Cholestan-26-oic acid

Hvdroxvlation pathway An alternative explanation for the bile acid synthetic defect in CTX has been proposed by Oftebro and colleagues which starts via 26-hydroxylation of 5P-cholestane-3a,7a,12a-triol (IX, Fig. lOa and 10b). In this pathway the mitochondrial fraction of both human and rat liver contains a 26-hydroxylase enzyme (63) which can convert 5P-cholestane-3a,7a,12a-triol (IX ) to 5P-cholestane-3a,7a,12a,26-tetrol (XI) (Fig. 10a and 10b ). This tetrol is oxidized to 3a,7a,12a-trihydroxy-5P-cholestan-26-oic acid (THCA, XII) by liver cytosol (2,64). Further hydroxylation at C-24 forms varanic acid (XIV) and its side chain is shortened with oxidation at C-24 to yield cholic acid (X,Fig. 10 a). These investigators demonstrated diminished mitochondrial 26-hydroxylation of 5p-cholestane-3a,7a,12a-triol and 5P-cholestane-3a,7a-diol, possible precursors for cholic acid and chenodeoxycholec acid in CTX liver. As a consequence, neither 26-hydroxylated intermediates can be formed so that total primary bile acid synthesis would be diminished. Accordingly, the accumulation of 5P-cholestane-3a,7a,12a,25-tetrol arises from 25-hydroxylation of 5P-cholestane-3a,7a,12a-triol by the alternative microsomal 25-hydroxylation mechanism. 

Hydroxylation pathway. The mitochondrial fraction of both human and rat liver contains a 26-hydroxylase enzyme (B23), which can convert 5fi-cholestane-3a, 7a, 12a-triol (V) to 5ff-cholestane-3a,7o,12o,26-tetrol (VII) (Fig. 4). This tetrol is oxidized to 3a,7a,12a-trihydroxy-5p-cholestan-26-oic acid (THCA, VIII) by liver cytosol (C5). Further hydroxylation at C-24 forms varanic acid (IX) and its side chain is then shortened with oxidation at C-24 to yield cholic acid (X) (Fig. 4). 

D., and Sharp, H. L., The metabolism of 3a,7a, 12a-trihydroxy-5R-cholestan-26-oic acid in two siblings with cholestasis due to intrahepatic bile duct abnormalities. /. Clin. Invest. 56, 577-587 (1975). 

Kase, F., Bjorkhem, I., and Pedersen, J. I., Formation of cholic acid from 3a,7a,12a-trihydroxy-5P-cholestan-26-oic acid by rat liver peroxisomes. J. lipid Res. 24,1560-1567 (1983). 

Deoxy-5j8-cyprinol, 5 -choIestane-3o,7a,12a,26-tetrol, is present in some species of frogs [9,19] and toads [9,33]. Two stereoisomers at C-25 of this bile alcohol have been prepared from (257 )- and (25S)-3a,7a,12a-trihydroxy-5)S-cholestan-26-oic acids, respectively [13]. 

SS)-3a,7a,12a-Trihydroxy-5)S-cholestan-26-oic acid 3/8,7a,l 2a-Trihydroxy-5)S-cholestan-26-oic acid 3a,7a,12a-Trihydroxy-5a-cholest-23-en-26-oic acid (25/ )-3a,7a.l2a-Trihydroxy-5/S-cholest-23-en-26-oicacid 3 a,7a. 12 a-T rihydroxy-5j8-cholest-24-en-26-oic acid 3a,7a,24-Trihydroxy-5 8-cholestan-26-oicacid 3a,12a,22-Trihydroxy-5/8-cholestan-26-oicacid 3 a,7a-Dihydroxy-5)8-cholestan-26-oic acid 3 a,7a-Elihydroxy-5j8-cholest-23-en-26-oic acid 3a,7a-Dihydroxy-5y8-choIest-24-en-26-oic acid 3a,7 8-Dihydroxy-5/ -cholestan-26-oic acid 3/8,7/3-Dihydroxy-5/S-cholestan-26-oic acid 3 a. 12 a- Dihydroxy-5/8-cholestan-26-oic acid 3/8,12 a-Dihydroxy-5/8-cholestan-26-oic acid 3a-Hydroxy-5)3-cholestan-26-oic acid 3/8-Hydroxy-5/8-cholestan-26-oic acid 3a,12a-Dihydroxy-7-oxo-5j8-cholestan-26-oic acid 7a,12a-Dihydroxy-3-oxo-5a-cholestan-26-oic acid 7a, 12 a-Dihydroxy-3-oxo-5)6-cholestan-26-oic acid... 

The bile of Arapaima gigas, whose major bde salts are sulfate esters of arapaimol-A and -B, contains small amounts of a higher bile acid, arapaimic acid [8]. Spectral analysis indicates that arapaimic acid is the bile acid corresponding to arapaimol-B, 2j8,3a,7a,12a-tetrahydroxy-5/ -cholestan-26-oic acid. 

The major bile acid in all turtles and tortoises examined is 3a,7a,12a,22-tetrahy-droxy-5/8-cholestan-26-oic acid [51,52]. This higher bile acid may be considered a characteristic bile acid of Testudinidae. 

Haslewood and Wootton isolated varanic acid, 3a,7a,12a,24-tetrahydroxy-5)S-cholestan-26-oic acid, from bile of the monitor lizard, Varanus niloticus [53]. Varanic acid was later found in bile of the frog, Bombina orientalis, where it occurs in the unconjugated form [18]. Une et al. synthesized 4 diastereoisomers at C-24 and C-25 of 3a,7a,12a,24-tetrahydroxy-5y3-cholestan-26-oic acid. Comparisons with these synthetic bile acids of known absolute configuration showed that the varanic acid in frog bile is (24i ,25S )-3a,7a,12a,24-tetrahydroxy-5jS-cholestan-26-oic acid [54]. 

In 1939, Kurauti and Kazuno isolated a major higher bile acid from the bile of the bullfrog, Rana catesbeiana [55]. This bile acid was later isolated from bile of Alligator mississippiensis [56]. Its structure was deduced as a 3a,7a,12a-trihydroxy-5 -cholestan-26-oic add by its degradation to cholic add [57] and by its conversion to the stem acid, 5)8-cholestan-26-oic acid [58]. 

Okuda et al. isolated a higher bile acid from the bile of Iguana iguana [66]. The structure was shown to be 3a,7 ,12a-trihydroxy-5a-cholestan-26-oic acid by lithium aluminum hydride reduction to 27-deoxy-5 -cyprinol [66]. The 5a-C27 bile acid was also partially synthesized from 27-deoxy-5a-cyprinol [67]. 3a,7a,12a-Trihydroxy-5a-cho estan-26-oic acid also occurs in bile of some species of frogs [18,19] and the alligator [68],... 

The 12-deoxy derivative of varanic acid, 3a,7a,24-trihydroxy-5)8-cholestan-26-oic acid was detected in the bile of Varanus monitor, as a minor companion of varanic acid, the major bile acid of this lizard [69]. 

The presence of small amounts of 3a,12 ,22-trihydroxy-5i8-cholestan-26-oic acid as taurine conjugate in Chelonia mydas was reported by Haslewood et al. [52]. This bile acid is the 7-deoxy derivative of 3 ,7a,12a,22-tetrahydroxy-5)8-cholestan-26-oic acid, the major bile acid of this turtle, and is most likely formed by bacterial 7 -dehydroxylation (Chapter 12). 

A trihydroxycholestenoic acid was isolated as a major bile acid from bile of the toad, Bufo vulgaris formosus [70] and was characterized as (25/ )-3a,7a,12o(-trihy-droxy-5/8-cholest-23-en-26-oic acid by oxidation to 23-norcholic acid and hydrogenation to (25/ )-3a,7a,12a-trihydroxy-5j8-cholestan-26-oic acid [71]. The 5a isomer, 3a,7a,12a-trihydroxy-5a-cholest-23-en-26-oic acid, was recently detected as a minor bile acid in toad bile [46]. The bile of Varanus monitor contains as minor constituents 3a,7a,12a-trihydroxy-5j8-cholest-23- and 24-enoic acids [69]. The acid has been synthesized prior to its detection in nature [72]. 

Three higher bile acids possessing a keto group were detected in the bile of Alligator mississippiensis, and characterized as 7a,12a-dihydroxy-3-oxo-5)8-cholestan-26-oic acid, 3a,12a-dihydroxy-7-oxo-5/3-cholestan-26-oic acid, and 7a,12a-dihy-droxy-3-oxo-5a-cholestan-26-oic acid [68]. Nothing is known about the formation of these bile acids. 

Although small amounts of the biliary bile acids, 3a,7 ,12a-trihydroxy- and 3a,la-dihydroxy-5)3-cholestan-26-oic acids, were detected, the major fecal bile acids were their 7-deoxy derivatives, 3 ,12 -dihydroxy- and 3a-hydroxy-5 8-cholestan-26-oic acids. Small amounts of 3j8,7a,12a-trihydroxy-5 -cholestan-26-oic acid, 3a,7 -, 3j8,7 -and 3jS,12a-dihydroxy-5)S-cholestan-26-oic acids, and 3 8-hydroxy-5j3-cholestan-26-oic acid were found as well [75]. Since intestinal bacterial in mammals are known to 7a-dehydroxylate C24 bile acids and to interconvert a- and j8-hydroxyl groups (Chapter 12), these C27 bile acids may be products of the intestinal flora in the alligator. 

In 1963, Carey and Haslewood isolated trace amounts of (25/ )-3a,7 ,12a-trihy-droxy-5 8-cholestan-26-oic acid from human fistula bile [96]. The stereochemistry at C-25 of this bile acid was recently confirmed by direct comparison with reference compounds of known absolute configuration [97], This trihydroxy-5j8-cholestanoic acid also occurs in baboon bile [98]. Hanson and Williams found the corresponding dihydroxy bile acid, 3 ,7a-dihydroxy-5 S-cholestan-26-oic acid, in human bile [99]. The occurrence of these higher bile acids, quantitatively of minor importance, is of interest because they are biosynthetic precursors of two primary bile acids of mammalian species, cholic acid and chenodeoxycholic acid, respectively (Chapter 9). 

Eyssen et al. found large quantities of 3 ,7a,12a-trihydroxy-5i8-cholestan-26-oic acid in the duodenal fluid from two unrelated infants with intrahepatic bile duct anomalies [100]. Hanson et al. have confirmed the presence of this bile acid in elevated amounts in bile, serum, urine, and feces from 2 siblings with the same syndrome [101], Considerable amounts of 3a,7a-dihydroxy-5 -cholestan-26-oic acid were found by Parmentier et al. in bile, serum, urine, and feces from 3 other children with the same syndrome [102], Varanic acid was also detected in the bile [102]. In addition, the serum of these patients contained significant amounts of an unusual C,9 bile acid and small amounts of 3a-hydroxy-5 8-cholestan-26-oic acid and 3 -hy-droxycholest-5-en-26-oic acid [102,103]. The structure of the C29 bile acid was shown to be 3a.7a,12a-trihydroxy-27a,27h-dihomo-5]3-cholestane-26,27h-dioic acid (3a,7a,12a-trihydroxy-27-carboxymethyl-5/3-cholestan-26-oic acid) [104]. 

Clayton et al. found small amounts of 2 tetrahydroxy C27 bile acids along with a number of C24 bile acids in gastric contents from neonates with high intestinal obstruction [110]. One of the higher bile acids was identified as 3a,7a,12 ,25-tetra-hydroxy-5 -cholestan-26-oic acid another was tentatively identified as 3a,7 ,12a,26-tetrahydroxy-5)8-cholestan-27-oic acid by its reduction with lithium aluminum hydride to 5]3-cyprinol (5j8-cholestane-3 ,7a,12a,26,27-pentol) [110]. 

Masui and Staple [126] and Gustafsson [127] isolated labeled varanic acid (3a,7 ,12a,24-tetrahydroxy-5 -cholestan-26-oic acid) (XII) after incubation of [ H]3 ,7 ,12a-trihydroxy-5j8-cholestan-26-oic acid with rat liver preparations. This acid is converted to cholic acid in rat and human in vivo [126,128] as well as in rat... 

The major pathway for the formation of chenodeoxycholic add is thought to be the same as that for cholic acid, with the exception that no 12a-hydroxylation occurs. Thus, 3a,7a-dihydroxy-5i8-cholestan-26-oic acid is a probable intermediate, and Hanson has shown that this acid can be made from cholesterol and is effidently converted to chenodeoxycholic acid but only to a very limited extent to cholic acid [131]. 

In the in vivo studies with labeled cholesterol [33] as well as mevalonate [157], the label was incorporated into the minor bile acids of the toad, cholic acid (XIV) and 3a,7a,12a-trihydroxy-5y3-cholestan-26-oic acid (X). In contrast, the major bile acids, 3a,7 ,12 -trihydroxy-5/3-cholest-22-ene-24-carboxylic acid and 3a,7a,12a-trihy-droxy-5)8-cholest-23-en-26-oic acid, did not become labeled and their biochemical origin is still obscure. The formation of labeled cholic acid and trihydroxy-5j8-cholestanoic acid suggests that in the toad 27-deoxy-5j8-cyprinol (VIII) is converted to cholic acid (XIV) via the C27 bile acid (X) by the same pathway as that in mammals. 

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. 

The two diastereoisomers of 3a,7a,12a-trihydroxy-5/3-cholestan-26-oic acid (4), isomeric at C-25, have been distinguished by AT-ray crystallographic analy-sis. " It should now be possible to establish the configurations of samples obtained from natural sources. The earlier literature contains apparently conflicting evidence, which may result from the possibility of equilibration at C-25 during hydrolytic steps in the isolation of the natural material, thought to be a key biosynthetic intermediate between cholesterol and cholic acid. 

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