UDP-glucuronyltransferase in perfused rat liver and in microsomes. Glucuronidation of bilirubin.

In perfused rat liver and in fistula rats the formation of bilirubin conjugates was studied after labeling with [14C]bilirubin,5-amino [14C]levulinic acid and [14C]hemin. The latter two compounds were used to study heme degradation to bilirubin from intrahepatic and extrahepatic sources, respectively. Bilirubin glucuronides were the major conjugates in fistula bile. In liver perfusion bile the proportion of non-glucuronide conjugates was increased. After a high dose of hemin (2.5 mumol) bilirubin glucuronides were decreased compared with other bilirubin conjugates both in fistula bile and in liver perfusion bile. In addition green pigments were formed. These alterations were reversed in chronically hemin-treated rats in which heme oxygenase had been induced. The interference of UDP-glucose and UDP-glucuronic acid with bilirubin glucuronidation and glucosidation was studied in liver microsomes. UDP-glucose did not affect bilirubin glucuronidation in native microsomes in which UDP-glucuronyltransferase activity is constrained. When this constraint was released by various treatments altering membrane structure UDP-glucose markedly inhibited bilirubin glucuronidation. However, under these conditions bilirubin glucosidation was unaffected by UDP-glucuronic acid. The results suggest that the release of the constraint of UDP-glucuronyltransferase in vivo may lead to a decrease of the proportion of bilirubin glucuronides to other bilirubin conjugates in bile.

Biliary excretion of foreign compounds. Benzene and its derivatives in the rat.

1. The extent of the excretion in the bile of the rat of benzene and 21 of its simple derivatives was studied. 2. Some 16 compounds of molecular weight less than 200, and including neutral molecules (benzene and toluene), aromatic acids, aromatic amines and phenols, were injected in solution intraperitoneally into biliary-cannulated rats. Metabolites in the bile were identified and estimated. The extent of biliary excretion of these compounds was low, i.e. 0-10% of the dose in 24hr., and most appeared in the bile mainly as conjugates. 3. The biliary excretion of six conjugates of molecular weight less than 300, including three glycine conjugates, one sulphate conjugate, one glucuronic acid conjugate and two acetyl derivatives, was low (less than 3% of the dose). 4. It is concluded that simple benzene derivatives of molecular weight less than about 300 are poorly excreted in rat bile.

Biliary excretion in foreign compounds. Species difference in biliary excretion.

1. The biliary excretion of injected [(14)C]aniline, [(14)C]benzoic acid, 4-amino-hippuric acid and 4-acetamidohippuric acid in six or eight species of animal (rat, dog, hen, cat, rabbit, guinea pig, rhesus monkey and sheep) was studied. 2. These compounds, with molecular weights in the range 93-236, are poorly excreted in the bile in all the species examined and, in effect, there is little significant species difference in the extent of their biliary excretion. 3. Compounds of higher molecular weight (355-495) were also studied, namely succinylsulphathiazole, [(14)C]stilboestrol glucuronide, sulphadimethoxine N(1)-glucuronide and phenolphthalein glucuronide. 4. With these compounds a clear species difference in the extent of biliary excretion was found, the rat, dog and hen being good excretors, the rabbit, guinea pig and monkey poor excretors, and the cat and sheep taking an intermediary position. 5. There was a general trend for biliary excretion to be higher in all species when the compounds were of higher molecular weight. 6. These results are discussed in their relation to species differences in drug metabolism.

Biliary excretion of [C]succinylsulphathiazole in the rat and rabbit.

1. After intravenous injection about 30% of the dose (20mg./kg.) of succinylsulphathiazole is excreted unchanged in the bile in 3hr. by the rat, whereas only about 1% is excreted by the rabbit. When the renal pedicles are ligated the biliary excretion of succinylsulphathiazole in the rat increases to about 80% of the dose, but in the rabbit under these conditions the biliary excretion is only 2% of the dose. 2. In the rat, the sulphonamide readily enters the liver and biliary excretion occurs against a concentration gradient from liver to bile; further, the excretory process can be saturated, and can be depressed by the simultaneous administration of phenolphthalein glucuronide or bile salts. 3. In the rabbit, these conditions have not been found; succinylsulphathiazole does not readily enter the liver from the plasma, there is no transfer of the drug from the liver cells to the bile against a concentration gradient, and no saturation or depression of the biliary excretion of succinylsulphathiazole is found. 4. It is suggested that two factors responsible, at least partly, for the low biliary excretion of succinylsulphathiazole in the rabbit are the poor entry of the sulphonamide into the liver in this species and a deficiency of the concentrative mechanism for its excretion in the bile.