Effective glucuronidation of 6 alpha-hydroxylated bile acids by human hepatic and renal microsomes.

The glucuronidation of bile acids is an established metabolic pathway in different human organs. The hepatic and renal UDP-glucuronyltransferase activities vary according to the bile acids concerned. Thus, hyodeoxycholic acid is clearly differentiated from other bile acids by its high rate of glucuronidation and elevated urinary excretion in man. To determine whether such in vivo observations are related to variations in bile acid structure, human hepatic and renal microsomes were prepared and time courses of bile acid glucuronidation measured with the bile acids possessing hydroxyl groups in different positions. Eleven [24-14C]bile acids were chosen or synthesized in respect of their specific combination of hydroxyl and oxo groups at the 3, 6, 7 and 12 positions and of their alpha or beta hydroxyl configurations. The results clearly demonstrate that bile acids with an hydroxyl group in the 6 alpha position underwent a high degree of glucuronidation. Apparent kinetic Km and Vmax values for UDP-glucuronyltransferase activities ranged over 78-66 microM and 1.8-3.3 nmol.min-1.mg-1 protein in the liver and over 190-19 microM and 0.5-9.2 nmol.min-1.mg-1 protein in the kidney. All the other bile acids tested, each of which possessed a 3 alpha-hydroxyl group and whose second or third hydroxyl was bound at the 6 beta, 7 or 12 positions, were glucuronidated to a degree far below that of the 6 alpha-hydroxylated bile acids. We conclude that an active and highly specific UDP-glucuronyltransferase activity for 6 alpha-hydroxylated bile acids exists in human liver and kidneys. Moreover, this activity results in the linkage of glucuronic acid to the 6 alpha-hydroxyl group and not to the usual 3 alpha-hydroxyl group of bile acids.

Glucuronidation of bile acids in human liver, intestine and kidney. An in vitro study on hyodeoxycholic acid.

The activities of UDP-glucuronyl transferase(s) in homogenates and microsomal preparations of human liver, kidney and intestine were tested with hyodeoxycholic acid (HDC). The various kinetic parameters of the UDC-glucuronidation were determined from time course experiments. In both liver and kidney preparations, HDC underwent a very active metabolic transformation: liver Km = 78 microM, Vmax = 3.3 nmol . min-1 . mg-1 protein; kidney Km = 186 microM, Vmax = 9.9 nmol . min-1 . mg-1 protein. To our knowledge this is the first observation of both an extensive and comparable bile acid glucuronidation occurring in renal and hepatic tissues.

Bioavailability, gastrointestinal transit, solubilization and faecal excretion of ursodeoxycholic acid in man.

The bioavailability of ursodeoxycholic acid (UDCA), a cholesterolic gallstone dissolving agent, has been analysed in seven healthy human volunteers. After absorption of a capsule containing a 500 mg dose, the time course of plasma concentrations of the drug presented a double peak profile over a 240 min period. In order to explain this result, a second group of five subjects bearing a four-way jejunal catheter fitted with an occluding balloon, received an oral dose of 250, 500 or 750 mg of the drug. Simultaneous analyses of plasma UDCA concentrations and jejunal UDCA contents were carried out. UDCA is poorly soluble in the gastro-duodeno-jejunal contents of fasted subjects since 21-50% of the ingested doses were recovered in solid form. The profile of plasma concentration paralleled the amount of soluble UDCA present in intestinal lumen. When jejunal contents were infused below the balloon a second plasma peak appeared in cases corresponding to ingestion of higher doses of UDCA. In conclusion, pharmacological doses of UDCA are not readily soluble in the stomach and intestine of a healthy fasting man. In consequence, the bioavailability of the drug varies with its progressive solubilization in the gastrointestinal tract. The present results suggest that repeated daily doses of UDCA should improve its bioavailability in treated gallstone patients.

Metabolism of beta-muricholic acid in man.

Labeled beta-muricholic acid was obtained from germfree rats given [24-14C]-chenodeoxycholic acid. It was crystallized with the same unlabeled bile acid extracted from germfree rat pooled biles. Five patients fitted with a T-tube after cholecystectomy were given orally 100 mg of the bile acid. Metabolites of beta-muricholic acid in bile, urine and feces were studied. Glyco- and tauro-beta-muricholic acid were the only metabolites detected in bile. The urinary bile acid pattern was complex and included free, glyco- and sulfoconjugated beta-muricholic acid, but no glucuronide was observed. Analysis of fecal bile acid showed very few metabolites: the 3 beta-epimer was identified; the 6 beta- and 7 beta-hydroxyls were apparently not transformed by human intestinal microflora.

Intestinal absorption, excretion, and biotransformation of hyodeoxycholic acid in man.

Five patients fitted with a biliary T-tube after cholecystectomy were given orally a tracer dose of [14C]hyodeoxycholic acid and 500 mg of the same unlabeled acid. Intestinal absorption and biotransformation, liver metabolism, bile secretion, fecal and urinary excretions of this acid or of its metabolites were studied. Hyodeoxycholic acid was well absorbed by the human intestine. It was not subjected to intestinal transformations and, particularly, did not produce a significant amount of lithocholic acid, which does not support the existence of intestinal bacterial 6 alpha-dehydroxylases. The percentage of hyodeoxycholic acid and of its metabolites recovered in bile varied from 11.5 to 31%. Two major metabolites were isolated from bile: glycohyodeoxycholic acid and hyodeoxycholic acid glucuronide. Analysis of urinary bile acids showed that a large proportion (30-84%) of the administered hyodeoxycholic acid was excreted by the kidney as a glucuronide. The large extent of both glucuronidation and urinary excretion of hyodeoxycholic acid is a unique example of bile acid metabolism and excretion in man.

Mechanism of ethacrynic acid-induced choleresis in the rat.

Ethacrynic acid (EA) was injected to rats with functional nephrectomy after a control period of steady-state bile flow sustained by taurocholate infusion. Biliary clearance of [14C]mannitol was measured in order to estimate canalicular bile flow and bile salt-independent fraction (BSIF). After EA infusion, bile flow increased by 56%; bile salt excretion rate decreased by 10%; electrolyte excretion rates all increased, principally Na+ and K+. Mannitol clearance increased in parallel with bile flow. The BSIF increased. EA was excreted into bile as a metabolite identified as the cysteine adduct of EA; its excretion rate was linearly correlated with the increment in bile flow. The results are consistent with the hypothesis that the biliary excretion of an EA derivative results in an osmotic water flow increasing the canalicular BSIF. Since EA ia a Na+-K+-ATPase inhibitor, it is necessary to reconsider the relationship between secretion of canalicular BSIF and active Na+ transport mediated by the Na+-K+-ATPase system.