Ursodeoxycholic acid 6/3-hydroxylation

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. 

Recent investigations into the mechanism of action of these bile acids indicate that ursodeoxycholic acid has certain advantages over chenodeoxycholic acid in the context of the overall homeostasis of cholesterol metabolism (F6). In contrast to chenodeoxycholic acid, ursodeoxycholic acid does not suppress bile acid synthesis (H7), possibly because the a-orientation of the 7-hydroxyl group of chenodeoxycholic acid is required to inhibit cholesterol 7a-hydroxylase activity. Thus, cholesterol breakdown into bile acids is not reduced by ursodeoxycholic acid. Other favorable factors are that ursodeoxycholic acid has a reduced capacity to solubilize cholesterol into micellar solution compared to chenodeoxycholic acid and intestinal cholesterol absorption is decreased by this bile acid (F6, H7). However, in gallbladder bile the relative limitation of ursodeoxycholic acid for micellar solubilization of cholesterol is compensated for by an ability to transport... 

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 physiological role for the conversion of chenodeoxychoUc acid to ursodeoxycholic acid by C. absonum or other intestinal bacteria is unknown. However, ursodeoxychoUc acid is more polar than chenodeoxycholic acid (Chapter 13) and should be less toxic to bacterial cell membranes than the latter. Indeed, C. absonum will readily grow in media containing 1 mM ursodeoxychoUc acid but not 1 mM chenodeoxychoUc acid [42]. Therefore, the epimerization of the axial 7 -hydroxyl group may represent a detoxication process for C. absonum and would provide a physiological explanation for these enzymes. 

A derivative of ursodeoxycholic acid with poly(ethyleneglycol) of M 200 was prepared in a way similar to that previously reported in the case of Ibuprofen derivatives. This derivative was esterifi ed only at one end, and, consequently, had a primary hydroxyl group at the opposite end ... 

More recently, Carrea s group [14] showed a similar transformation (a/(3 inversion) at the C-7 hydroxyl position of different steroid derivatives. Steroids hydroxylated at C-7 are not so commonly found in nature and are mainly related to the bile acid family. Nevertheless, they do have important pharmaceutical applications due to their ability to dissolve cholesterol gallstones, thus avoiding surgery [15]. This property seems to be displayed to a greater extent by ursodeoxycholic acid 52. 

The first studies of specificity were carried out using cholate, the glycine and taurine conjugates and taurine conjugates of the dihydroxy bile acids cheno-deoxycholate and ursodeoxycholate. Kramer and colleagues prepared plasma membrane vesicles from rat liver and compared bile-acid transport with values from CHO cells stably expressing NTCP. This work established that transport by the liver enzyme was maximal when 2 hydroxyls were present,... 

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). 

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