In vivo quantification of renal glucuronide and sulfate conjugation of 1-naphthol and p-nitrophenol in the rat

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.

Specific estrogen sulfotransferase (SULT1E1) substrates and molecular imaging probe candidates

This work focuses on the development of specific substrates for estrogen sulfotransferase (SULT1E1) to produce molecular imaging probes for this enzyme. SULT1E1 is a key enzyme in estrogen homeostasis, playing a central role in the prevention and development of human disease. In vitro sulfation assays showed alkyl and aryl substitutions to a fused heterocyclic system modeled after beta-naphthol (betaN), based on compounds that interact with the estrogen receptor, rendered several molecules with enhanced specificity for SULT1E1 over SULT1A1*1, SULT1A1*2, SULT1A3, and SULT2A1. Several 6-hydroxy-2-arylbenzothiazoles tested demonstrated excellent affinity--V(max)/K(m) ratios-and specificity for SULT1E1. K(m) values ranged from 0.12-2.36 microM. A strong correlation was observed between polarity of the 4'-sustituent on the 2-aryl moiety (Hammett sigma(p)) and the log(V(max)/K(m)) (r = 0.964). Substrate sensitivity is influenced by the acidity of the 6-phenolic group demonstrated by correlating its (1)H NMR chemical shift (delta(OH)) with the log(V(max)/K(m)) (r = 0.963). Acidity is mediated by the electron withdrawing capacity of the 4'-substituent outlined by the correlation of the C-2 (13)C NMR chemical shift (delta(C2)) with the log(V(max)/K(m)) (r = 0.987). 2-[4-(Methylamino)phenyl]-6-hydroxybenzothiazole (2b) was radiolabeled with carbon-11 ((11)C-(2b)) and used in vivo for microPET scanning and tissue metabolite identification. High PET signal was paralleled with the presence of radiolabeled (11)C-(2b)-6-O-sulfate and the SULT1E1 protein detected by western blot. Because this and other members of this family presenting specificity for SULT1E1 can be labeled with carbon-11 or fluorine-18, in vivo assays of SULT1E1 functional activity are now feasible in humans.

Sulfate conjugating and transport functions of MDCK distal tubular cells

BACKGROUND

Transfected Madin-Darby canine kidney (MDCK) cells (of distal tubular origin) have been used to study transport of organic anions. These cells have not been shown to possess sulfate-conjugating activity. Neither has transport activity been demonstrated in nontransfected MDCK cells.

METHODS

Polarized and monolayers of nontransfected MDCK type II cells were incubated with prototype substrates of phenolsulfotransferase (PST) and sodium sulfate in the absence or presence of known inhibitors of multidrug resistance protein (MRP): (3-3-(2-(7-chloro-2-quinionlinyl) ethenyl)phenyl)(3-dimethylamino-3-oxopropyl)thio)methyl)thio) propanoic acid (MK571), cyclosporin A (CsA), and probenecid. Effects of glutathione (GSH) and buthionine sulfoximine (BSO), potential modulators of the organic anion transporting protein/polypeptide (OATP) isoform, OATP1 were also examined. Sulfated conjugates were identified by high-performance liquid chromatography (HPLC)-radiometry or HPLC-fluorimetry.

RESULTS

Uptake, sulfate conjugation, and efflux of the sulfated conjugates of harmol, p-nitrophenol, N-acetyldopamine and acetaminophen were demonstrated. Activities in MDCK type II cells were higher than those in HepG2, human fetal liver, and Chang liver cells. A significant decrease in extracellular with a reciprocal increase in intracellular harmol sulfate was observed with MK571, CsA, and probenecid and with preloading of glutathione. Depletion of intracellular glutathione by BSO had the opposite effects.

CONCLUSIONS

Normal (nontransfected) MDCK type II cells provide a suitable system for the study of the physiologic processes of uptake, sulfate conjugation, and transport of sulfated conjugates in kidney cells. Based on the action of specific inhibitors and modulators of MRP2 and OATP1, it was concluded that MRP2-like and OATP1-like transporters are possibly responsible for the transport of sulfated conjugates.

Comparative study of toxicity of 4-nitrophenol and 2,4-dinitrophenol in newborn and young rats

The toxicities of 4-nitrophenol and 2,4-dinitrophenol in newborn and young rats was examined and the susceptibility of newborn rats was analyzed in terms of presumed unequivocally toxic and no observed adverse effect levels (NOAELs). In the 18-day repeated dose newborn rat study, 4-nitrophenol was orally given from Day 4 to Day 21 after birth but did not induce any toxicity up to 160 mg/kg in the main study, although it induced death in one of six males at 160 mg/kg, and three of six males and one of six females at 230 mg/kg in a prior dose-finding study. In the 28-day repeated dose oral toxicity study starting at 6 weeks of age, 4-nitrophenol caused the death of most males and females at 1,000 mg/kg but was not toxic at 400 mg/kg except for male rat-specific renal toxicity. As unequivocally toxic levels were considered to be 230 mg/kg/day in newborn rats and 600 to 800 mg/kg/day in young rats, and NOAELs were 110 mg/kg/day in newborn rats and 400 mg/kg/day in young rats, the susceptibility of the newborn to 4-nitrophenol appears to be 2.5 to 4 times higher than that of young animals. In the newborn rat study of 2,4-dinitrophenol, animals died at 30 mg/kg in the dose-finding study and significant lowering of body and organ weights was observed at 20 mg/kg in the main study. In the 28-day young rat study, clear toxic signs followed by death occurred at 80 mg/kg but there was no definitive toxicity at 20 mg/kg. As unequivocally toxic levels and NOAELs were considered to be 30 and 10 mg/kg/day in newborn rats and 80 and 20 mg/kg/day in young rats, respectively, the toxicity of 2,4-dinitrophenol in newborns again seems to be 2 to 3 times stronger than in young rats. Abnormalities of external development and reflex ontogeny in the newborn were not observed with either chemical. Based on these results, it can be concluded that the toxic response in newborn rats is at most 4 times higher than that in young rats, at least in the cases of 4-nitrophenol and 2,4-dinitrophenol.

Soy isoflavone conjugation differs in fed and food-deprived rats

An experiment clarifying the influence of food deprivation on the isoflavone conjugation pattern in rats was conducted. Food-deprived and fed rats were administered daidzein and genistein at 7.9 mcmol/kg body, and changes in their plasma metabolites (i.e., free compounds, sulfates, glucuronides, sulfates/glucuronides) were measured quantitatively as a function of time. In the food-deprived group, total plasma daidzein and genistein reached maximum concentrations of 20.9 +/- 4.4 and 11.4 +/- 3.1 mcmol/L, respectively, 10 min after administration, whereas in the fed group, the maxima were 2.4 +/- 0.8 mcmol/L for daidzein after 2 h and 1. 8 +/- 0.2 mcmol/L for genistein after 4 h. In both groups, there were significantly more daidzein sulfates than genistein sulfates. Moreover, depriving rats of food before daidzein and genistein administration significantly increased plasma isoflavone sulfates with simultaneous significant decreases in plasma isoflavone glucuronides compared with fed rats. Additionally, nonconjugated daidzein and genistein appeared in plasma of food-deprived rats for 1 h after administration. Plasma concentrations of conjugates having both sulfate and glucuronide moieties were not significantly different between the groups.

Conjugation of 1-naphthol in primary cell cultures of rat ovarian cells

The present study concerns conjugation of 1-naphthol in primary cultures of rat ovarian cells. Two phase II enzymes catalyzing conjugation, i.e. phenol sulfotransferase (P-SULT) and phenol UDP-glucuronosyltransferase (P-UGT), were measured using 1-naphthol as substrate. The rates of conjugation by the different cell types of the rat ovary were the same at low concentrations and short incubation times. However, after 20 h of incubation the rate of conjugation in cells isolated from ovaries enriched in corpora lutea (CL) exceeded the rate in cells isolated from ovaries enriched in preovulatory follicles. In addition, when the granulosa cells were removed from the preovulatory follicles, the rate of conjugation was 1.7-fold higher, i.e. in the theca/stroma cells. When the cells were incubated with 1-[14C]naphthol and conjugates were subsequently separated by thin-layer chromatography, naphthyl glucuronide was the only conjugate observed. Pentachlorophenol (PCP), a commonly used inhibitor of P-SULT, inhibited 1-naphthol conjugation 50% in cell cultures, as well as in microsomal preparations. alpha-Naphthoflavone (ANF) and ellipticine (ELP), both cytochrome P450 (CYP) inhibitors, affected the conjugation of 1-naphthol in different ways; ANF did not affect P-UGT activity in microsomal preparations, but inhibited 1-naphthol conjugation in cell cultures by as much as 90%. On the other hand, ELP inhibited the conjugation of 1-naphthol up to 99% in the cell cultures, but only 75% in microsomal fractions. Testosterone (TST) and estradiol inhibited this activity approximately equal 50% in both of these experimental systems. Clomiphene citrate (CLF), a drug used to induce ovulation and demonstrating both estrogenic and antiestrogenic effects, did not influence the conjugation of 1-naphthol significantly in the cell cultures. The present findings demonstrate that P-UGT is by far the major enzyme conjugating 1-naphthol in the rat ovary and that commonly used inhibitors of P-SULT and CYPs also inhibit P-UGT activity, either directly or via other mechanisms.

The presence of 1-naphthol in the urine of industrial workers exposed to naphthalene

1-Naphthol at concentrations ranging from 0.4 to 34.6 mg/l was found in urine collected directly after the end of the work shift from a group of industrial workers employed in distillation of naphthalene oil. The maximum excretion was found one hour after the end of the shift and the mean excretion rate was 0.57 mg/h. Coke plant workers exposed to naphthalene and other aromatic and polycyclic hydrocarbons also had 1-naphthol in their urine. Mean values were 0.89 mg/l (working with new technology) and 4.86 mg/l (working with old technology) and the excretion rates were 0.19 and 0.31 mg/h respectively. The maximum excretion was shifted to two to three hours after the end of the exposure. For non-exposed subjects the mean urinary 1-naphthol concentration was 120 micrograms/l and the excretion rate was 7.0 micrograms/h.

Propofol metabolism in man during the anhepatic and reperfusion phases of liver transplantation

1. An i.v. dose of 14C-propofol (0.53 mg/kg) was administered to three male and three female patients during the anhepatic phase of liver transplantation, which lasted 30-56 min after dosing. Arterial and venous blood samples, bile (T-tube drainage) and urine were collected at various times afterwards and submitted to h.p.l.c. and radioassay or specific fluorescence detection for the unchanged drug. 2. Extrahepatic metabolism was apparent during the anhepatic phase, since at 30 min post-dose, unchanged propofol comprised only 42-89% of the blood radioactivity. 3. Examination of the plasma radioactivity during the anhepatic phase in two subjects showed evidence of propofol glucuronide and 4-quinol sulphate, confirming extrahepatic metabolism of the drug. Quinol glucuronides were only detected in the liver reperfusion phase. 4. There was no evidence that the lungs contribute to the extrahepatic metabolism of propofol, since drug concentrations in the arterial blood were not less than in central venous samples. 5. During the first 24 h period, urine collected from five patients contained 7-74% dose, whilst the bile contained 0.1-0.9%. In three patients with normal renal function recovery in urine was 66-74% dose. Examination of urinary radioactivity in one subject showed the main component to be propofol glucuronide during the anhepatic phase.

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.

Glucuronidation and sulfation in the rat in vivo. The role of the liver and the intestine in the in vivo clearance of 4-methylumbelliferone

The role of the liver in the conjugation of 4-methylumbelliferone (4MU), mainly glucuronidation, was investigated in the rat in vivo. The liver extracted 4MU almost completely (97%) during steady-state infusion, as measured by the difference between 4MU concentration in portal and hepatic venous blood. Previously, it was shown that the intestinal region extracts 40% of the 4MU of the incoming arterial blood. The liver and the gastrointestinal region are so efficient that their conjugation activity can account for total body clearance of 4MU (50-60 ml/min per kg). These results and other evidence on extrahepatic conjugation of phenolic substrates suggest that glucuronidation may be limited to the liver, (the kidney) and the gastrointestinal region, while sulfation may occur more widespread throughout the body. Protein binding studies showed the sulfate conjugate to be even more protein-bound than unconjugated 4MU, while 4MU glucuronide was much less bound to rat plasma proteins.

1-Naphthol conjugation in isolated cells from liver, jejunum, ileum, colon and kidney of the guinea pig

Cells from liver, jejunum, ileum, colon and kidney of the guinea pig were freshly prepared by standard isolation procedures. Cells were incubated in the presence of several concentrations of 1(14C)-naphthol, and formation of 1-naphthol glucuronide and 1-naphthol sulphate was assessed at various times by thin layer chromatography. Cells from all five tissues conjugated 1-naphthol. The velocity of conjugation was fastest in jejunals cells (2.9 nmole/mg prot X min) followed by cells from liver, ileum, colon and kidney (0.2 nmole/mg prot X min). The apparent Km-values for glucuronidation ranged from 17 microM to 32 microM, and those for sulphation from 15 microM to 35 microM. Each tissue had a specific conjugation pattern. The two extremes were kidney cells, which had a glucuronidation/sulphation ratio in excess of 10, and colon cells which had a ratio of 0.38. The data suggest that these tissues possess different levels of 1-naphthol-conjugating enzymes, which resemble with regard to their apparent substrate affinities.