Development of highly sensitive methods for the determination of activated carboxylic acids in biological specimens

In the human body, endogenous biologically active substances and xenobiotics that contain a carboxy group are known to transform various metabolites. Commonly, these carboxylic acids are activated to form the corresponding intermediates, such as coenzyme A (CoA) thioesters, key intermediates in reactions that result in the transfer of an acyl group, including conjugation with amino acids, chiral inversion of 2-methyl branched carboxylic acids, and β-oxidation and elongation of fatty acids. During the formation of CoA thioesters, acyl adenylates have also been identified as intermediates. These activated forms of carboxylic acid generally have high chemical reactivities and play important roles in a variety of biochemical processes, such as energy transduction, biosynthesis, and metabolism. On the other hand, conjugation with glucuronic acid takes place through carboxy groups as well as hydroxy groups, amino groups, and mercapto groups. Glucuronidation, which converts lipophilic substances into water-soluble forms, plays a significant role in the metabolism and disposition of xenobiotics and endogenous compounds. This conjugation process, which is catalyzed by UDP-glucuronosyltransferase (UGT), has long been considered to be an important detoxification mechanism. Recent observations, however, indicate that certain carboxylic acid substrates of UGTs become more toxic upon glucuronidation [1]. Furthermore, recent evidence indicates that ester-type glucuronides, namely acyl glucuronides, react with nucleophiles, such as amino groups on some proteins, to produce covalently bound adducts [2, 3]. These reactions involve the displacement of the glucuronic acid moiety with the nucleophile. Alternatively, such adducts also form after the migration of an acyl group on the sugar moiety and tautomerization for conversion into the aldose form, which is readily condensed with nucleophiles [4, 5]. The structures of these adducts are similar to those of hapten-carrier protein conjugates, which are used for immunizations with small molecules. The formation of these adducts has attracted attention as a possible explanation for hypersensitivity and adverse reactions to carboxylic acids [6, 7]. Bile acids are endogenous carboxylic acid derivatives with a steroid nucleus and a side chain at the C-17β position. The comFocusing Review