Binding of magnesium in a mutant Escherichia coli alkaline phosphatase changes the rate-determining step in the reaction mechanism

To investigate the role of magnesium at the M3 site in Escherichia coli alkaline phosphatase, site-specific mutagenesis was used to substitute Glu-322, a ligand of the Mg2+ with either aspartic acid (E322D) or alanine (E322A). The residual Mg2+ content of the E322D enzyme is about 16-fold lower than that of the wild-type enzyme, and both mutant enzymes exhibit extremely poor catalytic activity compared to the wild-type enzyme. Mg2+ is a strong activator of the E322D enzyme. The hydrolysis activity of the E322D enzyme maximally stimulated by Mg2+ is 60% of that of the wild-type enzyme. Under conditions that measure the sum of hydrolysis and transphosphorylation activities, the kcat of the E322D enzyme in the presence of 500 mM Mg2+ is 2.6-fold higher than the kcat observed for the wild-type enzyme. Zn2+ also activates the E322D enzyme, although it is not as strong an activator as Mg2+. Competition experiments suggest that the activation of the E322D enzyme by Mg2+ and Zn2+ results from binding of either of these metals to the same site on the enzyme. High concentrations of the substrate p-nitrophenyl phosphate inhibit the activity of the E322D enzyme; however, high concentrations of Mg2+ can overcome this inhibition. Stopped-flow experiments indicate that the rate-limiting step of the nonstimulated E322D enzyme at pH 8.0 differs from that of the wild-type enzyme and involves the breaking of the covalent bond between the enzyme and phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Amount of G-protein alpha-subunit in rat white adipocytes: lack of difference between subcutaneous and visceral fat

It has been the purpose of this study to examine possible differences in the amount of stimulatory (Gs) and inhibitory (Gi) G-protein alpha-subunits (measured with a quantitative enzyme-linked immunosorbent assay in fat cell membrane preparation) between subcutaneous and intra-abdominal regions in rats. The lipolytic response to isoproterenol and the number of beta-adrenergic binding sites were also examined. These parameters were all evaluated simultaneously in subcutaneous (inguinal), epididymal and perirenal fat samples collected from six male Sprague-Dawley rats. The membrane contents of the Gs and Gi alpha-subunits were similar in the three depots. Moreover, no difference was found among the different regions with regard to isoproterenol-stimulated glycerol release and beta-adrenoceptor number, expressed per cell number. In conclusion, the present study shows for the first time in rats that the abundance of inhibitory and stimulatory G-protein alpha-subunits is similar in subcutaneous and in visceral adipocytes. Moreover, the number of beta-adrenoceptors and the lipolytic response to isoproterenol do not show significant variations with the anatomical site. As the present results are apparently in contrast with those obtained previously in human adipocytes, there is a possibility that the different results observed in rat and in human fat cells could be explained by species differences.

Solution structure of tertiapin determined using nuclear magnetic resonance and distance geometry

The solution structure of tertiapin, a 21-residue bee venom peptide, has been characterized by circular dichroism (CD), two-dimensional nuclear magnetic resonance (NMR) spectroscopy, and distance geometry. A total of 21 lowest error structures were obtained from distance geometry calculations. Superimposition of these structures shows that the backbone of tertiapin is very well defined. One type-I reverse turn from residue 4 to 7 and an alpha-helix from residue 12 to 19 exist in the structure of tertiapin. The alpha-helical region is best defined from both conformational analysis and structural superimposition. The overall three-dimensional structure of tertiapin is highly compact resulting from side chain interactions. The structural information obtained from CD and NMR are compared for both tertiapin and apamin (ref. 3), another bee venom peptide. Tertiapin and apamin have some similar secondary structure, but display different tertiary structures.

Passive cigarette smoke, coal heating, and respiratory symptoms of nonsmoking women in China

In this study we evaluated data from a sample of 973 never-smoking women, ages 20-40, who worked in three similar textile mills in Anhui Province, China. We compared prevalence rates of respiratory symptoms across homes with and without coal heating and homes with different numbers of smokers. Multiple logistic regression models that controlled for age, job title, and mill of employment were also estimated. Respiratory symptoms were associated with combined exposure to passive cigarette smoke and coal heating. Effects of passive cigarette smoke and coal heating on respiratory symptoms appeared to be nearly additive, suggesting a dose-response relationship between respiratory symptoms and home indoor air pollution from these two sources. The prevalence of chest illness, cough, phlegm, and shortness of breath (but not wheeze) was significantly elevated for women living in homes with both smokers and coal heating.

Programmable drug delivery from an erodible association polymer system

An erodible association polymer system based on blends of cellulose acetate phthalate (CAP) and Pluronic F127, a block copolymer of poly(ethylene oxide) and poly(propylene oxide), has been investigated for its applicability to rate-programmed drug delivery. The compatibility and thermal properties were characterized by DSC and FTIR. Results from the thermal analysis indicate that the blends are compatible above 50% CAP, as revealed by a single composition-dependent glass transition temperature (Tg). The existence of molecular association through intermolecular hydrogen bonding between the carboxylic acid and the ether oxygen groups is supported by the observation of an upward shift in the IR carbonyl stretching frequency at increasing Pluronic F127 concentrations. Using theophylline as a model drug, the in vitro polymer erosion and drug release characteristics of the present polymer system were evaluated at different buffer pH's on a rotating-disk apparatus. The results show that the rates of both polymer erosion and drug release increase with the Pluronic F127 concentration in the blend. Further, at pH 4, the polymer erosion is minimal and the theophylline release appears to be governed mainly by diffusion through the polymer matrix. In contrast, at pH 7.4, the theophylline release is controlled primarily by the polymer surface erosion. To demonstrate the unique approach to programmed drug release based on the concept of nonuniform initial drug distribution, pulsatile patterns of drug release have been achieved successfully from the present surface-erodible polymer system using a multilaminate sample design with alternating drug-loaded layers.(ABSTRACT TRUNCATED AT 250 WORDS)

A specific activator of the ATP-inhibited K+ channels in guinea pig ventricular cells

The specific activity of a pyridylcyanoguanidine, P-1075, on cardiac potassium current inhibited by internal ATP (KATP) was examined using whole-cell and single-channel recording techniques. At 10 microM, the compound markedly abbreviated ventricular cell action potentials. The effect was reversible by 1 microM glyburide. Whole-cell current experiments showed that the compound elicited a time-independent outward current which had a linear current-voltage relationship between -70 to 0 mV. This current was inhibited by glyburide. Elevation of intracellular ATP from 1 to 5 mM strongly reduced the effect of P-1075 (50 microM) on the current, from 25.5 +/- 1.8 to 3.4 +/- 0.6 pA/pF. Removal of internal ATP, which caused KATP channel to "rundown", markedly diminished the drug effect, suggesting phosphorylation of the channel may be necessary for drug action. Dose-response effect of P-1075 on KATP channel had a Kd of 34.5 microM and a Hill coefficient of 2.0 for the 1 mM ATP experiment and a corresponding 36.6 microM and 2.2 for the 5 mM ATP experiment, implying probably two drug receptor sites. Single-channel recordings indicated that P-1075 increased KATP channel open probability. The complex interaction of ATP and P-1075 with the whole-cell current can be explained by a scheme developed by other investigators based on single-channel results. The compound, at doses of up to 25 microM, did not affect the inwardly and outwardly rectifying K+ currents nor the L-type Ca++ current. We conclude that P-1075 is a specific activator of phosphorylated KATP channels.

Influenza A virus haemagglutinin polymorphism: pleiotropic antigenic variants of A/Shanghai/11/87 (H3N2) virus selected as high yield reassortants

Genetic reassortment of the A/Shanghai/11/87 (H3N2) variant of influenza A virus with A/PR8/34 (H1N1) virus [the standard donor of high yield (hy) genes for influenza vaccine viruses] resulted in the isolation of two reassortants with differing H3 haemagglutinin (HA) phenotypes, X-99 and X-99a. The two HA phenotypes were derived from individual subpopulations of the H3N2 wild-type virus during the reassortment event. The HA mutants and their respectively derived reassortants (identical in RNA genotype) differed in antigenicity, replication characteristics, yield in chick embryos and haemagglutinin gene sequence. Despite antigenic differences in reactions to polyclonal rabbit antisera of 60%, both X-99 and X-99a, the hy reassortants, were equally immunogenic and protective in BALB/c mice to challenge by parental wild-type virus. Differences in HA phenotype were related to a Ser to Ile change at amino acid position 186. These findings emphasize the polymorphism of influenza virus strains as well as the need for caution in selection of vaccine strains from among antigenically distinct viral subpopulations.

Effects of acupuncture on somatosensory evoked potentials: a review

Although acupuncture has a long history of analgesic effects, the mechanisms underlying its effects are still unclear. Somatosensory evoked potential (SEP) methodology has been adopted in the research of acupuncture since the 1970s. In research on the effects of acupuncture on the conventional SEP, variable results have been observed, and two different opinions concerning the presence or absence of acupuncture effects on the conventional SEP are discussed. Since the conventional SEP is mediated mainly by fast conducting sensory nerve fibers, the conventional SEP methodology, especially that for recording short-latency SEP, may be inadequate for studying acupuncture mechanisms. In the case of the long-latency cortical SEP, there are too few data available to judge the effects of acupuncture analgesia (AA). Studies on the effects of AA on pain SEPs demonstrated that AA has a suppressive effect on amplitude of the pain SEP (and affecting the latency as well in some experiments) in both animals and humans, being accompanied by increased pain threshold. Thus, acupuncture seems to have analgesic effects that are probably related to activation of the antinociceptive system, and application of the pain SEP methodology to the study of mechanisms of AA may be promising.

Cerebrovasodilation elicited by fastigial stimulation is preserved under deep halothane anesthesia

We studied the effect of halothane anesthesia on the increases in cerebral blood flow (CBF) and arterial pressure (AP) elicited by electrical stimulation of the cerebellar fastigial nucleus (FN). Rats were anesthetized (0.75-2% halothane), instrumented for continuous recording of AP, and ventilated. The FN was stimulated through stereotaxically implanted microelectrodes. In CBF experiments the elevations in AP resulting from FN stimulation were eliminated by spinal cord transection at C1. After cord transection AP was maintained by intravenous phenylephrine. CBF or cerebral glucose utilization (CGU) was measured by laser-Doppler flowmetry or the 2-deoxyglucose method, respectively. FN stimulation produced increases in CBF that were graded with the intensity (10-150 microA) or frequency (10-150 Hz) of stimulation. At 1% halothane, FN stimulation (100 microA; 75 Hz; n = 8) increased CBF by 123 +/- 16%. The elevations in CBF were attenuated by increasing levels of halothane anesthesia in a dose-dependent manner. At halothane concentrations of 1.5 and 2% the CBF response to FN stimulation (100 microA; 75 Hz) was reduced by 58 +/- 6 and 77 +/- 4%, respectively (p < 0.05 from 0.75% halothane; analysis of variance and Tukey's test). In contrast, the increases in CBF elicited by hypercapnia were not attenuated (P > 0.05 from 0.75% halothane). At 1% halothane, FN stimulation did not change CGU in neocortex (frontal cortex: unstimulated 48 +/- 6, mumol.100 g-1.min-1, FN stimulation: 47 +/- 11; P > 0.05; n = 5/group). In the group of rats in which the pressor response was studied (n = 7), halothane produced a dose-dependent attenuation of the elevations in AP. The degree of attenuation of the AP response was comparable to that of the CBF response (P > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Defining topological equivalences in protein structures by means of a dynamic programming algorithm

An automatic algorithm for defining topological equivalences in protein structures is presented. The algorithm is based on a dynamic programming technique and self-consistent scoring method. We have used it to align pairs of similar protein structures of several protein families and to identify recurrent structural domains in aspartic proteinase 2APR. Its ability to find suboptimal paths permits a thorough comparison of proteins at each level in the hierarchy of the protein structure: secondary structure, super-secondary structure, domain and entire globular structure. The algorithm has been extended to the structure alignment of ribonucleic acid and can be extended to the structure alignment of any linear polymer.

A new method of RNA preparation for detection of hepatitis A virus in environmental samples by the polymerase chain reaction

A new method based on RNA preparation from shellfish by polyethylene glycol (PEG) precipitation and trichloroacetic acid (TCA) extraction was developed for the detection of hepatitis A virus (HAV) by the polymerase chain reaction (PCR). The extraction provides high yield and the extracted RNA is undegraded. This method proved to be particularly useful for detection of RNA viruses from environmental samples by PCR.

Genetic and antigenic analyses of influenza A (H1N1) viruses, 1986-1991

Eighteen strains of human influenza A (H1N1) viruses isolated between August 1986 and January 1991 were analyzed in this study. Examination of the total viral genome of 12 strains by T1 mapping revealed that considerable genetic heterogeneity exists among these viruses. Partial sequencing of each of the non-HA RNA segments of 4 viruses having divergent T1 oligonucleotide maps indicated that only one was a reassortant virus that had genes from both the influenza A (H1N1) and (H3N2) subtypes. This reassortant obtained its PB2 gene from a virus of the H3N2 subtype and the other 7 RNA segments from an H1N1 parent. Sequencing studies of the HA1 domains of the hemagglutinin (HA) genes of these 18 strains revealed that although these viruses are antigenically similar to the reference strains A/Taiwan/1/86 and A/Singapore/6/86, 7 conserved amino acid substitutions that are shared by recently isolated H1N1 viruses have occurred in the main stream of evolution of the H1N1 subtype. Our data indicate that: (1) Genetic reassortment continues to contribute to genetic variability of H1N1 viruses. (2) Genetic variants of non-reassortant H1N1 viruses are co-circulating in the world. (3) The HA's of recent H1N1 viruses are related to those of the 1986 reference strains. (4) Although there has been little detectable antigenic variability, the HA genes of human epidemic influenza A (H1N1) viruses have continued to evolve at an evolutionary rate similar to that for the H1N1 and H3N2 viruses analyzed previously.

Role of nitric oxide synthase-containing vascular nerves in cerebrovasodilation elicited from cerebellum

We studied whether the increases in cortical cerebral blood flow (CBF) elicited by stimulation of the cerebellar fastigial nucleus (FN) are attenuated by systemic administration of inhibitors of nitric oxide synthase (NOS) and, if so, whether NOS-containing perivascular nerves arising from the sphenopalatine ganglia (SPG) are the source of NO during FN stimulation. Rats were anesthetized (1-3% halothane) and artificially ventilated. The FN or the pontine reticular formation (PRF) was stimulated electrically through a stereotaxically implanted microelectrode. To eliminate the elevation in arterial pressure (AP) elicited by FN or PRF stimulation the cervical spinal cord was transected and AP was maintained by intravenous phenylephrine. CBF was measured by a laser-Doppler probe placed over the parietal cortex. Systemic administration of the NOS inhibitor N omega-nitro-L-arginine methyl ester (L-NAME; 5-40 mg/kg) reduced resting CBF, an effect that was maximal at 10 mg/kg (-30 +/- 4%; n = 6; P < 0.003, analysis of variance). L-NAME, but not its inactive isomer D-NAME, attenuated the increases in CBF elicited by FN stimulation or hypercapnia in a dose-dependent fashion (10-40 mg/kg). At 40 mg/kg, the response to FN stimulation was reduced by 80 +/- 6% (n = 6; P < 0.05) and that to hypercapnia was reduced by 70 +/- 9% (P < 0.05). In contrast, the increases in CBF elicited by PRF stimulation were not affected (10-40 mg/kg; P > 0.05; n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

Postreceptor events involved in the up-regulation of beta-adrenergic receptor mediated lipolysis by testosterone in rat white adipocytes

In the previous studies we have shown that testosterone increases lipolytic responsiveness to catecholamines in rat white adipocytes, and that is associated with an up-regulation of beta-adrenergic receptor density. However, the postreceptor events involved in the testosterone induced enhancement of beta-adrenergic receptor activated lipolysis in these cells have not been adequately studied, and were therefore investigated in the present study. Male Sprague Dawley rats were divided into three groups: control, castrated, and castrated treated with testosterone. The beta-adrenergic receptor-mediated cAMP accumulation, measured with RIA after isoproterenol (a beta-adrenergic agonist) stimulation was decreased in castrated rats, and reversed by testosterone treatment, suggesting a testosterone effect at or proximal to adenylate cyclase. However, no differences between the groups were found in abundance of G alpha protein messenger RNAs (G alpha s, G alpha i-1, and G alpha i-2) as analyzed by Northern blot and a solution hybridization RNase protection assay, or in G protein mass measured with a quantitative enzyme-linked immunosorbent assay in fat cell membrane preparation. Lipolysis stimulated by N6-monobutyryl-cAMP was reduced in castrated rats and recovered by testosterone treatment, suggesting that components distal to the adenylate cyclase, i.e. protein kinase A (PKA) and/or hormone sensitive lipase (HSL) also are involved in testosterone regulation of lipolysis. In conclusion, these and previous results suggest that the testosterone-induced increase in lipolytic response to catecholamines in rat white adipocytes is mediated through several events including an increased beta-adrenergic receptor density, probably an increased adenylate cyclase activity and an increased protein kinase A/hormone sensitive lipase activity at the postreceptor level with apparent absence of effect on the expression of G-proteins.

Nitric oxide synthase-containing neural processes on large cerebral arteries and cerebral microvessels

We studied whether neural processes containing nitric oxide synthase (NOS) are associated with large cerebral arteries and/or intraparenchymal microvessels. The presence of NOS-positive nerves on large cerebral arteries was examined in whole-mount preparations processed for NADPH diaphorase histochemistry, a procedure that stains NOS-containing neurons. The association between NOS-containing neural processes and intracerebral microvessels was studied by electron microscopy in ultrathin brain sections reacted with antibodies against NOS. A dense perivascular plexus of NADPH diaphorase positive axons was observed in the anterior portion of the circle of Willis and its branches while in the basilar artery the innervation was less dense. Lesions of the major sources of perivascular innervation of the cerebral arteries indicated that these nerve fibers arise from the sphenopalatine ganglia. Within the brain parenchyma, NOS immunoreactivity was observed in dendrites and axonal terminals closely associated with the basal lamina of arterioles and capillaries. We conclude that NOS-containing nerves of peripheral origin innervate large cerebral arteries while NOS-containing neural processes of central origin, especially dendrites, are closely associated with cerebral arterioles and capillaries. The presence of NOS in perivascular dendrites raises the possibility that these structures are a major source of NO during neural activity. These findings, collectively, provide morphological evidence supporting the hypothesis that NOS neurons participate in the mechanisms that match neural activity to cerebral blood flow.

Optical perceptron learning for binary classification with spatial light rebroadcasters

Binary classification of an object in a two-dimensional image is considered. A spatial light rebroadcaster is shown to be advantageous for learning in this case because it can store the weights and permit upward and downward adjustments. Two learning algorithms, based on the perceptron, are considered. A modification of the perceptron algorithm is developed so that only positive weights are needed. This is convenient because light intensity is positive only. The modified algorithm is shown to converge in a finite number of steps for positive linear separable classes. Optical experiments show the classification of four characters in two groups, in which alternative groupings are used to show robustness. In the second group of experiments the complements of the two-dimensional characters are used, and the convergence is equally fast. Adding the results from the original and complementary patterns provides a discrimination superior to that obtained using either on its own.

Requirement for posttranslational processing of Rac GTP-binding proteins for activation of human neutrophil NADPH oxidase

Rac1 and Rac2 are closely related, low molecular weight GTP-binding proteins that have both been implicated in regulation of phagocyte NADPH oxidase. This enzyme system is composed of multiple membrane-bound and cytosolic subunits and when activated catalyzes the one-electron reduction of oxygen to superoxide. Superoxide and its highly reactive derivatives are essential for killing microorganisms. Rac proteins undergo posttranslational processing, primarily the addition of an isoprenyl group to a carboxyl-terminal cysteine residue. We directly compared recombinant Rac1 and Rac2 in a human neutrophil cell-free NADPH oxidase system in which cytosol was replaced by purified recombinant cytosolic components (p47-phox and p67-phox). Processed Rac1 and Rac2 were both highly active in this system and supported comparable rates of superoxide production. Under different cell-free conditions, however, in which suboptimal amounts of cytosol were present in the assay mixture, processed Rac2 worked much better than Rac1 at all but the lowest concentrations. This suggests that a factor in the cytosol may suppress the activity of Rac1 but not of Rac2. Unprocessed Rac proteins were only weakly able to support superoxide generation in either system, but preloading of Rac1 or Rac2 with guanosine 5'-O-(3-thio-triphosphate) (GTP gamma S) restored activity. These results indicate that processing is required for nucleotide exchange but not for interaction with oxidase components.

Potential role for scavenger receptors of human monocytes in the killing of Schistosoma mansoni

Human low-density lipoproteins (LDL) bind specifically and saturably to the surface of the trematode parasite, Schistosoma mansoni, in vitro. Here we have tested whether human monocytes process the bound LDL. Monocytes obtained by leukapheresis generate H2O2, kill schistosomula, and were seen here endocytosing fluorescently labeled human LDL that was bound to the surface of the parasites. Compounds known to inhibit uptake of LDL via the scavenger receptor, namely, acetylated LDL, polyinosinic acid, dextran sulfate, fucoidan, and polyvinyl sulfate, inhibited both endocytosis of LDL and cell-mediated killing. Non-functional analogs of these inhibitors, namely, polycytidylic acid and dextran, did not inhibit either endocytosis or killing. Monocytes obtained from whole blood after venipuncture neither killed the parasite nor endocytosed LDL from the worm surface. Thus, human monocyte killing of schistosomula may involve removal of LDL from the parasite surface via scavenger receptors.

Magnesium in the active site of Escherichia coli alkaline phosphatase is important for both structural stabilization and catalysis

Site-specific mutagenesis was used to explore the roles of the side chains of residues Lys-328 and Asp-153 in Escherichia coli alkaline phosphatase. The D153H enzyme exhibits a 3.5-fold decrease in activity at pH 8.0 compared to that of the wild-type enzyme, while a double mutant D153H/K328H exhibits a 16-fold decrease in activity under these conditions. However, the Km values for both enzymes, employing the substrate p-nitrophenyl phosphate, are lower than the value for the wild-type enzyme. The Ki for phosphate, which is pH- and Mg(2+)-dependent, is decreased for the D153H enzyme and increased for the D153H/K328H enzyme. Relative to the wild-type enzyme, both mutant enzymes bind Mg2+ more weakly and undergo a time-dependent activation induced by Mg2+. The half-time of the activation process is independent of the Mg2+ concentration, indicating that the activation most probably involves a conformational change. The pH versus activity profiles of both enzymes are altered relative to that of the wild-type enzyme and exhibit greatly enhanced activity, relative to that of the wild-type enzyme, at high pH values. The pre-steady-state kinetics for the D153H and D153H/K328H enzymes exhibit a transient burst of product formation at pH 8.0, under conditions at which the wild-type enzyme exhibits no transient burst, indicating that at pH 8.0 the hydrolysis of the covalent enzyme-phosphate complex is rate-determining and not the release of phosphate from the noncovalent enzyme-phosphate complex as is observed for the wild-type enzyme. Therefore, these mutations are directly influencing catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of NADPH diaphorase in neurons of the rostral ventral medulla: possible role of nitric oxide in central autonomic regulation and oxygen chemoreception

We studied whether neurons containing nitric oxide synthase (NOS) are localized to the rostral ventrolateral medulla (RVM) and, if so, whether they are distinct from the adrenergic neurons of the C1 group. NOS-containing neurons and/or C1 neurons were visualized using NADPH diaphorase histochemistry and phenylethanolamine N-methyltransferase (PNMT) immunohistochemistry, respectively. A column of NADPH diaphorase-positive neurons, extending 2 mm in the rostrocaudal plane, was observed lateral to the inferior olive and medial to the C1 neurons. Double labelling studies showed that NADPH diaphorase-positive neurons were not immunoreactive for PNMT, indicating that the two enzymes were localized in the different cells. Furthermore, only a small fraction of NADPH diaphorase neurons were retrogradely labelled after injections of fluorogold into the thoracic cord. We conclude that the RVM contains a well-defined group of neurons endowed with NOS that are distinct from the adrenergic neurons of the C1 group and have only limited monosynaptic projections to the spinal cord. Since the RVM is involved in the control of arterial pressure and in oxygen-conserving reflexes, the findings raise the possibility that nitric oxide participates in central autonomic regulation and oxygen chemoreception.

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.