A microbial enzyme, called bilirubin reductase, is believed to be responsible for the yellow tint of urine.
- Bilirubin is a pigment produced by the breakdown of red blood cells.
- Once in the intestine, it is transformed into urobilin, namely a molecule responsible for the yellow color of urine.
- This microbial enzyme is often absent in newborns, which may contribute to infant jaundice.
“It is remarkable that an everyday biological phenomenon has remained unexplained for so long, and our team is excited to be able to explain it.” This was stated by Brantley Hall, professor in the department of cell biology and molecular genetics at the University of Maryland (United States). Along with other scientists, they identified the microbial enzyme responsible for the yellow hue of urine. To achieve this discovery, they carried out work, the results of which were published in the journal Nature Microbiology.
Bilirubin reductase is transformed into urobilin which colors the urine
When red blood cells break down after their six-month lifespan, a bright orange pigment called “bilirubin” is produced. The latter is generally secreted into the intestine, where it is intended to be excreted, but can also be partially reabsorbed. Excessive reabsorption can cause bilirubin to build up in the blood and cause jaundice, which results in yellowing of the skin and eyes. Once in the intestine, the resident flora can convert bilirubin into other molecules.
“Gut microbes encode the enzyme bilirubin reductase which converts bilirubin into a colorless byproduct called ‘urobilinogen’. This substance then spontaneously breaks down into a molecule called ‘urobilin’, which is responsible for the yellow color of urine that we all know”, Brantley Hall said.
Infant jaundice: this microbial enzyme is absent in babies
According to the authors, bilirubin reductase is present in almost all healthy adults, but it is often absent in newborns and people with chronic inflammatory bowel diseases. Thus, they speculate that the absence of bilirubin reductase may contribute to infant jaundice and the formation of pigmented gallstones.
“Now that we have identified this enzyme, we can begin to study how bacteria in our gut influence circulating bilirubin levels and related health problems like jaundice. This discovery lays the foundation for a better understanding of the axis intestine-liver”, concluded Xiaofang Jiang, co-author of the work, in a statement.
