Competition between two enzymes for substrate removal in liver: modulating effects due to substrate recruitment of hepatocyte activity

Impact of route-dependent phase-II gut metabolism and enterohepatic circulation on the bioavailability and systemic disposition of resveratrol in rats and humans: A comprehensive whole body physiologically-based pharmacokinetic modeling

Resveratrol, a natural polyphenolic phytoalexin, is a dietary supplement that improves the outcomes of metabolic, cardiovascular, and other age-related diseases due to its diverse pharmacological activities. Although there have been several preclinical and clinical investigations of resveratrol, the contributions of gut phase-II metabolism and enterohepatic circulation to the oral bioavailability and pharmacokinetics of resveratrol remain unclear. Furthermore, a physiologically-based pharmacokinetic (PBPK) model that accurately describes and predicts the systemic exposure profiles of resveratrol in clinical settings has not been developed. Experimental data were acquired from several perspectives, including in vitro protein binding and blood distribution, in vitro tissue S9 metabolism, in situ intestinal perfusion, and in vivo pharmacokinetics and excretion studies. Using these datasets, an in-house whole-body PBPK model incorporating route-dependent phase-II (glucuronidation and sulfation) gut metabolism and enterohepatic circulation processes was constructed and optimized for chemical-specific parameters. The developed PBPK model aligned with the observed systemic exposure profiles of resveratrol in single and multiple dosing regimens with an acceptable accuracy of 0.538-0.999-fold errors. Furthermore, the model simulations elucidated the substantial contribution of gut first-pass metabolism to the oral bioavailability of resveratrol and suggested differential effects of enterohepatic circulation on the systemic exposure of resveratrol between rats and humans. After partial modification and verification, our proposed PBPK model would be valuable to optimize dosage regimens and predict food-drug interactions with resveratrol-based natural products in various clinical scenarios.

Disposition of the Emerging Brominated Flame Retardant, 2-Ethylhexyl 2,3,4,5-Tetrabromobenzoate, in Female SD Rats and Male B6C3F1 Mice: Effects of Dose, Route, and Repeated Administration

2-Ethylhexyl-2,3,4,5-tetrabromobenzoate (EH-TBB; MW 549.92 g/mol; CAS 183658-27-7) is a brominated component of flame retardant mixtures used as substitutes for some PBDEs. EH-TBB is added to various consumer products, including polyurethane foams, and has been detected in humans. The present study characterized the fate of EH-TBB in rodents. [¹⁴C]-labeled EH-TBB was absorbed, metabolized, and eliminated via the urine and feces following single administrations of 0.1-100 µmol/kg (∼0.05-55 mg/kg) or repeated administration (0.1 µmol/kg/day × 5-10 days) by gavage to female Hsd:Sprague DawleySD (SD) rats. Cumulative excretion via feces increased (39-60%) with dose (0.1-10 µmol/kg) with corresponding decreases in urinary excretion (54 to 37%) after 72 h. Delayed excretion of [¹⁴C]-radioactivity in urine and feces of a 100 µmol/kg oral dose was noted. Recovery was complete for all doses by 72 h. IV-injected rats excreted more of the 0.1 µmol/kg dose in urine and less in feces than did gavaged rats, indicating partial biliary elimination of systemically available compound. No tissue bioaccumulation was found for rats given 5 oral daily doses of EH-TBB. Parent molecule was not detected in urine whereas 2 metabolites, tetrabromobenzoic acid (TBBA), a TBBA-sulfate conjugate, and a TBBA-glycine conjugate were identified. EH-TBB and TBBA were identified in extracts from feces. Data from gavaged male B6C3F1/Tac mice indicated minimal sex- or species differences are likely for the disposition of EH-TBB. Approximately 85% of a 0.1 µmol/kg dose was absorbed from the gut. Overall absorption of EH-TBB is expected to be even greater at lower levels.

Upregulation of UGT2B4 Expression by 3'-Phosphoadenosine-5'-Phosphosulfate Synthase Knockdown: Implications for Coordinated Control of Bile Acid Conjugation

During cholestasis, the bile acid-conjugating enzymes, SULT2A1 and UGT2B4, work in concert to prevent the accumulation of toxic bile acids. To understand the impact of sulfotransferase deficiency on human hepatic gene expression, we knocked down 3'-phosphoadenosine-5'-phosphosulfate synthases (PAPSS) 1 and 2, which catalyze synthesis of the obligate sulfotransferase cofactor, in HepG2 cells. PAPSS knockdown caused no change in SULT2A1 expression; however, UGT2B4 expression increased markedly (∼41-fold increase in UGT2B4 mRNA content). Knockdown of SULT2A1 in HepG2 cells also increased UGT2B4 expression. To investigate the underlying mechanism, we transfected PAPSS-deficient HepG2 cells with a luciferase reporter plasmid containing ∼2 Kb of the UGT2B4 5'-flanking region, which included a response element for the bile acid-sensing nuclear receptor, farnesoid X receptor (FXR). FXR activation or overexpression increased UGT2B4 promoter activity; however, knocking down FXR or mutating or deleting the FXR response element did not significantly decrease UGT2B4 promoter activity. Further evaluation of the UGT2B4 5'-flanking region indicated the presence of distal regulatory elements between nucleotides -10090 and -10037 that negatively and positively regulated UGT2B4 transcription. Pulse-chase analysis showed that increased UGT2B4 expression in PAPSS-deficient cells was attributable to both increased mRNA synthesis and stability. Transfection analysis demonstrated that the UGT2B4 3'-untranslated region decreased luciferase reporter expression less in PAPSS-deficient cells than in control cells. These data indicate that knocking down PAPSS increases UGT2B4 transcription and mRNA stability as a compensatory response to the loss of SULT2A1 activity, presumably to maintain bile acid-conjugating activity.

Interplay of transporters and enzymes in drug and metabolite processing

This review highlights the "interplay" between enzymes and transporters, essential components of eliminating organs for drug removal. The understanding of the interplay is important in terms of deciphering the change of one eliminatory pathway on compensatory mechanisms in drug disposal, and, ultimately, their importance in drug-drug interactions. Controversy existed on the explanation underlying the interplay between transporters and enzymes in the Caco-2 cell monolayer or cell culture systems, but less so on eliminating organs such as the intestine and liver. For the Caco-2 system, the increase in the mean residence time (MRT) accompanying increased secretion had been construed as the basis for increased metabolism. We hold the opposite view and assert that increased secretion should evoke a decrease in metabolism due to the competition between the enzyme and apical efflux transporter for the drug within the cell. To illustrate this point, simulations on the MRT, fraction of dose metabolized (f(met)) and the extraction ratio (ER) as defined by various investigators under linear and nonlinear metabolic conditions were compared to observed data and the trends upon induction/inhibition of secretion. The conclusion is that the f(met) is the more appropriate index to reflect the extent of metabolism in transporter-enzyme interplay, since the parameter captures drug metabolism in the cell when its contents in the apical, cell, and basolateral compartments or the entire dose is considered to be available for metabolism. This parameter for metabolism (f(met)) bears a reciprocal relationship to the secretory intrinsic clearance and is in concordance with the notion that both the enzyme and apical transporter compete for the cellular substrate within. For the liver and intestine, several physiologically based pharmacokinetic (PBPK) models that contain transporters and enzymes were utilized, together with the solved equations for the area under the curve (AUC), metabolic, excretory, and total clearance (CL) to shed meaningful insight of how the inhibition of one pathway can result in a higher AUC and therefore a reduced total clearance for drug, but a higher apparent clearance of the alternate pathway; induction of the same pathway would lead to an increased total clearance but decreased drug AUC, and reduced clearance of the alternate pathway. The use of an increased MRT to explain increased extents of metabolism upon increased apical excretion is not tenable in these organs or "open systems" since the MRT of drug in the cell is reduced with irreversible loss from biliary excretion or hastened gastrointestinal transit of the secreted drug in the lumen. Data in the literature for the Caco-2 system, knockout animals and organ perfusion systems were discussed in relation to these concepts on clearance based on fundamental, pharmacokinetic theory. The shortcomings in data interpretation were discussed. The general conclusion is that a reciprocal relationship exists between the clearances related to enzymes and apical transporters due to their competition for the substrate within the cell, and is a relationship independent of the MRT of drug in the system.

Formed and preformed metabolites: facts and comparisons

The administration of metabolites arising from new drug entities is often employed in drug discovery to investigate their associated toxicity. It is expected that administration of metabolites can predict the exposure of metabolites originating from the administration of precursor drug. Whether exact and meaningful information can be obtained from this has been a topic of debate. This communication summarizes observations and theoretical relationships based on physiological modelling for the liver, kidney and intestine, three major eliminating organs/tissues. Theoretical solutions based on physiological modelling of organs were solved, and the results suggest that deviations are expected. Here, examples of metabolite kinetics observed mostly in perfused organs that did not match predictions are provided. For the liver, discrepancies in fate between formed and preformed metabolites may be explained by the heterogeneity of enzymes, the presence of membrane barriers and whether transporters are involved. For the kidney, differences have been attributed to glomerular filtration of the preformed but not the formed metabolite. For the intestine, the complexity of segregated flows to the enterocyte and serosal layers and differences in metabolism due to the route of administration are addressed. Administration of the metabolite may or may not directly reflect the toxicity associated with drug use. However, kinetic data on the preformed metabolite will be extremely useful to develop a sound model for modelling and simulations; in-vitro evidence on metabolite handling at the target organ is also paramount. Subsequent modelling and simulation of metabolite data arising from a combined model based on both drug and preformed metabolite data are needed to improve predictions on the behaviours of formed metabolites.

Effects of protein calorie malnutrition on the pharmacokinetics of ketamine in rats

The effect of protein calorie malnutrition (PCM) on the pharmacokinetics of ketamine (KET) enantiomers has been investigated. Six control and six PCM rats were administered 85 mg/kg racemic KET by intramuscular injection, and plasma concentrations of (S)- and (R)-KET, norketamine (NKET), and 5,6-dehydronorketamine (DNK) were measured using enantioselective gas chromatography. Pharmacokinetic profiles were analyzed using standard noncompartmental and compartmental modeling methods. The volumes of distribution were similar between control and PCM rats for (S)- and (R)-KET. However, total clearance of both KET enantiomers was decreased, resulting in an increase in systemic exposure (p < 0.05). The KET absorption rate was also increased in PCM rats. A decrease in the clearance of both NKET enantiomers led to a significant increase in exposure in PCM rats (p < 0.005), and modeling results could not exclude the possibility that PCM induced an increase in the fraction of KET following the NKET pathway, which may further contribute to this increase in exposure. An increase in exposure to DNK enantiomers was also evident in PCM animals compared with controls [p < 0.005 (DNK1); N.S. (DNK2)], which was in concordance with the decrease in apparent clearance values. These results show that PCM significantly alters the pharmacokinetics of KET and several of its metabolites.

THE ROLES OF TRANSPORTERS AND ENZYMES IN HEPATIC DRUG PROCESSING

A simple, physiological model was used to illustrate the competing nature of transporters and metabolic enzymes in hepatic drug processing. Enalapril, a drug whose basolateral influx and canalicular efflux are mediated by rat organic anion-transporting polypeptide 1 (Oatp1) and rat multidrug resistance-associated protein 2 (Mrp2), respectively, and metabolism by the carboxylesterases, was enlisted as the example to illustrate how the transport and intrinsic clearances are inter-related in the estimation of the hepatic and metabolic, and excretion clearances. Moreover, simulations were performed to explore the effects of inhibitors or inducers of transporters/enzymes to unravel the compensatory changes of alternate pathways. Generally speaking, inhibition of one pathway led to an apparent increase in the alternate (competing) pathway and total hepatic clearance was decreased; induction would lead to an apparent decrease in the alternate pathway and an increase in total hepatic clearance. A reduction in influx clearance brought about parallel decreases in the biliary and metabolic clearances, whereas a reduction in efflux basolateral clearance evoked similar increases in biliary and metabolic clearances. However, the steady-state tissue concentration (C(L,ss)) or area under the tissue concentration-time curve (AUC(L)) was reliant only on the unbound fraction in liver, and the secretory and metabolic intrinsic clearances and not the influx and efflux clearances. Variations in the influx and efflux intrinsic clearances evoked temporal changes in the tissue concentration-time profile but not the AUC(L) or C(L,ss). The pharmacokinetic theory developed offers data interpretation from literature reports on P-glycoprotein and cytochrome P450 substrates with mdr1a/1b knockout versus wild-type mice, and rat liver perfusion studies, with and without the use of inhibitors. In some cases, critiques on data interpretation were made.

Monitoring in situ liver metabolism in rats using microdialysis. Comparison of microdialysis mass-transport model predictions to experimental metabolite generation data

The generation of metabolites from two model compounds, phenacetin and acetaminophen, included in the perfusion fluid of a microdialysis probe implanted into rat liver was studied. When 60 microM phenacetin was included in the perfusion fluid using a flow rate of 1.0 microL/min, acetaminophen and acetaminophen sulfate were recovered at concentrations that ranged between 0.4 and 1.6 microM. Acetaminophen sulfate ([AS]gain) diffused back into the microdialysis probe on a micromolar percentage basis of 8.9+/-2.4% (n = 3) when acetaminophen was passed through the probe at a concentration between 11 and 12 microM. When 220-240 microM acetaminophen was passed through the probe, the percentage of acetaminophen sulfate recovered was 4.8+/-1.4% (n = 3) (P < 0.1 compared to the 11 microM group). No acetaminophen glucuronide was detected in the dialysate samples. A mathematical model that describes mass transport in microdialysis sampling was used to predict the concentration of metabolite that could be recovered into the dialysate after the loss of a substrate compound that undergoes metabolism. The model predicts a metabolite recovery of 23.6% using estimates for phenacetin metabolism and 21.5% using estimates for acetaminophen metabolism. The results presented here indicate that microdialysis has potential to be used to study local in situ metabolism and with further refinements of the microdialysis mass-transport model may be used to estimate in vivo metabolic formation rates.

Organ clearance concepts: new perspectives on old principles

The removal capacity of an eliminating organ by metabolism and/or excretion is often expressed as its clearance. Metabolic and excretory clearances are considered to be mutually independent, and the sum of these constitute the whole organ clearance. The influence of metabolism on estimates of the excretory clearance and vice versa was examined for the liver and kidney with physiologically based models. Mass transfer first-order rate equations describing transport and removal were derived. Upon inversion of the matrices originating from the coefficients of these equations, the area under the curve (AUC) and clearance (dose/AUC) were obtained with the liver or kidney as the eliminating organ. A more complex solution was found to exist for the kidney since glomerular filtration, secretion; reabsorption, and intrarenal metabolism were present. To ascertain the effect of excretion on estimates of the metabolic clearance as well as the effect of metabolism on estimates of the excretory clearance, intrinsic clearances for excretion or metabolism were set to zero. Clearance values were found to be altered when alternate pathways were present. Whereas excretory clearance estimates were consistently reduced in the presence of metabolism, metabolic clearance estimates were affected differentially by excretion and varied according to the site of metabolism. Excretion reduced metabolic clearance estimates when metabolism occurred intracellularly. If metabolism occurred intraluminally (e.g., on the renal brush border or luminal membrane), the metabolic clearance estimate could become higher since the substrate was made available to the enzymes following its excretion. As expected, these changes depended on the relative magnitudes of the intrinsic clearances for metabolism and excretion. The above theory was applied to the elimination of enalapril which is both metabolized and excreted by the perfused rat liver and kidney preparations. Data obtained in these studies were consistent with a set of published physiologic parameters denoting transfer and intrinsic clearances. Perturbations on clearance estimates were studied by setting the metabolic/excretory intrinsic clearance to zero, then to some finite value. In liver, the avid hepatocellular metabolism of enalapril reduced biliary clearance by 73%. For the kidney, the fractional excretion (FE or unbound excretory clearance/glomerular filtration rate) was decreased modestly (from 0.64 to 0.44) with intracellular esterolysis, whereas if metabolism had occurred intraluminally, FE would have been significantly decreased (from 1.8 to 0.45). Simulation results show clearly that clearance estimates are affected by the presence of alternate removal pathways, and question the well-established principle the metabolic and excretory clearance estimates are independent of each other.

Membrane transport in hepatic clearance of drugs. II: Zonal distribution patterns of concentration-dependent transport and elimination processes

PURPOSE

The objective of the present simulation study was to investigate the effects of hepatic zonal heterogeneity of membrane transporter proteins and intrinsic elimination activities on hepatic clearance (CL) and drug concentration gradient profiles in the sinusoidal blood and hepatocytes.

METHODS

The model used in the simulations assumes an apparent unidirectional carrier-mediated transport and a bidirectional diffusion of substrates in the hepatic sinusoidal membrane as well as a nonlinear intrinsic elimination. Three different distribution patterns of the transporter and the metabolizing enzyme along the sinusoidal flow path were used for the simulations. The effects of changes in the Michaelis-Menten parameters for those nonlinear processes, and in the unbound fractions of the drug in blood and tissue components were investigated.

RESULTS

Significant differences in CL occurred when the distribution patterns of the transporter and/or the metabolizing enzyme activities were altered under nonlinear conditions. The highest CL values were observed when the transporter and the metabolizing enzyme had similar distribution patterns within the liver acinus, while opposite distribution patterns produced the lowest CL values. Tissue concentration profiles were significantly affected by the distribution patterns of the transporter, but the changes in blood concentration profiles were relatively small. Altering protein binding in blood produced significant changes in CL, and blood and tissue concentration gradients, while altering protein binding in tissue affected only drug accumulation patterns within hepatocytes, regardless of the distribution patterns of the transporter or the metabolizing enzyme.

CONCLUSIONS

The present simulations demonstrate that hepatic zonal heterogeneities in the transporter and the metabolizing enzyme activities can significantly influence hepatic clearance and/or drug concentration gradient profiles in the sinusoidal blood and hepatocytes.

Hepatic conjugation/deconjugation cycling pathways. Computer simulations examining the effect of Michaelis-Menten parameters, enzyme distribution patterns, and a diffusional barrier on metabolite disposition

Conjugation/deconjugation cycling plays an important role in the physiologic regulation of the concentration of endogenous compounds that form conjugated metabolites. Less is known concerning the deconjugation of xenobiotics. The model compound p-nitrophenol (pNP) is conjugated to sulfate and glucuronide metabolites which can also undergo hydrolysis, via separate enzyme systems, to regenerate pNP. In the present investigation, computer simulations were performed using literature values for the KM and Vmax for each of the four enzyme systems involved in net pNP conjugation. The apparent sulfation rate, apparent glucuronidation rate, and the extraction ratio (E) of pNP were each examined (i) as a function of pNP concentration, (ii) following alterations in the KM and Vmax values for the deconjugation enzymes, (iii) after modulating the enzyme distribution patterns along the liver flow path for both the conjugating and deconjugating enzymes, and (iv) in the presence of drug metabolite diffusional barriers for membrane transport. Results of these simulations demonstrated that changes in the KM or Vmax for deglucuronidation produced changes not only in net glucuronidation but also in net sulfation. Overall extraction (E) of the parent compound was only affected when glucuronidation was an important pathway, i.e., at higher pNP concentrations. Similar results were observed with changes in desulfation, with desulfation having the greatest effects at low pNP concentrations where sulfation represents the predominant metabolic pathway. Changes in the enzyme distribution patterns for the deconjugation pathways showed that the greatest influence on net conjugation rates occurred when hydrolase enzyme activity was distributed downstream from the respective forward reaction. In the presence of a diffusional barrier for metabolite transport (i.e., when the diffusional clearance was one tenth of blood flow), net metabolism of parent was diminished with E decreasing from 0.74, in the absence of a barrier, to 0.23, since the generated metabolite remained, to a great extent, within hepatocytes and underwent a more pronounced hydrolysis. In the presence of diffusional barriers for uptake of the conjugated metabolites, the lowest drug extraction and metabolite formation rates were observed when the distribution of the conjugation and deconjugation pathways across the liver were the same. Therefore, the effects of deconjugation on hepatic drug removal and metabolite formation are highly dependent on the enzymatic parameters of both the forward and reverse reactions, the parent drug concentration, the enzyme distribution patterns, and the presence of diffusional barriers for metabolite membrane transport. Since a change in the deconjugation of one metabolite can influence the net formation of not only itself but also other metabolites, and overall drug extraction, evaluation of conjugation/deconjugation cycling represents an important consideration in pharmacokinetic studies involving physiological-, pathological-, or pharmacological-induced alterations in conjugate formation.

Benzene: a case study in parent chemical and metabolite interactions

Benzene, an important industrial solvent, is also present in unleaded gasoline and cigarette smoke. The hematotoxic effects of benzene in humans are well documented and include aplastic anemia and pancytopenia, and acute myelogenous leukemia. A combination of metabolites (hydroquinone and phenol for example) is apparently necessary to duplicate the hematotoxic effect of benzene, perhaps due in part to the synergistic effect of phenol on myeloperoxidase-mediated oxidation of hydroquinone to the reactive metabolite benzoquinone. Since benzene and its hydroxylated metabolites (phenol, hydroquinone and catechol) are substrates for the same cytochrome P450 enzymes, competitive interactions among the metabolites are possible. In vivo data on metabolite formation by mice exposed to various benzene concentrations are consistent with competitive inhibition of phenol oxidation by benzene. In vitro studies of the metabolic oxidation of benzene, phenol and hydroquinone are consistent with the mechanism of competitive interaction among the metabolites. The dosimetry of benzene and its metabolites in the target tissue, bone marrow, depends on the balance of activation processes such as enzymatic oxidation and deactivation processes such as conjugation and excretion. Phenol, the primary benzene metabolite, can undergo both oxidation and conjugation. Thus, the potential exists for competition among various enzymes for phenol. However, zonal localization of Phase I and Phase II enzymes in various regions of the liver acinus regulates this competition. Biologically-based dosimetry models that incorporate the important determinants of benzene flux, including interactions with other chemicals, will enable prediction of target tissue doses of benzene and metabolites at low exposure concentrations relevant for humans.

Concentration-dependent metabolism of diazepam in mouse liver

Previous mouse liver studies with diazepam (DZ), N-desmethyldiazepam (NZ), and temazepam (TZ) confirmed that under first-order conditions, DZ formed NZ and TZ in parallel. Oxazepam (OZ) was generated via NZ and not TZ despite that preformed NZ and TZ were both capable of forming OZ. In the present studies, the concentration-dependent sequential metabolism of DZ was studied in perfused mouse livers and microsomes, with the aim of distinguishing the relative importance of NZ and TZ as precursors of OZ. In microsomal studies, the Kms and Vmaxs, corrected for binding to microsomal proteins, were 34 microM and 3.6 nmole/min per mg and 239 microM and 18 nmole/min per mg, respectively, for N-demethylation and C3-hydroxylation of DZ. The Kms and Vmaxs for N-demethylation and C3-hydroxylation of TZ and NZ, respectively, to form OZ, were 58 microM and 2.5 nmole/min per mg and 311 microM and 2 nmole/min per mg, respectively. The constants suggest that at low DZ concentrations, NZ formation predominates and is a major source of OZ, whereas at higher DZ concentrations, TZ is the important source of OZ. In livers perfused with DZ at input concentrations of 13 to 35 microM, the extraction ratio of DZ (E[DZ]) decreased from 0.83 to 0.60. NZ was the major metabolite formed although its appearance was less than proportionate with increasing DZ input concentration. By contrast, the formation of TZ increased disproportionately with increasing DZ concentration, whereas that for OZ decreased and paralleled the behavior of NZ. Computer simulations based on a tubular flow model and the in vitro enzymatic parameters provided a poor in vitro-organ correlation. The E[DZ], appearance rates of the metabolites, and the extraction ratio of formed NZ (E[NZ, DZ]) were poorly predicted; TZ was incorrectly identified as the major precursor of OZ. Simulations with optimized parameters improved the correlations and identified NZ as the major contributor of OZ. Saturation of DZ N-demethylation at higher DZ concentrations increased the role of TZ in the formation of OZ. The poor aqueous solubility (limiting the concentration range of substrates used in vitro), avid tissue binding and the coupling of enzymatic reactions in liver, favoring sequential metabolism, are possible explanations for the poor in vitro-organ correlation. This work emphasizes the complexity of the hepatic intracellular milieu for drug metabolism and the need for additional modeling efforts to adequately describe metabolite kinetics.

Dose-, route-, and sex-dependent urinary excretion of phenol metabolites in B6C3F1 mice

Phenol is the major oxidized metabolite of benzene, a known human leukemogen and ubiquitous environmental pollutant. Unlike benzene, phenol does not induce tumors in mice following oral exposure; benzene also exhibits sex-related differences in genotoxicity to bone marrow cells that are not observed following phenol administration. We studied the urinary excretion of phenol metabolites in mice as a means to further investigate the metabolic basis for differences in benzene- and phenol-induced toxicity. Male and female B6C3F1 mice (n = 3/group) were exposed to 15, 40, 100, or 225 mumol [14C]phenol/kg by i.v. tail vein injection (6 microCi/mouse). First-pass intestinal metabolism of phenol was evaluated by comparison of urinary excretion of phenol metabolites following i.v. administration with additional groups of male mice that received the same dose levels by oral gavage. Mice were placed in glass metabolism cages, and urine was collected over dry ice for 48 h. Urinary metabolites were separated by high-pressure liquid chromatography (HPLC) and quantified by liquid scintillation spectrometry. Urinary excretion of conjugated metabolites of phenol was dose-dependent in both male and female mice administered phenol by i.v. injection or gavage. The major urinary metabolites of phenol were phenol sulfate (PS), phenol glucuronide (PG), and hydroquinone glucuronide (HQG). Sulfation was the dominant pathway at all dose levels, but decreased as a percent of the excreted dose with a concomitant increase in glucuronidation as the dose level increased. Male mice consistently excreted a higher proportion of phenol as the oxidized conjugated metabolite, HQG, compared to female mice, suggesting that male mice oxidize phenol to hydroquinone more rapidly than female mice. Increased oxidation of phenol to hydroquinone by male mice compared to female mice is consistent with both the greater sensitivity of male mice to the genotoxic effects of benzene and the greater potency of hydroquinone compared to phenol as a genotoxicant. Intestinal conjugation of phenol prior to absorption was significant only at low doses and thus alone does not provide an explanation for the lack of carcinogenicity of phenol in bioassays conducted at much higher dose levels.

Transport, binding, and metabolism of sulfate conjugates in the liver

Sulfate conjugates are a heterogeneous class of polar, anionic metabolites that result from the conjugation of endogenous and exogenous compounds. Sulfate conjugates exhibit a high degree of binding to albumin, the extent of which usually exceeds those of their parent compounds. Preponderant direct and indirect evidence suggests that sulfation activity is slightly higher in the periportal than in the perivenous (centrilobular) region of the liver, but recent immunohistochemical studies imply that specific isoforms of the sulfotransferases may also be preferentially localized in the perivenous region. Entry of sulfate conjugates into the liver cell is poor unless discrete carriers are present. Although known transport carriers exist for the sulfated bile acids, the specificity of the carriers for drug sulfate conjugates is presently unknown. The removal of sulfates is usually by way of biliary excretion while, on occasion, sulfates can be desulfated and participate in futile cycling with their parent compounds. The binding, transport, and hepatic elimination of various drug sulfate conjugates are examined. Non-recirculating studies carried out in the perfused rat liver with the multiple indicator dilution technique under varying input sulfate conjugate concentrations have provided essential information on the effects of vascular (red blood cells and plasma protein) binding on transport and removal of the conjugates. These studies clearly demonstrate the need to study protein binding, transmembrane transfer characteristics across the liver basolateral (sinusoidal) and canalicular membranes, and enzyme zonation in a distributed-in-space fashion in order to properly define the handling of sulfate conjugates in the liver.

Nonlinear protein binding and enzyme heterogeneity: effects on hepatic drug removal

The kinetics of substrate removal by the liver and the resulting nonlinear changes in unbound fraction along the flow path at varying input drug concentrations were examined by a model simulation study. Specifically, we varied the binding association constant, KA, and the Michaelis-Menten constants (Km and Vmax) to examine the steady state drug removal (expressed as hepatic extraction ratio E) and changes in drug binding for (i) unienzyme systems and (ii) simple, parallel metabolic pathways; zonal metabolic heterogeneity was also added as a variable. At low KA, E declined with increasing input drug concentration, due primarily to saturation of enzymes; only small differences in binding were present across the liver. At high KA, a parabolic profile for E with concentration was observed; changes in unbound fraction between the inlet and the outlet of the liver followed in parallel fashion. Protein binding was the rate-determining step at low input drug concentrations, whereas enzyme saturation was the rate-controlling factor at high input drug concentration. Heterogeneous enzymic distribution modulated changes in unbound fraction within the liver and at the outlet. Despite marked changes in unbound fraction occurring within the liver for different enzymic distributions, the overall transhepatic differences were relatively small. We then investigated the logarithmic average unbound concentration and the length averaged concentration as estimates of substrate concentration in liver in the presence of nonlinear drug binding. Fitting of simulated data, with and without assigned random error (10%), to the Michaelis-Menten equation was performed; fitting was repeated for simulated data obtained with presence of a specific inhibitor of the high-affinity, anteriorly distributed pathway. Results were similar for both concentration terms: accurate estimates were obtained for anterior, high affinity pathways; an overestimation of parameters was observed for the lower affinity posteriorly distributed pathways. Improved estimations were found for posteriorly distributed pathways upon inhibition with specific inhibitors; with added random error, however, the improvement was much decreased. We applied the method for fitting of several sets of metabolic data obtained from rat liver perfusion studies performed with salicylamide (SAM) (i) without and (ii) with the presence of 2,6-dichloro-4-nitrophenol (DCNP), a SAM sulfation inhibitor. The fitted results showed that SAM sulfation was a high-affinity high-capacity pathway; SAM glucuronidation was of lower affinity but comparable capacity as the sulfation pathway, whereas SAM hydroxylation was of lower affinity and lower capacity.

Hepatic modeling of metabolite kinetics in sequential and parallel pathways: salicylamide and gentisamide metabolism in perfused rat liver

Previous data on salicylamide (SAM) metabolism in the perfused rat liver had indicated that SAM was metabolized by three parallel (competing) pathways: sulfation, glucuronidation, and hydroxylation, whereas sequential metabolism of the hydroxylated metabolite, gentisamide (GAM), was solely via 5-glucuronidation to form GAM-5G. However, under comparable conditions, preformed GAM formed mainly two monosulfate conjugates at the 2- and 5-positions (GAM-2S and GAM-5S); 5-glucuronidation was a minor pathway. In the present study, the techniques of normal (N) and retrograde (R) rat liver perfusion with SAM and mathematic modeling on SAM and GAM metabolism were used to explore the role of enzymic distributions in determining the dissimilar fates of GAM, as a generated metabolite of SAM or as preformed GAM. Changes in the steady-state extraction ratio of SAM (E) and metabolite formation ratios between N and R perfusions were used as indices of the uneven distribution of enzyme activities. Two SAM concentrations (134 and 295 microM) were used for single-pass perfusion: the lower SAM concentration exceeded the apparent Km for SAM sulfation but was less than those for SAM glucuronidation and hydroxylation; the higher concentration exceeded the apparent Km's for SAM sulfation and glucuronidation but was less than the Km for hydroxylations. Simulation of SAM metabolism data was carried out with various enzyme distribution patterns and extended to include GAM metabolism. At both input concentrations, E was high (0.94 at 134 microM and 0.7 at 295 microM) and unchanged during N and R, with SAM-sulfate (SAM-S) as the major metabolite and GAM-5G as the only detectable metabolite of GAM. Saturation of SAM sulfation occurred at the higher input SAM concentration as shown by a decrease in E and a proportionally less increase in sulfation rates and proportionally more than expected increases in SAM hydroxylation and glucuronidation rates. At both SAM concentrations, the steady-state ratio of metabolite formation rates for SAM-S/SAM-G decreased when flow direction changed from N to R. An insignificant decrease in SAM-S/SAM-OH was observed at the low input SAM concentration, due to the small amount of SAM-OH formed and hence large variation in the ratio among the preparations, whereas at the high input SAM concentration, the decrease in SAM-S/SAM-OH with a change in flow direction from N to R was evident. The metabolite formation ratio, SAM-G/SAM-OH, however, was unchanged at both input concentrations and flow directions.(ABSTRACT TRUNCATED AT 400 WORDS)

The multiple-indicator dilution technique for characterization of normal and retrograde flow in once-through rat liver perfusions

The technique of normal and retrograde rat liver perfusion has been widely used to probe zonal differences in drug-metabolizing activities. The validity of this approach mandates the same tissue spaces being accessed by substrates during both normal and retrograde perfusions. Using the multiple-indicator dilution technique, we presently examine the extent to which retrograde perfusion alters the spaces accessible to noneliminated references. A bolus dose of 51Cr-labeled red blood cells, 125I-albumin, 14C-sucrose and 3H2O was injected into the portal (normal) or hepatic (retrograde) vein of rat livers perfused at 10 ml per min per liver. The outflow perfusate was serially collected over 220 sec to characterize the transit times and the distribution spaces of the labels. During retrograde perfusion, red blood cells, albumin and sucrose profiles peaked later and lower than during normal perfusion, whereas the water curves were similar. The transit times of red blood cells, albumin and sucrose were longer (p less than 0.005), whereas those for water did not change. Consequently, retrograde flow resulted in significantly larger sinusoidal blood volumes (45%), albumin Disse space (42%) and sucrose Disse space (25%) than during normal flow, whereas the distribution spaces for total and intracellular water remained unaltered. The distension of the vascular tree was confirmed by electron microscopy, by which occasional isolated foci of widened intercellular recesses and spaces of Disse were observed. Cellular ultrastructure was otherwise unchanged, and there was no difference found between normal and retrograde perfusion for bile flow rates, AST release, perfusion pressure, oxygen consumption and metabolic removal of ethanol, a substrate with flow-limited distribution, which equilibrates rapidly with cell water (hepatic extraction ratios were virtually identical: normal vs. retrograde, 0.50 vs. 0.48 at 6 to 7.4 mM input concentration). These findings suggest that the functional and metabolic capacities of the liver remain unperturbed during retrograde perfusion, rendering the technique suitable for the investigation of zonal differences in drug-metabolizing enzymes.

Competing pathways in drug metabolism. II. An identical, anterior enzymic distribution for 2- and 5-sulfoconjugation and a posterior localization for 5-glucuronidation of gentisamide in the rat liver

Gentisamide (2,5-dihydroxybenzamide, GAM), a substrate that forms two monosulfates at the 2 and 5 positions (GAM-2S and GAM-5S) and a monoglucuronide at the 5 position (GAM-5G), was delivered at 8 or 80 microM by normal (N) and retrograde (R) flows to the once-through rat liver preparation. At the lower (8 microM) input concentration, ratios of conjugate formation rate, GAM-5S/GAM-5G and GAM-2S/GAM-5G, were decreased significantly (4.01 +/- 1.42 to 2.93 +/- 0.99, and 1.13 +/- 0.65 to 0.66 +/- 0.41, respectively) whereas a small but significant increase in the steady-state extraction ratio, E (0.89 +/- 0.029 to 0.94 +/- 0.016), was observed upon changing the flow direction from N to R. At the higher input GAM concentration (80 microM), conjugate formation rate ratios were relatively constant for GAM-5S/GAM-5G (1.18 +/- 0.08 to 1.11 +/- 0.12) and GAM-2S/GAM-5G (0.33 +/- 0.05 to 0.31 +/- 0.05) upon changing flow direction from N to R, despite a slight increase in E from 0.87 +/- 0.023 to 0.91 +/- 0.016 was observed. These results suggest that the sulfotransferase activities responsible for 2- and 5-sulfoconjugations are identically distributed and localized anterior to 5-glucuronidation activities during a normal flow of substrate into the rat liver (entering the portal vein and exiting the hepatic vein), and that the presence of uneven distribution of conjugation activities is discriminated only at the lower input drug concentration. At high concentration (greater than Km for all systems), saturation of all pathways occurs, and other anteriorly/identically distributed competing pathways would fail to perturb downstream intrahepatic drug concentrations and the resultant conjugation rates. The lack of change in metabolic profiles renders the condition unsuitable for examination of uneven distribution of enzymes in the liver. These observations were generally predicted by theoretical enzymic models of consistent distribution patterns. Because 2- and 5-sulfation were mediated by systems of similar Km but different Vmax values, two possibilities, the same isozyme of sulfotransferase being involved in the formation of two enzyme-substrate complexes to form two distinctly different products or two isozymes of sulfotransferases of identical distribution, were discussed.