More than 125 years ago, scientists identified urobilin as the yellow pigment in urine, but they didn't know what was responsible for the urobilin's production. Now, researchers at the University of Maryland and National Institutes of Health have found that the microbial enzyme bilirubin reductase is responsible for giving urine its yellow hue.

This unraveling of the enzyme could help researchers learn more about gut health, inflammatory bowel disease and jaundice.  "It's remarkable that an everyday biological phenomenon went unexplained for so long, and our team is excited to be able to explain it," Brantley Hall, an assistant professor in the University of Maryland's Department of Cell Biology and Molecular Genetics, said in a news statement.

Urine is a combination of water, electrolytes and waste that your kidneys filter out from your blood. It turns out urine's color is connected to the body's red blood cells, researchers said. As red blood cells degrade, a bright orange pigment called bilirubin is produced. The pigment is typically secreted into the gut, where it can then be excreted or partially reabsorbed. Once the bilirubin arrives in your gut, the microorganisms in your intestines can convert the bilirubin into other molecules, the study found.

"Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colorless byproduct called urobilinogen," Hall, the study's lead author, said. "Urobilinogen then spontaneously degrades into a molecule called urobilin, which is responsible for the yellow color we are all familiar with."

The study authors said that before their research, scientists thought there were multiple enzymes involved, rather than a single enzyme.