SARS-CoV-2 disease (COVID-19) has affected over 22 million patients worldwide as of August 2020. As the medical community seeks better understanding of the underlying pathophysiology of COVID-19, several theories have been proposed. One widely shared theory suggests that SARS-CoV-2 proteins directly interact with human hemoglobin (Hb) and facilitate removal of iron from the heme prosthetic group, leading to the loss of functional hemoglobin and accumulation of iron. Herein, we refute this theory. We compared clinical data from 21 critically ill COVID-19 patients to 21 non-COVID-19 ARDS patient controls, generating hemoglobin-oxygen dissociation curves from venous blood gases. This curve generated from the COVID-19 cohort matched the idealized oxygen-hemoglobin dissociation curve well (Pearson correlation, R2 = 0.97, P

Carbon monoxide (CO) remains the most common cause of human poisoning. The consequences of CO poisoning include cardiac dysfunction, brain injury, and death. CO causes toxicity by binding to hemoglobin and by inhibiting mitochondrial cytochrome c oxidase (CcO), thereby decreasing oxygen delivery and inhibiting oxidative phosphorylation. We have recently developed a CO antidote based on human neuroglobin (Ngb-H64Q-CCC). This molecule enhances clearance of CO from red blood cells in vitro and in vivo. Herein, we tested whether Ngb-H64Q-CCC can also scavenge CO from CcO and attenuate CO-induced inhibition of mitochondrial respiration. Heart tissue from mice exposed to 3% CO exhibited a 42 ± 19% reduction in tissue respiration rate and a 33 ± 38% reduction in CcO activity compared with unexposed mice. Intravenous infusion of Ngb-H64Q-CCC restored respiration rates to that of control mice correlating with higher electron transport chain CcO activity in Ngb-H64Q-CCC–treated compared with PBS-treated, CO-poisoned mice. Further, using a Clark-type oxygen electrode, we measured isolated rat liver mitochondrial respiration in the presence and absence of saturating solutions of CO (160 μm) and nitric oxide (100 μm). Both CO and NO inhibited respiration, and treatment with Ngb-H64Q-CCC (100 and 50 μm, respectively) significantly reversed this inhibition. These results suggest that Ngb-H64Q-CCC mitigates CO toxicity by scavenging CO from carboxyhemoglobin, improving systemic oxygen delivery and reversing the inhibitory effects of CO on mitochondria. We conclude that Ngb-H64Q-CCC or other CO scavengers demonstrate potential as antidotes that reverse the clinical and molecular effects of CO poisoning.

Neuroglobin (Ngb) is a highly conserved hemoprotein of uncertain physiological function that evolved from a common ancestor to hemoglobin and myoglobin. Ngb shows a six‐coordinate heme iron with proximal and distal histidines in the heme pocket directly bound to the heme iron however coordination of the sixth ligand is reversible. Deoxygenated human neuroglobin reacts with nitrite to form nitric oxide (NO). This reaction is regulated by redox sensitive surface thiols, cysteine 55 and 46, which regulate the open probability of heme pocket, nitrite binding and NO formation. Replacement of the distal histidine by leucine (H64L) leads to a stable five‐coordinated geometry; the Ngb H64L mutant reduces nitrite to NO approximately 2000‐times faster than the wild type, while mutation of either C55 or C46 to alanine stabilizes the six‐coordinate structure and slows the reaction. Using lentivirus expression systems we show that the nitrite‐reductase activity of Ngb inhibits cellular respiration via NO binding to cytochrome c oxidase and confirm that the six‐to‐five coordinate status of Ngb regulates intracellular hypoxic NO signaling pathways. These studies suggest that Ngb may function as a physiological oxidative stress sensor and a post‐translationally redox regulated nitrite reductase, that generates NO under six‐to‐five coordinate heme pocket control.

Aims: Nitrite is reduced to nitric oxide (NO) under physiological and pathological hypoxic conditions to modulate angiogenesis and improve ischemia-reperfusion injury. Although adult mammals lack the ability to regenerate the heart after injury, this is preserved in neonates and efforts to reactivate this process are of great interest. Unlike mammals, the adult zebrafish maintain the innate ability to regenerate their hearts after injury, providing an important model to study cardiac regeneration. We thus explored the effects of physiological levels of nitrite on cardiac and fin regeneration and downstream cellular and molecular signaling pathways in response to amputation and cryoinjury. Results: Nitrite treatment of zebrafish after ventricular amputation or cryoinjury to the heart in hypoxic water (*3 parts per million of oxygen) increases cardiomyocyte proliferation, improves angiogenesis, and enhances early recruitment of thrombocytes, macrophages, and neutrophils to the injury. When tested in a fin regeneration model, neutrophil recruitment to the injury site was found to be dependent on NO. Innovation: This is the first study to evaluate effects of physiological levels of nitrite on cardiac regeneration in response to cardiac injury, with the observation that nitrite in water accelerates zebrafish heart regeneration. Conclusion: Physiological and therapeutic levels of nitrite increase thrombocyte, neutrophil, and macrophage recruitment to the heart after amputation and cryoinjury in zebrafish, resulting in accelerated cardiomyocyte proliferation and angiogenesis. Translation of this finding to mammalian models of injury during early development may provide an opportunity to improve outcomes during intrauterine fetal or neonatal cardiac surgery. Antioxid. Redox Signal. 32, 363-377.

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