Slipping Past UGT1A1 and Multidrug Resistance-Associated Protein 2 in the Liver: Effects of Steric Compression and Hydrogen Bonding on the Hepatobiliary Elimination of Synthetic Bilirubins

Abstract

ABSTRACT:The hepatobiliary metabolism and excretion of three isomeric bilirubin analogs with propanoic replaced by benzoic acid side-chains were studied in the rat. Despite their isomeric relationship and similar constitutions, the three analogs were metabolized quite differently from each other and from bilirubin. In the di-o-benzoic compound, steric hindrance involving the phenyl groups reinforces intramolecular hydrogen bonding of the two carboxyl groups. This compound is considerably less polar than bilirubin on reversephase high-performance liquid chromatography and, like bilirubin, was not excreted in bile in UGT1-deficient (Gunn) rats. But, quite unlike bilirubin, it was not glucuronidated or excreted in bile in normal rats. In contrast to both bilirubin and the di-o-benzoic isomer, the more polar m-and p-isomers, in which intramolecular hydrogen bonding of carboxyl groups is sterically difficult, were excreted rapidly in bile in unchanged form in both normal and Gunn rats. However, only one of them, the di-m-isomer, was excreted rapidly and unchanged in bile in rats (TR ؊ rats) congenitally deficient in the canalicular ATP-binding cassette transporter Mrp2. The marked differences in hepatobiliary metabolism of the three isomers studied can be rationalized on the basis of their computed three-dimensional structures and minimum-energy conformations and the remote effects of steric compression on intramolecular hydrogen bonding.Bilirubin (1) (Fig. 1) is formed in mammals by reduction of biliverdin (2), end product of most heme catabolism. It is insoluble in water, lipophilic, extensively protein-bound in plasma, and, unlike biliverdin, requires phase-2 metabolism, mainly to mono-and diglucuronides, for elimination. Once used as a liver function test (Harrop and Barron, 1931;Kornberg, 1942), bilirubin is a constituent of several Asian traditional medicines (McDonagh, 1990), and its antioxidant/anti-inflammatory properties are currently attracting attention (Stocker et al., 1987;Stocker, 2004; Ollinger et al., 2007a,b). Historically, bilirubins have been useful for investigating mechanisms of acyl glucuronidation, membrane transport, and hepatobiliary excretion. Bilirubin and its two monoglucuronides are isozyme-specific substrates for the glucuronosyl transferase UGT1A1 (Owens et al., 2005), and its three acyl glucuronides are classic examples of compounds that depend wholly on the ATP-binding cassette transporter MRP2 (ABCC2) for elimination in bile (Nies and Keppler, 2007). Deficiencies of UGT1A1 cause neonatal jaundice, Gilbert's and CriglerNajjar syndromes (Kaplan and Hammerman, 2005), and congenital deficiency of MRP2 underlies the rare Dubin-Johnson syndrome (Nies and Keppler, 2007). Studies on bilirubin glucuronides led to the discovery that acyl glucuronides of many drugs are reactive, potentially toxic, metabolites (Smith et al., 1986), and studies on bilirubin analogs have revealed how subtle changes in molecular conformation can profoundly influence hepatic metabolism (McDonagh and Lightner, 1991;...

Homorubins and homoverdins

“…Reversed phase performance liquid chromatography (HPLC) [10, 11] likewise similarly revealed very similar retention times for 2 and mesobilirubin. Homorubin 1 , while exhibiting the expected chromatographic behavior for a nonpolar rubin, appears to be slightly more polar than 2 ; yet, all these data (Table 6) point to good intramolecular hydrogen bonding in 1 and 2 , as is well-known for mesobilirubin.…”

“…Yellow 1 and 2 preferentially adopt folded, intramolecularly hydrogen-bonded conformations and exhibit a lipophilicity comparable to that of mesobilirubin-XIIIα. Like the last, hepatobiliary elimination of homorubin 1 in Sprague-Dawley rats [10, 11, 54] succeeds by formation of mono- and diglucuronides, as well (surprisingly) as intact [16]. The reddish b -homoverdins ( 3 and 4 ) corresponding to 1 and 2 , which can exist as (10 Z ) or (10 E ) diastereomers, prefer the latter and are stabilized by intramolecular hydrogen bonds.…”

“…1C), named “ridge-tile” [3], which minimizes non-bonded steric destabilizing interactions and found in the crystal [3–5] and solution [6–9]. It is more stable than all others, and as such it plays a dominating role in the pigment’s physico-chemical properties and metabolism [1, 10–14]. Analogs of bilirubin with vinyl groups reduced to ethyls, e.g., mesobilirubin-XIIIα (Fig.…”