Lithocholic acid 7«-hydroxylation

Problem 27.9 Lithocholic acid is an A-B cis steroid found in human bile. Draw lithocliolic acid showing chair conformations as in Figure 27.11, and tell whether the hydroxyl group at C3 is axial or equatorial. 

Bile acids within the enterohepatic circulation that undergo absorption in the terminal ileum encounter a relatively low number of species and population of bacteria and return to the liver in portal blood relatively unchanged. However, the approximately 5% of the bile-acid pool that enters the colon provides substrate for the extensive microbial population that deconjugate and oxidise hydroxyl groups leading to formation of the secondary bile acids deoxycholic and lithocholic acids that are the major bile acids in faeces. 

A and B are in cis position relative to each other (see p. 54). One to three hydroxyl groups (in a position) are found in the steroid core at positions 3, 7, and 12. Bile acids keep bile cholesterol in a soluble state as micelles and promote the digestion of lipids in the intestine (see p.270). Cholic add and cheno-deoxychoMc acid are primary bile acids that are formed by the liver. Their dehydroxylation at C-7 by microorganisms from the intestinal flora gives rise to the secondary bile acids lithocholic acid and deoxycholic acid. 

Bacteria in the intestine can remove glycine and taurine from bile salts, regenerating bile acids. They can also convert some of the primary bile acids into "secondary" bile acids by removing a hydroxyl group, producing deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid (Figure 18.11). 

The C>4 bile acids arise from cholesterol in the liver after saturation of the steroid nucleus and reduction in length of the side chain to a 5-carbon add they may differ in the number of hydroxyl groups on the sterol nucleus. The four acids isolated from human bile include cholic acid (3,7,12-tiihydroxy), as shown in Fig. 1 deoxycholic acid (2,12-dihydroxy) chenodeoxycholic acid (3,7-dihydroxy) and lithocholic acid (3-hydroxy). The bile acids are not excreted into the bile as such, but are conjugated through the C24 carboxylic add with glycine or... 

Bile Acids and Alcohols. Bile acids have been detected in all vertebrates that have been examined and are a result of cholesterol metabolism. The C24 acid, 5(8-cholatiie acid (9) is the structural derivative uf the majority of bile acids in vertebrates. Most mammalian bile acids have a cis-fused A-B ring junction resulting in a nonplanar steroid nucleus. Bile acids, like sterols, typically contain a C3a-hydroxyl group (lithocholic acid 3a-hyroxycholanic acid. 

Scalia and Games developed a packed column SFC method for the analysis of free bile acids cholic acid (CA), chenodeoxycholic acid (CDCA), deoxycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA) [32]. The baseline separation of all five bile acids was achieved on a packed phenyl column with a methanol-modified carbon dioxide in less than 4 min. The elution order showed a normal-phase mechanism because the solutes eluted in the order of increasing polarity following the number of hydroxyl groups on the steroid nucleus. The method was also applied to the assay of UDCA and CDCA in capsule and tablet formulations. The method was found to be linear in the range 1.5-7.5 ng/ml (r > 0.99, n = 6). The average recoveries (n= 10) for UDCA and CDCA were 100.2% with a RSD of 1.7% and 101.5% with a RSD of 2.2%, respectively. The reproducibility of the method was less than 1.5% (n = 10) for both UDCA and CDCA. 

The fungus Helirastylum piriforme ATCC 8992 is cultivated for 3 d at 28 "C in a medium (pH 6.8-7.0) consisting of 3.5 % glucose. 2% peptone and 0.3 % corn steep liquoT in tap water, on a rotary shaker. The wet mycelium harvested from the culture broth (100 ml) is suspended in 100 mL of deionized water and 50 mg (0.13 mmol) of finely divided lithocholic acid (3) are added. The mixture is incubated for 2 d at 28 JC on a shaker. Extraction of the incubation mixture with EtOAc yields a mixture of hydroxylation products. The principal component 4 is purified by column chromatography on silica gel and isolated by crystallization from EtOAc and then from acetone yield 20 mg ( 40%) mp 171.5- 172.5 C. 

A modified Huang-Minlon reduction was found advantageous in the conversion of methyl eholate to lithocholic acid without purification of intermediates. The 3-hydroxyl group (equatorial) is selectively succinoylated, the hydroxyl functions... 

In the intestine, the vitamin D receptor also funetions as a receptor for lithocholic aeid, a bile aeid produced by the action of intestinal baeteria on endogenously produced ehenoeholie aeid. Lithocholic acid is hepato-toxic and may be a carcinogen. In the intestine, vitamin D receptors, activated by vitamin D and possibly lithocholic acid, induce both liver and intestine cytochrome P450 3A oxidase (CYP 3A). This enzyme metabolizes lithoeholie aeid to inactive hydroxylated forms. The proeess of inducing CYP 3A might... 

Bile acids contain hydroxyl groups, which are usually substituted at positions, C-3, C-7, or C-12 of the steroid nucleus. The three major bile acids found in man are 3a,7a,12a-trihydroxy-5P-cholan-24-oic acid 3a,7a-dihydroxy-5p-cholan-24-oic add and 3a,12a-dihydroxy-5p-cholan-24-oic acid. Because of the complexities of steroid nomenclature, bile acids are nearly always referred to by trivial names. 11108, the three major human bile acids are named cholic acid, chenodeoxycholic acid, and deoxycholic acid, respectively, and their chemical structures are shown in Fig. 1. Human bile does, however, contain small amounts of other bile acids, such as lithocholic acid (3a-hydroxy-5P-cholan-24-oic add) and ursodeoxycholic add (3a,7p-dihydroxy-5p-cholan-24-oic acid) (see Fig. 1). 

Other precursors of the muricholates via 6)8-hydroxylation include 5)8-cholanic acid [78], lithocholic acid [84-86], 7-oxolithocholic acid [95,96], and ursodeoxycholic acid (3a,7j6-dihydroxy-5/3-cholanic acid) [97], The rat metabolized 12a-hydroxy-5/S-cholanic acid to 6/S,12a-dihydroxy-5)S-cholanic acid [83] and a small amount of 6)3,7a,12a-trihydroxy-5 -cholanic acid [98]. 3a,6)8,12a-Trihydroxy-5)8-cholanic acid was isolated from urine of surgically jaundiced rats after administration of de-oxycholate [99]. A series of bile acids from rat bile of unconfirmed structures but containing the 6/3,7/3-diol will be reviewed in Section II1.3. 

The intestinal microflora of man and animals can biotransform bile acids into a number of different metabolites. Normal human feces may contain more than 20 different bile acids which have been formed from the primary bile acids, cholic acid and chenodeoxycholic acid [1-5], Known microbial biotransformations of these bile acids include the hydrolysis of bile acid conjugates yielding free bile acids, oxidation of hydroxyl groups at C-3, C-6, C-7 and C-12 and reduction of oxo groups to give epimeric hydroxy bile acids. In addition, certain members of the intestinal microflora la- and 7j8-dehydroxylate primary bile acids yielding deoxycholic acid and lithocholic acid (Fig. 1). Moreover, 3-sulfated bile acids are converted into a variety of different metabolites by the intestinal microflora [6,7]. 

Exact positions for hydroxyl glucuronides are given when known. Abbreviations LA, lithocholic acid atRA, all frams-retinoic acid. 

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