Figure 1

Molecular mechanisms through which melatonin can protect against COVID-19

• Melatonin as an anti-oxidant:

The fatal consequences due to infection of alveolar tissue with SARS-CoV 2 are associated with oxidative stress characterized by elevated levels of ROS or RNS and melatonin is the promising endogenous antioxidant. But melatonin is observed to be lost with age. One molecule of melatonin can bind with 10 free radicals maximally. Melatonin scavenges the hydroxyl radical(•OH) i.e. a potent mediator of peroxidation of membrane lipids, carbonylation of proteins, and DNA damage, which results in the damage of alveolar tissue. It is also known to detoxify cell-damaging radicals like superoxide anion radical(O₂•^–) nitric oxide (NO•). Moreover, SARS-CoV induces oxidative stress; which in turn, induces  PLA2G2D phospholipase synthesis; at an abnormally high concentration which can reduce the immunity against a viral antigen, thereby making the biochemical situation more lethal to the human. Also, melatonin inhibits inducible nitric oxide synthase(iNOS) and stimulates several antioxidant enzymes like superoxide dismutase, glutathione, catalase, glutathione peroxidase, and glutathione reductase supporting its indirect antioxidant action. On the other hand, Melatonin is also a good preventor of fibrosis which is the most fatal consequence in COVID-19 patients.

Melatonin is further observed to reduce oxidative stress by recycling the glutathione(GSH) level in the body which gets reduced during the excess oxidative stress condition. During this, oxidation of GSH to its dimerized form occurs, GSSG. Melatonin stimulates the Glutathione reductase(GR) enzyme which quickly converts GSSG to form GSH(through the reduction/hydrogenation of disulphide linkages) again by the action of the rate-limiting enzyme, gamma-glutamylcysteine synthetase. Furthermore, Melatonin can donate one or more electrons to free radicals detoxifies them. In this situation, melatonin becomes a free radical itself, which doesn’t possess any toxicity to cells.  Metabolites that are formed during this process are Cyclic3-hydroxymelatonin (c3OHM), N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK), and N1-acetyl-5-methoxykynuramine (AMK). This oxidation process results in the formation of several hydroxylated products which proves the binding of melatonin with diamagnetic hydroxyl radicals, that is indicating towards the construction of never-ending antioxidant cascade in the cells of the mammalian system.

• The inhibitory action of melatonin against ROS generation:

 ROS generation is a major factor for lung injury during COVID-19 infection as a result of mitochondria, where the molecular oxygen(O₂) is reduced to water via electron transport chain(ETC), which is the main source of ROS generation due to its primary target of ATP production. In mitochondria, there is several sites where superoxide radical(O₂•^–) are produced including complex I, complex III, glycerol 3-phosphate dehydrogenase, , pyruvate dehydrogenase, and 2-oxoglutarate dehydrogenase. Mn-SOD(Mn-Superoxide Dismutase) (mitochondrial)and Cu-Zn-SOD(Cytosolic) converts the superoxide radical into hydrogen peroxide(H₂O₂). The H₂O₂ in the mitochondrial membrane is also converted into hydroxyl radical(OH•) via a Fenton reaction catalyzed by mitochondrial aconitase. The cytosolic catalytic cycle is another site for the production of ROS.

During COVID-19 infection, increased oxidative stress inhibits mitochondrial Kreb’s cycle enzymes, and ETC associated enzymes of the lung that results in free electron leakage and increases the ROS level. As a result, the intensity of alveolar tissue damage will increase due to the persistence of inflammatory signaling.

Several studies had shown that Melatonin and its metabolites not only stimulate the activity of SOD for scavenging the ROS generation but also stimulates the proper activities of ETC associated enzymes and Kreb’s cycle enzymes(at nM to µM concentration in vitro system) and blocks the free-electron leakage and the formation of excess ROS.

• Melatonin also induces the antioxidant enzyme encoding genes through G-protein coupled receptor(GPCR):

Melatonin binds to its transmembrane bound GPCR(comprised of α, β and γ subunits )(MT1 and MT2). Upon binding, the Gα sub-unit dissociates from GPCR transmembrane receptors and activate phospholipase C-β, to phosphorylate and activate the JAK-Erk complex, which in turn increase the Nrf2 expression level through SIRT-1 activation, upon entering into the nucleus which increases the expression of the genes encoding the antioxidant enzymes.

• Melatonin and its anti-inflammatory action:

Inflammation of alveoli of the lung is one of the major hallmarks of COVID-19 patients. In the case of coronavirus, its ssRNA is first detected by TLR7(Toll-like receptor 7) and is then directed for lysosomal degradation. It then activates the Type 1-Interferon reaction and releases many other pro-inflammatory cytokines [IL-1β, IL-2, IL-6, IL-8, both IFN-α/β, Tumor necrosis factor(TNF), C-C motif chemokine 3(CCL3), CCL5, CCL2, etc]. But in some cases, excessive generation of pro-inflammatory cytokines might lead to ‘cytokine storm’ causing host cell damage and thus, inducing acute lung injury(ALI)/acute respiratory distress syndrome(ARDS) or even death. Therefore, to decrease mortality, the inflammation needs to be suppressed. The anti-inflammatory properties of melatonin suppress this following consequences of inflammation:

1. TLR recognizes PAMP(pathogen-associated molecular pattern), thereby initiates the signaling cascade to create inflammatory responses by the induction of the gene encoding TLR2, TLR4, TLR9. Melatonin downregulates the expression of the mentioned TLR encoding genes to suppress inflammatory responses.
2. At the lower concentration, melatonin reduces the expression of pro-inflammatory cytokines TNF-α, IL-1β, IL-6, and IL-8(which are induced during COVID-19 infection) and increases the rate of expression of anti-inflammatory cytokine IL-10.
3. Melatonin also suppresses activation of NF-κβ, which would otherwise mediate inflammation, by downregulating its activation in T-cells and lung cells.
4. Elevated activity of NOD-like receptor 3(NLRP3) inflammasome leads to amplification of inflammatory responses. Melatonin can repress NLRP3 inflammasome activation.

Discussion:

             The endogenous melatonin may possess the ability to protect ourselves, but its concentration is not sufficient to combat against inflammation-mediated ROS generation, so if the people take the melatonin tablet(an available form is “Meloset-3 mg”), then it can protect human being by the mechanisms as mentioned above. On the other hand, some recent research works showed that melatonin inhibits the electron leakage from the ETC is generally accompanied by an increase in the activity of cardiac mitochondrial ATP synthase activity resulting in the higher synthesis of ATP, i.e. enabling higher energy to sustain life without any side effects. So, by enhancing the energy metabolism in heart, melatonin provides not only the support life of COVID-19 patients but also provides the time to the medical practitioners for providing proper treatment and healthy life to the SARS-COV2 infected patients. Therefore, it further suggests that melatonin due to its high redox potential produces a beneficiary effect not only on the mitochondrial complexes but on the overall physiological system and thereby able to protect the human being from the present global life-threatening situation.