production in living organisms. Dietary vitamin C, largely contained in fruits and vegetables, can reduce the nitrite in saliva to produce NO in the oral cavity when chewing foods. In the stomach, salivary nitrite can also be reduced to NO by vitamin C secreted from the epidermal cells of the stomach. The strong acidic pH of gastric juice facilitates the chemical reduction of salivary nitrite to produce NO. Vitamin C contributes in multiple ways to the host innate immune system as a first-line defense mechanism against pathogens. Highlighting chemical NO production by vitamin C, we suggest that controversies on the therapeutic effects of vitamin C in previous clinical trials may partly be due to less appreciation of the pleiotropic functions of vitamin C as a universal bioreductant.

Keywords: COVID-19; SARS-CoV-2; antiviral activity; l-ascorbic acid: Linus Pauling; nitric oxide; nitrite; vitamin C.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
NO and RNS in COVID-19. NO and its derived reactive molecules are frequently referred to as “reactive nitrogen species” (RNS). Peroxynitrite (ONOO−) is a reaction product between NO and superoxide (O2−). RNS potentially mediate the oxidation, nitration, nitrosation and nitrosylation of biomolecules. Those reactions exhibit both beneficial and harmful effects. NO2-Tyr, nitro-tyrosine; 8-NO2-cGMP, 8-nitroguanosine 3′,5′-cyclic monophosphate; NO2-FA, nitro-fatty acids; RS-NO, S-nitrosothiol; GS-NO, S-nitrosoglutathine.
Figure 2
Figure 2
Diversity of NO generation mechanisms. NO displays multiple physiological functions in humans, such as vasodilation effects. It has also been suggested that NO inhibits the replication of viruses, including SARS-CoV, thereby preventing viral infection. There are two distinct mechanisms for NO synthesis, namely, NOS-dependent and NOS-independent NO generating mechanisms. Regardless of the pathways, inorganic nitrate (NO3−) and/or nitrite (NO2−) are produced as the oxidation product of NO. In humans, NO3− is supplied by daily diets, including green vegetables or seaweeds, which may help to support NO bioavailability. NO, nitric oxide; iNO, inhaled nitric oxide; NOS, nitric oxide synthase; oxy-Hb, oxy-hemoglobin; deoxy-Hb, deoxy-hemoglobin; XOR, xanthine oxidoreductase; NO2−, inorganic nitrite; NO3−, inorganic nitrate; AsA, l-ascorbic acid (vitamin C).
Figure 3
Figure 3
Nitrate and vitamin C in vegetables and fruits. On Earth, plants and soil bacteria sustain the global nitrogen (N) cycle, which is essential for recycling the N contained in biomolecules, including proteins and nucleic acids. NO is involved in this global cycle through the denitrification activities of soil bacteria. Both denitrifying and nitrifying bacteria produce inorganic nitrite (NO2−) as an intermediate metabolite [124,125,126]. NO3− is absorbed by the roots, followed by being delivered to the green leaves where photosynthetic CO2 assimilation takes place. NO3− is reduced to NO2− in the cytosol and further reduced to glutamate using light energy. In addition to nitrogen assimilation, chloroplasts are involved in the synthesis of cysteine using light energy. During the three major assimilation metabolisms, ROS, RNS, and RSS are produced as the byproducts [127], which is a strong reason why green leaves contain potent antioxidant systems [128]. In plant cells, NO is produced by reductive mechanisms [91,129,130]. Ascorbate accumulates in fruits and leaves. In leaves, ascorbate is present at high concentrations in the chloroplast stroma, such as 25 mM [131]. Ascorbate in the chloroplast is essential to detoxify ROS produced during the photosynthetic electron transport. Ascorbate is used to remove H2O2 by ascorbate peroxidase (APX), and the oxidized ascorbate monodehydroascorbate (MDA) and dehydroascorbate (DHA) are quickly reduced to regenerate ascorbate using light energy. The inset shows a schematic diagram for the water‒water cycle [132]. Ascorbate also accumulates in fruits at high concentrations. In tomato fruits, the ascorbate content in red fruits is higher than that in green fruits [133]. The localization of ascorbate in a tomato fruit is shown in yellow. AsA, l-ascorbic acid (vitamin C); MDA, monodehydroascorbate; DHA, dehydroascorbate; tAPX, thylakoidal ascorbate peroxidase; sAPX, stromal ascorbate peroxidase; MDAR, monohydroascorbate reductase; DHAR, dehydroascorbate reductase; GR, glutathione reductase; SOD, superoxide dismutase; Fd, ferredoxin, PSI, photosystem I complex; PSII, photosystem II complex; NR, nitrate reductase.
Figure 4
Figure 4
Possible functions of orally-taken ascorbate and nitrate in the prevention and mitigation of COVID-19. Plants (vegetables and fruits) contain abundant nitrate (NO3−) and vitamin C (l-ascorbate) in their tissues. When chewing those diets with saliva, NO can be generated by the reductive mechanism. Organic acids contained in fruits and vegetables facilitate the chemical production of NO in the oral cavity. The oral cavity harbors a huge diversity of symbiotic bacteria that can reduce the nitrate secreted from saliva glands to nitrite (NO2−). If powdery vitamin C is taken orally, a high concentration of NO could be generated in the oral cavity by the reductive mechanism. When saliva, including nitrite, is mixed with gastric juice, similarly, chemical NO generation occurs. The strong acidity of the gastric juice facilitates chemical NO generation. NO3− is absorbed in the small intestine and is concentrated in the saliva glands. The vitamin C in plasma is incorporated into cells by sodium-dependent Vit C transporters (SVCTs) [20]. In parallel, dehydroascorbic acid (DHA), an oxidized form of vitamin C, is taken up through glucose transporters (GLUTs) [20]. Since glucose competes with DHA on transporters [163], vitamin C availability in cells may be limited in high sugar diets and high blood sugar conditions, a potential reason for the pathological severity of COVID-19 in diabetes patients [164]. Ascorbate and DHA show direct and indirect pharmaceutical effects that are sometimes opposed. AsA, l-ascorbic acid or vitamin C; DHAR, dehydroascorbic acid reductase; GSH, reduced form of glutathione; GSSG, oxidized form of glutathione; Cys, l-cysteine.
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