Bile acids, lower vertebrates

Bile acids are C-24 to C-28 carboxylic acids with a steroid nucleus containing hydroxylic substituents and part or all of the side chain of 5 -cholestane (Fig. 1). The A/B ring junction is cis (Sp hydrogen) in the bile acids of most higher vertebrates, although A/B trans (alio) bile acids (5a hydrogen) occur in lower vertebrates (HIO). Bile acids do not occur in invertebrates. Most unconjugated bile acids have 24 carbon atoms with the basic structure of 5p-... 

Ill) Occurrence and structure of primitive bile acids in lower vertebrates... 

Most primitive bile acids found in lower vertebrates possess the carbon skeleton of cholesterol. Differences within these C27 bile acids are associated with number, position, and configuration of hydroxyl groups and with side-chain unsaturation. In addition, some C2g bile acids have been found in bile of a few species of amphibians. Cholenoic acid derivatives have not been found in lower vertebrates. A list of primitive bile acids in lower vertebrates is given in Table 2. 

The natural distribution and the chemical structure of bile alcohols and primitive bile acids indicate that these compounds found in lower vertebrates are evolutionary precursors of the common bile acids found in mammalian species. Haslewood proposed that the mechanism of conversion of cholesterol to the common bile acids in mammals is a recapitulation of the evolution of bile salts and thus would entail the intermediary formation of bile alcohols and primitive bile adds similar to or the same as those found in lower species [120]. Thus, studies have been carried out to test whether the naturally occurring bile alcohols and primitive bile acids are intermediates in the biosynthetic pathway between cholesterol and the C24 bile acids in mammals. There is no doubt that such studies contributed to the elucidation of the sequence of reactions in the biosynthesis of the mammalian C24 bile acids. 

Studies on the biosynthesis of bile alcohols and primitive bile acids in lower vertebrates will provide additional information on the formation of the common bile acids in mammals and will help to clarify the molecular evolution of bile salts. However, such studies have been carried out only in a few species of lower vertebrates. 

The conversion of cholesterol to primitive bile acids in lower vertebrates was first demonstrated in 1959 by Briggs et al. in Alligator mississippiensis [163]. [26- C]Cholesterol was given to a bile fistula alligator, and two labeled products were isolated from bile. The major product was identified as 3a,7a,12a-trihydroxy-5j8-cholestan-26-oic acid, the major bile acid of the reptile, and the minor one was identical with the second bile acid, later identified as 3a,7a-dihydroxy-5 -cholestan-26-oic acid. 

Bile adds components of bile which serve as emuldfying agents. They are steroid carboxylic adds linked to taurine or glycine by a peptide bond. According to the constituent amino add, they are called glycocholic or taurocholic acids. Ihe former predominate in human and bovine bile, the latter in canine bile. B. a. are characteristic of mammals in lower vertebrates the same function is performed by Bile alcohols (see). In mammals, the pattern of B.a. is often spedfic for the spedes. 

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