Activation of hepatic microsomal glucuronyl transferase by bilirubin

In rat liver microsomal preparations, bilirubin markedly stimulated the glucuronidation of morphine and p-nitrophenol catalyzed by UDPglucuronyltransferase (UDPGT, EC 2.4.1.17). The activation was not due to contamination of bilirubin with bile acids. At equimolar concentrations, the activating effect of bilirubin was greater than that produced by deoxycholate, a detergent well known as an activator of UDPGT. Other results suggest that bilirubin activation of UDPGT is similar to that produced by detergents. In $\text{in vivo}$ experiments, the rate of urinary excretion of morphine glucuronide in rats treated with bilirubin was twice that of control animals. These results suggest that bilirubin may be a physiologic activator of UDPGT activity.