Conjugated bile acids metabolism

Mass spectrometry has become an indispensable method for the analysis of bile acids by virtue of its power to identify, assign structure and quantify free or conjugated bile acids, either pure or in mixtures. It is useful not only to study the metabolism of bile acids but also for the detection and diagnosis of metabolic diseases. Indeed, numerous metabolic diseases resulting from an alteration of the conversion of cholesterol to bile acids have been described, including peroxisomal disorders resulting in a block of (3-oxidation of the lateral chain and other enzyme deficiencies interfering with the biochemistry of the side chain or the steroid nucleus. 

Hardison, W. G-, and Grundy, S. M. l 83). tffcci of bile acid conjugation pattern on bile acid metabolism in normal humans. GusfrOfMffwtogy 84, 617-620. 

The bile acids are 24-carbon steroid derivatives. The two primary bile acids, cholic acid and chenodeoxycholic acid, are synthesized in the hepatocytes from cholesterol by hy-droxylation, reduction, and side chain oxidation. They are conjugated by amide linkage to glycine or taurine before they are secreted into the bile (see cholesterol metabolism. Chapter 19). The mechanism of secretion of bile acids across the canalicular membrane is poorly understood. Bile acids are present as anions at the pH of the bile, and above a certain concentration (critical micellar concentration) they form polyanionic molecular aggregates, or micelles (Chapter 11). The critical micellar concentration for each bile acid and the size of the aggregates are affected by the concentration of Na+ and other electrolytes and of cholesterol and lecithin. Thus, bile consists of mixed micelles of conjugated bile acids, cholesterol, and lecithin. While the excretion of osmotically active bile acids is a primary determinant of water and solute transport across the canalicular membrane, in the canaliculi they contribute relatively little to osmotic activity because their anions aggregate to form micelles. 

Another type of conjugation occurring with bile acids involves the hydroxyl groups. These may form sulfate esters or conjugates with glucuronic acid, usually at the 3a position. The formation of sul te esters was first demonstrated as a possible bile acid metabolic pathway in man by Palmer and Bolt (P3). These bile acids can be detected as excretion products in the urine (A6). 

The effects of pharmacological agents on bile acid metabolism may be assessed in several ways. The total excretion of bile acids in the bile or feces can be measured, the spectrum of bile acids or their conjugates can be determined, or the synthesis of bile acids can be assessed. To date, most experimental approaches have involved the first of these possibilities. 

TABLE V. Metabolism of Free and Conjugated Bile Acids by Microorganisms Belonging Within Genera Often... 

In addition, three types of lipophilic conjugates have been found in pyrethroid metabolism studies (Fig. 4). They are cholesterol ester (fenvalerate) [15], glyceride (3-PBacid, a common metabolite of several pyrethroids) [16], and bile acid conjugates (fluvalinate) [17]. It is noteworthy that one isomer out of the four chiral isomers of fenvalerate yields a cholesterol ester conjugate from its acid moiety [15]. This chiral-specific formation of the cholesterol ester has been demonstrated to be mediated by transesterification reactions of carboxylesterase(s) in microsomes, not by any of the three known biosynthetic pathways of endogenous cholesterol esters... 

D. J. Sweeney, S. Barnes, R. B. Diasio, Formation of Conjugates of 2-Fluoro-/3-alanine and Bile Acids during the Metabolism of 5-Fluorouracil and 5-Fluoro-2-deoxyuridine in the Isolated Perfused Liver , Cancer Res. 1988, 48, 2010-2014. 

Serum ALP and total bilirubin (unconjugated and conjugated fractions) are traditionally used to monitor cholestatic injury. The ALP families of enzymes are zinc metalloproteases that are present in nearly all tissues. In the liver, ALP is immu-nolocalized to the microvili of the bile canaliculus [124]. Increased synthesis of ALP and its release into the circulation occurs within hours of cholestatic injury [129]. Serum assays of 5 -nucleotidase (5 -NT) or y-glutamyltransferase activity (GGT) are used to confirm the liver as the specific origin for the elevation of ALP. Increases in serum bilirubin or bile acids are usually the result of bile retention subsequent to impaired bile flow, increased production associated with accelerated erythrocyte destruction, or altered bilirubin metabolism [129]. 

Before the bile acids leave the liver, they are conjugated to I molecule of either glycine or taurine (an end-product of cystene metabolism) by an amide bond between the carboxyl group of tit J bile acid and the amino group of the added compound. These netfl... 

As already pointed out, cysteine may be metabolized to pyruvate, or it can be oxidized to cystine. It can also be converted to taurine, NH3+-CH2-CH2-S03. Taurine is obtained by oxidizing the -SH group of cysteine and losing the carboxyl group of decarboxylation. Taurine is quite abundant in most tissues and is said to be the most abundant "amino acid" of the human organism. One of its functions is to conjugate primary bile acids (Chapter 19). 

The usefulness of low-energy MS/MS has been shown in the diagnosis of metabolic diseases by analysis of underivatized bile acids, conjugated or not, in complex biological samples such as urine or serum. A series of neutral loss and precursor ion scans was... 

Libert, R., Hermans, D., Draye, J.R et al. (1991) Bile acids and conjugates identified in metabolic disorders by fast atom bombardment and tandem mass spectrometry. Clin. Chem., 37, 2102-10. 

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