Hydrogen sulfide deserves its bad reputation. It’s famous for filling the air of high school chemistry labs with the smell of rotten eggs. One strong whiff of the noxious gas can knock you to the ground. Too much can kill you.
Aside from its odorous infamy, hydrogen sulfide, or H2S, has long been considered biologically unimportant. But it turns out that H2S produced in the body manages many major biological functions. Some preclinical data show that H2S can ameliorate the side effects of anti-inflammatory drugs. And just last December, scientists reported that relatively high doses of H2S extend the life span of the minute roundworm Caenorhabditis elegans. Perhaps, such research suggests, the Fountain of Youth is a chamber of that stinky gas.
In fact, it now seems, you can’t live without H2S. At the same time, true to reputation, its effects are not always benign. In recent years scientists have compiled growing evidence that H2S can both harm and help the body, influencing aspects of biology from immunology to inflammation to cell proliferation. H2S affects illnesses spanning hypertension, diabetes, atherosclerosis, and neurological disorders, including strokes. Figuring out how H2S wields such power will provide insights into basic biology, could suggest therapeutic agents for many diseases, and may explain why garlic protects the heart.
Scientists have known for decades that the body produces H2S but didn’t realize its importance beyond its toxicity. Then a new story about the body’s gases began in the early 1980s, with the discovery that nitric oxide, or NO, performed the job of helping cells communicate. Years later, a second bodily gas, carbon monoxide, or CO, revealed its ability to transmit signals between cells as well. These gases and other substances were christened “gasotransmitters.” H2S is the most recent member to join the club.
“Gasotransmitters show a new way for scientific discovery to unveil the secrecy of ourselves,” says Rui Wang of Lakehead University in Thunder Bay, Ontario, Canada. “This is a revolution and it’s just beginning.”
Wang first encountered H2S around 1989 during graduate work at the University of Alberta. A toxicologist wanted to understand why leaks of that gas from oil fields posed such a quick danger to oil workers.
By 1996, scientists had begun to understand that the amount of H2S in the body is important. For example, they had figured out that both low and high doses of H2S could affect nerve cells. It occurred to Wang that this gas could join the gasotransmitter family. And then all sorts of questions popped into his head. “Why do we make H2S? If we are what we smell, then why aren’t we smelly?” he remembers wondering.
To answer those questions, workers in Wang’s lab isolated the gene for CSE (cystathionine-lyase), an enzyme that regulates H2S levels in mammals (except in the brain, where the enzyme doing that job is cystathionine-synthase).
Other scientists had already found CSE in organs such as the liver, indicating that cells had a mechanism to produce H2S. Wang’s team cloned the CSE gene from vascular smooth muscle cells and found it to be identical to the CSE previously cloned from the liver. He reasoned that if blood vessels also produce the enzyme for making H2S, cells in blood vessels must need daily doses.
Wang’s team then investigated rats and cultures of aortic cells to understand what H2S concentrations do to the cardiovascular system. The work, published in the EMBO Journal in 2001, showed that the presence of H2S in a rat’s circulatory system relaxed cells in the aortic tissue and lowered the rat’s blood pressure—but only when the H2S concentration occurred in the natural range.
The team also figured out that H2S opens a type of potassium channel, a group of proteins anchored on the cell membrane that provide a pathway for potassium ions flowing in and out of the cell. This flow signals the cell to do something: contract, secrete, or move.
Good gas, bad gas
Research about H2S started with understanding toxic effects from environmental exposure. Now, studies are looking inside, aiming to understand why the body makes its own H2S, and why that H2S can have both positive and negative effects.
For instance, rat studies show a connection between elevated H2S levels and diabetes.
In rat cells that mimic human pancreas cells, high levels of H2S suppress insulin secretion. Because people with type 1 diabetes—the genetic form—make a lot of the enzyme CSE, thereby elevating H2S, Wang speculates that too much H2S can partially explain why people with type 1 diabetes don’t release insulin. H2S effects may thus help explain the origins of diabetes itself.
In other areas, the effects of H2S are more mixed. Some animal studies show a relationship between H2S and the brain in cases of ischemic stroke—the kind with the blood clot. Whether the connection is helpful or harmful remains unclear.
Inflammation also exemplifies both sides of H2S. Philip K. Moore, now of King’s College London, and his team were first to show that H2S worsened inflammation in mice. In work published in the Journal of the Federation of American Societies for Experimental Biology (FASEB), the team injected mice with lipopolysaccharide (LPS), a toxic component of some bacterial cell walls, to induce inflammation in lungs. Moore’s team could lessen inflammation in the lungs and liver by preinjecting mice with DL-propargylglycine, a chemical that blocks CSE. To find a more direct link between high H2S and worsened inflammation, the team injected mice with sodium hydrosulfide (NaHS), essentially adding H2S to the body. Injecting NaHS worsened inflammation to the same degree LPS did.
Other work, published by Moore and his collaborators in the January 2008 issue of Shock, reveals the anti-inflammatory side of H2S. This project showed that H2S protects lung function in mice with pancreatitis (an inflamed pancreas). People with bad cases of pancreatitis also suffer lung inflammation.
And in animals with the equivalent of inflammatory bowel disease, giving H2S–releasing drugs can reduce inflammation.
So it’s not clear whether H2S is pro-inflammatory or anti-inflammatory. “This sounds odd, but the same is true for nitric oxide, which can be both,” comments Moore. “It may simply be that high levels of H2S are pro- and low levels are anti-inflammatory, but I would bet it would turn out to be more complex than that.”
Nematodes love H2S
While sorting through the good and bad effects of H2S on inflammation will help both basic science and therapeutics, Mark Roth of the Fred Hutchinson Cancer Research Center in Seattle has his sights on the Fountain of Youth.
Roth never wondered much about H2S. He manipulates oxygen metabolism to put animals in suspended animation. He stopped the metabolic rate of nematodes and fish so that they didn’t consume oxygen and didn’t produce carbon dioxide.
He could suspend fish and worm metabolisms and bring them back again by incrementally removing and then adding oxygen. But for an animal as large as a mouse, removing oxygen would be lethal. So he put mice in a hybernationlike state by lowering oxygen demand. For that he needed something toxic. He settled on H2S.
H2S could help mammals survive an otherwise lethal lack of oxygen, Roth showed in work published in Shock in April 2007. Exposing mice to H2S reduced their ability to use oxygen by a factor of 10, which in turn reduced their heart rate and their breathing rate and slowed their movement to a state of deep hibernation. The mice survived this way for 6 hours with no behavioral defects.
“I thought, ‘Well jeez Louise, if you can do that, why not do that for people who are suffering their own problems, such as having no blood in their bodies because they were just shot by someone in Iraq?'” Roth recalls.
Indeed, the U.S. military thought so too, and the Defense Advanced Research Projects Agency, or DARPA, funded Roth to use rats to mimic a battlefield situation. Roth saved the rats under certain conditions of blood loss by exposing them to H2S and lowering their need for oxygen.
Roth wants to extend the rat work to the battlefield and help injured soldiers. He patented a way to bottle H2S as a liquid for intravenous use. Soldiers with severe blood loss could be treated with an IV of H2S, possibly lowering their need for oxygen until enough blood could be transfused. The work is in clinical trials in Australia.
Roth’s current work, published in December 2007 in Proceedings of the National Academy of Sciences (PNAS), suggests that beyond battlefield healing, H2S has Fountain of Youth possibilities.
To dissect genetic pathways allowing H2S to help animals survive a lethal lack of oxygen, Roth and colleagues turned to the roundworm C. elegans. They grew the nematodes from larvae to sexual maturity and beyond in a chamber of 50 parts per million H2S, the OSHA limit in which people can work for 8 hours a day. Since C. elegans is barely larger than the period at the end of this sentence, at the very least Roth figured that the worms would be sluggish, just like the mice.
But instead, the worms wiggled. They reached sexual maturity and produced offspring at the same rate per hour as did animals raised in room air. They withstood high heat.
And they lived 70 percent longer than worms raised in room air. Why? “I don’t know the answer to that yet,” Roth says.
The paper does hint at a mechanism. Worms raised in H2S and that lacked the SIR-2 gene, or silent information regulator number 2, had life spans matching their room-air counterparts. Michael Crowder, of Washington University Medical School in St. Louis, says that C. elegans helped researchers establish the link between SIR-2 and life span regulation of multicellular organisms.
Always a smelly legacy
Roth and others are optimistic that H2S compounds will save lives and produce anti-inflammatory drugs with relatively few side effects, but it’s still early for basic research on H2S. Researchers need to figure out how H2S fits in among the thousands of mediators that contribute to inflammation. Mammals constantly produce H2S and both of its regulating enzymes. Researchers need to know—on a real-time basis—what triggers the enzymes to release H2S. Researchers also want to know whether this release could be controlled.
Complicating matters, H2S easily diffuses anywhere in the body and likely works through a variety of mechanisms. Also, depending on the circumstances, cells can be exposed to H2S alone or in combination with NO or CO, so scientists need to figure out how these gases work together. Also, some work shows H2S may affect genes. How many genes, and whether H2S does so alone or in combination with other gases, is unknown. An accurate way of monitoring real-time releases of H2S from single cells would give scientists exact amounts of H2S surrounding cells. “For H2S there really is no accurate way of checking the levels in the precise environment of a cell,” says Moore of King’s College. With accurate measurements of a single cell, scientists could determine any differences between what happens in a single cell and then what occurs in the same cell type in the body.
David Kraus and his colleagues at the University of Alabama at Birmingham developed their own sensor that measures H2S in solutions mimicking real cellular conditions. They used this sensor to figure out how H2S and garlic work with red blood cells to protect the heart.
Researchers already knew that garlic compounds and H2S provide similar benefits in the heart—for example, preventing clots and lowering blood pressure. In work published in December 2007 in PNAS, Kraus’ team used its new sensor to watch the garlic compounds be converted into H2S as soon as they came in contact with red blood cells.
“It occurred to us that the cardiovascular effects of the garlic compounds may be mediated by H2S, so we had to show that the garlic compounds were metabolized into H2S. When we did that, we found that the effects of the garlic compounds were identical to those of H2S,” says Kraus. Which just further goes to show that whether it’s beneficial or detrimental, H2S remains stinky.
Jeanne Erdmann is a freelance writer in Missouri.