Properties of bile acids

Table 1.1 Some of the biochemical properties of bile acids.

Biochemical Properties of Bile Acids and Their Effects on the GI Tract... 

Hofmann, A.F., and A. Roda. 1984. Physicochemical properties of bile acids and their relationship to biological properties An overview of the problem. J Lipid Res 25 1477. 

Yoon, YJ., Hagey, L.R., Hofmann, AJ ., Gurantz, D., Michelotti, E.L., Steinbach, J. H. Effect of side-chain shortening on the physiologic properties of bile acids Hepatic transport and effect on bihary secretion of 23-nor-ursodeoxycholate in rodents. Gastroenterology 1986 90 837-852... 

Fini A and Roda A, Chemical properties of bile acids. IV. Acidity constants of glycine-conjugated bile acids, /. Lipid Res., 28(7), 755-... 

Publication of a book devoted entirely to the chemistry, physiology, and metabolism of bile acids indicates a renascence in interest in these poly-functional detergents. Here we will summarize present views on the physical and physiological properties of bile acids in relation to their chemical structure which bear on their participation in the intestinal absorption of fat, their enterohepatic circulation, and their influence on electrolyte and water absorption by the colon. Several of these topics are considered in detail elsewhere in this book, as well as in recent reviews (1-4). This chapter will focus on our own studies but will also emphasize areas in which information is needed. 

In addition to changing the physical properties of bile acids, conjugation also alters their physiological properties. On the basis of extremely limited evidence, it seems likely that the bile acid pool of animals with exclusively taurine conjugates is maintained chiefly by active absorption from the ileum. 

Besides influencing the critical micellar temperature, the number and position of hydroxy groups also influence the critical micellar concentration. The critical micellar concentration of dihydroxy bile acids is significantly below that of trihydroxy acids (1,63), but no values have been reported for monohydroxy acids. Conjugation with glycine or taurine does not appear to have any particular effect on those properties of bile acids which are related to the steroid nucleus, provided that experiments are carried out at pH sufficiently alkaline that all bile acid molecules present are ionized (64). 

Therefore, I shall concentrate on only those bile acids that have been reasonably well studied from a physicochemical point of view and which have some relation to physiology and biochemistry of living things. Because the specific physical characteristics of the bile acids and their alkaline metal salts vary considerably with the number of hydroxyl groups present on the steroid nucleus, I will present a fairly detailed description of the physicochemical properties of cholanic acid (no hydroxyl groups), monohydroxy, dihydroxy, and trihydroxy bile acids. Since the triketo bile acid (dehydrocholic acid) has been used widely as a choleretic, its properties will also be discussed. Unfortunately, many interesting bile acids and bile alcohols isolated from a variety of vertebrates (29-32) have not been studied physicochemical ly. However, knowing their molecular structure, many of the properties of these compounds can be deduced by comparison with the known properties of bile acids discussed in this chapter. 

The topic of bile acids has been the subject of a number of general reviews and books (28, 29, 30, 33, 34). Certain reviews have dealt, in part, with physicochemical aspects of the bile salts (35-39) or have mentioned physicochemical properties only briefly (28, 30, 34, 40). A review dealing specifically with the physicochemical properties of bile salts and their relation to physiologic function was published in 1967 (1). This chapter will be limited to a discussion of the physicochemical properties of bile acids and their salts. (For physiologic correlations, see 1, 8, 9, 10, 41, and 197.)... 

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