Extract:The bilirubin-UDP-glucuronyl transferase (BGT) activity in liver of newborn rats was measured to determine if unconjugated bilirubin could be the trigger substance of glucuronyl transferase activity. Comparison was made among normal rats, newborn rats, and rats of mothers that had been given varying doses of unconjugated bilirubin during pregnancy. Three groups of pregnant Wistar rats were used for the experiments. Group I received bilirubin by intravenous injection once a day during the last five days of pregnancy. Group II received a suspension of bilirubin by intraperitoneal injection during the sixteenth to twenty-first day of pregnancy. Group III served as controls. Activity was determined by a modification of the method of Lathe and Walker [14]. Total bilirubin was determined in the incubates according to the method of Fog and Bakken [9], while conjugated and unconjugated bilirubin were determined by methods described by Fog and Bakken [10]. BGT activity was measured as bilirubin conjugated per gram wet weight liver per hour (table I). Normal newborn rats were unable to conjugate bilirubin at birth, while newborn rats of mothers that had been injected with 150 mg bilirubin for more than 24 hours showed BGT activity at birth. Newborn rats of mothers that had received 150 mg bilirubin less than 24 hours before delivery showed no BGT activity in liver at birth, but the enzyme was activated earlier after birth in these rats than it was in normal rats. The results show that unconjugated bilirubin triggers UDP-glucuronyl transferase and thus functions as an activator for excretion of bilirubin. Speculation:The finding that unconjugated bilirubin functions as a trigger for bilirubin-UDP-glucuronyl transferase activity does not explain the molecular mechanism that leads to increased enzyme activity. Further studies are needed to show whether the increased level of activity after birth is due to a new production of ribonucleic acid (RNA), to activation of a protein already present, or to inactivation or withdrawal of an inhibitor. The possible relation between the level of bilirubin in the serum and the conjugating capacity in the newborn should also be studied. © International Pediatrics Research Foundation, Inc. 1969. All Rights Reserved.
