Hydrogen sulfide, or H₂S, is normally toxic, but in small amounts it plays a role in cardiovascular health.

In the new study, chemists developed a chemical probe that reacts and lights up when live human cells generate hydrogen sulfide, says chemist Alexander R. Lippert, Southern Methodist University, Dallas. The discovery allows researchers to observe the process through a microscope.

The researchers captured on video the successful chemical probe at work, said Lippert, an assistant professor in the SMU Department of Chemistry.

"We made a molecular probe that, when it reacts with hydrogen sulfide, forms a fluorescent compound that can be visualized using fluorescence microscopy," Lippert said. "This is the first time that endogenously generated hydrogen sulfide has been directly visualized in a living system. This confirms a lot of hypotheses that scientists have, but no one had the tools to directly detect it in real time."

H₂S is one of several small gaseous molecules increasingly recognized as key signaling molecules in the body. For example, H₂S helps reduce high blood pressure. Scientists discovered in the past decade that cells in the human body generate small quantities of H₂S molecules, which in turn deliver information to proteins. The proteins act on the information to perform critical functions in the body.

Previously, scientists couldn't observe H₂S being generated in live cells. As a result, researchers faced challenges when studying hydrogen sulfide in living systems, Lippert said. The new discovery now provides a tool to view directly how and when hydrogen sulfide is generated, he said. Lippert and study co-author chemist Vivian S. Lin made the discovery.

Discovery provides research tool for scientists to observe H₂S in live cells

"Having the tools to do this in living systems is going to open up a lot of possibilities and experiments for scientists," Lippert said. "As a tool, this will allow researchers to ask questions that weren't possible before."

Lippert's real-time video features live human cells, taken from the lining of blood vessels and treated with the chemical probe and with a protein known to promote cell growth. Once the cells start generating H₂S, they behave like squiggly fluorescent green worms.

The researchers' scientific article, "Cell-trappable fluorescent probes for endogenous hydrogen sulfide signaling and imaging H₂O₂-dependent H₂S production," was published online in the Proceedings of the National Academy of Sciences.

Lippert and Lin authored the research with Christopher J. Chang, principal investigator. Lin is a PhD candidate at the University of California at Berkeley. Chang is with the Howard Hughes Medical Institute, University of California at Berkeley. Lippert and Lin carried out the research in Chang's UC Berkeley laboratory.

Discovery can help scientists attack diseases such as cancer

H₂S -- along with nitric oxide, carbon monoxide and others in this emerging class of gaseous signaling molecules -- assists the body's large proteins.

Large proteins do much of the functional work in the body, such as digesting the food we eat and harnessing the energy in the oxygen we breathe. Their size, however, forces them to move slowly inside the cell. In contrast, H₂S and other small gaseous molecules diffuse quickly and easily across cellular membranes, enabling them to travel much faster and rapidly deliver information that mediates critical functions, such as blood pressure regulation, Lippert said.

For their experiments, Lippert and Lin placed living endothelial cells cultured from the internal lining of a blood vessel into a petri dish under a microscope.

Lippert and Lin then added a chemical solution containing an azide-functionalized organic molecule that they'd synthesized to act as a molecular probe. They gave the cells time to absorb the probe, then added a protein solution known to stimulate blood vessel formation. As the cells initiated blood vessel formation, H2S was generated. In reaction, the scientists observed a steady increase in the probe's fluorescence.

"Essentially we're observing the initial events that lead to the building of new blood vessels, a process that's active in babies as they develop, or in women during their menstruation cycles," Lippert said. "We see the cells get really bright as they start moving around and ruffling their membranes. That's the H₂S being formed. In the control group, which weren't stimulated with the growth protein, they don't get any brighter and they don't move around."

The discovery provides new insights that can help scientists attack diseases, such as cancer, by starving the nutrient supply to a tumor, Lippert said.

"When tumors grow they need a lot of blood support because they need the nutrients to support their rapid growth," he said. "If you can stop blood vessel formation you could starve the tumor and the tumor will die. So inhibiting H₂S formation might be a way to treat cancer using this method."