What is Bile Acid?
Bile acids are the main components of bile and are synthesized in the liver from cholesterol through a series of enzymatic reactions. Bile acids can be divided into free bile acids and conjugated bile acids according to their structures.
Free bile acids include bile acids, goose deoxycholic acid, deoxycholic acid and lithophanic acid. The above free bile acids are combined with glycine or taurine respectively to form various corresponding conjugated bile acids, including glycocholic acid, taurocholic acid, glycocholic goose deoxycholic acid and taurocholic goose deoxycholic acid.
Conjugated bile acids are more water-soluble and generally exist in the body as sodium salts, which are more stable than free bile acids.
Bile acids can also be divided into primary and secondary bile acids according to their origin.
Bile acids synthesized directly from cholesterol in hepatocytes are called primary bile acids, which include bile acids, goose deoxycholic acid and their combination products with glycine or taurine.
Primary bile acids are secreted into the intestine and undergo 7α dehydroxylation by intestinal bacteria to form secondary bile acids. They mainly include deoxycholic and lithic bile acids and their binding products in the liver, which are combined with glycine or taurine, respectively. Thus, gut microbes can regulate the production of secondary bile acids and thus influence signaling.
Bile acids are also an important component of the intestinal mucosa, linking the intestinal microbiota to liver and intestinal metabolism, thereby influencing gastrointestinal motility, intestinal permeability, and the development of cancer.
Bile acids can influence the composition of intestinal microbes, and intestinal microbes can influence the total amount and composition of the bile acid pool.
The Main Physiological Functions of Bile Acids
Promote the digestion and absorption of lipids
The steric conformation of bile acids has both hydrophilic and hydrophobic sides. This structural feature gives bile acids strong interfacial activity and becomes a strong emulsifier, which can reduce the interfacial tension between oil/water phases, emulsify lipids into fine microclusters, increase the attachment area of lipids and lipase, and facilitate the digestion of lipids.
The digestion products of lipids can in turn combine with bile acid salts and converge into phospholipids etc. to form fine mixed microclusters, which facilitate the passage through the surface water layer of small intestinal mucosa and promote the absorption of lipids.
Maintaining the dissolved state of cholesterol in bile to inhibit cholesterol precipitation
About 99% of cholesterol is excreted from the body via the intestine with bile, of which 1/3 is in the form of bile acids and 2/3 is in the direct form. Cholesterol in bile is insoluble in water and works synergistically with bile acids and lecithin to disperse cholesterol into soluble microclusters, making it less likely to precipitate out.
Whether cholesterol precipitates from bile depends on the appropriate ratio of bile acids and lecithin to cholesterol in the bile. If there is too little bile acid or lecithin in the liver, or too much cholesterol, cholesterol precipitation is likely to occur and gallstones are formed.
The separation of bile acids is particularly complicated by the presence of structural analogs (as isomers) and the polarity differences between unconjugated and conjugated bile acids. There are limitations in analyzing these compounds by gas chromatography (GC) or liquid chromatography (LC) alone. the GC method usually requires a stepwise derivatization process, and a certain amount of bile acids is lost in each step of the derivatization. In addition, the determination of the composition or concentration of conjugated bile acids by GC analysis requires the extraction of several aliquots of the sample separately. Although the LC analysis method can detect both conjugated and unconjugated bile acids, the analysis time for a single shot is up to 30 min.
Creative Proteomics provides animal target metabolomics services. UPLC-MS/MS technology combined with bile acid isotope standards allows simultaneous analysis of multiple bile acids and their adducts in biological matrices, enabling the analysis and quantification of bile acid species and significantly improving the accuracy of qualitative and quantitative analysis.
