Determination of bilirubin in liver homogenate with diazotized p-iodoaniline. Improvement of color recovery by incorporation of antioxidant (2,6-di-tert-butyl-p-cresol) in the reaction system

Investigation of the role of reactive oxygen species in bilirubin metabolism in the Gunn rat

It has been previously established that the attenuation of hepatic lipid peroxidation by a fat-free diet is accompanied by a marked rise in plasma bilirubin in Gunn rats. Present in vitro studies confirmed that microsomal lipid peroxidation caused the concurrent degradation of added bilirubin but failed to show that microsomal superoxide, hydroxyl radical or hydrogen peroxide would degrade bilirubin. Moreover, although injection of vitamin E completely inhibited microsomal lipid peroxidation and bilirubin degradation it had no effect on plasma bilirubin. No evidence has therefore been obtained that in Gunn rats, in the absence of bilirubin glucuronidation, that reactive oxygen species provide a significant physiological pathway of bilirubin disposal.

Dietary modulation of plasma bilirubin and of hepatic microsomal lipid peroxidation in the Gunn rat

An in vitro assay for the simultaneous measurement of lipid peroxidation (LPO) and bilirubin degradation (BRD) activities in rat liver microsomes has been developed; a good correlation between the 2 activities was observed (r = 0.78). In the Gunn rat a lipid free diet caused an increase in plasma bilirubin (62.4 +/- 25.8%, n = 6) and a concomitant decrease in both hepatic microsomal LPO and BRD to zero. In contrast, on a 25% lipid diet there was a decrease in plasma bilirubin (46.1 +/- 3.6%; n = 8) associated with an increase in LPO (1.26 +/- 0.11 nmol/min/mg protein, and BRD (0.21 +/- 0.6 nmol/min/mg protein). Therefore, in the absence of bilirubin glucuronidation, dietary modulation of plasma bilirubin and lipid peroxidation appear to be closely associated.

Effects of ursodeoxycholate on maximal biliary secretion of bilirubin in the rat

The effect of sodium ursodeoxycholate (0.5 and 1.0 mumol/min/100 g) on the maximal biliary secretion (Tm) of bilirubin and on the concentration of bilirubin in liver and plasma at the end of a bilirubin load was studied in Wistar rats. Administration of ursodeoxycholate at 0.5 mumol/min/100 g caused a 0.8-fold increase in bile flow and a significant increase in the bilirubin Tm (+24%). This was associated with a significant reduction of liver and plasma bilirubin concentrations (-16% and -17%, respectively). Bilirubin UDP-glucuronosyltransferase activity was not significantly enhanced. There was a significant increase in the biliary excretion of bilirubin conjugates (+30%) and in the diconjugates/monoconjugates ratio in bile (+31%). When ursodeoxycholate was given at 1.0 mumol/min/100 g, it produced a 1.7-fold increase in bile flow, but the bilirubin Tm was significantly reduced (-21%). Liver bilirubin concentrations were decreased (-20%) and there was a significant enhancement in total pigment concentration in plasma (+19%). Both the excretion of unconjugated bilirubin and that of bilirubin conjugates were significantly reduced (-60% and -18%, respectively). There was a significant decrease in the bilirubin-UDP glucuronosyltransferase activity and the diconjugates/monoconjugates ratio in bile (-27% and -27%, respectively). These results indicate that ursodeoxycholate is able to increase maximal bilirubin secretion only when administered at low doses and that infusion at higher rates can significantly interfere with different steps in the hepatobiliary transport of the pigment.

Principles of a competitive binding assay for the study of the interactions of poorly water-soluble ligands with their soluble binding partners. Application to bilirubin with the use of Sephadex G-10 as a competitive adsorbent

1. A generally applicable method is described for obtaining experimental data on the interactions between a poorly water-soluble ligand and soluble binding factors, with the use of chemically inert solid adsorbent. The equilibrium distribution of the ligand between the liquid phase containing the soluble binders and the adsorbent must be measured and knowledge of the binding isotherm of the adsorbent is required. Procedures are given for the calculation of the binding parameters. 2. The method has been applied to quantify the interactions of bilirubin with serum and liver cytosol from the rat, Sephadex G-10 serving as the competing adsorbent. Reversible adsorption keeps the concentration of the free ligand low, thereby preventing formation of colloidal bilirubin. The sensitivity of the procedure accommodates the rather high binding affinities by which bilirubin generally interacts with its specific binding proteins. 3. The binding activities of serum and liver cytosol are of comparable magnitude. Binding of bilirubin by rat serum can be described by two independent binding sites, the affinities of which differ by two orders of magnitude. Only the site with the higher affinity appears to be of physiological importance. The major bilirubin-binding sites of rat liver cytosol seem to contribute equally to the overall binding activity of this preparation, provided that GSH is present.

Enzymatic oxidation of bilirubin by intestinal mucosa

Bilirubin oxidase, an aerobic enzyme which degrades bilirubin 'in vitro' to colourless diazo-negative compounds, including propentdyopents and trace amounts of biliverdin, has been demonstrated in homogenates of rat intestine, kidney and liver. The enzyme in the intestinal mucosa has been partially characterised and appears to be mitochondrial in origin; maximal activity was detected in the jejunum. Intestinal bilirubin oxidase has a mean activity of 0.51 +/- 0.03 (S.D.) nmol bilirubin degraded/min per mg protein. Similar bilirubin oxidase activities were found in the tissue of Sprague-Dawley and Gunn rats. The role of the enzyme 'in vivo' remains to be determined.

Enzymic oxidation of unconjugated bilirubin by rat liver

The presence of the enzyme bilirubin oxidase, which degrades bilirubin in vitro, was demonstrated in the liver. Subcellular-fractionation experiments indicate that bilirubin oxidase is located in both the inner and outer membranes of the mitochondria. The mean rate of the reaction is 1.57 +/- 0.38 (S.D.) nmol of bilirubin degraded/min per mg of mitochondrial protein (munits/mg of protein). With respect to the overall breakdown of bilirubin, the enzyme has a Km' of 136 microM-bilirubin and a Vmax.' of 9.13 munits/mg of protein. Its activity is influenced by the ionic strength of the media and is inhibited by KCN, thiol reagents, NADH and albumin. The enzyme is aerobic, and between 1 and 1.5 mol of O2 are consumed per mol of bilirubin degraded. The products of the reaction include propentdyopents. The hepatic bilirubin oxidase activity of the jaundiced Gunn-rat liver is not significantly different from that of the Sprague-Dawley rat, and it is not induced by beta-naphthoflavone.

Enhancement by secretin of the apparently maximal hepatic transport of bilirubin in the rat

The effect of secretin (0.4 C.U. per hr per 100 gm body weight) on bile flow and the apparent maximal hepatic transport of bilirubin (Tm) was investigated in the rat. When secretin was administered during an already established bilirubin-Tm condition, it increased bile flow and bilirubin-Tm by 15 to 20% over a 30- to 50-min period. Enhancement of bilirubin output correlated with augmented flow and was sustained by an increased rate of excretion of monoglucuronides. When secretin was given for 90 min before bilirubin loading, it enhanced biliary bilirubin concentration and output, largely as diglucuronides. Bilirubin-Tm correlated positively with glururonyltransferase activity in liver homogenates. In the isolated perfused rat liver, injection of secretin in the portal cannula failed to produce choleresis. Bilirubin uridine diphosphate-glucuronyltransferase activity was lower than in intact rats and higher after treatment than in controls. The effect of secretin had an early effect on hepatocytic bile flow and a later effect on conjugation. Maximal hepatic bilirubin output was modulated both by flow and conjugation rate; the two mechanism may act independently.

Hepatic accumulation and intracellular binding of conjugated bilirubin

After the intravenous injection of unconjugated [(3)H]bilirubin into normal Sprague-Dawley and Wistar R rats, radiolabeled bile pigments rapidly accumulated in the liver. By 1.5 min after injection, an average of 36% of the injected isotope was present in liver homogenates. Between 3 and 15 min, 37-64% of the total intrahepatic radiolabeled bilirubin was conjugated, as demonstrated by extraction of label into the polar phase of a solvent partition system. This indicates both rapid conjugation, and accumulation of conjugated bilirubin within the liver cell. Fluorometric determination of the dissociation constants of purified bilirubin and its mono- and diglucuronides for homogeneous preparations of two human and four rat glutathione S-transferases, including ligandin, revealed avid binding of all three bile pigments to this class of proteins. Hence, the observation that the intrahepatic bile pigment pool contains substantial amounts of conjugated bilirubin can be attributed to the high binding affinities observed. Thin-layer chromatographic analysis of the (3)H-pigments produced by p-iodoaniline diazotization of homogenates and cytosol demonstrated that the intrahepatic pool of conjugated bilirubin was almost exclusively monoglucuronide. Examination of radiolabeled bilirubin conjugates excreted in bile during the first 20 min after injection of [(3)H]bilirubin showed no preferential excretion of diglucuronide. These studies indicate that (a) both bilirubin and its monoglucuronide accumulate within the liver cell as ligands with the glutathione S-transferase; and (b) bilirubin diglucuronide does not significantly accumulate within the general intrahepatocellular pool of protein-bound bile pigments. The latter observation is compatible with the formation and excretion of bilirubin diglucuronide directly from the canalicular pool of the liver cell.

Sites of competition in the selective hepatic uptake of rifamycin-SV, flavaspidic acid, bilirubin, and bromsulphthalein

In rats both rifamycin-SV and flavaspidic acid impaired the disappearance of bromsulphthalein (BSP) from plasma. The addition of rifamycin-SV to rat liver supernatant containing BSP did not displace BSP from ligandin or Z protein unless it was added in very high concentration. In similar studies in vitro with flavaspidic acid, BSP was displaced from Z protein even at low concentrations of flavaspidic acid, whereas there was only a minor effect on the binding to ligandin. The intravenous administration of rifamycin to Gunn rats raised the plasma bilirubin concentration slightly after 30 minutes whereas flavaspidic acid was without significant effect. At 30 minutes the amount of bilirubin in the liver was markedly reduced by rifamycin and moderately depressed by flavaspidic acid. Although both the rifamycin and flavaspidic acid lowered the bilirubin in cell sap, the main effect of flavaspidic acid appeared to be on binding to Z protein. Rifamycin reduced the amount bound to both ligandin and Z protein. It is suggested that flavaspidic acid acted on the hepatic uptake of bilirubin and BSP predominantly by competing for binding to Z protein. Rifamycin-SV acted at a different site, probably blocking uptake at the plasma membrane.