Determination of affinity of monoclonal antibodies against human IgG

Deconvolution of antibody affinities and concentrations by non-linear regression analysis of competitive ELISA data

Physiological responses of the adaptive immune system are polyclonal in nature whether induced by a naturally occurring infection, by vaccination to prevent infection or, in the case of animals, by challenge with antigen to generate reagents of research or commercial significance. The composition of the polyclonal responses is distinct to each individual or animal and changes over time. Differences exist in the affinities of the constituents and their relative proportion of the responsive population. In addition, some of the antibodies bind to different sites on the antigen, whereas other pairs of antibodies are sterically restricted from concurrent interaction with the antigen. Even if generation of a monoclonal antibody is the ultimate goal of a project, the quality of the resulting reagent is ultimately related to the characteristics of the initial immune response. It is probably impossible to quantitatively parse the composition of a polyclonal response to antigen. However, molecular regression allows further parameterization of a polyclonal antiserum in the context of certain simplifying assumptions. The antiserum is described as consisting of two competing populations of high- and low-affinity and unknown relative proportions. This simple model allows the quantitative determination of representative affinities and proportions. These parameters may be of use in evaluating responses to vaccines, to evaluating continuity of antibody production whether in vaccine recipients or animals used for the production of antisera, or in optimizing selection of donors for the production of monoclonal antibodies.

Computational models in immunological methods: an historical review

The utilization of computational models in immunology dates from the birth of the science. From the description of antibody-antigen binding to the structural models of receptors, models are utilized to bring fundamental understandings of the processes together with laboratory measurements to uncover implications of these data. In this review, an historical view of the role of computational models in the immunology laboratory is presented, and short mathematical descriptions are given of fundamental assays. In addition, the range of current uses of models is explored -- especially as seen through papers which have appeared in the Journal of Immunological Methods from volume 1 (1971/1972) to volume 208 (1997). Each paper which introduced a new mathematical, statistical, or computer simulation model, or introduced an enhancement to an instrument through a model in those volumes is cited and the type of computational model noted.

Adaptive regulation of taurine and beta-alanine uptake in a human kidney cell line from the proximal tubule

1. The underlying mechanisms involved in the adaptive regulation of beta-amino acid uptake in the human proximal tubule were examined by use of an immortalized human embryonic kidney epithelial cell line (IHKE). 2. The results indicated that the adaptive response to maintain whole-body taurine homeostasis occurs predominantly via changes in the activity of the high-affinity taurine transport system by alterations in the uptake capacity and with an unaffected half-saturation constant. An adaptive response was not observed for the structurally related beta-alanine. 3. Only colchicine, which interferes with microtubule organization, was capable of blocking the response to alterations of taurine in cell medium, whereas inhibition of protein and nucleic acid synthesis by cycloheximide and actinomycin D, respectively, did not change the adaptive pattern. 4. Phorbol 12-myristate 13-acetate (PMA), mimicking the effects of diacylglycerol, induced inhibition of both beta-alanine and taurine uptake. By contrast, the Ca2(+)-ionophore A23187, mimicking the effects of IP3, only stimulated the uptake of taurine but not the influx of beta-alanine. However, the effect of PMA down-regulation and A23187 up-regulation was rapid and short-lived in contrast to the adaptive response, suggesting that the inositol phospholipid pathway involving diacetylglycerol and IP3 is less likely to be linked directly to the adaptive regulation, but rather plays a role in short-term regulation.

Uptake of neutral alpha- and beta-amino acids by human proximal tubular cells

The transport characteristics of amino acids in primary cell cultures from the proximal tubule of human adults (AHKE cells) were examined, using alpha-aminoisobutyric acid (AIB) and beta-alanine as representatives of alpha- and beta-amino acids, respectively. The Na(+)-gradient dependent influx of AIB occurred by a single, saturable transport system, whereas the Na(+)-gradient dependent uptake data for beta-alanine could be described in terms of two-independent transport components as well as one-transport one-leak model with identical kinetic constants for the high-affinity system. Competition experiments revealed that all the neutral amino acids tested reduced the uptake of AIB, whereas there was no effect of taurine, L-aspartic acid, and L-arginine. By contrast, the influx of beta-alanine was only drastically reduced by beta-amino acids, whereas the inhibition by neutral alpha-amino acids was relatively low. Nor did L-arginine and L-aspartic acid affect the uptake of beta-alanine into AHKE cells. Comparison with the results obtained for normal (NHKE) and immortalized (IHKE) embryonic cells suggested an unaltered expression of the types of transport carriers for neutral alpha- and beta-amino acids in the embryonic and AHKE cells. However, the uptake capacity of the above-mentioned transport proteins was relatively smaller in the embryonic kidney compared with the adult human kidney, which may explain, at least partly, the phenomenon of physiologic amino aciduria in neonates.

Taurine and beta-alanine transport in an established human kidney cell line derived from the proximal tubule

The transport mechanisms of taurine and beta-alanine by an immortalized human embryonic kidney epithelial cell line (IHKE) were examined. The uptake of these beta-amino acids was characterized by two Na(+)-dependent transport components, whereas an inwardly directed H(+)-gradient only stimulated amino acid influx to a small extent and in the absence of sodium. Competition experiments revealed that taurine and beta-alanine drastically reduced the uptake of one another by the high-affinity Na(+)-dependent transport system. However, some alpha-amino acids could also compete with the beta-amino acids, but with a low affinity. Examinations of the effect of different anions on the Na(+)-dependent uptake of taurine at a low amino acid concentration (240 nM) revealed a specific requirement for Cl-, whereas Cl- had no measurable effect at a higher concentration (1.0 mM) of taurine. In addition, activation of taurine transport as a function of Na+ and Cl- concentration indicated a probable coupling ratio of 3 Na+/1 Cl-/1 taurine for the high-affinity carrier. Finally, cellular regulation of taurine transport was indicated by the finding that pretreatment with taurine containing media decreased the activity of the taurine transporter(s).

A comparative study on the uptake of alpha-aminoisobutyric acid by normal and immortalized human embryonic kidney cells from proximal tubule

We investigated whether an immortalized human kidney epithelial cell line (IHKE), compared with normal embryonic cells (NHKE), can be used as a representative system with which to characterize the transport of neutral amino acids in the proximal tubule of the human kidney. The IHKE cell line, immortalized by treatment with NiSO4, exhibited microvilli and enzyme markers specific for highly specialized tubule cells. The Na(+)-dependent uptake of alpha-aminoisobutyric acid (AIB) by IHKE and NHKE cells occurred by means of a single transport system with identical half-saturation constants, but the capacity for uptake was higher in the immortalized cells. Proton-dependent influx of AIB was also mediated by a single transport component with similar uptake characteristics in both types of cells. Imposition of an H(+)-gradient to a Na(+)-gradient reduced the sodium dependent uptake of AIB with the exception of short incubation time (1 min), where addition of a proton gradient produced a marked increase in the Na(+)-dependent influx of AIB in NHKE but not in IHKE cells. Competition experiments revealed that the Na(+)-dependent uptake at 50 microM AIB was reduced by neutral alpha-amino acids in the two cell lines. L-Glutamate, L-aspartate, L-arginine and the beta-amino acid taurine had no effect. Only in the IHKE cell line did addition of 5 mM L-lysine produce a slight inhibition. Except for L-proline all of the neutral and acidic amino acids tested reduced the H(+)-dependent uptake of AIB in the IHKE cell line. By contrast, addition of L-aspartate did not influence the transport of AIB in NHKE cells. L-Arginine, but not L-lysine decreased the influx in both cell lines. We conclude that the IHKE cell line has retained the capability to accumulate AIB by transport protein(s) similar to those present for neutral alpha-amino acids in NHKE cells.

Calculation of the affinity constant KASS for solid phase antigen. A model system using monoclonal antibodies against the cat allergen Fel d I

In this report a procedure is described to calculate the affinity constant of an antibody for solid-phase Ag. KSP (KASS solid phase) was defined as the reciprocal of the concentration of Ag required for half saturation of the Ab-binding sites, extrapolated to an infinitely small concentration of Ab (semi-saturation plot). Using this procedure, the affinity of IgE antibodies can be measured without interference from 'invisible' IgG antibodies. As a model system, mAbs against the cat allergen Fel d I were used. Serial dilutions of Fel d I-Sepharose were incubated with serial dilutions of mAb Fd1a and Fd1b, with or without rabbit antibodies as 'invisible' antibodies. The Ab-binding capacity of Sepharose-coupled Fel d I was low: 4.15% and 2.13% of the expected value for mAb Fd1a and Fd1b, respectively, and this must be taken into account when calculating KSP. The K values of mAb Fd1a and Fd1b, calculated from the gamma axis intercept of the semi-saturation plot, were 85 (pmol/test)-1 and 65 (pmol/test)-1 respectively. Using the semi-saturation plot, KSP of mAb Fd1a was not affected by the presence of rabbit antibodies against Fel d I, confirming the applicability of the procedure for measuring the KSP of IgE in patient sera. For one cat-allergic patient the KSP of IgE and IgG4 for Fel d I were calculated and found to be 62 (pmol/test)-1 and 147 (pmol/test)-1 for IgE and IgG4 respectively.

Cooperativity in the antibody binding to surface-adsorbed antigen

The binding to surface-adsorbed antigen of monoclonal mouse IgG-antibodies (mAbs), with two different affinities to dinitrophenyl (DNP), was measured by a calibrated ELISA. The concentration-dependence of antibody binding to surface-bound antigen of different epitope densities was analysed using Scatchard plots. The dissociation of bound tritium-labelled antibodies was measured in the presence of unlabelled antibodies in the bulk. At low surface concentration of bound anti-DNP, both high-affinity mAb and low-affinity mAb show a positive cooperativity in the binding reaction to antigen of high epitope density. Using antigen of lower epitope densities, the positive cooperativity is more pronounced for low-affinity clones. At higher surface concentrations of bound anti-DNP, the Scatchard plots indicate a negative cooperativity of binding, which is also implied by the increased dissociation found in the presence of antibodies in solution. The study confirms previous findings that the binding of antibodies to surface-bound antigen not only depends on intrinsic antibody affinity measured in solution. Other factors, such as self-interaction, also affect the heterogeneous binding reaction.

L-tryptophan uptake by segment-specific membrane vesicles from the proximal tubule of rabbit kidney

1. The mechanism of the renal transport of L-tryptophan by basolateral and luminal membrane vesicles prepared from either the pars convoluta or the pars recta of the rabbit proximal tubule was studied. The uptake of L-tryptophan by basolateral membrane vesicles from the pars convoluta was found to be an Na(+)-dependent transport event. The Na(+)-conditional influx of the amino acid was stimulated in the presence of an inwardly directed H+ gradient. Lowering the pH without an H+ gradient had no effect, indicating that L-tryptophan is co-transported with H+. 3. On the other hand, no transient accumulation of L-tryptophan was observed in the presence or absence of Na+ in basolateral membrane vesicles from the pars recta. 4. In luminal membrane vesicles from the pars recta, the transient Na(+)-dependent accumulation of L-tryptophan occurred via a dual transport system. In addition, an inwardly directed H+ gradient could drive the uphill transport of L-tryptophan into these vesicles in both the presence and the absence of an Na+ gradient. 5. By contrast, the uptake of L-tryptophan by luminal membrane vesicles from the pars convoluta was a strictly Na(+)-dependent and electrogenic transport process, mediated by a single transport component. 6. Investigation of the coupling ratio in luminal membrane vesicles suggested that 1 Na+:1 L-tryptophan are co-transported in the pars convoluta. In the pars recta, examination of the stoichiometry indicated that approx. 1 H+ and 2 Na+ (high affinity) or 1 Na+ (low affinity) are involved in the uptake of L-tryptophan.

Evaluation of labelled monoclonal antibodies by simultaneous estimation of the association constant, the immunoreactive fraction, and the number of effective binding sites on the specific target

The reaction between a labelled monoclonal antibody (MoAb) and its specific target is characterised by three parameters: the association constant (Ka) of the labelled MoAb, the number (N) of effective binding sites on the specific target, and the immunoreactive fraction (F) of the labelled MoAb preparation. Immunological binding parameters are usually estimated graphically, by fitting the experimental data to linear equations derived from the first order law of mass action (FLMA) at equilibrium. However, only two parameters can be estimated simultaneously in a two-dimensional plot. Consequently, graphical estimation of Ka, F and N must be performed stepwise, using at least two different plots. The three parameters are interdependent, and therefore a stepwise estimation procedure might give suboptimal results. In order to investigate whether this is a problem of practical significance in the evaluation of labelled MoAbs, a computerised iterative nonlinear least squares (INLSQ) method was applied to estimate the three parameters simultaneously. The binding parameters in reactions between different 125I-labelled MoAbs and different types of targets were significantly changed when a graphical procedure was replaced by the computerised INLSQ method, and the goodness of fit to FLMA was improved. Hence, the nonlinear least squares method is the preferred procedure. Values were affected when only a subset of the data was included in the estimation procedure, indicating some heterogeneity even in these presumably homogeneous MoAb reactions. The radiolabelling procedure was presumed to be the main reason for this heterogeneity.

Measurement of affinity of viral monoclonal antibodies using Fab'-peroxidase conjugate. Influence of antibody concentration on apparent affinity

The binding affinity of a monoclonal antibody to tobacco mosaic virus (TMV) was studied using a Fab'-peroxidase conjugate. Measurement of the enzymatic activity allowed the determination of the amount of free antibody present after ultracentrifugation of virus-antibody complexes at equilibrium. The method was very sensitive and allowed measurements over a 1000-fold range of antibody concns. The calculated affinity constant decreased about 25 fold when the antibody concn used in the binding assay was increased from 30 ng/ml to 35 micrograms/ml.

Renal transport of taurine in luminal membrane vesicles from rabbit proximal tubule

The uptake of taurine by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule was examined. In pars convoluta, the transport of taurine was characterized by two Na(+)-dependent (Km1 = 0.086 mM, Km2 = 5.41 mM) systems, and one Na(+)-independent (Km = 2.87 mM) system, which in the presence of an inwardly directed H(+)-gradient was able to drive the transport of taurine into these vesicles. By contrast, in luminal membrane vesicles from pars recta, the transport of taurine occurred via a dual transport system (Km1 = 0.012 mM, Km2 = 5.62 mM), which was strictly dependent on Na+. At acidic pH with or without a H(+)-gradient, the Na(+)-dependent flux of taurine was drastically reduced. In both kind of vesicles, competition experiments only showed inhibition of the Na(+)-dependent high-affinity taurine transporter in the presence of beta-alanine, whereas there was no significant inhibition with alpha-amino acids, indicating a beta-amino acid specific transport system. Addition of beta-alanine, L-alanine, L-proline and glycine, but not L-serine reduced the H(+)-dependent uptake of taurine to approx. 50%. Moreover, only the Na(+)-dependent high-affinity transport systems in both segments specifically required Cl-. Investigation of the stoichiometry indicated 1.8 Na+: 1 Cl-: 1 taurine (high affinity), 1 Na+: 1 taurine (low affinity) and 1 H+: 1 taurine in pars convoluta. In pars recta, the data showed 1.8 Na+: 1 Cl-: 1 taurine (high affinity) and 1 Na+: 1 taurine (low affinity).

Na(+)- and H(+)-gradient-dependent transport of alpha-aminoisobutyrate by luminal membrane vesicles from rabbit proximal tubule

1. The characteristics of renal transport of alpha-aminoisobutyrate (AIB) by luminal membrane vesicles isolated from either the proximal convoluted part (pars convoluta) or the proximal straight part (pars recta) of rabbit proximal tubule were investigated. 2. Transport of AIB in vesicles from pars convoluta was mediated by both Na(+)-dependent and Na(+)-independent systems, which in the presence of an inwardly directed H+ gradient can drive the uphill transport of AIB into these vesicles. 3. By contrast, in luminal membrane vesicles from pars recta, transient accumulation of AIB was only dependent on Na+. Lowering pH without a H+ gradient (pHi = pH0 = 5.5) completely abolished the Na(+)-dependent transient accumulation of AIB in these vesicle preparations. 4. Attempts to determine the stoichiometry of both the Na(+)-AIB and H(+)-AIB transporters located in these two segments of proximal tubule suggested that one Na+ and one H+ ion may be involved in the transport of AIB. 5. Sodium-dependent uptake of AIB in vesicles from pars convoluta was competitively inhibited by L-serine and L-phenylalanine, whereas the presence of L-proline, L-alanine and glycine had no significant effect. By contrast, the H(+)-gradient-dependent uptake of AIB was drastically reduced (30% of the control value) by L-proline, L-alanine and glycine, while L-serine and L-phenylalanine had no significant effect. 6. On the other hand, pars recta vesicles exhibited a different transport specificity. L-Phenylalanine, L-serine, L-alanine and glycine, but not L-proline competitively inhibited the uptake of AIB, providing evidence for the existence of a common transport system for AIB, L-phenylalanine, L-serine, L-alanine and glycine in this segment of rabbit proximal tubule.

The influence of epitope density on the estimation of the affinity of antibody for complex antigens

The influence of the epitope density of the antigen on antibody affinity values determined by fluid- and solid-phase immunoassays was assessed. The affinity of the interaction of a panel of monoclonal anti-DNP antibodies of different affinities (as determined by equilibrium dialysis) for DNP-protein conjugates of various hapten substitution ratios was used as the test system. The results obtained showed that the epitope density of the antigen markedly influences the observed affinity values obtained by both experimental approaches. However, the monoclonal antibodies were ranked in affinity terms by both assays in a similar order to that given by equilibrium dialysis. It is concluded that provided due care is exercised in choosing an appropriate epitope density for the test antigen, these methods can be used to provide rapid estimations of average antibody affinities.

Electrogenic uptake of D-imino acids by luminal membrane vesicles from rabbit kidney proximal tubule

Some characteristics of electrogenic uptake of D-proline and hydroxy-D-proline by luminal membrane vesicles isolated either from pars convoluta or from pars recta of rabbit proximal tubule were indirectly studied by the spectrophotometric method. In vesicles from pars convoluta, the uptake of D-imino acids was mediated by both Na+-dependent and Na+-independent, but electrogenic processes. Indirect evidence for coupling between D-imino acids and H+ fluxes was obtained by the following observations: (1) Addition of the H+ ionophore (FCCP) to the vesicle-dye (3,3'-diethyloxadicarbocyanine iodide) suspension completely abolished the Na+-independent electrogenic uptake of D-proline and hydroxy-D-proline by membrane vesicles from pars convoluta. (2) Addition of a relatively low concentration of D-proline in the incubation system decreased the H+-gradient dependent renal uptake of radioactive L-proline to approx. 60% of the control value. By contrast, the uptake of D-proline in vesicles from pars recta was strictly Na+-dependent, since no transient depolarization of membrane vesicles was ever observed in the absence of Na+. A comparison between the transport characteristics of D-imino acids and their naturally occurring L-isomers indicated that these compounds probably share common transport systems located along the proximal tubule of rabbit kidney.

Demonstration of H+- and Na+-coupled co-transport of beta-alanine by luminal membrane vesicles of rabbit proximal tubule

1. The characteristics of renal transport of beta-alanine by luminal membrane vesicles isolated from either the proximal convoluted part (pars convoluta) or the proximal straight part (pars recta) of rabbit proximal tubule were investigated. 2. In vesicles from pars convoluta two transport systems have been characterized: (1) a Na+-dependent system with intermediate affinity (half-saturation 2.7 mM), and (2) a Na+-independent system, which in the presence of a H+ gradient (extravesicular greater than intravesicular) can drive the uphill transport of beta-alanine into these vesicles. This is the first demonstration of H+-beta-alanine co-transport across luminal membrane of rabbit kidney proximal convoluted tubule. 3. By contrast, in membrane vesicles from pars recta, transport of beta-alanine was strictly dependent on Na+ and occurred via a dual transport system, namely a high-affinity (half-saturation 0.16 mM) and a low-affinity system (half-saturation 9.3 mM). 4. The demonstration of competition between the Na+-gradient-dependent uptake of beta-alanine and taurine, without appreciable inhibition by alpha-amino acids in vesicles from pars convoluta as well as from pars recta, strongly suggests that the luminal membrane of proximal tubule has transport systems for the reabsorption of beta-amino acids which are distinct from alpha-amino acid transport systems.

Characteristics of D-alanine transport by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule

Uptake of D-alanine against a concentration gradient has been shown to occur with isolated luminal-membrane vesicles from pars convoluta or pars recta of rabbit proximal tubule. Renal D-alanine transport systems, displaying the following characteristics, were shown: (1) In vesicles from pars convoluta, the uptake of D-alanine was mediated by both Na+-dependent and Na+-independent transport processes. It was found that an inwardly directed H+-gradient could drive the transport of D-alanine into the vesicles both in the presence and absence of Na+. Thus, in addition to Na+, the transport of D-alanine is influenced by the H+-gradient. (2) In vesicles from pars recta, the transient accumulation of D-alanine was strictly dependent on Na+, since no 'overshoot' was ever observed in the absence of Na+. Although the Na+-dependent uptake of D-alanine was stimulated at acid pH, H+ did not substitute for Na+, as it apparently does in pars convoluta, but instead potentiated the Na+ effect. (3) Addition of L-alanine to vesicle preparations, both from pars convoluta and from pars recta, specifically inhibited renal uptake of D-alanine. A comparison between the transport characteristics of D- and L-alanine indicated that these two isomers of alanine probably share common transport systems located along the proximal tubule of rabbit kidney.

Transport of pyruvate by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule

The characteristics of renal transport of pyruvate by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule were studied. It was found that the uptake of pyruvate in these vesicle preparations occurred by means of multiple transport systems. An electrogenic and Na+-requiring system confined to pars convoluta, exists for transport of pyruvate with an intermediate affinity, KA = 0.71 +/- 0.08 mM. In vesicles from pars recta, the uptake of pyruvate was mediated by a dual transport system with a high (KA1 = 0.30 +/- 0.05 mM) and low affinity (KA2 = 5.75 +/- 0.82 mM). The relation of these three pyruvate transport systems to the transport of other monocarboxylates and dicarboxylates was determined by examination of the inhibitory effect of L-lactate, beta-hydroxybutyrate and L-malate on the renal uptake of pyruvate. It was found that L-malate efficiently reduced the uptake of pyruvate by vesicles from pars convoluta, while addition of monocarboxylates (L-lactate, beta-hydroxybutyrate) has no effect on the transport of pyruvate in this region of proximal tubule. Furthermore, it was observed that Na+-dependent uptake of L-malate was specifically inhibited to about the same extent by 1 mM pyruvate. These findings strongly suggest that pyruvate and L-malate are taken up by the same transport system in vesicles from pars convoluta, which is different from that of L-lactate and beta-hydroxybutyrate. Experiments designed to investigate the substrate specificity for the high and low affinity system for pyruvate in pars recta, revealed that monocarboxylates (L-lactate, beta-hydroxybutyrate) substantially inhibit pyruvate transport at the low, but not the high substrate concentration, indicating that all monocarboxylates compete for the same high-affinity transport system. By contrast, L-malate at low concentrations preferentially inhibited the low-affinity system for pyruvate. This observation led us to suggest that L-malate and pyruvate share a common transport system in pars recta with low affinity for pyruvate and high affinity for L-malate.

Estimation of association constants of 42 monoclonal antibodies to human IgG epitopes using a fluorescent sequential-saturation assay

We measured the association constant (Ka) for 42 murine monoclonal antibodies (MAbs) to human IgG epitopes. Included are antibodies, previously evaluated in an IUIS/WHO collaborative study, to various epitopes on the four subclasses of human IgG - IgG Fc, IgG Fab, kappa, and lambda - and to selected IgG allotopes. We used a sequential-saturation immunofluorescent assay and interactive computer program to determine the Ka by Scatchard analysis. Kas ranged from unmeasurably low by this method (approximately 10(6) L/M) to 3.8 X 10(9) L/M. Some class specific MAbs had large Ka differences for different subclasses and some subclass specific MAbs had large Ka differences for molecules of the same subclass but of different light-chain types.

Renal transport of neutral amino acids. Cation-dependent uptake of L-alanine by luminal-membrane vesicles

The characteristics of L-alanine transport in luminal-membrane vesicles isolated either from whole cortex or from pars convoluta or pars recta of rabbit proximal tubules were studied by a rapid filtration technique and by a spectrophotometric method. Uptake of L-alanine by vesicles from whole cortex was mediated by both Na+-dependent and Na+-independent, but electrogenic, processes. The nature, mechanism and tubular localization of the transport systems were studied by the use of vesicles derived from pars convoluta and pars recta. In vesicles from pars recta transport of L-alanine was strictly dependent on Na+ and occurred via a dual transport system, namely a high-affinity (half-saturation 0.14 mM) and a low-affinity system (half-saturation 9.6 mM). The cation-dependent but Na+-unspecific transport system for L-alanine was exclusively localized to the pars convoluta, which also contained an Na+-preferring system of intermediate affinity (half saturation 2.1 mM). A closer examination of the mechanism of transport of L-alanine in vesicles from pars convoluta revealed that an H+ gradient (extravesicular greater than intravesicular) can drive the transport of L-alanine into the vesicles both in the presence and in the absence of Na+. The physiological importance of various L-alanine transporters is briefly discussed.

Prostaglandin-E2 receptors in the rat kidney: biochemical characterization and localization

A radiohistochemical technique yielding data on the distribution and characteristics of PGE2-receptor binding in tissue sections is described. The binding of tritiated PGE2 (3H-PGE2) to slide-mounted tissue sections had all the characteristics associated with ligand-receptor interactions: it was saturable, of high affinity and displayed high specificity for PGE2 binding. From the binding curves a Hill coefficient of 1.1 was calculated which suggests the presence of a homogeneous receptor population. Pretreatment with indomethacin for four days resulted in a 66% increase in maximal binding capacity (Bmax) without any change in affinity. The distribution of receptor was mapped in rats with and without indomethacin pretreatment and compared to the distribution of Tamm-Horsfall glycoprotein, a specific marker for the thick ascending limb of Henle. In both groups the PGE2 receptor showed striking regional variation. In the untreated group the distribution of PGE2 receptors was similar to that of the thick ascending limb of Henle, with maximal density in the outer medullary zone. After indomethacin pretreatment, however, a striking increase in inner medullary binding was observed together with increased binding in the cortex. Thus, in accordance with the main action of PGE2 being regulation of renal water and sodium excretion, we found the highest receptor density in areas of the kidney dominated by the thick ascending limb of Henle and collecting tubules, and much less binding to glomeruli and cortical vessels. In order to investigate characteristics and distribution of PGE2 receptor binding, however, it was mandatory that endogenous prostanoid synthesis is blocked.

Determination of intrinsic affinity constants of monoclonal antibodies against carcinoembryonic antigen

A 2-phase method is described for the determination of the intrinsic affinity constants (K-values) for the interaction between monoclonal antibodies (MAbs) against carcinoembryonic antigen (CEA) and CEA. The Mabs were coupled covalently to CNBr-activated paper discs. MAb coupled discs and a 2-fold dilution series of 125I-CEA were incubated at 20 degrees C until equilibrium was reached. Nonlinear curve-fitting was used for estimation of K-values and different calculation models were thereby tested. The K-values for 12 different anti-CEA MAbs were determined to be 0.3-52 X 10(6) M-1, which is 1-2 orders of magnitude lower than the values obtained in a previous study with some of these MAbs using the ammonium sulphate method to separate free from bound antigen. The K-values obtained by the paper disc method are probably better estimates of the intrinsic association constant than those obtained previously. There are two main reasons for this: (1) free and bound antigen are separated from each other under physiological conditions and (2) the opportunity for multipoint interaction between MAb and CEA is minimized when the MAb is coupled to a rigid carrier substance. Thus, even when the MAb reacts with greater than or equal to 2 epitopes in CEA, as seems to be the case with several of our anti-CEA MAbs, the intrinsic K-value should be obtained. The fundamental validity of 2-phase assays, sometimes questioned in the literature, is demonstrated.

Na+-H+ exchange in luminal-membrane vesicles from rabbit proximal convoluted and straight tubules in response to metabolic acidosis

Na+-H+-exchanger activity of pars convoluta and pars recta luminal-membrane vesicles prepared from the proximal tubule of acidotic and control rabbits were assayed by a rapid-filtration technique and an Acridine Orange method. Both experimental approaches revealed the existence of an antiporter, sensitive to metabolic acidosis, in pars convoluta membrane vesicles. Kinetic data, obtained with the pH-sensitive dye, showed that the Km for Na+ transport was unchanged by acidosis, whereas Vmax. for exchanger activity was increased, on an average, by 44%. The fluorescence method, in contrast with the rapid-filtration technique, was able to detect exchanger activity in pars recta membrane vesicles. The Km value for the antiporter located in pars recta is comparable with that calculated for pars convoluta membrane vesicles. By contrast, the Vmax. of this exchanger is only about 25% of that found for pars convoluta. Furthermore, metabolic acidosis apparently does not increase Na+-H+-exchanger activity of pars recta luminal-membrane vesicles.

Nuclear uptake of 1,25-dihydroxy[3H]-cholecalciferol in peripheral blood monocytes

The active vitamin D metabolite, 1,25-dihydroxycholecalciferol induces differentiation of monocytes into macrophages. The pharmacological induction of differentiation of primitive, rapidly proliferating cell lines into more mature cells with lower proliferative potential is a new dimension in the treatment of myeloproliferative disorders, which may prove to be an important alternative to more traditional regimens. Furthermore, the cell primarily engaged in bone resorption--the osteoclast--represents another differentiated form of mononuclear phagocytes, and 1,25-dihydroxycholecalciferol increases the number of osteoclasts. Since the cellular action of 1,25-dihydroxycholecalciferol is exerted mainly through its binding to nuclear receptors, a detailed knowledge of ligand-receptor interactions is mandatory for future work in this area. In order to investigate the interaction between 1,25-dihydroxycholecalciferol and its receptor in mononuclear cells, the nuclear uptake of the hormone was studied using a whole cell assay. The nuclear uptake of 1,25-dihydroxy[3H]cholecalciferol in human monocytes at physiological temperature and pH was saturable, specific, and fully reversible. When eight normal individuals were investigated, the maximal binding capacity (Bmax) was 0.4-8.4 fmol/10(6) cells and the dissociation constants (Kd) were 0.12-0.45 nmol/l. The characterization of the nuclear uptake of 1,25-dihydroxy[3H]cholecalciferol in intact human monocytes shows that it is mediated by binding of the ligand to a specific nuclear receptor. The binding to the nuclear receptor is the result of the passage of ligand across the cytoplasmic membrane and of the cytoplasmic transport of ligand. In contrast to conventional receptor assays in hypertonic cellular extracts, this system provides information on the role of the cytoplasmic membrane in relation to the nuclear uptake of 1,25-dihydroxycholecalciferol, which may be closer to in vivo cellular conditions.

Mechanisms of uptake of ketone bodies by luminal-membrane vesicles

The energetics and location of renal transport of acetoacetate, beta-hydroxybutyrate, alpha-hydroxybutyrate and gamma-hydroxybutyrate by luminal-membrane vesicles from either whole cortex or pars convoluta or pars recta of rabbit proximal tubule were studied. Addition of either acetoacetate or beta-hydroxybutyrate or its analogues to dye-membrane-vesicle suspensions in the presence of Na+ gradient (extravesicular greater than intravesicular) resulted in absorbance changes indicative of depolarizing event(s). Valinomycin enhanced the Na+-dependent uptake of monocarboxylic acids, provided a K+ gradient (intravesicular greater than extravesicular) was present. By contrast, Na+-dependent uptake of these compounds was nearly abolished by ionophores that permit Na+ to pass through the luminal-membrane via another channel, either electrogenically (e.g. gramicidin D) or electroneutrally (e.g. nigericin). These results established that the Na+-dependent transport of ketone bodies and analogues by luminal-membrane vesicles is an electrogenic process. Eadie-Hofstee analysis of saturation kinetic data suggested the presence of multiple transport systems in vesicles from whole cortex for these compounds. Tubular localization of the transport systems was studied by the use of vesicles derived from pars convoluta and pars recta. In pars recta uptake of all these compounds was mediated by means of a single high affinity common transport system. Uptake of these compounds by vesicles from pars convoluta was carried out via a relatively low affinity but common transport system. The physiological importance of the transport systems is discussed.

Serine uptake by luminal and basolateral membrane vesicles from rabbit kidney

The mechanism of renal transport of L- and D-serine by membrane vesicles prepared from either whole cortex, pars convoluta or pars recta of rabbit proximal tubule was studied by a rapid filtration technique and by a spectrophotometric method using a potential-sensitive carbocyanine dye. Transport studies carried out with different salt gradients and by employing various ionophores showed that uptake of both L- and D-serine by luminal membrane vesicles from whole cortex was mediated by an Na+-dependent and electrogenic transport process. Eadie-Hofstee analysis of experimental data, obtained under extravesicular greater than intravesicular NaCl gradients, revealed the existence of multiple transport systems for L-serine but only one system for the D-isomer. The value of KA (the concentration producing a half-maximal optical response) for the D-serine transport system was calculated to be approximately 30 mM. Luminal membrane vesicles from pars convoluta take up both L- and D-serine by a single and common transport system. KA values for L- and D-serine transport were calculated to be 3.7 and 30 mM, respectively. Luminal membrane vesicles from pars recta take up L-serine by means of two transport systems, one of high affinity (KA = 0.37 mM) and the other of low affinity (KA = 10 mM). By contrast, no D-serine transport by these membrane vesicles could be detected. Uptake of L-serine by basolateral membrane vesicles is Na+ independent and electroneutral. Filtration studies showed that the transport is saturable (Km = 25-30 mM) and is inhibited by the presence of L-phenylalanine (but not by D-serine), indicating carrier-mediated uptake of L-serine.

Isolation and partial purification of dicarboxylic acid binding protein from luminal-membrane vesicles of rabbit kidney cortex

A specific dicarboxylic acid binding protein was isolated by solubilizing highly purified renal luminal-membrane vesicles with the non-ionic detergent C12E8 , followed by affinity chromatographic procedures. SDS-polyacrylamide gel electrophoresis of the samples containing dicarboxylic acid binding protein showed a single sharp band of an apparent molecular weight of 50 000. After treatment with mercaptoethanol the protein was split in two subunits of apparent molecular weights of 35 000 and 15 000. By analytical ultracentrifugation the minimal molecular weight of the dicarboxylic acid binding protein preparation was calculated to be 54 000. Binding of the radioactive succinate and L-malate to the dicarboxylic acid binding protein preparation as studied by equilibrium dialysis showed saturation phenomenon and was specifically inhibited by addition of D-malate. The dissociation constants for succinate (0.18 mM) and L-malate (0.33 mM) calculated from the binding data agree extremely well with the apparent Km values for these organic acids found in transport studies utilizing intact luminal-membrane vesicles.

Renal transport of neutral amino acids. Demonstration of Na+-independent and Na+-dependent electrogenic uptake of L-proline, hydroxy-L-proline and 5-oxo-L-proline by luminal-membrane vesicles

Uptake of L-proline, hydroxy-L-proline and 5-oxo-L-proline by luminal-membrane vesicles isolated either from whole cortex or from pars convoluta or pars recta of proximal tubules was studied by a spectrophotometric method. Uptake of L-proline and hydroxy-L-proline by vesicles from whole cortex was mediated by both Na+-dependent and Na+-independent, but electrogenic, processes, whereas transport of 5-oxo-L-proline in these vesicles was strictly Na+-dependent. Eadie-Hofstee analysis of saturation-kinetic data suggested the presence of multiple transport systems in luminal-membrane vesicles from whole renal cortex for the uptake of all these amino acids. Tubular localization of the transport systems was studied by the use of vesicles derived from pars convoluta and from pars recta. In pars recta transport of all three amino acids was strictly dependent on Na+ and occurred via a high-affinity system (half-saturation: 0.1-0.3 mM). Cation-dependent but Na+-unspecific transport of low affinity for L-proline and hydroxy-L-proline was exclusively localized to the pars convoluta, which also contained a Na+-preferring system of intermediate affinity (half-saturation: L-proline, 0.75 mM; hydroxy-L-proline, 1.3 mM). 5-Oxo-L-proline was transported by low-affinity and Na+-dependent systems in both pars convoluta and pars recta. Competition experiments revealed that transport systems for L-proline and hydroxy-L-proline are common, but indicated separate high-affinity transport systems for 5-oxo-L-proline and L-proline in luminal-membrane vesicles from pars recta. The physiological importance of the presence of various neutral amino acid-transport systems in different segments of the proximal tubule is discussed.

Renal transport of neutral amino acids. Tubular localization of Na+-dependent phenylalanine- and glucose-transport systems

The transport properties for phenylalanine and glucose in luminal-membrane vesicles from outer cortex (pars convoluta) and outer medulla (pars recta) of rabbit kidney were studied by a spectrophotometric method. Uptake of phenylalanine as well as of glucose by the two types of membrane vesicles was found to be Na+-dependent, electrogenic and stereospecific. Na+-dependent transport of L-phenylalanine by outer-cortical membrane vesicles could be accounted for by one transport system (KA congruent to 1.5 mM). By contrast, in the outer-medullary preparation, L-phenylalanine transport occurred via two transport systems, namely a high-affinity system with K1A congruent to 0.33 mM and a low-affinity system with K2A congruent to 7 mM respectively. Na+-dependent uptake of D-glucose by pars convoluta and pars recta membrane vesicles could be described by single, but different, transport systems, namely a low-affinity system with KA congruent to 3.5 mM and a high-affinity system with KA congruent to 0.30 mM respectively. Attempts to calculate the stoichiometry of the different Na+/D-glucose transport systems by using Hill-type plots revealed that the ratio of the Na+/hexose co-transport probably is 1:1 in the case of pars convoluta and 2:1 in membrane vesicles from pars recta. The Na+/L-phenylalanine stoichiometry of the pars convoluta transporter probably is 1:1. Both the high-affinity and the low-affinity Na+-dependent L-phenylalanine transport system of pars recta membrane vesicles seem to operate with a 1:1 stoichiometry. The physiological importance of the arrangement of low-affinity and high-affinity transport systems along the kidney proximal tubule is discussed.

Immunochemistry of scorpion alpha-toxins: purification and characterization of two functionally independent IgG populations raised against toxin II of Androctonus australis Hector

We report the isolation and characterization of two IgG populations specific to two synthetic peptides corresponding to two antigenic sites of toxin II of the North African scorpion Androctonus australis Hector. Firstly, thanks to the use of: (1) antigenic homology studies between toxin II of A. australis Hector and toxin III of Buthus occitanus tunetanus, (2) chemical modification of toxin II of A. australis Hector, and (3) prediction of the localization of the four major antigenic sites of scorpion alpha-toxins by the method developed by Hopp and Woods [Proc. natn. Acad. Sci. U.S.A. 78, 3824-3828 (1981)], we have established that the region around the disulfide bridge between cysteines 12 and 63 as well as the stretch of residues 50-59 probably each enclosed an antigenic site. Secondly, the synthetic replicates of these regions linked to Sepharose allowed us to isolate, by immunoaffinity chromatography, two IgG populations from the whole anti-toxin II of A. australis Hector IgGs. Finally, each of these two IgG populations was shown to be specific to one antigenic site as evidenced by the multideterminant effect on the slopes of binding curves developed by Berzofsky et al. [Biochemistry 15, 2113-2121 (1976)]. Furthermore, these two IgG populations were found to be functionally independent and this could be related to the fact that the two regions carrying the two antigenic sites are not close to each other in space and that there is neither steric hindrance nor cooperative effects between them. The association constant of these site-specific IgG populations was calculated and found to be equal to 1.18-5.14 X 10(9) l/mole for IgG anti-site 1 and 1.16-5.62 X 10(9) l/mole for IgG anti-site 2 respectively by Sips [J. chem. Phys. 16, 490-495 (1948)], Scatchard [Am. N.Y. Acad. Sci. 51, 660-772 (1949)] and Steward and Petty [Immunology 23, 881-887 (1972)] representations. The index of heterogeneity of 0.9 for anti-P1 and anti-P2 indicates the purification of essentially homogeneous affinity IgG populations.

Comparative mapping of opioid receptors and enkephalin immunoreactive nerve terminals in the rat hippocampus. A radiohistochemical and immunocytochemical study

Opioid receptors can be localized to the hippocampal formation of the rat by autoradiography. The binding of 3H-enkephalinamide to fixed and mounted tissue sections has all the characteristics associated with binding to opioid receptors. It is saturable, of high affinity and displays stereospecificity. The opioid receptor distribution shows striking regional variation throughout the hippocampal formation. Areas with high density include the pyramidal cell layer of both regio superior (CA1) and regio inferior (CA3), stratum moleculare of the hippocampus, the cell layer of subiculum, the superficial part of presubiculum and the deep layer (VI) of the medial and lateral entorhinal cortices. Areas with low to medium densities include regions corresponding to the dendritic field of the pyramidal cells (str. oriens, str. radiatum and the mossy fiber zone), the dentate granule cell layer and the molecular layer of the dentate area. Enkephalin-like immunoreactivity is detected in both intrinsic neuronal systems: 1) the mossy fibers which terminate on the proximal part of the CA3 pyramidal cell dendrites and on CA4 pyramidal cells, 2) cell bodies with multiple short processes, probably interneurons, dispersed throughout the hilus of the dentate area, the pyramidal cell layer of hippocampus, the str. radiatum, and occasionally in the str. moleculare and in the str. oriens, and extrinsic neuronal systems: 1) the lateral perforant path and 2) the lateral temporo-ammonic tract. Thus, the hippocampus contains intrinsic systems of enkephalin-like immunoreactive nerve terminals which may exert their effect on the opioid receptors with a localization corresponding to the pyramidal cells and their apical dendrites. Extrinsic enkephalinergic systems corresponding to the terminal fields of the lateral perforant path and the temporoammonic tract, both of entorhinal origin, may influence the opioid receptors located in the molecular layer of the dentate area, and in the molecular layer of the hippocampus and the subiculum. Thus, the enkephalin-like immunoreactive nerve terminals are all located in areas which contain opioid binding sites. This suggests that the "opioid peptide-opioid receptor" systems may regulate hippocampal neuronal activity via neurotransmission or neuromodulation. However, a high or medium number of opioid binding sites occur over the pyramidal cell bodies and the dentate granule cell bodies, and these opioid binding sites are not in close contact with the major enkephalinergic systems.(ABSTRACT TRUNCATED AT 400 WORDS)