Can carbon monoxide unlock new pathways in inflammation therapy?

Carbon monoxide (CO) is a colourless, odourless and tasteless gas which is a major product of the incomplete combustion of carbon-containing compounds. The toxic identity CO stems from its strong affinity for the haemoglobin in our blood which is around 300 times as strong as the affinity of oxygen. As a result, once the gas is inhaled, CO binds to the haemoglobin instead and reduces the amount of oxygen our blood can transport, which can cause hypoxia (low levels of oxygen in tissue) and dizziness, eventually leading to death. However, an intriguing fact is that CO is also endogenously produced in our body, due to the degradation of haem in the blood. Moreover, recent prospects for CO indicate that it may even be developed as an anti-inflammatory drug.

How CO is produced in the body

See Figure 1

Haem is a prosthetic (non-peptide) group in haemoglobin, where the oxygen binds to the iron in the molecule. When red blood cells reach the end of their lifespan of around 120 days, they are broken down in a reaction called haemolysis. This occurs in the bone marrow by macrophages that engulf the cells, which contain the necessary haem-oxygenase enzyme.

Haem-oxygenase converts haem into CO, along with Fe2+ and biliverdin, the latter being converted to bilirubin for excretion. The breakdown of haem is crucial because the molecule is pro-oxidant. Therefore, free haem in the blood can lead to oxidative stress in cells, potentially resulting in cancers. 

Haem degradation also contributes to the recycling of iron for the synthesis of new haem molecules or proteins like myoglobin. This is crucial for maintaining iron homeostasis in the body.

The flow map illustrates haemolysis and the products produced, which either protect cells from further stress or result in cell injury. CO can go on to induce anti-inflammatory effects- see Figure 2.

Protein kinases and CO

Understanding protein kinases is crucial before exploring carbon monoxide (CO) reactions. Protein kinases phosphorylate (add a phosphate group to) proteins using ATP. Protein kinases are necessary to signal the release of a hormone or regulating cell growth. Each kinase has two regulatory (R) subunits and two catalytic (C) subunits.

ATP as a reactant is usually sufficient for protein kinases. However, some kinases require additional mitogens – specific activating molecules like cytokines (proteins regulating immune cell growth), that are involved in regulating cell division and growth. Without the activating molecules, the R subunits bind tightly to the C subunits, preventing phosphorylation.

Research on obese mice showed that CO binding to a Mitogen-Activated Protein Kinase (MAPK) called p38 inhibits inflammatory responses. This kinase pathway enhances insulin sensitivity, reducing obesity effects. The studies used gene therapy, modifying haem-oxygenase levels in mice. Mice with reduced haem-oxygenase levels had more adipocytes (fat-storing cells) and increased insulin resistance, suggesting CO treatment potential for chronic obstructive pulmonary disease (COPD), which causes persistent lung inflammation and results in 3 million deaths annually.

Carbon-monoxide-releasing molecules

As a result of these advancements, specific CO-releasing molecules (CORMs) have been developed to release carbon monoxide at specific doses. Researchers are particularly interested in the ability of CORMs to regulate oxidative stress and improve outcomes in conditions during organ transplantation, and cardiovascular diseases. Advances in the design of CORMs have focused on improving their stability, and targeted release to specific tissues or cellular environments. For instance, CORMs based on transition metals like ruthenium, manganese, and iron have been developed to enhance their efficacy and minimize side effects. This is achieved through carbon monoxide forming a stable ‘ligand’ structure with metals to travel in the bloodstream. Under an exposure to light or a chemical, or even by natural breakdown, these structures can slowly distribute CO molecules.

Although the current research did not find any notable side effects within mouse cells, this does not reflect the mechanisms in human organ systems, therefore there is still a major risk of incompatibility due to water insolubility and toxicity issues. These problems could lead to potentially lead to disruption in the cell cycle, which may promote neurodegenerative diseases.

Conclusion: the future of carbon monoxide

Carbon monoxide has transitioned from being a notorious toxin to a valuable therapeutic agent. Advances in CO-releasing molecules have enabled its safe and controlled use, elevating its anti-inflammatory and protective properties to treat various inflammatory conditions effectively. This shift underpins the potential of CO to revolutionise inflammation therapy. It is important to remember that while carbon monoxide-releasing molecules (CORMs) have potential in controlled therapeutic settings, carbon monoxide gas itself remains highly toxic and should be handled with extreme caution to avoid serious health risks.

Written by Baraytuk Aydin

REFERENCES

Different Faces of the Heme-Heme Oxygenase System in Inflammation - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/The-colorimetric-actions-of-the-heme-HO-system-heme-oxygenase-mediated-heme-degradation_fig3_6531826 (accessed 11 Jul, 2024).

Nath, K.A. (2006) Heme oxygenase-1: A provenance for cytoprotective pathways in the kidney and other tissues, Kidney International. Available at: https://www.sciencedirect.com/science/article/pii/S0085253815519595 (Accessed: 12 July 2024).

Gáll, T. et al. (2020) ‘Therapeutic potential of carbon monoxide (CO) and hydrogen sulfide (H2S) in hemolytic and hemorrhagic vascular disorders—interaction between the heme oxygenase and H2S-producing systems’, International Journal of Molecular Sciences, 22(1), p. 47. doi:10.3390/ijms22010047.

Venkat, A. (2024) Protein kinase, Wikipedia. Available at: https://en.wikipedia.org/wiki/Protein_kinase (Accessed: 12 July 2024).  

Goebel, U. and Wollborn, J. (2020) Carbon monoxide in intensive care medicine-time to start the therapeutic application?! - intensive care medicine experimental, SpringerOpen. Available at: https://icm-experimental.springeropen.com/articles/10.1186/s40635-020-0292-8 (Accessed: 07 July 2024).

Bansal, S. et al. (2024) ‘Carbon monoxide as a potential therapeutic agent: A molecular analysis of its safety profiles’, Journal of Medicinal Chemistry, 67(12), pp. 9789–9815. doi:10.1021/acs.jmedchem.4c00823.

DeSimone, C.A., Naqvi, S.L. and Tasker, S.Z. (2022) ‘Thiocormates: Tunable and cost‐effective carbon monoxide‐releasing molecules’, Chemistry – A European Journal, 28(41). doi:10.1002/chem.202201326.