Biliary excretion of a stretched bilirubin in UGT1A1-deficient (Gunn) and Mrp2-deficient (TR-) rats

Homorubins and Homoverdins

The syntheses are described for centrally expanded bilirubin analogs: b -homorubins with propionic and butyric acid groups in the positions corresponding to the propionic acids of bilirubin. Their syntheses were accomplished by coupling two equivalents of a reactive monopyrrole (5-(bromomethylene)pyrrolin-2-one) to a dipyrrylethane. The corresponding b -homoverdins and dehydro- b -homoverdins were prepared by dehydrogenating the rubins or their dimethyl esters using DDQ. As supported by NMR measurements and molecular mechanics calculations, the homorubins are found to engage in conformation-determining intramolecular hydrogen bonding between the dipyrrinone and carboxylic acid moieties. Likewise, the homoverdins are believed to favor intramolecularly hydrogen-bonded conformations.

Hepatic metabolism and biliary excretion of silymarin flavonolignans in isolated perfused rat livers: role of multidrug resistance-associated protein 2 (Abcc2)

Silymarin, an extract from seeds of Silybum marianum, is used by 8 to 33% of patients to self-treat chronic viral hepatitis C in the United States. Studies in humans and rodents suggest that biliary excretion of glucuronide and sulfate conjugates is the major route for silymarin's elimination. To determine the role of multidrug resistance-associated protein 2 (Mrp2) (Abcc2) in the biliary excretion of silymarin, the hepatobiliary disposition of the six major silymarin flavonolignans was studied using isolated perfused livers (IPRLs) from wild-type (WT) and Mrp2-deficient (TR(-)) Wistar rats. For all the flavonolignans, approximately 96% of the dose was cleared from perfusate within 30 min in both WT and TR(-) livers, and <5% of parent was recovered in bile or perfusate by the end of the perfusion. In WT livers, the percentage of dose excreted as conjugates into bile varied for each flavonolignan (silychristin, 51.6 +/- 9.3%; silydianin, 101.5 +/- 28.3%; silybin A, 21.0 +/- 8.3%; silybin B, 31.7 +/- 13.2%; isosilybin A, 50.5 +/- 23.6%; isosilybin B, 42.8 +/- 19.3%). Among the flavonolignans, only silydianin was primarily glucuronidated and almost completely recovered in bile. In TR(-) livers, biliary excretion of flavonolignan conjugates was reduced 80 to 92%, with 30 to 83% of each flavonolignan conjugate recovered in perfusate compared with only 5 to 30% at 90 min. Biliary excretion of glucuronide and sulfate conjugates of all the flavonolignans were reduced by 94 to 98% and 73 to 84%, respectively, in TR(-) IPRLs. These data indicate a primary role for Mrp2 in the biliary elimination of silymarin flavonolignan conjugates.

Toward an amphiphilic bilirubin: the crystal structure of a bilirubin e-isomer

A new bilirubinoid analog (1) with two methoxy beta-substituents on the lactam ring of each dipyrrinone was synthesized and examined spectroscopically. It is more soluble in CH3OH and CHCl3 than bilirubin, which is insoluble in CH3OH but soluble in CHCl3. The solubility of 1 is approximately 10 microg/mL in CH3OH (vs < or =1 microg/mL for bilirubin) and approximately 3 mg/mL in CHCl3 (vs approximately 0.6 mg/mL for bilirubin). Vapor pressure osmometry indicates that 1, like bilirubin, is monomeric in CHCl3, and NMR studies show that the most stable structure has the syn-4Z,syn-15Z configuration, with the pigment's dipyrrinones engaged in intramolecular hydrogen bonding to the propionic acid carboxyl groups. And, like bilirubin, Z,Z-1 adopts a conformation that is bent in the middle into a ridge-tile shape. For the first time, a crystal structure of a bilirubin E-isomer has been obtained. Crystallization of 1 under dim room lighting gave an X-ray quality crystal of the anti-4E,syn-15Z-(photo) isomer, in which only the Z-dipyrrinone half is engaged in intramolecular hydrogen bonding to a propionic acid. Hydrogen bonding is nearly completely disengaged in the E-dipyrrinone half; yet, the ridge-tile conformation persists.

Influence of Conformation and Intramolecular Hydrogen Bonding on the Acyl Glucuronidation and Biliary Excretion of Acetylenic Bis-Dipyrrinones Related to Bilirubin

Glucuronidation and transporter-mediated efflux into bile are important in the elimination of xeno- and endobiotics, including the natural biladienone pigment bilirubin. The mechanisms of these processes and the structural factors that dictate whether cholephilic compounds are excreted directly in bile or require prior glucuronidation are poorly understood. To investigate effects of molecular shape and intramolecular hydrogen bonding on the interplay between direct excretion and glucuronidation in the liver, we studied a series of novel synthetic exploded and homologated bilirubin analogues. These include dicarboxylic mono- and diacetylenic tetrapyrroles with linear shapes that are unable to adopt the folded ridge-tile conformations that are crucially important in bilirubin metabolism. Intramolecular hydrogen bonding was varied by adjusting the alkyl chain lengths of the pendent carboxyl groups, and preferred conformations were predicted by molecular dynamics calculations. Metabolism studies were done in rats, including Gunn rats, congenitally deficient in UGT1 glucuronosyl tranferases, and TR- rats, deficient in the canalicular transporter Mrp2 (Abcc2). The results show strikingly that minor, seemingly inconsequential, changes in constitution, amplified by their influence on hydrogen bonding and molecular conformation, can profoundly influence competing clearance pathways in the liver, an effect that is unlikely to be restricted to bis-dipyrrinone carboxylic acids. Exposed carboxyl groups seem to favor the direct route of elimination, whereas the potential for carboxyl infolding by hydrogen bonding seems to favor glucuronidation. The results also show that molecular shape is less important in the hepatic glucuronidation and biliary excretion of bilirubin and of this series of acids than the capacity for intramolecular hydrogen bonding.

Hepatobiliary excretion of dipyrrinone sulfonates in Mrp2-deficient (TR(-)) rats

The biliary excretion of the sodium salts of 8-(2-ethanesulfonic acid)-3-ethyl-2,7,9-trimethyl-1,10-dihydro-11H-dipyrrin-1-one (xanthosulfonic acid) and a fluorescent analogue (8-desethyl-N,N'-carbonyl-kryptopyrromethenone-8-sulfonic acid) was compared in Mrp2-deficient (TR(-)) and normal rats. Both organic anions were excreted rapidly in bile in Mrp2-deficient rats, but the biliary excretion of the fluorescent sulfonate was impaired relative to normal controls. The rat clearly has efficient Mrp2-independent mechanisms for biliary efflux of these anions that are not used by bilirubin or its mono- and diglucuronides.

Hepatobiliary excretion of biliverdin isomers and C10-substituted biliverdins in Mrp2-deficient (TR(-)) rats

Multidrug resistance protein 2 (Mrp2) is considered the major mammalian membrane transporter of non-bile salt organic anions from liver to bile. Using Mrp2-deficient rats, we show that the protein is not essential for biliary excretion of biliverdin, its IIIalpha and XIIIalpha isomers, mesobiliverdin XIIIalpha or biliverdins bearing bulky lipophilic groups that are not reduced by biliverdin reductase in vivo. Yet, Mrp2 deficiency does retard the biliary excretion of these verdins to different degrees. The data indicate that there are Mrp2-independent mechanisms in the rat for biliary excretion of dicarboxylate organic anions related to biliverdin.