Ever thought of waking through the sanitizer-washed floors, past the Dettol-smelling walls of a hospital only to encounter the stench of rotten egg? If you have never thought of this then better be ready for it, because researchers around the world are accumulating evidences for the role and beneficial effects of the nasty, rotten egg smelling Hydrogen Sulfide,H2S.
Hydrogen Sulfide is one of the spiteful smelling gases and it is extremely noxious. This may be the main reason why scientific world never thought of even its presence in human body, although its toxic effects were known for centuries. Even though the physiological role and mechanism of action of other gases like Nitric oxide (NO) and Carbon Monoxide (CO) were known for some time, the discovery of H2S in the body was a surprise to the scientific world. Since then this area has been under hot pursuit.
But, how and why H2S has a role in normal human physiology? To answer this we have to page back to the early days on earth, some 250 million years, when the outlook towards life was very different from now. The Permian era was going to end and the massive eruption of volcanoes tremendously increased the levels of Carbon Dioxide in the atmosphere. This made it hard for oxygen requiring forms of life to survive. But anaerobic organisms like green sulfur bacteria thrived and increased the concentration of hydrogen sulfide. The only option for aerobic organisms was to adapt to the existing conditions and thus in course of time, those who can tolerate these conditions survived and reproduced. So the presence of H2S in human body has a strong evolutionary history.
The quest for the presence of H2S in human began with works of Rui Wang from Lakehead University, Ontario. He first discovered the presence of H2S in the blood vessel walls of rats. He showed in his subsequent studies that H2S is produced in blood vessels and an enzyme cystathionine gamma-lyase(CSE) combined L-cysteine to produce H2S and two other compounds, ammonium and pyruvate. As the studies progressed it seemed as though H2S, like Niric Oxide (NO), was a regulator of blood pressure. Later studies in mice and animal tissues proved this to be correct and shortly after this the mechanism of action of H2S was elucidated. In our body H2S acts as a “Gasotransmitter”, like NO and CO. But the mechanism of action of H2S is totally different from NO and CO. NO is important in dilation of blood vessels and it acts through an enzyme called Guanylyl cyclase. But H2S accomplishes the same effect by altering the membrane permeability of certain substances- or to be more precise, of Calcium by activating KATP (Potassium ATP) channels that control flow of potassium ions in and out of the membranes of a cell. The electric potential created by this flow limits the flow of Calcium ions, a constraint that relaxes the muscles and dilates the vessels. From the studies using CSE knockout mice it was concluded that NO works in larger vessels and H2S works in small vessels.
The ability of H2S to control blood pressure ignited interest among cardiovascular scientists who were looking for novel ways to protect heart against damages caused by oxygen deprivation. The studies were positive and the findings suggested that H2S could be used to prevent or treat hypertension, heart attacks and strokes in humans,
The gas was also found to help regulate metabolism-the processes that regulate the use of energy in the body. It was found that H2S can produce “H2S Hibernation”, a state of suspended animation, where the heart rate is dropped instantaneously. This could be used to transport a patient from the site of car accident or a person having heart attack to hospital. By this way we can “buy time”. This also helps to keep one alive in case if he wants organ transplantation and there is a delay in getting the perfect match. It can also be used to keep the organs viable until it reaches the patient.
Even though this is the case, everything that H2S touches does not turn gold. H2S is found to be an important player in Type 1 Diabetes. When the H2S produced in pancreatic Beta cells are in surplus, the gas kills most of the insulin producing Beta cells, leaving behind only too little of the cells that can produce insulin. It is also not clear whether the “H2S Hibernation” can put life on hold while preserving important brain functions like memory and reason. Some of the positive effects produced in rats and mice were for unidentified reasons have not been reported in larger animals like sheep. So the jury is still out on various aspects of H2S and its effects on physiology.
But the ongoing work on H2S is still young and the chances seems good that it will eventually lead to treatment for some diseases whose cure have thus far eluded us.
For more information refer Scientific American India, March 2010 or www.sciam.co.in and the gasotransmitter role of hydrogen sulfide, Wang R. Antioxid Redox Signal. 2003 Aug;5(4):493-501.