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K S Pang

Publications and source records attributed to K S Pang.

At least 91 records · Page 5Linked to original sources
PubMed1989

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)

Animals↗
PubMed1989

Viability of the vascularly perfused, recirculating rat intestine and intestine-liver preparations.

Function and stability of vascularly perfused, recirculating in situ rat intestine (I) and intestine-liver (IL) preparations were evaluated in fasted and nonfasted rats because these techniques may be readily applied in drug metabolism studies. The rat intestine was perfused with blood medium (7.5 ml/min) via the superior mesenteric artery, with the venous outflow draining into the portal vein, which, together with hepatic arterial flow (2.5 ml/min), constituted the total blood flow (10 ml/min) to the liver. Maintenance of intestinal membrane integrity was observed. Rapid [14C]glucose absorption against a concentration gradient and a lack of [3H]-polyethylene glycol 4000 (PEG 4000, less than 4%) and Evans blue absorption by the recirculating I and IL preparations resulted after bolus injections of these markers into the pyloric end of the duodenum. Other indexes that revealed stable intestinal and liver functions were the following: preservation of reservoir perfusate volume, constancy in perfusion pressure, bile flow, and hemoglobin concentrations, evidence of intestinal glucose utilization and liver glucose production, and a lack of significant leakage of serum glutamic oxalic transaminase. The intestine and liver consumed oxygen at relatively constant rates, but the consumption rates for the fasted tissues (I or L) were significantly higher than those for nonfasted tissues. These results indicate that the vascularly perfused I and IL preparations were maintained in a viable and stable state for a 2-h perfusion period.

Animals↗
PubMed1989

Renal handling of enalapril and enalaprilat: studies in the isolated red blood cell-perfused rat kidney.

An isolated recirculating or single pass red cell-perfused rat kidney preparation (IPK) was used to examine the differential handling of renal metabolites. In single pass experiments, enalapril was primarily metabolized to its polar, dicarboxylic acid metabolite, enalaprilat, and its fractional excretion (FE) was less than unity, suggesting net reabsorption. Its steady-state extraction ratio decreased from 0.3 to 0.2 at concentrations of 1.06 to 12.7 microM, due to a saturation of enzymes for esterolysis. Enalaprilat administered to the IPK was excreted into urine in a concentration-independent (0.41-35.3 microM) fashion, with FE values approximating unity, suggesting net filtration. Differences in handling were observed for enalaprilat, as a metabolite formed from enalapril and as an administered (preformed) species in the single pass IPK, when tracer concentrations of [14C]enalapril and [3H]enalaprilat were given simultaneously. A comparison made between steady-state extraction ratio Ess[mi] [generated metabolite]/glomerular filtration rate (GFR) and Ess[pmi] [preformed metabolite]/GFR, respectively, revealed a 2-fold difference. The finding suggests the presence of a barrier for entry of enalaprilat into the kidney. Or else, in absence of the barrier, the opposite would be observed, that is, Ess [pmi]/GFR greater than Ess [mi]/GFR because preformed enalaprilat, in contrast to generated enalaprilat, undergoes filtration and utilizes facilitative transport carriers at the basolateral membrane. In recirculating IPKs which received simultaneously a tracer bolus dose of [14C]enalapril and [3H]enalaprilat, the FE values for generated [14C]enalaprilat were high and variable, decreasing with perfusion time and exceeding those for preformed [3H]enalaprilat, which approached unity with perfusion time. The variable FE values for [14C]enalaprilat are due to time-dependent contributions of circulating enalaprilat (which behaves identically to preformed enalaprilat) and the intrarenally generated enalaprilat. Hence, with renal drug metabolism, the conventional method of estimating urinary clearance (or Fe[mi]) for the metabolite [(total) excretion rate/midpoint plasma FE[mi] metabolite concentration] results in a greater metabolite clearance than that predicted from the administration of preformed metabolite.

Animals↗
PubMed1988

Effects of perfusate flow rate on measured blood volume, disse space, intracellular water space, and drug extraction in the perfused rat liver preparation: characterization by the multiple indicator dilution technique.

The effect of hepatic blood flow on the elimination of several highly cleared substrates was studied in the once-through perfused rat liver preparation. A constant and low input concentration of ethanol (2.0 mM), [14C]-phenacetin and [3H]-acetaminophen (0.36 and 0.14 microM, respectively), or meperidine (8.1 microM) was delivered once-through the rat liver preparation in five flow periods (greater than 35 min each); control flow periods at 12 ml/min were interrupted by flow changes to 8 or 16 ml/min. The steady-state hepatic availabilities (F or outflow survivals) at 12 ml/min were ethanol, 0.075 +/- 0.038; [14C]-phenacetin, 0.15 +/- 0.059; [3H]-acetaminophen, 0.34 +/- 0.051; meperidine, 0.047 +/- 0.017. Flow-induced changes were different among the compounds: with reduced flow (8 ml/min), F was decreased for ethanol (0.061 +/- 0.032) and [3H]-acetaminophen (0.28 +/- 0.051), as expected, but was increased for [14C]-phenacetin (0.20 +/- 0.068) and meperidine (0.05 +/- 0.03); with an elevation of flow (to 16 ml/min), F was increased for all compounds, as expected of shorter sojourn times: ethanol, 0.13 +/- 0.065; [14C]-phenacetin, 0.22 +/- 0.062; [3H]-acetaminophen, 0.43 +/- 0.063; meperidine, 0.055 +/- 0.022. A marked increase in F for ethanol had occurred when flow changed from 12 to 16 ml/min due to nonlinear metabolism; the latter was confirmed by a reduction in the extraction ratios at increasing concentrations (1.8 to 11.4 mM); this condition was not present for the other compounds. In order to explain the observations, we used the multiple indicator dilution technique to investigate the flow-induced behaviors of tissue distribution spaces of vascular and intracellular references in the perfused rat liver preparation.

Acetaminophen↗
PubMed1988

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.

Animals↗
PubMed1988

Primary, secondary, and tertiary metabolite kinetics.

Because of the propensity of nascently formed metabolites towards sequential metabolism within formation organs, theoretical and experimental treatments that achieve mass conservation must recognize the various sources contributing to primary, secondary, and tertiary metabolite formation. A simple one-compartment open model, with first-order conditions and the liver as the only organ of drug disappearance and metabolite formation, was used to illustrate the metabolism of a drug to its primary, secondary, and tertiary metabolites, encompassing the cascading effects of sequential metabolism. The concentration-time profiles of the drug and metabolites were examined for two routes of drug administration, oral and intravenous. Formation of the primary metabolite from drug in the gut lumen, with or without further absorption, and metabolite formation arising from first-pass metabolism of the drug and the primary metabolite during oral absorption were considered. Mass balance equations, incorporating modifications of the various absorption and conversion rate constants, were integrated to provide the explicit solutions. Simulations, with and without consideration of the sources of metabolite formation other than from its immediate precursor, were used to illustrate the expected differences in circulating metabolite concentrations. However, a simple relationship between the area under the curve of any metabolite, M, or [AUC (m)], its clearance [CL(m)], and route of drug administration was found. The drug dose, route, fraction absorbed into the portal circulation, Fabs, fraction available of drug from the liver, F, availabilities of the metabolites F(m) from formation organs, and CL(m) are determinants of the AUC(m)'s. After iv drug dosing, the area of any intermediary metabolites is determined by the iv drug dose divided by the (CL(m)/F(m] of that metabolite. When a terminal metabolite is not metabolized, its area under the curve becomes the iv dose of drug divided by the clearance of the terminal metabolite since the available fraction for this metabolite is unity. Similarly, after oral drug administration, when loss of drug in the gut lumen does not contribute to the appearance of metabolites systematically, the general solution for AUC(m) is the product of Fabs and oral drug dose divided by [CL(m)/F(m)]. A comparison of the area ratios of any metabolite after po and iv drug dosing, therefore, furnishes Fabs. When this fraction is divided into the overall systemic availability or Fsys, the drug availability from the first-pass organs, F, may be found.(ABSTRACT TRUNCATED AT 400 WORDS)

Administration, Oral↗
PubMed1988

Combined hepatic arterial-portal venous and hepatic arterial-hepatic venous perfusions to probe the abundance of drug metabolizing activities: perihepatic venous O-deethylation activity for phenacetin and periportal sulfation activity for acetaminophen in the once-through rat liver preparation.

Although normal-retrograde liver perfusion allows definition of relative distribution patterns of multienzyme systems involved in parallel, competing and sequential metabolic pathways, the technique fails to describe localization patterns for unienzyme systems. A novel method of perfusion, which can describe the relative zonal enrichment of drug metabolizing activity of unienzyme systems in periportal region and in the entire liver, is described. The rat liver was perfused once-through with combined hepatic arterial (2 ml/min/liver-containing drug) and portal or hepatic venous (10 ml/min/liver, without drug) flows [HAPV and HAHV]. Because of known enrichment of phenacetin O-deethylation and acetaminophen sulfation activities in the perihepatic venous and periportal regions, respectively, metabolic data derived with these compounds were used for validation of the method. The overall steady-state hepatic extraction ratio of [3H]phenacetin during HAPV (0.88-0.98) was decreased markedly during HAHV (0.06-0.47), but was reduced to a lesser extent for preformed [3H]acetaminophen (0.57-0.86 for HAPV to 0.23-0.62 for HAHV). Because a comparison of metabolic activities encountered by substrate(s) [estimated from extraction ratios] required normalization to the intracellular water spaces accessed during both HAPV and HAHV, the multiple indicator dilution technique (a bolus injection of noneliminated reference materials: 51Cr-labeled RBC, 125I-labeled albumin, [14C]sucrose and 3H2O) into the hepatic artery during both HAPV and HAHV was used, with outflow profiles from the hepatic vein or portal vein characterized by rapid serial collections. Sinusoidal blood volume, albumin accessible Disse space, sucrose accessible Disse space and the estimated intracellular water space reached during HAHV were 48, 56, 53 and 33% those during HAPV. The ratios of intrinsic clearances per volume of cell water space for HAHV/HAPV, representing periportal activity/whole liver activity were: phenacetin O-deethylation, 0.34 and acetaminophen sulfation, 1.13. These findings validate the use of HAHV and HAPV perfusion for the examination of relative abundances of enzymic activities across the liver acinus for substrates metabolized by unienzyme systems.

Acetaminophen↗
PubMed1988

Competing pathways in drug metabolism. I. Effect of input concentration on the conjugation of gentisamide in the once-through in situ perfused rat liver preparation.

Sulfation and glucuronidation are two parallel pathways for the metabolism of phenolic substrates. Gentisamide (GAM) was used as a model compound to examine the effects of parallel competing pathways on drug disappearance and metabolite formation in the once-through perfused rat liver preparation. GAM was found to form one glucuronide (GAM-5G) and two sulfate (GAM-2S and GAM-5S) conjugates. These GAM conjugates were biosynthesized in recirculating rat liver preparations, and were isolated by preparative high-performance liquid chromatography. Specific incorporation of 35S-sodium sulfate and [14C]glucose into GAM sulfate and glucuronide conjugates revealed corresponding elution patterns as labeled GAM metabolites. Their identities were characterized by enzymatic and acid hydrolyses and by NMR spectroscopy. Gentisamide-5-sulfate (GAM-5S) and gentisamide-5-glucuronide (GAM-5G) are major metabolites, and gentisamide-2-sulfate (GAM-2S) is a minor metabolite. Single-pass rat liver perfusions were used to examine the effect of stepwise increases/decreases of input GAM concentration (CIn) on the extraction ratio (E) of GAM and formation of metabolites. The E of GAM remained constant (about 0.89) at input concentrations from 0.9 to 120 microM and decreased at CIn greater than 120 microM. Metabolite patterns, however, changed with GAM CIn, even when E was constant at CIn up to 120 microM. GAM-5S was present as the major metabolite of GAM at all GAM CInS in most liver preparations but the proportions of GAM-5S and GAM-2S decreased at increasing CIn; the proportion of GAM-5G, a minor metabolite at low CIn, increased with increasing CIn. Biliary excretion rates at steady state accounted for 5.3 +/- 2.7% (mean +/- S.D.) of the input rate: GAM-5G was the predominant metabolite found. Fitting the metabolic data (sum of the rates of efflux in bile and perfusate at steady state) and the logarithmic average drug concentration at the various CIn to the Michaelis-Menten equation furnished parameter estimates for the three metabolic pathways. The estimated Km and Vmax values were quite comparable: for GAM-2S formation, 22 microM and 287 nmol/min; for GAM-5S formation, 26 microM and 978 nmol/min; for GAM-5G formation, 71 microM and 1062 nmol/min, indicating that sulfation and glucuronidation are effective competing pathways of each other. In viewing the fate of GAM over the CIn range, a changing metabolic fate of GAM over a constant E was noted at CIn less than 120 microM: the sum of the rates of metabolite formation was proportional to CIn.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Determination of diazepam and its metabolites by high-performance liquid chromatography and thin-layer chromatography.

A sensitive, simple high-performance liquid chromatographic assay, capable of simultaneously measuring diazepam, its active metabolites oxazepam, temazepam and N-desmethyldiazepam and two phenyl hydroxylated metabolites, 4'-hydroxy-N-desmethyldiazepam and 4'-hydroxydiazepam, is described. The assay is easily modified to include separation of additional metabolite(s), e.g. oxazepam glucuronide(s). A thin-layer chromatographic assay, which resolves diazepam, the active metabolites and the two phenyl hydroxylated derivatives in one solvent system, is also reported. Application of these procedures to the quantitation of diazepam and its metabolites was shown, after delivery of diazepam (5 micrograms/ml or 16 microM) at a constant flow-rate (10 ml/min per liver) through the single-pass perfused rat liver preparation. Blood perfusion medium and bile were analysed for parent drug and metabolites before and after enzyme hydrolysis. These assay methods are found to be particularly pertinent and useful in providing a more comprehensive metabolic profile of diazepam metabolism, especially when aromatic hydroxylation pathways predominate.

Animals↗
PubMed1987

High-performance liquid chromatographic method for the quantitation of salicylamide and its metabolites in biological fluids.

A high-performance liquid chromatographic (HPLC) assay is described for the determination of salicylamide (SAM) and its metabolites in biological fluids obtained from an in situ rat liver preparation and rat in vivo. SAM and its six metabolites, salicylamide sulphate, salicylamide glucuronide, gentisamide (GAM), gentisamide 2-sulphate, gentisamide 5-sulphate and gentisamide 5-glucuronide, were separated on a mu Bondapak C18 column with 0.085 M potassium dihydrogenphosphate pH 3.35, as mobile phase and by using a flow gradient. Enriched recovery of SAM and GAM from blood was achieved by extraction followed by a second HPLC procedure with a mobile phase consisting of acetic acid and methanol. Furthermore, the reproducibility and the stability of the samples under assay conditions were investigated.

Animals↗
PubMed1987

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

Modulating effects of competing pathways, exemplified by sulfation (high affinity-low capacity) and glucuronidation (low affinity-high capacity), on drug disappearance and metabolite formation were investigated in a simulation study. The phenomenon of substrate recruitment of hepatocyte activity in drug removal and metabolite formation was shown with respect to inlet substrate concentration, and drug processing from inlet to outlet by enzyme systems localized differentially along the sinusoidal flow path in liver. Three enzymic distribution models: (A) sulfation and glucuronidation evenly distributed in liver, (b) sulfation occurring exclusively in the first half of the liver and glucuronidation in the second half, and (C) glucuronidation solely in the first half and sulfation in the second half, were described. The influence of Km and Vmax of the competing pathway, including enzyme induction (increase in Vmax), on any given pathway was also explored. Competing pathways exert their effects on other given pathways by modulating intrahepatic drug concentration from the inlet to outlet of the liver. When a competing pathway is similarly distributed or is at an anterior location to another pathway, the former pathway effectively reduces intrahepatic drug concentrations which reach downstream hepatocytes for recruitment of activity. For example, when glucuronidation activity is anterior to sulfation activity (defined with respect to flow direction), sulfation is without an effect on glucuronidation, but glucuronidation exerts a maximal influence over sulfation rates (Model C). When glucuronidation is in direct competition with sulfation (Model A) or is posteriorly distributed to sulfation (Model B), saturation of the high-affinity sulfation pathway leads to greater fluxes of substrate available downstream for glucuronidation. This results in an apparent compensatory increase in glucuronidation with reduced sulfation capacity, which occurs at input concentrations greater than the Km for sulfation but less than the Km for glucuronidation. This compensation pattern is more prominent for highly extracted compounds where both sulfation and glucuronidation are effective pathways in drug removal, and where large intrahepatic drug concentration gradients are expected. Since the physiologic description of intrahepatic drug concentration is often described by a concentration gradient from the inlet to outlet of the liver, the logarithmic average concentration has been used to estimate the mean liver concentration in the determination of kinetic constants for enzymic reactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Glucuronidase↗
PubMed1987

Stereoselective glutathione conjugation and amidase-catalyzed hydrolysis of alpha-bromoisovalerylurea enantiomers in isolated rat hepatocytes.

alpha-Bromoisovalerylurea (BIU) is used as model substrate for studies on the pharmacokinetics of glutathione conjugation in vivo. Its metabolism in isolated rat hepatocytes is presently studied. A major part of the substrate was conjugated with glutathione, but also amidase-catalyzed hydrolysis occurred, resulting in the products urea and alpha-bromoisovaleric acid (BI). The amidase activity was located in the microsomal fraction of the rat liver. The product of hydrolysis, BI, also was conjugated efficiently with glutathione. In glutathione-depleted hepatocytes, no glutathione conjugates but only urea and BI were formed. A pronounced stereoselectivity in the metabolism of the BIU enantiomers was observed: (R)-BIU was conjugated with glutathione much faster than (S)-BIU. (S)-BIU was hydrolyzed substantially in the cells and the glutathione conjugate of the hydrolytic product, (S)-BI, could be detected. At high BIU concentrations (500 microM of the racemate) intracellular glutathione was seriously depleted; then, the cosubstrate availability most likely was the rate-limiting factor in the conjugation of BIU with glutathione. More urea was formed from (racemic) BIU in isolated rat hepatocytes in the present study than in the perfused liver and the intact rat in previous studies. This in vivo-in vitro difference is tentatively assigned to differences in glutathione availability in these systems. The results suggest that BI may also be a useful model substrate to study the kinetics of glutathione conjugation in vivo and in vitro.

Amidohydrolases↗
PubMed1986

An enzyme-distributed system for lidocaine metabolism in the perfused rat liver preparation.

The influence of enzymic distribution on lidocaine metabolism was investigated in the once-through perfused rat liver preparation. Low input concentrations of 14C-lidocaine (1-2 microM) and preformed monoethylglycine xylidide (MEGX; 2.3-2.8 microM) were delivered by normal and retrograde flow directions to the liver preparations at 10 ml/min per liver. Upon reversal of normal to retrograde delivery of lidocaine, the rates at which lidocaine, MEGX, and glycine xylidide (GX) left the liver almost doubled, whereas the rates of appearance of (total) hydroxylated lidocaine and MEGX in bile and perfusate increased to lesser extents. Upon reversal of normal to retrograde delivery of preformed MEGX, the rates of appearance of MEGX and GX were virtually unchanged. Computer simulations on lidocaine and preformed MEGX metabolism were performed on both evenly distributed ("parallel tube" model) and enzyme-distributed systems. An even or parallel distribution of N-deethylation and hydroxylation activities for lidocaine metabolism failed to predict the observed increased hepatic availability of lidocaine. Rather, the distribution of a low-affinity, high-capacity N-deethylation system anterior to a high-affinity, low-capacity hydroxylation system for lidocaine metabolism adequately predicted the increased hepatic availability of lidocaine. Further extension of these consistent enzyme-distributed models on the metabolism of lidocaine metabolites suggests that the N-deethylation and hydroxylation activities for the metabolism of lidocaine, MEGX, 3-hydroxyidocaine, and 3-hydroxy MEGX are not identically distributed. When these enzyme-distributed models were appraised with reference to the "parallel tube" and "well-stirred" models of hepatic drug clearance, predictions from these enzyme-distributed models proved to be superior to the "parallel tube" and "well-stirred" models for the present data on lidocaine metabolites with normal and retrograde perfusions. Previously published data on lidocaine and MEGX metabolism after inputting 4 micrograms/ml (17 microM) lidocaine at flow rates of 10, 12, 14, and 16 ml/min were reexamined with respect to the adequacy of these enzyme-distributed models. They were found to be superior to the evenly-distributed or "parallel tube" model in predicting hepatic availability of lidocaine and the rate of appearance of MEGX. However, the enzyme-distributed systems were not as consistent as the "well-stirred" model in predicting lidocaine hepatic availability in these flow experiments.

Animals↗
PubMed1985

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.

Animals↗
PubMed1985

A review of metabolite kinetics.

The importance of metabolites as active and toxic entities in drug therapy evokes the need for an examination of metabolite kinetics after drug administration. In the present review, emphasis is placed on single-compartmental characteristics for a drug and its primary metabolites under linear kinetic conditions. The determination of the first-order elimination rate constants for drug and metabolite are also detailed. For any ith primary metabolite mi formed solely in liver, kinetic parameters with respect to primary metabolite formation under first-order conditions require a comparison of the areas under the metabolite concentration-time curve after drug and preformed metabolite administrations. These area ratios hold regardless of the number of noneliminating compartments for the drug and metabolite. These parameters include fmi and gmi, the fractions of total body clearance that respectively furnishes mi to the general circulation and forms mi, and hmi, the fraction of hepatic clearance responsible for the formation of mi. Moreover, the fraction of dose dmi converted to form mi is defined with respect to the route of drug administration. The inherent assumption of these estimates, however, requires that the extent of sequential elimination of the generated mi be identical to the extent of metabolism of preformed mi. Discrepancies have been found, and may be attributed mostly to the uneven distribution of drug-metabolizing activities as well as to the presence of diffusional barriers. Other linear systems that involve mi formation from multiple organs are briefly described.

Animals↗
PubMed1985

Disposition of enalapril in the perfused rat intestine-liver preparation: absorption, metabolism and first-pass effect.

A new procedure, namely the in situ perfused rat intestine-liver preparation, was introduced to examine the roles of the intestine and the liver in the elimination of enalapril, a new angiotensin-converting enzyme inhibitor. The in situ perfused rat intestine preparation was used to determine the rate and extent of enalapril absorption after an-intraduodenal dose. In the former technique, enalapril in blood perfusate (10 ml/min) was delivered via the superior mesenteric artery into the once-through perfused rat intestine-liver preparation, with sampling effected in reservoir, portal vein and hepatic vein. The ease of sampling, proximal and distal to the intestine and liver, allowed the direct estimation of the extraction ratios by the intestine and the liver. The steady-state intestinal extraction ratio of enalapril was small (0.04 +/- 0.066) compared to that for the liver (0.74 +/- 0.06), indicating that the liver was responsible for most of the hydrolytic conversion of enalapril to its pharmacologically active diacid metabolite, enalaprilat. Moreover, no trend in the values of the extraction ratios by both organs was apparent among the input concentrations of enalapril (0.55, 2.6 and 13.3 microM) used. Portal venous plasma consisted mainly of enalapril and was devoid of enalaprilat, whereas both enalapril and enalaprilat were detected in bile and hepatic venous plasma. With the latter technique, an intraduodenal injection of a tracer dose of [14C]enalapril (0.14-0.39 mumol) was made close to the pyloric sphinctor, whereas the intestine preparation was recirculated (7.5 ml/min) with blank perfusate.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetaminophen↗
PubMed1985

Alteration of transit time and direction of flow to probe the heterogeneous distribution of conjugating activities for harmol in the perfused rat liver preparation.

Previous studies in the once-through perfused rat liver preparation have shown that the techniques of normal and retrograde delivery of substrate and computer simulation of enzyme distributions along the sinusoidal flow path in liver were useful in delineating the relative distributions of sulfation and glucuronidation activities for harmol metabolism (Pang et al., J. Pharmacol. Exp. Ther. 224: 647-653, 1983). The observed steady-state hepatic extraction ratios of harmol and the steady-state formation rates of harmol sulfate and harmol glucuronide were consistent with three enzyme-distribution models which described a proximal localization of sulfation activity and a more distal distribution of glucuronidation activities. The present study was a further refinement on the definition of the distribution of these activities. The experimental approach used included the perturbation of delivery of harmol (10 microM) at 8, 12 and 16 ml/min by normal flow, and the alternation of normal and retrograde directional flows for delivery of harmol (10 microM) at constant hepatic blood flows (8, 12 or 16 ml/min) in the single-pass perfused rat liver preparation. The observed steady-state sulfation and glucuronidation rates were compared against predicted values afforded by models previously shown to be adequate and additional enzyme-distributed models. The observed and predicted data point to a periportal distribution of sulfation activity and an even distribution of glucuronidation activity for the metabolism of harmol.

Alkaloids↗