Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

Abstract

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Keywords: bile acid analysis; bile acid metabolism; bile acid physical chemistry; bile acid transport; enterohepatic circulation.

Fig. 1.

Participants in the first symposium in the United States dealing with bile acid metabolism. The meeting was organized by Leon Schiff, a clinical hepatologist and took place in September 1967. First row, left to right: M. Shiner, L. Schiff (organizer), G. Haslewood, M. Siperstein, S. Tabaqchali, D. Small. Second row, left to right: L. Lack, L. Schoenfield, P. Nair, J. Carey, A. Hofmann. Third row, left to right: M. Stanley, S. Hsia, unidentified, N. Javitt, H. Danielsson, J. Dietschy, S. Emerman. Fourth row, left to right: F. Kern, J. Sjövall, B. Drasar, R. Palmer, J. Senior, N. Myant, J. Wilson, E. Staple. Proceedings of the meeting were published in 1969 (480).

Fig. 2.

Numbering system of bile acids and bile alcohols, and frontal view of CDCA. The numbering system was developed in the 1930s when the steroid structure was finally established.

Fig. 3.

Molecular structure of cholesterol (above) and CDCA (below) showing the changes in the cholesterol molecule when a C₂₄ bile acid is formed. The changes are: 1) reduction of the double bond to give a 5β (A/B cis) A/B ring juncture; 2) a modified β-oxidation of the side chain to remove three carbon atoms and convert the terminal carbon to a carboxyl group; and 3) epimerization of the 3β-hydroxy group to a 3α-hydroxy group. For a discussion of the enzymes involved, see (155).

Fig. 4.

Depictions of the two epimers of decalin, a saturated C₁₀ hydrocarbon that is used to illustrate the two A/B ring juncture epimers of bile acids. When the two bridgehead hydrogen atoms are across from each other, the juncture is called trans; when they are on the same side of the ring juncture, the juncture is called cis. When bile acid structure is shown in frontal view (Fig. 2), the only indication of the stereochemistry of the A/B ring juncture is the orientation of the hydrogen atom, i.e., whether α or β. However, in bile acids, the substituent at the upper bridgehead carbon atom is the C-19 methyl group. Often bile acid structure is shown without any indication as to the orientation of the hydrogen atom at C-5. When this is done, it is understood that the molecule is A/B cis, as 5β (A/B cis) bile acids are much more common in nature than 5α (A/B trans) bile acids. A/B trans bile acids are termed “allo” bile acids.

Fig. 5.

Silhouette of a C₂₄ conjugated bile acid molecule emphasizing its possession of a hydrophobic side (β face) and a hydrophilic side (α face). Common sites of hydroxylation in addition to the default structure (3α, 7α) are at C-6, C-12, and C-16; all of these are on the hydrophilic side of the molecule. Modified from Roda et al. (360).

Fig. 6.

Time course in healthy subjects of levels in peripheral venous plasma of immunoreactive conjugates of cholic acid (cholyl conjugates) and those of CDCA (here termed “chenyl” conjugates) in response to meals; meals are indicated by the thick vertical arrows. Levels of CDCA conjugates rise sooner than those of cholyl conjugates, indicating more proximal absorption. Levels of CDCA conjugates are also higher than those of cholyl conjugates despite similar proportions in bile because of lower fractional extraction by the liver. From (216).

Fig. 7.

Schematic depiction of the work of L. Lack and I. Weiner on conjugated bile acid (here, taurocholate) absorption by everted sacs of rat intestine, showing preferential and active absorption by the distal ileum. At time zero, bile acid concentrations are identical on both sides of the intestine. Modified from (229).

Fig. 8.

Depiction of cholesterol and bile acid metabolism as well as the enterohepatic circulation by Bergström, Danielsson, and Samuelsson in 1959 (8). In the accompanying text, they note that the mass of cholesterol entering the small intestine in bile is greater than that of dietary cholesterol. The figure shows neither the spillover of absorbed bile acids into the systemic circulation nor selective absorption of conjugated bile acids by the terminal ileum. Values for biliary bile acid secretion and fecal bile acid excretion are about 2-fold higher than those obtained in more recent studies.

Fig. 9.

Depiction of the use of triangular coordinates to plot biliary lipid composition and thereby to define the saturation of cholesterol in bile. The triangle is derived from a plane parallel to the base indicating a constant water composition (10% solids, 90% water) in the tetrahedron consisting of the four biliary components (water, lecithin, cholesterol, and bile acids). The micellar zone is shown in black. Bile having a lipid composition falling within in the micellar zone is unsaturated. If the composition is above the upper limit of the micellar zone, the sample is supersaturated in cholesterol and at risk for cholesterol gallstone formation (376). If biliary lipids are known, the percent saturation can also be obtained from tables published by Carey and colleague (393, 394).

Fig. 10.

Molecular arrangement of the drum-shaped mixed micelle micelle originally proposed by Small, Penkett, and Chapman (369) and modified by Carey (330), as compared with the radial shell model proposed by Nichols and Ozarowski (371). The radial shell model was confirmed by Hjelm and colleagues using small angle neutron scattering (372, 403, 404) and also by molecular dynamic studies of Marrink and Mark (373).

Fig. 11.

Phase diagram of the water-monoolein (monooleylgl­ycerol)-taurocholate system showing the large micellar area (cross-hatched), thus depicting the excellent solubilizing potency of bile acids for polar lipids. Liquid crystal states are indicated by vertical and horizontal cross-hatching. Modified from Svärd et al. (398).