Poisonous gas saves lives

Hydrogen sulfide, a deadly gas best known for its smell reminiscent of fried eggs, plays key roles in the body. This finding may lead to the development of new treatments for heart attack victims, and more

By Rui Wang

Imagine that you enter an intensive care room in a hospital, although its walls are decorated with hand sanitizers and every surface is carefully scrubbed with it to prevent contamination and you are surrounded by the smell of boiled eggs. This scenario may be inconvenient, but the foul-smelling toxic gas hydrogen sulfide (H2S) may well become a regular fixture in hospitals in the future. Over the past ten years, scientists have discovered that H2S is actually essential for several processes occurring in the body, including the regulation of blood pressure and metabolism. Our findings indicate that if we use it properly, the gas could, among other things, benefit heart attack victims and stabilize trauma victims until they reach the operating room or until they receive a blood transfusion.

the scent of poison
Scholars have known the toxic effect of H2S on humans for centuries. Today it is considered the first occupational hazard in oil or gas wells, along the transport pipelines and in the processing and refining plants. The human nose can detect H2S in concentrations of 0.0047 parts per million (ppm). At a concentration of 500 ppm it harms breathing. Exposure to 800 ppm for five minutes causes death. Yet, paradoxically, we need H2S to survive.

To understand why the human body came to rely on this smelly gas, we have to go back 250 million years, to a time when the outlook for life on Earth was quite bleak. The Permian era was coming to an end, and the greatest extinction event of all time was underway. At that time, according to accepted extinction theory, the emission of carbon dioxide due to massive volcanic eruptions in Siberia caused a chain of environmental changes that lowered the concentration of oxygen in the oceans to a dangerous level that ultimately led to widespread mortality [see "From the Deep the Evil Will Rise" by Peter Ward, Scientific American Israel, February-March 2007].

This change in the chemical composition of the oceans is bad news for marine animals that breathe oxygen, i.e. for aerobic production. But anaerobic organisms, which do not need oxygen, called green sulfur bacteria, thrived in the low oxygen concentrations. The success of these bacteria made the oceans even more hostile to most of their aerobic inhabitants because they produced massive amounts of hydrogen sulfide. Eventually, according to the theory, the deadly gas bubbled from the oceans into the air and killed land plants and animals. At the end of the Permian extinction, 95% of marine species and 70% of terrestrial species became extinct.

The importance of H2S in human physiology is probably an inheritance from that ancient period. The creatures that survived the catastrophe were the only ones that could deal with hydrogen sulfide and in some cases even consume it, and humans maintained a certain affinity for this gas.

follow the nose
Hydrogen sulfide is not the only toxic gas that works in the human body. In the 80s, researchers began to uncover evidence that nitric oxide (NO) is produced in the body in low concentrations, where it acts as a signaling molecule that affects cell behavior. Research that led to the Nobel Prize in Physiology or Medicine in 20 showed that nitric oxide dilates blood vessels, regulates the immune system, transmits signals between nerve cells, and more. Carbon monoxide (CO), an odorless and colorless gas often called the "silent killer," has similar effects.

Since I had studied CO and NO, I was convinced that the body must be producing and using additional gases as signal transmitters. Until 1998 I engaged in constant brainstorming as to the identity of these gases. That summer the idea flashed in my mind. After a busy day at work, I came home and my nose smelled bad. In the end I located the source in the glass case where all my family treasures are displayed. The smell came from a cracked and rotten egg, one of the Easter eggs that my eldest daughter dyed at school. Because of this, I began to wonder if this gas of boiled eggs, hydrogen sulfide, is also produced by the organs and tissues in our body.

It turned out that scientists have shown that H 2S is indeed present in the body. Its activity in the brain was particularly prominent, and they hypothesized that it serves as a signaling or protective agent. H 2S is also produced in blood vessels. Because my work on CO and NO focused on their effect on the cardiovascular system, I decided to find out if H2S acts as a signaling molecule in them. This was a good place to start: a series of experiments revealed considerable activity.

The first tests I conducted with my colleague did find small amounts of the gas in the blood vessel walls of rats. Since the physiology of rodents is very similar to that of humans, the discovery meant that human blood vessels must also produce the gas. This was indeed an encouraging finding, but to determine whether H2S is important for the body's activity, it is not enough to show its presence in blood vessel walls.

The next step was to understand how the H2S we found is produced in the body. We decided to examine an enzyme known as cystathionine-gamma-lyase (CSE), which helps produce the gas in bacteria. Previous studies to ours have documented the presence of CSE in the liver, where it coordinates the building process of several sulfur-containing amino acids that serve as the building blocks of proteins. Researchers have found messenger RNA molecules encoding CSE in blood vessels, but no one knew whether active CSE, identical to that found in the liver, is also present in blood vessels. After we cloned the protein from the blood vessels, we discovered for the first time that it uses an amino acid known as L-cysteine ​​for the production of H2S and two other compounds, ammonium and pyruvate. Since NO causes vasorelaxation, we hypothesized that H2S might play a similar role. Experiments we conducted confirmed the assumption: when we soaked blood vessels in H2S solution, they expanded.

It began to get the impression that H2S regulates blood pressure, similar to NO. But the molecular mechanism underlying this phenomenon was still unknown. Clues eventually came from our studies of single cells taken from animal blood vessels. The results, published in 2001, were surprising. While NO relaxes blood vessel walls by activating the enzyme guanylyl cyclase found in smooth muscle cells, H2S manages to do the same in a completely different pathway. In fact, H2S activates proteins called KATP channels that control the flow of potassium ions out of smooth muscle cells. The movement of the ions produces an electric current that limits the amount of calcium ions that are able to enter the cells. This restriction relaxes the muscle and dilates the blood vessels.

In the next step, we progressed from single cells to animals, and injected the rats with H2S solution. We found that their blood pressure dropped, probably because the gas opened up the arteries and made the blood flow easier. Accumulating evidence has shown that H2S relaxes blood vessels and therefore participates in blood pressure control. But we were not sure that adding the gas to the blood vessels really simulates what happens when blood vessels produce H2S themselves.

In order to better evaluate the roles of the gas, in 2003 my colleague and I developed a line of transgenic mice that lack the CSE enzyme and are therefore unable to produce H2S in the blood vessels. Over the next five years, we collaborated with the research groups of Solomon Schneider of Johns Hopkins University and Lingyun Wu of the University of Saskatchewan in Canada to study these mice. Our efforts paid off and in 2008 we published an article in the journal Science detailing our findings. As the transgenic mice aged, their blood vessels narrowed and they developed much higher than normal blood pressure (as measured by tiny inflatable cuffs attached to the mice's tails). But when we injected the mice with H2S, their blood pressure decreased.

The research on the transgenic mice proved beyond doubt that hydrogen sulfide plays a vital role in the cardiovascular system. He also solved an ancient mystery. Years after the Nobel laureate's research on NO, researchers knew that the full extent of vasodilation could not be attributed to this signaling gas. First, in animals that were genetically engineered to not produce NO in the endothelial cells of the blood vessel walls, peripheral blood vessels (those that do not lead directly to the heart from it) could still relax. But what causes relaxation in the absence of NO?

Our research indicates that the mysterious factor is most likely H2S. Although we initially showed that CSE, the enzyme that produces H2S, is found in smooth muscle cells, later studies conducted on endothelial cells taken from mice, cows, and humans showed that these cells also contain CSE, even in greater amounts than the smooth muscle cells. It is not yet clear how the responsibility for vasorelaxation is divided between NO and H2S, although there is evidence that NO does most of the work in the large vessels while H2S is responsible for the small vessels.

A cure for everything?
The discovery that H2S is produced in blood vessels and helps regulate blood pressure caught the eye of many other researchers who were looking for new ways to protect the heart from damage caused by oxygen deprivation, as happens when a blood clot prevents the blood from supplying oxygen to the heart and leads to the death of heart tissue (heart attack). . In 2006, Gary P. Baxter, now at Cardiff University in Wales, and his colleagues reported that when a rat's heart is first exposed to a saline solution that simulates blood supply and then the solution is withheld from the heart to simulate a heart attack, the H2S gas reduces the degree of damage to the heart muscle if it is delivered to the heart before that prevent the supply of the saline solution. And David Leffer of Emory University showed a year later that transgenic mice that produce more H2S in the heart cope better with clot-induced oxygen deprivation and are more resistant to the damage that often occurs when blood flow to tissues is restored (a condition known as reperfusion injury).

Such findings indicate that H2S may be used in the prevention or treatment of hypertension, heart attacks and strokes in humans. Moreover, the ability of the gas to relax blood vessels means that it can also be used for other problems involving blood vessels, including erectile dysfunction. Penile erection is achieved by dilating blood vessels. In fact, Viagra works by prolonging the effect of NO on the penis, where the gas relaxes blood vessels and therefore increases blood flow. Research suggests that H2S may have a similar effect, but more research is needed to determine its exact role in penile tissue. (CO is also produced in the penis, but it encourages ejaculation and not erection).

H2S is not only found in the cardiovascular system. It is also produced in the nervous system, but not by CSE but by an enzyme known as cystathionine beta synthase. It is not clear what exactly his role is there. Some studies claim that it regulates neural activity by making neural circuits more responsive or less responsive to various stimuli. It may participate in a process known as long-term potentiation, which improves communication between cells and may therefore encourage learning and memory. Also, the gas increases the levels of the antioxidant substance glutathione in nerve cells, so it may protect these cells from strain. And it may also help the body feel pain so we can respond accordingly.

More than that, the gas seems to help regulate metabolism, those chemical processes responsible for energy use and synthesis in the body. In an amazing series of experiments, Mark B. Roth of the University of Washington and his colleagues provided low concentrations of H2S to mice to reduce metabolism and consequently delayed the progression of certain diseases. The heart rate of the animals immediately dropped to half, and they entered a state of hibernation (suspended animation) where the metabolism slowed down to a level that allowed them to exist on a "diet" of H2S and oxygen without visible negative effects. During the "H2S coma" the body apparently maintains a basic metabolism that protects vital organs from damage until the energy supply returns to its normal level. Within 30 minutes of ceasing H2S inhalation, the animals returned to a normal metabolic rate [see "Living in Delay" by Mark B. Roth and Todd Nistoll, Scientific American Israel, October-November 2005].

If it turns out that H2S anesthesia is effective and safe in humans, it will be a welcome addition to emergency medicine. Inhalation of H2S at the scene of a car accident or by a person having a heart attack may provide the time required to successfully transport the patient to a hospital in order to save his life. H2S may also keep the patient alive but in a coma until the required organ is received (the gas may even extend the life time of the donated organs). Also, in areas of war or natural disasters, it will be possible to use H2S treatment, which may reduce the need for blood transfusions until there are enough blood doses available. In 2008, Roth and colleagues reported that rats that inhaled H2S after losing 60% of their blood survived better than untreated rats. Only 25% of the treated rats died due to the trauma compared to 75% of the untreated rats who died.

safe optimism
But not everything H2S touches turns to gold. For example, scientists have not yet decided whether the gas aggravates inflammation or soothes it. Research in my lab and others shows that the gas is a key player in juvenile diabetes, which leaves people dependent on insulin injections to survive. H2S is produced, among other things, in insulin-producing cells in the pancreas called beta cells. These cells produce H2S in animals with juvenile diabetes at higher than normal levels. The excess gas has two negative effects. First, it kills many beta cells, leaving too few cells to produce the amount of insulin the body needs to break down glucose for energy. Second, it interferes with the release of insulin from the remaining beta cells. In other words, H2S may be partially to blame for the lack of insulin in the blood in juvenile diabetes.

Moreover, some of the positive effects of H2S documented in rats and mice have not been replicated in larger mammals. For example, a study carried out by a French research group in 2007 failed to put sheep that inhaled the gas into a state of partial coma like the rodents did. And in another study, the metabolic rate of piglets given H2S increased instead of decreasing.

Also, it is not clear if the H2S coma, when it can be induced, damages brain activity. Although laboratory tests have not revealed such problems in treated animals, it is difficult to detect changes in brain activity in laboratory animals. We have to wait and see if H2S coma can suspend life and at the same time preserve vital brain functions, such as memory and reason.

However, the tremendous medical potential of H2S arouses great interest in the pharmaceutical industry. Several companies are already developing products designed to inject doses of H2S into the body. For example, CTG Pharma in Italy produced several compounds that combine non-steroidal anti-inflammatory drugs (called NSAIDS) and H2S. Experiments on animals show that these drugs are effective in treating inflammation of the nervous system or digestive system, erectile problems, heart attacks and pathological changes in the structure of blood vessels. At the same time, the Ikaria company in New Jersey, of which Roth is one of the founders, recently started Phase II clinical trials, that is, efficacy trials, which tested an injectable form of H2S designed for people who have had a heart attack or are undergoing heart-lung surgery.

Despite the natural tendency of people to avoid exposure to H2S, the studies conducted during the last ten years make it clear that this gas plays a vital role in the health of the heart and probably also in the health of the brain and other organs. And it is assumed that it has additional functions that we have not yet identified. These breakthroughs will guide physiologists in developing a new concept of human health. Research on H2S is still in its infancy, but there is a good chance that it will eventually lead to the treatment of diseases that currently have no answer.