Bile salts and emulsion of fats

See also Adipocytes, Fat Absorption and Transport, Bile Salts and Emulsion of Fats, Mobilization of Stored Fat, Energy Storage, Triacylglycerol Synthesis, Action of Insulin, Lipids... 

See also Figure 19.23, Bile Salts and Emulsion of Fats, Bile Acids (from Chapter 19), Fats, Glycine, Taurine, Glycocholate, Taurocholate, Cholic Acid... 

Biosynthesis, Bile Salts and Emulsion of Fats, Cholesterol... 

The digestion and absorption of fat is considerably more complex than that of carbohydrate or protein because it is insoluble in water, whereas almost aU enzymes catalyse reactions in an aqueous medium. In such media, fat can form small droplets, an emulsion, which is stable in this medium. Formation of an emulsion is aided by the presence of detergents these possess hydrophobic and hydrophilic groups, so that they associate with both the fat and the aqueous phases. Such compounds are known as emulsifying agents and those involved in digestion are mainly the bile salts and phospholipids. 

Pancreatic lipase, in the presence of bile salts and coUpase, acts at the oil-water interface of the triglyceride emulsion to produce fatty acids and 2-monoacylglycerols. Cohpase is secreted in pancreatic juice as an inactive proenzyme, which is converted to the active form by trypsin. Other significant enzymes involved in the breakdown of fats within the intestinal lumen are cholesterol ester hydrolase, phospholipase A, and a nonspecific bile salt-activated lipase. 

Steroids are lipids found in living systems that all have the ring system shown in Figure 3.8 for cholesterol. Steroids occur in bile salts, which are produced by the liver and then secreted into the intestines. Their breakdown products give feces its characteristic color. Bile salts act on fats in the intestine. They suspend very tiny fat droplets in the form of colloidal emulsions. This enables the fats to be broken down chemically and digested. 

Finally, we should not forget to mention nature s own emulsions and foams. Examples of natural emulsions are rubber latex produced by Hevea brasiliensis, fat droplets in milk stabilized by proteins, fats from the diet emulsified in the duodenum and stabilized by bile acids and salts, and blood too may be regarded as an emulsion. Where (micro)organisms living in an aqueous environment produce gaseous metabolites, foams are often formed. This may be desirable, as in beer, or undesirable as in installations for the treatment of wastewater. 

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