Re understanding of bile acids - surface activity of bile acids

1、 The structure of bile acids

Bile acid is a steroid compound with bile acid as its parent nucleus, consisting of a steroid skeleton consisting of three hexagonal rings (six carbon sterol ring), one pentagonal ring (five carbon sterol ring), and one short aliphatic side chain (as shown in Figure 1). The A and B rings in the molecule are connected in reverse, causing the overall shape to bend, resulting in the formation of a pore like structure of the entire bile acid molecule (as shown in Figure 2). The A-side hydroxyl group (up to three) has hydrophilicity; The three methyl groups connected to the β surface have hydrophobicity, with α and β pointing towards concave and convex surfaces, respectively; Therefore, two opposing surfaces with hydrophilicity and hydrophobicity can be distinguished. Based on this special molecular structure, bile acid molecules have surface activity and form micelles or mixed micelles in the intestine, promoting the digestion and absorption of lipid substances.

2、 HLB value of bile acids

Bile acids in the body mostly exist in the form of sodium or potassium salts, with a hydrophilic lipophilic balance (HLB) value of approximately 18; The HLB value of the mixture of bile salts and lysophosphatids is approximately 10. Both bile acid salts and mixtures of bile salts and lysophosphatids can form micelles with hydrophobic surfaces facing inward and hydrophilic surfaces facing outward, encapsulating insoluble lipid substances at the center of the micelles.

3、 Critical Micelle Concentration (CMC) of Bile Acids

The CMC value of most bile acids is between 1-20mmol/L, but this value largely depends on the type of bile acid, ionic strength, etc. When the critical micelle concentration (CMC) is exceeded, bile acids aggregate and transform from monomeric molecules to simple micelles. Simple micelles can dissolve cholesterol, phospholipids, and other types of lipids.

4、 Micelles of bile acids

The hydrophobic portion of each bile acid molecule faces inward and the hydrophilic group faces outward, while the hydrophobic portion of the lipid at the center of the micelle is dissolved. The simple micelles of bile acids look like disks (see Figure 5) with a diameter of approximately 3nm. After adding phospholipids to simple micelles, mixed micelles are formed with a diameter of about 4-8nm and three times more soluble cholesterol than simple micelles. The mixed micelles appear cylindrical, but when the ratio of lipids to bile acids is high, the micelles may elongate and become worm like.

In bile, mixed micelles are mainly composed of bile acids, phospholipids, and cholesterol. In the small intestine, mixed micelles are mainly composed of bile acids, monoglycerides, and fatty acids; Fat soluble dietary components such as fat soluble vitamins and cholesterol can also dissolve into mixed micelles. Micellar dissolution increases the water concentration of fatty acids and monoglycerides by approximately 1000 times.

Figure 5 Bile acid micelles

5、 Summary

Both free and bound bile acids contain hydrophilic and hydrophobic groups within their molecules, hence the stereochemistry of bile acids has both hydrophilic and hydrophobic sides. This unusual structure enables bile acids to exhibit strong interfacial activity, reducing the surface tension between oil and water, forming micelles to dissolve lipid substances. This plays a crucial role in the digestion and absorption of lipid substances, as well as maintaining the dissolution of cholesterol in bile.