Metabolism and function of bile acids

Bile acids make up a group of sterol-derived compounds that act as detergents in the intestine to facilitate the digestion and absorption of fats and fat-soluble molecules. In mammalian species, the cholesterol side chain is trimmed to yield C24-sterol derivatives. In other vertebrate species, the hydroxylation of the side chain does not lead to its removal and the products of the biosynthetic pathway are referred to as bile alcohols. Invertebrate species do not synthesize sterol bile acids. Over the last few years, much information has been gained about the function of bile acids and the mechanisms that regulate their synthesis. The focus of this chapter is to provide a general overview of bile acid biochemistry and to review recent discoveries that have advanced our understanding of bile acid metabolism and function in mammals. The concept of bile was developed around the late 1600s to mid 1700s. It was early in the 1800s when bile solutes were crudely isolated. Among the components identified were the amino acid taurine (identified in ox bile, hence its name), cholesterol and a nitrogenous acid. The term 'cholic acid' was initially applied to the acidic component but this was changed to the generic term 'bile acid' shortly after. By the mid 1800s, taurineand glycine-conjugated bile acids could be distinguished and it was also around this time that the idea that bile acids were responsible for solubilizing cholesterol in bile emerged. Nearly half a century ago, it became evident that bile acids are synthesized from cholesterol [1]. Bile acids are the major solutes in bile. The typical mammalian bile is comprised of about 82% water, 12% bile acids, 4% phospholipids (mostly phosphatidylcholines), 1% unesterified cholesterol and the remaining 1% as assorted solutes (including proteins).