Hydroxylation of the steroid nucleus

Evidence for a specific nuclear hydroxylation of a bile acid is frequently associated with similar information pertinent to another position of the bile acid. Thus, the topics in this section are titled separately, but evidence may also be cited under headings for other nuclear positions e.g. )8-muricholic acid, a 3a,6yS,7 8-trihydroxy 

In a similar study of the metabolism of [24- C]12a-hydroxy-5)8-cholanic acid given intraperitoneally to male rats with bile fistulas [83], the identified biliary metabolites were 7a,12a-dihydroxy-5)8-cholanic acid (26%), deoxycholic acid (18%), cholic acid (15%), 6)8,12a-hydroxy-5)8-cholanic acid (0.8%), and 12% of unchanged 12a-hydroxy-5)8-cholanic acid. Thus, 5)8-cholanic acid and 12a-hydroxy-5)8-cholanic acids are hydroxylated in vivo preferentially in the order 7a, 3a, and 6)8. The preference for 7a-hydroxylation may be related to the concentration and properties of the active enzyme. Although no in vitro studies have been carried out, these studies infer the ability of hepatic tissue to provide characteristic 3a-hydroxy bile acids from derivatives devoid of a C-3 oxygen. How often this activity is required is questionable, because of the abundance of 3-hydroxylated sterol derivatives provided to and by the liver. 

The conversion of lithocholate to 3a,6/8-dihydroxy-5/8-cholanate by the rat [84-86], mouse [87] and chicken [88] confirmed the presence of an active hepatic 6)8-hydrox-ylase. Earlier studies reported the metabolism of chenodeoxycholate [89,90] in the rat and mouse [81,91,92] to a-murocholic acid (3a,6)8,7a-trihydroxy-5)8-cholanic acid) and to )8-muricholic acid [89,90,93,94] (3a,6)8,7)8-trihydroxy-5)8-cholanic acid). 

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. 

An uncertainty also exists with human hepatic 6a-hydroxylase. Hyodeoxycholate has not been isolated from human bile. Despite the above-reported conversion of taurolithocholate to taurohyodeoxycholate [112], investigators have been unable to demonstrate hydroxylation of conjugated or free mono- or dihydroxy bile acids with 

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Early in vitro studies showed that mitochondria from livers of hyperthyroid rats did not oxidize cholesterol-26- C to C02 at a faster rate than similar preparations from normal animals (12). A more recent study (13) led to the conclusion that the effects of thyroid hormones on bile acid metabolism must take place at a biosynthetic step preceding side-chain oxidation, perhaps involving hydroxylation of the steroid nucleus. However, it must be realized that the normal substrate for side-chain oxidation leading to the formation of cholic acid from cholesterol is not cholesterol itself but presumably 3a,7afl2a-trihydroxy-5/5-cholestane (14,15), and the substrate for the side-chain oxidation leading to chenodeoxycholate is, presumably, 3a,7a-dihydroxy-5/5-cholestane (16). Thus results of in vitro experiments in which cholesterol is employed as the substrate must be interpreted with caution, since mitochondria do not have the enzyme system required for formation of the triol and diol from cholesterol. 

Concomitantly, additional transformations may occur at other parts of the steroid nucleus, e.g., 1,2-dehydrogenation, 6 - or 9a-hydroxylation. ... 

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 salts are substances derived from sterols, which make up a substantial part of the solid matter in bile and which play a central role in lipid absorption, by virtue of their surface-active properties. The structure and properties of these salts have been reviewed by Haslewood (305) and Heaton (316). Bile salts essentially have molecules of detergent type hydrocarbon, with a fat-dissolving part and a polar, water-attracting part. The fat-dissolving part consists of the bulk of the steroid nucleus. The hydroxyl groups are so distributed that hydration can readily take place the remainder of the molecule will dissolve the fatty phase. Emulsification of fat/water complexes can thus occur easily. The terms bile acid and bile salt are used somewhat interchangeably in the literature. 

The enhancement of nasal absorption of insulin by hydrophobic bile salts has also been investigated. It was found that minor differences in the number, position, and orientation of the nuclear hydroxyl groups as well as alterations to side-chain conjugation can improve the adjuvant potency of bile salts. Moreover, the absorption of insulin positively correlated with an increase in the hydrophilicity of the steroid nucleus of the bile salts. In the presence of bile salts, nasal absorption of insulin reached peak levels within about lOmin, and some 10-20% of the dose was found to have been absorbed into the circulation. Marked increases in serum insulin levels were seen with sodium deoxycholate, the most lipophilic of the bile salts, whereas the least elevation—as well as least lowering of blood glucose levels—was seen with the most hydrophobic bile salt, sodium ursodeoxycholate [63],... 

Substituted cardenolides or cardenolides with epimeric configurations at several positions of the steroid nucleus have been made by appropriate modifications of known syntheses. In one of these, the steroid (510) was hydroxylated at C(14) by a microbiological method, acetylated, and reduced catalytically to the A/B-c/s-steroid (511). Sodium borohydride reduction gave the 3a-alcohol, which after acetylation and acid hydrolysis was transformed into the 3,14-di-... 

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). 

Figure 6.35). The proposed mechanism for the dominant reaction involves an unremarkable hydroxylation at C17 of the steroid nucleus (Figure 6.36). This is then followed by an attack of the ferric peroxo moiety on the carbonyl to yield a species that fragments to an alkoxy radical and a one-electron oxidized ferryl species. The alkoxy radical subsequently decomposes to produce acetic acid and a carbon radical that recombines with the ferryl species to yield a gem-diol, which dehydrates to the C17 carbonyl of the product. This mechanism is in accord with a wealth of labeling studies and can be modified simply to explain the origin of the other observed products. 

Much of the work on steroids has been involved with identifying carbonyl and hydroxyl groups, as well as unsaturations in various parts of the steroid nucleus. Jones et al. (1955) and Jones and Herling (1956) have given much information on correlations of steroid structures with their spectra. Some of the more important correlations are given here. 

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