Characteristics of renal UDP-glucuronyltransferase

Scaling factors for the in vitro-in vivo extrapolation (IV-IVE) of renal drug and xenobiotic glucuronidation clearance

AIM

To determine the scaling factors required for inclusion of renal drug glucuronidation clearance in the prediction of total clearance via glucuronidation (CLUGT ).

METHODS

Microsomal protein per gram of kidney (MPPGK) was determined for human 'mixed' kidney (n = 5) microsomes (MKM). The glucuronidation activities of deferiprone (DEF), propofol (PRO) and zidovudine (AZT) by MKM and paired cortical (KCM) and medullary (KMM) microsomes were measured, along with the UGT 1A6, 1A9 and 2B7 protein contents of each enzyme source. Unbound intrinsic clearances (CLint,u,UGT ) for PRO and morphine (MOR; 3- and 6-) glucuronidation by MKM, human liver microsomes (HLM) and recombinant UGT1A9 and 2B7 were additionally determined. Data were scaled using in vitro-in vivo extrapolation (IV-IVE) approaches to assess the influence of renal CLint,u,UGT on the prediction accuracy of the calculated CLUGT values of PRO and MOR.

RESULTS

MPPGK was 9.3 ± 2.0 mg g(-1) (mean ± SD). The respective rates of DEF (UGT1A6), PRO (UGT1A9) and AZT (UGT2B7) glucuronidation by KCM were 1.4-, 5.2- and 10.5-fold higher than those for KMM. UGT 1A6, 1A9 and 2B7 were the only enzymes expressed in kidney. Consistent with the activity data, the abundance of each of these enzymes was greater in KCM than in KMM. The abundance of UGT1A9 in MKM (61.3 pmol mg(-1) ) was 2.7 fold higher than that reported for HLM.

CONCLUSIONS

Scaled renal PRO glucuronidation CLint,u,UGT was double that of liver. Renal CLint,u,UGT should be accounted for in the IV-IVE of UGT1A9 and considered for UGT1A6 and 2B7 substrates.

Differential disposition of intra-renal generated and preformed glucuronides: studies with 4-methylumbelliferone and 4-methylumbelliferyl glucuronide in the filtering and nonfiltering isolated perfused rat kidney

Objectives

This study was designed to investigate the renal disposition of 4-methylumbelliferone (4MU) and 4-methylumbelliferyl glucuronide (4MUG) to characterise the contribution of excretion and metabolic clearance to total clearance in the kidney.

Methods

The isolated perfused kidney (IPK) from the male Sprague–Dawley rat was used in filtering and non-filtering mode to study the renal disposition of 4MU, renally generated 4MUG and preformed 4MUG. Perfusate and urine (filtering IPK only) was collected for up to 120 min and 4MU and 4MUG in perfusate and urine were determined by HPLC. Analytes were also measured in kidney tissue collected at 120 min. Non-compartmental analysis was used to derive pharmacokinetic parameters.

Key findings

The concentration of 4MU in perfusate declined with a terminal half-life of approximately 120 min following administration to the filtering IPK and nonfiltering IPK. There was a corresponding increase in the concentration of 4MUG. Metabolic clearance of 4MU accounted for 92% of total renal clearance. After bolus dosing of preformed 4MUG in the perfusion reservoir of the filtering IPK, the perfusate concentration declined with the terminal half-life of approximately 260 min. The renal excretory clearance of preformed 4MUG accounted for 96% of total renal clearance. 4MU was extensively metabolized by glucuronidation in the filtering and nonfiltering IPK, and the total renal clearance of 4MU was far greater than its renal excretory clearance. This indicated that glucuronidation was the major elimination pathway for 4MU in the kidney.

Conclusions

The data confirmed an important role for the kidney in the metabolic clearance of xenobiotics via glucuronidation and signalled the lack of impact of impaired glomerular filtration on renal drug metabolism.

Human Renal Cortical and Medullary UDP-Glucuronosyltransferases (UGTs): Immunohistochemical Localization of UGT2B7 and UGT1A Enzymes and Kinetic Characterization ofS-Naproxen Glucuronidation

There is currently little information regarding the localization of UDP-glucuronosyltransferases (UGTs) in human renal cortex and medulla, and the functional contribution of renal UGTs to drug glucuronidation remains poorly defined. Using human kidney sections and human kidney cortical microsomes (HKCM) and human kidney medullary microsomes (HKMM), we combined immunohistochemistry to investigate UGT1A and UGT2B7 expression with in vitro microsomal studies to determine the kinetics of S-naproxen acyl glucuronidation. With the exception of the glomerulus, Bowman's capsule, and renal vasculature, UGT1A proteins and UGT2B7 were expressed throughout the proximal and distal convoluted tubules, the loops of Henle, and the collecting ducts. Additionally, UGT1A and UGT2B7 expression was demonstrated in the macula densa, supporting a potential role of UGTs in regulating aldosterone. Consistent with the immunohistochemical data, S-naproxen acyl glucuronidation was catalyzed by HKCM and HKMM. Kinetic data were well described by the two-enzyme Michaelis-Menten equation. K(m) values for the high-affinity components were 34 +/- 14 microM (HKCM) and 45 +/- 14 microM (HKMM). Fluconazole inhibited the high-affinity component establishing UGT2B7 as the enzyme responsible for S-naproxen glucuronidation in cortex and medulla. The low-affinity component was relatively unaffected by fluconazole (<15% inhibition), supporting the presence of other UGTs with S-naproxen glucuronidation capacity (e.g., UGT1A6 and UGT1A9) in cortex and medulla. We postulate that the ubiquitous distribution of UGTs in mammalian kidney may buffer physiological responses to endogenous mediators, but at the same time competitive xenobiotic-endobiotic interactions may provide an explanation for the adverse renal effects of drugs, including nonsteroidal anti-inflammatory drugs.

Repeated exposures to heroin and/or cadmium alter the rate of formation of morphine glucuronides in the rat

After absorption, heroin is transformed into mono-acetyl-morphine and then into morphine. Morphine, in turn, is metabolized to morphine-3-glucuronide (M3G), an inactive compound, and morphine-6-glucuronide (M6G), a potent opioid agonist. Thus, changes in the rate of formation of M6G may alter the pharmacological consequences of a treatment with heroin or morphine. In this study, we investigate the effect of repeated exposures (10 daily i.p. injections) to heroin, morphine, cadmium (which has been previously shown to inhibit M3G formation in vitro), or heroin + cadmium on morphine glucuronidation both in vivo and ex vivo (i.e., microsomal preparation obtained from rats treated in vivo). Repeated heroin (2.5, 5.0, and 10 mg/kg) increased plasma levels of M6G (which was undetectable in all other groups) and reduced those of M3G. Also, the microsomal preparations obtained from the liver of repeated heroin rats, when incubated with morphine, yielded significant amounts of M6G (which was undetectable in all other groups) and decreased levels of M3G relative to the control groups. These effects were reversible upon discontinuation of heroin administration. In contrast, repeated morphine (10, 20, and 40 mg/kg) only slightly reduced M3G formation at the dose of 40 mg/kg. Repeated cadmium (5, 15, and 45 microg/kg) reduced the rate of M3G formation without inducing M6G synthesis. The effects of the repeated coadministration of heroin (10 mg/kg) and cadmium (15 microg/kg) were virtually identical to those of repeated heroin alone. In summary, repeated exposure of rats to heroin can shift morphine glucuronidation toward the formation of the active metabolite M6G.

Human uridine diphosphate-glucuronosyltransferase UGT2B7 conjugates mineralocorticoid and glucocorticoid metabolites

Mineralocorticoid and glucocorticoid hormones are metabolized as glucuronide conjugates. Using labeled [(14)C]uridine diphosphate glucuronic acid and microsomal preparations from human embryonic kidney 293 cells stably expressing the different human and monkey uridine diphosphate glucuronosyltransferase (UGT)2B enzymes, it is demonstrated that the two human allelic variants UGT2B7H((268)) and UGT2B7Y((268)) conjugate aldosterone, its A-ring reduced metabolites (5alpha-dihydroaldosterone and 3alpha,5beta-tetrahydroaldosterone), and both 5alpha- and 5beta-tetrahydrocortisone epimers. The two variants of UGT2B4 also glucuronidate tetrahydroaldosterone, whereas all enzymes tested were inefficient to produce cortisol glucuronide derivatives. Kinetic analyses reveal that UGT2B7 polymorphisms glucuronidate mineralocorticoids with a 5.5- to 20-fold higher affinity than glucocorticoids. For the first time, a significant difference between the two allelic variants of UGT2B7 is described, because UGT2B7H((268)) possesses an 11-fold higher aldosterone glucuronidation efficiency (ratio Vmax((app.))/Km((app.))) than UGT2B7Y((268)). RT-PCR experiments demonstrate the expression of UGT2B7 in human kidney and in renal proximal tubule epithelial cells, suggesting that mineralocorticoids and glucocorticoids are metabolized in their target tissue. Measurement of aldosterone glucuronidation and normalization with the UGT2B protein contents in monkey tissues demonstrate that liver and kidney glucuronidate this hormone with a similar velocity. Immunohistochemical studies performed in monkey kidney cortex reveal a restrictive expression of UGT2B proteins in the epithelial cells of the proximal tubules. Because expression of the mineralocorticoid receptor was detected in the distal tubule epithelial cells, the present data suggest a two-cell mechanism of aldosterone action and metabolism in the kidney.

Glucuronidation of morphine in human kidney microsomes

The UDP-glucuronyltransferase (GT) activity with morphine as substrate was measured in microsomes prepared from seven human adult kidneys including one whose cortex and medulla had been separated. The formation of morphine-3- and 6-glucuronides (M3G and M6G) was determined by HPLC methods. All kidney specimen formed M3G and M6G when incubated with morphine and UDP-glucuronic acid (UDPGA). The estimated Vmax and Km of morphine to form M3G and M6G were 2.2 and 0.18 nmol per mg protein and min. and 8.2 and 4.6 mM, respectively. The corresponding values for UDPGA were 1.3 and 0.13 nmol per mg protein and min. and 5.4 and 4.1 mM, respectively. The GT activity in cortex was 2.8-fold higher than in the medulla. The human kidney thus contains glucuronyltransferase activities towards morphine, which is in contrast to previous findings in the rat.

The effect of dietary protein levels on xenobiotic biotransformations in F344 male rats

Diets containing protein levels of 8, 12, and 22% were fed for 14 days to Fischer 344 male weanling rats. Enzyme activities in liver and kidney were measured using several substrates for each of the following enzyme systems: Cytochrome P-450-dependent monooxygenase, ester hydrolysis, and conjugations with sulfate, glutathione, and glucuronic acid. Enzyme activities, for the various mechanisms, decreased from 15 to 65% with decreased dietary protein, with the exception of glucuronide and sulfate conjugation, which increased with decreased dietary protein. In vivo effects were evaluated by measuring hexobarbital sleeping time, procaine paralysis time, and bromobenzene hepatotoxicity. Increased dietary protein shortened hexobarbital sleeping time and procaine paralysis time, and increased procaine lethality, whereas low protein intake protected against bromobenzene hepatotoxicity. These data suggest that typical laboratory diets (22-25% protein) may provide artificially altered activities of xenobiotic biotransformation in rats, relative to nutritionally adequate (12% protein) diets.

The activity of 1-naphthol-UDP-glucuronosyltransferase in the brain

Cerebral microsomes catalysed efficiently the glucuronidation of 1-naphthol, this formation of glucuronide being activated by treatment with Triton X-100 or digitonin. Activated microsomes from the brain of the rat conjugated 1-naphthol with an apparent Km of 95 microM and a Vmax of 5.47 nmol/hr mg protein at 30 degrees C. Microsomal uridine diphosphate (UDP)-glucuronosyltransferase activity in brain towards 1-naphthol was not significantly induced by pretreatment of animals with 3-methylcholanthrene or phenobarbital. These data suggest that UDP-glucuronosyltransferases in brain are different from the hepatic enzymes with regard to biochemical parameters and in response to inducers of drug metabolism. The hepatic UDP-glucuronosyltransferase deficiency in Gunn rats was also observed in the brain.

Purification and properties of rat kidney UDP-glucuronosyltransferase

Rat kidney microsomes catalysed the glucuronidation of 1-naphthol, 4-nitrophenol, bilirubin and beta-estradiol. Unlike rat hepatic microsomes, UDP-glucuronosyltransferase activity towards morphine and testosterone was not detectable. Treatment of rats with beta-naphthoflavone resulted in a 3-fold induction of renal UDPGT activity towards 1-naphthol, 4-nitrophenol and phenol, and a 2-fold induction of bilirubin and beta-estradiol glucuronidation. No induction of renal UDPGT was observed after phenobarbital treatment, but renal bilirubin UDPGT activity was specifically induced after treatment of rats with clofibrate. UDPGT activity was purified from rat kidney by a combination of ion-exchange chromatography, gel filtration and affinity chromatography on UDP-hexanolamine Sepharose. One major protein-staining polypeptide was observed on silver-stained SDS-polyacrylamide gels, of molecular weight 55,000 Da, and a minor band of 54,000 Da was also present. Indeed, immunoblot analysis of purified renal UDPGTs with anti-rat liver UDPGT antibodies revealed two immuno-reactive polypeptides of molecular weight 55,000 and 54,000 Da. The highly purified preparations catalysed the glucuronidation of 1-naphthol and bilirubin. Glucuronidation of bilirubin by purified renal UDPGT preparations required the presence of phospholipid, the activity being further enhanced by incubation with rat lung microsomes. The data presented indicate that two UDPGT isoenzymes have been copurified.

Immunochemical and functional characterization of UDP-glucuronosyltransferases from rat liver, intestine and kidney

Glucuronidation of various substrates in hepatic, intestinal and renal microsomes of control, phenobarbital (PB), 3-methylcholanthrene (3MC) and Aroclor-1254 (A1254) pretreated rats was investigated. UDPGT activities tested could be divided in four groups on the basis of their tissue distribution and induction by PB or 3MC in liver microsomes. GT1 activities (1-naphthol, benzo(a)pyrene-3,6-quinol) are induced by 3MC in liver microsomes and are present in all tissues investigated. GT2 activities (morphine, 4-hydroxybipheynl) are induced by PB in liver microsomes and appear to be restricted to the liver and the intestine. UDPGT activity towards bilirubin, although induced by PB, can be detected in hepatic, intestinal and renal microsomes. UDPGT activity towards fenoterol is restricted to the liver and intestine and is not induced by PB, 3MC or A1254. The presence of inducible immunoreactive UDPGT isoenzymes in microsomes of liver, intestine and kidney of control and induced rats was demonstrated by immunoblot analysis using rabbit anti-rat liver-GT1 antibodies. Induction of both 54 and 56 kDa polypeptides in hepatitis, intestinal and renal microsomes by 3MC or A1254 was observed. Purification of UDPGT (1-naphthol as substrate) from intestinal microsomes to apparent homogeneity yielded a polypeptide with an apparent molecular weight of 54-56 kDa. The results indicate that 54 and 56 kDa UDPGT polypeptides are the major A1254 inducible isoenzymes in intestinal and renal microsomes. An increase in immunoreactive protein is correlated with a biochemically measurable increase in glucuronidation capacity for GT1 substrates.