Medicine

New bile acids discovered

Print edition : April 10, 2020

Structure of cholic acid, the primary bile acid, showing relationship to other bile acids.

Thanks to a new discovery made by scientists at Michigan State University (MSU), led by Robert Quinn, and published in the latest issue of “Nature”, the textbook description of the role of bile acids and their role in the human digestive system is likely to change.

Much of our knowledge about bile has not changed. Bile is produced in the liver, stored in our gall bladder and injected into our intestine when we eat, where it breaks down fats.

The first bile acid was discovered in 1848, and Heinrich Otto Wieland, the German chemist who revealed the structure of bile acids, won the 1927 Nobel Prize for Chemistry for his work that he began in 1912. He had isolated three bile acids and discovered that they had similar structures and were structurally related to cholesterol.

“Since then, our understanding of the chemistry of bile production in the liver was that the cholesterol backbone of the bile acid structure is linked to the amino acids glycine or taurine to produce our primary bile acids,” said Quinn, the lead author of the study. “It begs the question of how the new bile acids we've discovered have remained hidden during the past 170 years of bile acid chemical research.”

These new bile acids are not produced by our enzymes; they are made by microbes in our gut. This discovery will change how medical textbooks address digestion, and it contributes to an ever-growing body of knowledge supporting the importance of the microbiome, the collective community of bacteria and other microorganisms living in our guts.

According to the MSU press release, Quinn’s team, comprising scientists from MSU, the University of California San Diego and a number of collaborating institutions, showed that microbes in the gut produce unique bile acids by conjugating the cholesterol backbone with myriad other amino acids. This represents a fifth mechanism of bile acid metabolism by the microbiome that greatly expands our understanding of mammalian bile, the release said.

While much of the study was conducted in mice, these novel bile acids were also found in humans. Interestingly, the study found that these acids are particularly abundant in the guts of people suffering from gastrointestinal diseases, such as Crohn’s disease and cystic fibrosis.

“These molecules can alter signalling pathways in the human gut that result in a reduction of overall bile acid production, representing a new mechanism where our gut bacteria can manipulate our own physiology,” Quinn said.

“Clearly, our understanding of these compounds is in its infancy. This exciting new discovery opens more questions than answers about these compounds and their role in our health,” he said.

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