Bilirubin, UDP-glucuronyl transferase of liver in postmature rats. A functional and morphologic comparison.

Postmaturity was induced in dated pregnancies in Wistar rats by daily injection of 25 units of chorionic gonadotrophin, beginning on day 18 of gestational life. Animals were either delivered operatively on day 23 of postconceptual age or allowed to deliver spontaneously on day 24 by stopping the gonadotrophin. The hepatic bilirubin UDP-glucuronyl transferase activity (E.C.2.4.1.17) was quantitated in these animals and compared with that of control animals of normal gestations operatively and spontaneously delivered of similar postconceptual age. Normal animals at birth have very little hepatic bilirubin UDP-glucuronyl transferase activity, but it rapidly achieves adult activities in the first 4 days of neonatal life. In contrast, the postmature animals of 23 and 24 days of postconceptual age exhibited marked suppression of the hepatic bilirubin UDP-glucuronyl transferase activity, but they exhibited rapid neonatal maturation after delivery. The ultrastructure of the hepatocytes from the postmature rats of 24 days postconceptual age exhibited the same paucity of smooth endoplasmic reticulum of 21-day rats, which is in striking contrast to of postnatal life. The findings are interpreted to suggest that the intrauterine milieu actually suppresses fetal maturation of liver function.

The presence of a microsomal UDP-glucuronyl transferase for bilirubin in homozygous jaundiced Gunn rats and in the Crigler-Najjar syndrome.

The infusion of a closely related derivative of bilirubin, its dimethyl diester (DME), into jaundiced (jj) Gunn rats were associated with biliary excretion of mono- and diglucuronides of bilirubin. In vitro incubation of DME with liver microsomes from jj rats demonstrated sequential demethylation and glucuronidation of DME. Liver microsomes from a patient with the Crigler-Najjar syndrome were unable to form glucuronides of bilirubin in vitro unless DME was used as substrate. The results suggest that the deficiency in Gunn rats and in the Crigler-Najjar syndrome may be due to a structural defect in the microsomal matrix which contains glucuronyl transferase. This interpretation envisions a microenvironment of the transferase enzyme which is either impermeable to bilirubin or induces conformational changes which interfere with glucuronidation.

Increased osmotic fragility of erythrocytes in chronically jaundiced rats after phototherapy.

Littermate homozygous (jj) and heterozygous (Jj) Gunn rats, were irradiated with blue fluorescent light for 18 hours continuously. The incident irradiance was 1.5 mWatts/cm2 in the 420--480 nm band pass. The influence of the irradiance on circulating erthrocytes was studied by testing their osmotic fragility before and after the irradiance. The non-jaundiced, Jj, animals did not exhibit any increase in the osmotic fragility of their erythrocytes. The osmotic fragility of the erythrocytes from jaundiced, jj, animals was the same as the Jj animals prior irradiance. However, the fragility of the erythrocytes from the jj animals was significantly increased after the 18 hours of irradiance. The results indicated that the photodynamic action of bilirubin may be present in vivo.

Hepatic bilirubin UDP-glucuronyltransferase in patients with sickle cell anemia.

In sickle cell anemia the shortened survival of red blood cells presents the liver with an augmented load of bilirubin for hepatic clearance. To determine the effects of this excessive bilirubin load on the microsomal conjugating enzyme, hepatic bilirubin UDP-glucuronyltransferase, levels of this enzyme were measured in liver biopsies from patients with sickle cell anemia and several comparison groups. UDP-glucuronyltransferase activity in 14 patients with sickle cell anemia was 2-fold greater (P less than 0.005) than in 14 nonjaundiced comparison patients without liver disease. The elevated UDP-glucuronyltransferase activity in sickle cell anemia was similar to that found in 10 patients who chronically ingested drugs (barbiturates or estrogens) known to increase UDP-glucuronyltransferase activity. These observations suggest enhanced conjugation of bilirubin in patients with sickle cell anemia may result from substrate (bilirubin) induction of UDP-glycuronyltransferase.

The influence of fatty acids on the binding of bilirubin to albumin.

The influence of free fatty acids (FFA's) on the albumin binding of bilirubin was studied in vitro in the plasma of infants with neonatal hyperbilirubineamia and in solutions employing crystalline albumin to which bilirubin and FFA (oleic acid) were added. The bilirubin saturation index (SI) was utilized to distinguish between that fraction of bilirubin bound at the primary (high-affinity) site of albumin and bilirubin bound at secondary sites from which it is easily dissociated by salicylate. The relative saturation of albumin with bilirubin was also measured by addition of salicylate to whole blood samples where bilirubin was also measured by addition of salicylate to whole blood samples where bilirubin dissociated from the albumin could be sequestered by the red cells. The present studies indicate that FFA's influence the binding of bilirubin in two ways. At molar ratios of FFA to albumin (2:1 to 4:1), the FFA's compete with bilirubin for binding at the high-affinity site so that a significant portion of the bilirubin is transported at secondary sites, making it susceptible to displacement by water-soluble organic anions. At high molar ratios (greater than 5:1) FFA's compete with bilirubin for albumin binding at the secondary sites as well. In contrast to crystalline albumin where the first two molar equivalents of FFA do not influence the binding of bilirubin to albumin, all FFA concentrations in hyperbilirubinemic plasma reduce the affinity of albumin for bilirubin at its high-affinity site even though there is a molar excess of albumin over bilirubin.