Why Is Urine Yellow? Scientists Find Enzyme That Gives Pee Its Yellowish Hue

A new study has identified the microbial enzyme that makes urine yellow.

Why Urine Is Yellow

This discovery was made by researchers from the National Institutes of Health and the University of Maryland. The findings were noted in the "BilR is a gut microbial enzyme that reduces bilirubin to urobilinogen" study.

The enzyme, known as bilirubin reductase, offers a way for further research into the role of the gut microbiome in ailments such as inflammatory bowel disease and jaundice.

Brantley Hall, the study's lead author and an assistant professor from the Department of Cell Biology and Molecular Genetics at the University of Maryland, explains that the discovery of the enzyme unravels the mystery regarding the yellow color of urine. Hall adds how remarkable it is that a common biological phenomenon has remained unexplained for such a long time.

When red blood cells end up degrading at the end of their lifespan of six months, a pigment that is bright orange, known as bilirubin, gets produced. This is typically secreted into the gut for excretion. However, it can also be reabsorbed partially. Too much reabsorption could lead to bilirubin buildup in the blood and lead to jaundice, which is a condition that makes the skin and eyes yellow. When it is in the gut, the flora can turn bilirubin into different molecules.

Hall explains that microbes in the gut encode the bilirubin reductase enzyme that turns bilirubin into a byproduct known as urobilinogen, which is colorless. This byproduct then degrades spontaneously into a molecule known as urobilin, which is the one that gives the familiar yellow color of urine.

While urobilin has been linked to the yellow color of urine for a long time, the enzyme discovery made by the researchers answers a pressing question that has left scientists eluded for more than a century.

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Health Implications

Beyond unraveling a scientific mystery, the findings could also have crucial health implications. The researchers discovered that the enzyme can be found in nearly all healthy adults. However, it is typically missing among newborns and those with inflammatory bowel disease.

Given these observations, they hypothesize that the enzyme's absence could be a factor in infant jaundice and pigmented gallstone formation.

Xiaofang Jiang, a co-author of the study and NIH Investigator, explains that since they have identified the enzyme, they can start looking into how gut bacteria affect the bilirubin levels that circulate and other related health conditions. The discovery then lays the groundwork for knowing more about the gut-liver axis.

Moreover, the discovery takes researchers a step closer to gaining a holistic understanding of the role of the gut microbiome in human health.

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