An Overview of Bile-Acid Synthesis, Chemistry and Function

DENNIS STAMP AND GARETH JENKINS

1.1 The Bile Acids

Bile acids (BAs) are a group of water-soluble steroids formed during the catabolism of cholesterol, and synthesised in the hepatocytes of the liver. The products, cholic acid (CA), and chenodeoxycholic acid (CDCA), are called primary bile acids. Figure 1.1 shows an overview of the pathways involved in these reactions. These primary BAs are then conjugated, mainly to either glycine or taurine. The conjugated BAs play a pivotal role in fat (and fat-soluble vitamin) digestion and absorption, reaching the colon via the gallbladder, bile duct, and duodenum. BAs are strongly cytotoxic, and are able to act as nuclear sensors, detecting and controlling their own concentrations within the body. Bile acids also play a major role in carcinogenesis of some tissues (liver, gallbladder, upper and lower GI tract). These roles will be described in the following pages and following chapters. BAs are stored in the gallbladder under extremely high concentration (>300 mM), achieved by a constant removal of water and electrolytes. About 5% of these bile acids go to the colon for excretion in the faeces, and since cholesterol is a precursor of BA, this is the only time cholesterol is excreted from the body (as bile). Also present in bile are:

(1) Bilirubin and other pigments resulting from haem catabolism,

(2) Heavy metals such as copper or iron, in excess of bodily needs, and

(3) Lipophilic steroids and drug metabolites that would be insoluble in the urine.

In the colon, deconjugation of the conjugated primary bile acids occurs via the action of bacterial enzymes, producing free bile acids. Furthermore, the enzymatic action of the bacterial flora converts the bile acids into secondary BAs, by removing the hydroxyl group from the 7th carbon atom on the molecule. The specific enzyme responsible is 7 alpha-dehydroxylase, which forms deoxycholic acid (DCA) from cholic acid, and lithocholic acid (LCA) from chenodeoxycholic acid. These secondary bile acids then pass into the portal vein and reach the liver, where they join new primary BAs, they are then reconjugated to glycine or taurine in the canaliculi of the liver, and are then stored in the gallbladder. This recycling of bile acids is known as the enterohepatic circulation and can occur 10 times every day. Transport across the canalicular membrane of the liver, is an ATP-dependent process, aided by the bile-salt excretion pump (BSEP) expression in the canalicular membrane. Conjugation increases the aqueous solubility of the bile acids, and renders these bile acids largely impermeable to the cell membranes of the intestine and duodenum; hence, they are unable to leave the intestinal lumen. This allows bile-acid levels to rise in the lumen, ultimately reaching sufficient concentrations to form micelles, which allow lipid emulsification and subsequent absorption.

Many other BAs are formed at lower levels both in the colon and liver by the bacterial flora and the conjugation with other biomolecules, but this chapter will focus on the more common bile acids; cholic and chenodeoxycholic acids (primary BAs), deoxycholic acid and lithocholic acids (secondary BAs), and their glycine and taurine conjugates. These are the main sub-types of bile acids, as seen in Table 1.1. There are some "minor" BAs that have significant importance. One is ursodeoxycholic acid (UDCA), which, as its name suggests, is abundant in bears, and much prized in Eastern medicine. Human bacterial flora can produce it as well, along with dozens of other BAs and their many isomers. Ursodeoxycholic acid plays a role in human cholesterol regulation, and its medical applications include dissolving gallstones and protecting cells from the harmful effects of other BAs like DCA in cholestatic diseases. When used medically, UDCA is not obtained from bears, but is synthesised from cholic acid, a byproduct obtained from the abattoir.

1.2 Conjugated Bile-Acid Biosynthesis

Figure 1.1 illustrates a condensed version of the classical pathway of bile-acid synthesis, a series of 12 enzymatic reactions that convert cholesterol, which is insoluble, into BAs, which are water soluble. The cholesterol is first converted to 7 alpha-hydroxy cholesterol, followed by the series of enzymatic transformations, eventually producing cholic and chenodeoxycholic acids (not all steps shown). The rate-limiting enzyme in this pathway is cholesterol 7 alpha-hydroxylase (CYP 7A1), which originates from microsomal cytochrome P-450 enzymes, expressed only in the liver hepatocytes.

Another indirect pathway (not shown in Figure 1.1) involves cholesterol reacting enzymatically with CYP 27A1, producing both 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestanoic acid (omitted from the diagram for simplification). This is followed by a series of reactions, ending in the production of chenodeoxycholic acid. The inner mitochondrial membranes are the main reaction site for this pathway. In the adrenal glands, steroid acute response protein (StAR) delivers cholesterol to the mitochondrial membrane. StAR is necessary for steroidogenesis, and thus may provide a reliable source of cholesterol for these reactions.

Another pathway of some importance occurs in the brain; this is the cholesterol 24-hydroxylase pathway. About 25% of the body's cholesterol exists in the plasma membranes of myelin sheaths. Here, the blood–brain barrier prevents cholesterol exchanges with the circulating lipoproteins, which makes it difficult for cholesterol to leave the brain. The cytochrome P-450 enzymes (CYP 46), expressed almost exclusively in the endoplasmic reticula of the brain, allows formation of 24-hydroxycholesterol.

It is impossible to determine the relative contributions of each of these pathways to total bile-acid biosynthesis, due to the nature of the data. Some values were obtained from patients whose gallbladders had been surgically removed; other patients would be atypical due to illness, and many data were obtained from experimental animals, which may metabolise these compounds differently from humans. Also, the exact order of many of the reactions is not known, since the intermediates may act as substrates for more than one enzyme. Further details for these reactions can be found in reviews by Chiang, Moore et al., and Fuchs et al.

1.3 Bile-Acid Regulation

1.3.1 Bile-Acid Receptors (FXR)

The following is a brief overview of events in the area of BA synthesis,...