Table 1.

Biological and therapeutic aspects of the three gasotransmitters: nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H₂S).

	NO	CO	H2S
Chemical nature	Reactive free radical, gas	Gas, not a free radical	Reactive gas, not a free radical
Main enzymatic biosynthetic route(s)	From L-arginine via nitric oxide synthases.	From heme via heme oxygenases.	From L-cysteine, via CSE, CBS and 3-MPST.
Metabolism, elimination	The main metabolites are nitrite and nitrate. Major elimination route for nitrite and nitrate: urine; minor elimination route for NO: exhaled air.	Transiently binds to hemoglobin to form CO-Hb. A small fraction of CO oxidizes to form CO2. Main elimination route: via exhaled air.	The main metabolites are thiosulfate, sulfite and sulfate. Main elimination route for sulfite, sulfate and thiosulfate: urine; minor elimination route for H2S: exhaled air.
Intracellular effector pathways	cGMP-dependent protein kinases; modulation of enzymatic activity by posttranslational protein modifications via nitrosylation; variable effects on enzymes via reactions with heme groups of various enzymes; inhibition of cytochrome c oxidase.	cGMP-mediated actions with lower potency; KCa channels; intracellular kinase pathways, inhibition of cytochrome c oxidase.	Activation (opening) of KATP channels; inhibition of cytochrome c oxidase, inhibition of phosphodiesterase, modulation of intracellular calcium, modulation of signal transduction processes.
Therapeutic effects as inhaled gas	Inhaled NO is an approved for the therapy of primary pulmonary hypertension of the newborn. Inhaled NO was shown to provide benefit in animal models of ARDS, pulmonary hypertension, sickle cell disease, circulatory shock, transplant rejection, vascular injury, heart failure and other diseases.	Inhaled CO has been tested in COPD patients and entered into clinical studies for the therapy of graft dysfunction after renal transplantation. Inhaled CO was shown to provide benefit in animal models of ARDS, circulatory shock, transplant rejection, vascular injury and other diseases.	In preclinical studies, inhaled H2S was shown to provide benefit in acute hypoxia, acute hemorrhagic shock, ventilator-induced lung injury and other diseases.
Therapeutic effects as liquid formulation	Not explored.	A CO-containing lavage fluid was found to produce benefit in preclinical models of postoperative ileus. CO- containing preservation fluids improve the function of transplants in preclinical studies.	Sodium sulfide for injection has entered Phase II clinical trials for cardioprotective effects. In preclinical studies, parenteral sodium sulfide or sodium hydrogen sulfide was shown to exert benefit in myocardial infarction, cardiopulmonary bypass, liver and kidney reperfusion injury, transplant rejection, ARDS and other diseases.
Prodrugs requiring conversion	Organic nitrates (nitrovasodilator compounds of various classes) are in routine clinical use in the therapy of cardiac ischemia, cardiac failure, hypertension and other diseases.	Methylene chloride (which is converted to CO in cells) is a preclinical research tool that provides benefit in animal models of organ failure and transplantation.	Garlic-derived endogenous polysulfides release H2S and exert vasodilatory and cardioprotective effects in preclinical models.
Spontaneously releasing chemical donors	NONOates, nitrosothiols, sydnonimines, and other classes of NO donors are in clinical use or in human trials. Preclinical studies demonstrate their efficacy in cardiac failure, hypertension, pulmonary hypertension, reperfusion injury, stroke, atherosclerosis and wound healing.	CORMs (Carbon Monoxide Releasing Compounds) demonstrate preclinical efficacy in animal models of reperfusion injury, ARDS, organ transplantation and other diseases. They also exert antimicrobial effects.	The water-soluble hydrogen sulfide-releasing molecule GYY4137 has been tested in animal models of hypertension and vascular disease.
Combined donors	NO-nonsteroidals, NO- steroids, NO-statins, NO- captopril and other combined NO donors exert benefits in various models of inflammation, cardiac and vascular disease; some of them have also been tested in clinical trials including naproxcinod (Phase III trials completed) for arthritis.	Not explored.	Combined compounds with a sulfide-releasing moiety (e.g. various sulfo- nonsteroidals) demonstrate beneficial effects in preclinical models of inflammation and cardiovascular disease.
Pharmacological stimulation of the second messenger pathways activated by the gaseotransmitter	PDE5 inhibitors are in clinical use for male erectile dysfunction, and have demonstrated therapeutic benefit in other cardiovascular diseases in preclinical and clinical studies. Riociguat and cinaciguat (direct activators and stimulators of the soluble guanylyl cyclase enzyme) have shown proof-of-concept efficacy in preclinical and clinical models of pulmonary hypertension and heart failure.		Not explored.
Supplementing substrate(s) or co-factors of the enzyme that produces the mediator	L-arginine exerts beneficial effects in a variety of cardiovascular diseases in preclinical models and in some of the exploratory clinical studies. L-citrulline has been shown to be effective in preclinical and clinical models of pulmonary hypertension. Supplementation of tetrahydrobiopterin improves endothelium-dependent relaxations in preclinical models of vascular disease.	Not explored.	Cysteine, the precursor of H2S in some cellular assays leads to H2S formation. Parenteral cysteine injection leads to an increase in the amount of exhaled H2S after parenteral administration in a rodent study.