Annals of Pharmacology and Pharmaceutics

Research Article
Published: 18 Jun, 2021

Inhibition of the Binding of Variants of SARS-CoV-2
Coronavirus Spike Protein to a Human Receptor by
Chlorine Dioxide
Norio Ogata* and Takanori Miura
Department of R&D, Taiko Pharmaceutical Co., Ltd, Japan

Abstract
Aim: COVID-19 caused by a new coronavirus, SARS-CoV-2, has become an ongoing worldwide
pandemic. A safe and potent virucidal disinfection system is urgently needed to protect the populationfrom the virus. Chlorine Dioxide (ClO2) is a powerful disinfectant that is known to inactivate bothviruses and bacteria. The aim of this study was to investigate whether chlorine dioxide inhibits thebinding of the receptor-binding domain of the Spike protein (S protein) from variant coronavirus(British and South African variants) to human receptor, Angiotensin-Converting Enzyme 2 (ACE2).
Materials and Methods: In vitro experiments to determine binding of the purified receptor-binding
domain of spike protein to ACE2 were performed in the presence of various concentrations ofchlorine dioxide. Purified spike proteins from the British and South African variants were used.
Spike protein coated onto a microtiter plate was treated with chlorine dioxide aqueous solution orchlorine dioxide spray solution.
Result: Binding of variant spike proteins was inhibited in a concentration-dependent manner (50%
Inhibitory Concentration (IC50) of 7.6 µmol/L and 5.8 µmol/L for the British and the South African
variants, respectively).
Conclusion: These findings show that chlorine dioxide aqueous solution can inactivate the bindingof the variant spike proteins to the human ACE2 receptor protein, indicating that this strategy may
be useful in blocking the transmission of variant SARS-CoV-2 viruses.

OPEN ACCESS*Correspondence:
Norio Ogata, Department of R&D,
Taiko Pharmaceutical Co., Ltd., 1-2-1
Hikaridai, Seikacho, 619-0237 Kyoto,
Japan, Tel: 81 774 98 2716; Fax: 81
774 98 2737;
E-mail: nogata7@yahoo.co.jp
Received Date: 21 May 2021
Accepted Date: 16 Jun 2021
Published Date: 18 Jun 2021

Keywords: Chlorine dioxide; COVID-19; SARS-CoV-2; Virus; Disinfection; IC50

Introduction

Citation:
Ogata N, Miura T. Inhibition of theBinding of Variants of SARS-CoV-2
Coronavirus Spike Protein to a Human
Receptor by Chlorine Dioxide. Ann
Pharmacol Pharm. 2021; 6(1): 1199.

Coronavirus Disease 2019 (COVID-19) is caused by Severe Acute Respiratory SyndromeCoronavirus 2 (SARS-CoV-2) and has become a serious health issue since its emergence in late 2019[1,2]. The outbreak of this disease originated in Wuhan, Hubei Province, China, before spreading
rapidly throughout the world [2]. The COVID-19 pandemic was declared an international
emergency in 2020 [3]. As of May 2021, global death toll due to COVID-19 is estimated to beapproximately 3.3 million [4]. SARS-CoV-2 is an enveloped virus and has a single-stranded
negative-sense RNA genome of 29.9 kilobases [5]. The virus attaches to the host cell surface of type
I and II pneumocytes, endothelial cells and ciliated bronchial epithelial cells [6]. Although several
vaccines against the disease have already been developed and administered [7], the pandemic has
not yet ended. Furthermore, currently variant types of SARS-CoV-2 virus have been reported
from England (B.1.1.7 variant) and South Africa (variant B.1.351) that appear to be spread more
readily than the original virus [8]. Both of these variants carry a common mutation (N501Y) in the
Receptor-Binding Domain (RBD) of the spike protein (S protein) [5]. This mutation is importantin binding of the spike protein to human cell surface receptor, Angiotensin-Converting Enzyme 2(ACE2), a type I transmembrane glycoprotein with carboxypeptidase activity [5].

Copyright © 2021 Norio Ogata. This is
an open access article distributed under
the Creative Commons Attribution
License, which permits unrestricted
use, distribution, and reproduction inany medium, provided the original work
is properly cited.

To control the spread of COVID-19, it is essential to develop and deploy safe but effective
disinfectants against the virus alongside novel antiviral drugs and vaccines [8]. While ethanol isknown to be effective against the virus [9], it cannot eradicate the virus present in the air. Chlorine
Dioxide (ClO2) (CD) is a water-soluble yellow gas at room temperature and is a stable free radical[10]. CD can be used as a gas or an aqueous solution to inactivate both viruses and bacteria [11-15].
The powerful disinfection action of CD against microbes is due to its strong oxidizing activity against
proteins [16]. Moreover, safe and permissible concentrations of CD have been well documented

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Norio Ogata, et al.,Annals of Pharmacology and Pharmaceutics[17]. Here, we demonstrate that CD aqueous solutions can inactivate
the binding of the spike proteins of both the British and South African
variant to human ACE2 in in vitro experiments, suggesting it may beeffective at inhibiting infection of the SARS-CoV-2 virus.

Materials and Methods
ChemicalsCD, prepared in our laboratory as described previously [16], wasdissolved in purified water and stored at 4ºC in a tightly capped amberbottle. The stock CD was diluted in purified water to the desiredconcentration immediately prior to use. A commercially available
CD spray solution sold under the brand name of Cleverin (Taiko
Pharmaceutical, Osaka, Japan) was also tested. The composition ofCleverin was 1.48 mmol/L CD, 66.34 mmol/L sodium chlorite, 8.70
mmol/L sodium dihydrogen phosphates, 0.24 % (weight/weight)
decaglycerol monolaurate, 0.06 % silicone and 98.97 % water, pH
6.01. A tightly capped bottle of the spray solution made 3 years ago,
kept at room temperature and protected from the light was used. The
CD concentration of the spray solution at the time of the experimentwas 1.80 mmol/L (121 ppm (weight/weight)). The spray solution
was diluted as required with purified water immediately prior to theexperiment.

Figure 1: Effects of chlorine dioxide (CD) aqueous solution on the bindingof the Receptor-Binding Domain (RBD) of variants of spike protein of SARSCoV-2 virus to human Angiotensin-Converting Enzyme 2 (ACE2) in in vitro
experiments. Binding of the spike protein of the British variant (filled circles)
and the South African variant (open circles) to ACE2 are shown. Each point
represents an average ± standard deviation of four experiments (n=4).

Binding assay
The spike protein binding assay kit was purchased from BPS
Bioscience (San Diego, California, USA). The kit codes for the British
and South African variant assay were 78140 and 78151, respectively.
The kits consisted of purified Receptor-Binding Domain (RBD) of
spike proteins derived from the corresponding variant virus. The kit
was used as recommended by the manufacturer with somemodifications. The spike protein RBD provided in the kit was firstcoated onto a 96-well microtiter plate from the kit as indicated by the
manual. A 50 µL aliquot of CD or CD spray solution diluted to an
appropriate concentration was placed in each well and incubated at
25ºC for 5 min. Next, a 20 µL aliquot of 10 mmol/L sodium thiosulfate
in Blocking Buffer 2 provided in the kit was added to each well toterminate the reaction of CD with the protein. Under this condition,
the CD is rapidly converted to ClO2- and becomes unreactive withproteins [16]. Next, a 35 µL aliquot of biotin-labeled ACE2 diluted to
1.5 µg/mL by Blocking Buffer 2 was added to each well. Biotin-labeled
ACE2 was then bound to streptavidin-labeled horseradish peroxidaseand detected using a substrate of the horseradish peroxidase suppliedin the kit. The Chemiluminescence signal was measured after 1 minusing a luminometer (model SH-9000; Corona Electric, Hitachinaka,Ibaraki, Japan). Each concentration of CD was assayed in four wells(n=4). Because the chemiluminescence intensity fluctuated between
experiments, the final data was normalized (i.e. 0 µg/ml CD as 100%).

Figure 2: Effects of the Chlorine Dioxide (CD) spray solution on the bindingof the Receptor-Binding Domain (RBD) of variants of spike protein of SARSCoV-2 virus to human Angiotensin-Converting Enzyme 2 (ACE2) in in vitro
experiments. Binding of the spike protein of the British variant (filled circles)
and the South African variant (open circles) to ACE2 are shown. Each point
represents an average ± standard deviation of four experiments (n=4).

Table 1: The concentration of Chlorine Dioxide (CD) aqueous solution required
to inhibit binding of the receptor-binding domain of variant SARS-CoV-2 spike
proteins (S proteins) to human receptor protein ACE2 by 50% (IC50) is shown.
IC50 (µmol/L)
CD aqueous solution against
7.6

South African variant

5.8

CD spray solution against
British variant

15.3

South African variant

4.7

Variants of spike protein Receptor-Binding Domain (RBD) were treated witheither Chlorine Dioxide (CD) aqueous solution or CD spray solution at various
concentrations, and then assayed for its binding ability to the receptor protein
ACE2

Results and Discussion
As shown in Figure 1, the intensity of the binding of human
ACE2 protein to the spike protein RBD decreased with increasing
concentrations of CD. The concentration of aqueous CD solution
required to give 50% binding inhibition (IC50) was 7.6 µmol/L and5.8 µmol/L for the British and South African variant, respectively(Table 1). An inhibitory effect of CD was also observed using the CD
spray solution (Figure 2). Specifically, IC50 for the binding of ACE2
to the spike protein RBD was 15.3 µmol/L and 4.7 µmol/L for the
British and South African variant, respectively (Table 1). Moreover,binding of the Wuhan strain spike protein RBD to ACE2 was also
inhibited by the CD aqueous solution with an IC50 of 6.5 µmol/L
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British variant(Ogata N, unpublished data). There were slight differences in theinhibitory effect between aqueous CD and the CD spray solution
in these experiments. The observed differences may be due to theeffect of other constituents in the spray solution, which are added toprolong the shelf life of the product. These results suggest all the CD
solutions are effective in inactivating the binding of the SARS-CoV-2
virus to human cell receptor ACE2. As such, CD is likely to have an
inhibitory action on the infection of the virus.
The spike protein RBD of SARS-CoV-2 virus is located in the S1
2

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Norio Ogata, et al.,Annals of Pharmacology and Pharmaceutics

subunit of the spike protein, and the region of RBD that interacts with
ACE2 consists of a small patch of 25 amino acid residues [18].

5. Naqvi AAT, Fatima K, Mohammad T, Fatima U, Singh IK, Singh A, et al.
Insights into SARS-CoV-2 genome, structure, evolution, pathogenesis andtherapies: Structural genomics approach. Biochim Biophys Acta Mol Basis
Dis. 2020;1866(10):165878.

The asparagine 501 (N501) residues, which is mutated to tyrosine
in both viral variants, forms part of the RBD patch region of the spike
protein [5]. The N501Y mutation is thought to result in enhanced
viral transmissibility by increasing the binding affinity to human
ACE2 [18]. Furthermore, it is worth noting that this mutation allows
the virus to escape from many antibodies raised against the virus [18],suggesting poor effectiveness of vaccines against these variant strainsof SARS-CoV-2 virus. We previously demonstrated that the bindingof the spike protein RBD of the Wuhan strain to human ACE2 isinhibited by CD aqueous solution [19]. These findings suggested CD
may be useful as a disinfectant against virus. Although the mechanismof this inhibition has not been elucidated, we speculated that it may
involve oxidization of the tyrosine 453 residue of the RBD of the
spike protein, which forms a hydrogen bond with ACE2 [19]. Indeed,
CD is known to oxidize tyrosine residues in proteins [16]. Because
asparagine 501 is mutated to tyrosine (Y) in the variant strains,treatment of the virus with CD may also oxidize 501Y. Given thatvaccines appear to have diminished effectiveness against the variant
viruses, disinfection systems employing CD may be invaluable. It
is also important to note that CD is a gas at room temperature andcan be used in this form to inactivate microbes floating in the air atconcentrations deemed safe to human health [17]. As such, gaseous
CD could be used to inactivate the SARS-CoV-2 virus floating in the
air of a crowded room.

6. Soni S, Jiang Y, Tesfaigzi Y, Hornick JL, Çataltepe S. Comparative analysis
of ACE2 protein expression in rodent, non-human primate, and humanrespiratory tract at baseline and after injury: A conundrum for COVID-19
pathogenesis. PLoS One. 2021;16(2):e0247510.
7. Wibawa T. COVID-19 vaccine research and development: ethical issues.
Trop Med Int Health. 2021;26(1):14-9.
8. Wang P, Nair MS, Liu L, Iketani S, Luo Y, Guo Y, et al. Increased resistanceof SARS-CoV-2 variants B.1.351 and B.1.1.7 to antibody neutralization.
BioRxiv. 2021;2021.01.25.428137.
9. Hirose R, Ikegaya H, Naito Y, Watanabe N, Yoshida T, Bandou R, etal. Survival of SARS-CoV-2 and influenza virus on the human skin:
Importance of hand hygiene in COVID-19. Clin Infect Dis. 2020;ciaa1517.
10. Shimakura H, Ogata N, Kawakita Y, Ohara K, Takeda S. Determinationof the structure of liquids containing free radical molecules: Intermolecular correlations in liquid chlorine dioxide. Molecular Physics.
2013;111(8):1015-22.
11. Ogata N, Shibata T. Protective effect of low-concentration chlorine dioxide
gas against influenza A virus infection. J Gen Virol. 2008;89(Pt 1):60-7.
12. Ogata N, Sakasegawa M, Miura T, Shibata T, Takigawa Y, Taura K, etal. Inactivation of airborne bacteria and viruses using extremely low
concentrations of chlorine dioxide gas. Pharmacology. 2016;97(5-6):301-6.
13. Ogata N. Inactivation of influenza virus haemagglutinin by chlorine
dioxide: Oxidation of the conserved tryptophan 153 residue in thereceptor-binding site. J Gen Virol. 2012;93(Pt 12):2558-63.

Conclusion
CD solution, whether in water or as a formulated spray solution
in water, was found to potently inhibit the binding of spike protein
RBD to human receptor protein ACE2 in a concentration-dependent
manner. These findings suggest CD may be invaluable for inhibiting
the infection of SARS-CoV-2 virus to human.

14. Morino H, Fukuda T, Miura T, Shibata T. Effect of low-concentration
chlorine dioxide gas against bacteria and viruses on a glass surface in wet
environments. Lett Appl Microbiol. 2011;53(6):628-34.
15. Morino H, Fukuda T, Miura T, Lee C, Shibata T, Sanekata T. Inactivationof feline calicivirus, a norovirus surrogate, by chlorine dioxide gas.
Biocontrol Sci. 2009;14(4):147-53.

Acknowledgement

16. Ogata N. Denaturation of protein by chlorine dioxide: Oxidative
modification of tryptophan andtyrosine residues. Biochemistry.
2007;46(16):4898-911.

The authors are employees of Taiko Pharmaceutical Co., Ltd. This
work was supported by the company.

References

17. Akamatsu A, Lee C, Morino H, Miura T, Ogata N, Shibata T. Six-month
low level chlorine dioxide gas inhalation toxicity study with two-week
recovery period in rats. J Occup Med Toxicol. 2012;7:2.

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