What is the biochemical reason why bile secretion is important for health?
Micelles are made up of amphipathic compounds such as bile acids, fatty acids and monoacylglycerols that interact leaving a relatively stable hydrophilic surface and hydrophobic interior (1). They form at certain temperature ranges when a mixture of lipids is present in concentrated amounts (1). Fatty acid and phospholipid micelles are spherical, but pure bile acid micelles are sandwich-shaped rectangles (1p1062).
During lipid digestion after hydrolysis of triacylglycerols by lipases, it’s up to the bile acid sandwiches to solubilize the spheres, thereby forming “mixed” micelles that appear not unlike rods (1p1062). These rods become longer as more lipids (including limited cholesterol) are solubilized (1p1062). The bile acid micelles form at concentrations of 2-5 mM and at pH values above pK, meaning in equilibrium with other micelles in solution (1p1061-2).
From the lumen, the micelles then transfer the lipids to the mucosal surface for absorption by diffusion (1p1063). Lipid-soluble vitamins A, D, E, and K are also transported within the micelles (1p1065). The delivery is dependent on bile acid micelles increasing effective concentration to create solute flux across the unstirred fluid layer (1p1063). Without bile acids, the absorption of triacylglycerols and the lipid-soluble vitamins would be reduced drastically (1p1063).
1. Devlin TM. Textbook of Biochemistry with Clinical Correlations. Philadelphia: Wiley-Liss, 2002.