The curvilinear Scatchard plot. Experimental artifact or receptor heterogeneity?

Effectively designed molecularly imprinted polymers for selective isolation of the antidiabetic drug metformin and its transformation product guanylurea from aqueous media

In the present study, two novel molecularly imprinted polymers (MIPs) with remarkable recognition properties for metformin and its transformation product, guanylurea, have been prepared for their selective, enrichment, isolation and removal from aqueous media. The prepared adsorbents were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and swelling experiments. The performance of the prepared MIPs was evaluated by various parameters including the influence of pH, contact time, temperature and initial compound concentration. The effects on the adsorption behavior of the removal process parameters were studied and the equilibrium data were fitted by the Langmuir and Freundlich models. Due to the imprinting effect, adsorption performance of MIPs was always superior to its corresponding NIP (non-imprinted polymer), with maximum adsorption capacity ∼80 mg g(-1) for both MIPs. Stability and reusability of the MIPs up to the 5th cycle meant that they could be applied repeatedly without losing substantial removal ability. In the next step, the prepared MIP nanoparticles were evaluated as sorbents in a dispersive solid phase extraction (D-SPE) configuration for selective enrichment and determination of metformin and guanylurea in different aqueous matrices. Under the working extraction conditions, the D-SPE method showed good linearity in the range of 50-1000 ng L(-1), repeatability of the extractions (RSD 2.1-5.1%, n=3), and low limits of detection (1.5-3.4 ng L(-1)). The expanded uncertainty of the data obtained was estimated following a bottom-up approach. The proposed method combined the advantages of MIPs and D-SPE, and it could become an alternative tool for analyzing the residues of METF and its transformation product GUA in complex water matrices, such as wastewaters.

BIOLOGICAL RECEPTORS: IMPLICATIONS OF RADIOLIGAND QUALITY IN BINDING ASSAYS

We tested the ability of different fitting procedures to discern differences in the binding affinity between the labeled and unlabeled ligand in a receptor assay, in order to obtain the correct value of the radiotracer specific activity. This point is critical for the correct conversion of the estimated binding constants to meaningful units. We simulated sets of receptor assays which considered distinct experimental situations, and fitted the data with two models considering either a unique or two different affinities for the two classes of ligand. We compared direct non-linear regression, linear robust regression on transformed data, and an experimental approach called pseudo-saturation (labeled plus unlabeled ligand in constant ratio) assay.

It emerges from the results that none of these methods would be sensitive enough, even in commonly acceptable error levels in the measured variables, to indicate the appropriate model with a reasonable certainty. The a priori consideration of a different binding capacity for both categories of the ligand gives an acceptable estimation of the parameters, regardless of the fact that the fitting criteria reflect a better fitting of the other model. We conclude that a two-affinity binding model should be routinely fitted to experimental data and the binding constants compared to the simple binding model when the specific activity of the radiotracer needs to be assessed.

Toxicokinetics, including saturable protein binding, of 4-chloro-2-methyl phenoxyacetic acid (MCPA) in patients with acute poisoning

Human data on protein binding and dose-dependent changes in toxicokinetics for MCPA are very limited. 128 blood samples were obtained in 49 patients with acute MCPA poisoning and total and unbound concentrations of MCPA were determined. The Scatchard plot was biphasic suggesting protein binding to two sites. The free MCPA concentration increased when the total concentration exceeded 239 mg/L (95% confidence interval 198–274 mg/L). Nonlinear regression using a two-site binding hyperbola model estimated saturation of the high affinity binding site at 115 mg/L (95%CI 0–304). Further analyses using global fitting of serial data and adjusting for the concentration of albumin predicted similar concentrations for saturable binding (184 mg/L and 167 mg/L, respectively) without narrowing the 95%CI. In 25 patients, the plasma concentration–time curves for both bound and unbound MCPA were approximately log-linear which may suggest first order elimination, although sampling was infrequent so zero order elimination cannot be excluded. Using a cut-off concentration of 200 mg/L, the half-life of MCPA at higher concentrations was 25.5 h (95%CI 15.0–83.0 h; n = 16 patients) compared to 16.8 h (95%CI 13.6–22.2 h; n = 10 patients) at lower concentrations. MCPA is subject to saturable protein binding but the influence on half-life appears marginal.

Insulin, synaptic function, and opportunities for neuroprotection

A steadily growing number of studies have begun to establish that the brain and insulin, while traditionally viewed as separate, do indeed have a relationship. The uptake of pancreatic insulin, along with neuronal biosynthesis, provides neural tissue with the hormone. As well, insulin acts upon a neuronal receptor that, although a close reflection of its peripheral counterpart, is characterized by unique structural and functional properties. One distinction is that the neural variant plays only a limited part in neuronal glucose transport. However, a number of other roles for neural insulin are gradually emerging; most significant among these is the modulation of ligand-gated ion channel (LGIC) trafficking. Notably, insulin has been shown to affect the tone of synaptic transmission by regulating cell-surface expression of inhibitory and excitatory receptors. The manner in which insulin regulates receptor movement may provide a cellular mechanism for insulin-mediated neuroprotection in the absence of hypoglycemia and stimulate the exploration of new therapeutic opportunities.

Quantification of PET studies with the very high-affinity dopamine D2/D3 receptor ligand [11C]FLB 457: re-evaluation of the validity of using a cerebellar reference region

The very high-affinity position emission tomography (PET) radioligand [(11)C]FLB 457 was developed in order to study extrastriatal tissues, where the density of dopamine D(2)/D(3) receptors is one to two orders of magnitude lower than in the striatum. The present study investigated the validity of using the cerebellum as a reference region. Ten healthy volunteers underwent a 90-min dynamic PET study after the bolus injection of [(11)C]FLB 457. The total volume of distribution (VD(t)) was estimated for the thalamus, hippocampus, frontal cortex, and cerebellum using a two-tissue compartmental model with a metabolite-corrected arterial plasma input function. VD(t) was sensitive to co-injected stable FLB 457 in all regions, including the cerebellum. Ex vivo saturation studies were also conducted in 17 rats where the dose of stable ligand was varied over five orders of magnitude. Specific binding was estimated to account for more than half of the rat cerebellar uptake of [(11)C]FLB 457, questioning the latter as an estimate of nonspecific binding in human PET studies. To check whether the cerebellum is a reference region, the binding potential (BP) was calculated either from the VD(t) ratio or using the simplified reference tissue model (SRTM). A non-negligible density of D(2)/D(3) receptors in the cerebellum was shown to lead to underestimation of BP as well as erroneous estimation of differential occupancies. Binging potential estimates from the SRTM were found to be sensitive to changes in cerebral blood flow, providing further evidence for caution in the use of the cerebellum as a reference region in measures of [(11)C]FLB 457 binding.

3-[(±)-2-Carboxypiperazin-4-yl]propyl-1-Phosphonic Acid Recognizes Two N-Methyl-D-Aspartate Binding Sites in Rat Cerebral Cortex Membranes

Binding of 3-[(+-)-2-carboxypiperazin-4-yl][3H]-propyl-1-phosphonic acid ([3H]CPP), a competitive inhibitor of N-methyl-D-aspartate (NMDA), has been studied in synaptic plasma membranes from rat cerebral cortex. Computer analysis of saturation and homologous displacement isotherms deriving from these plasma membranes indicated the existence of two binding sites: a specific, saturable, high-affinity binding site with a pKD value of 7.53 +/- 0.03 (29.5 nM) and a maximum binding value (Bmax) of 2.25 +/- 0.36 pmol/mg of protein, and a low-affinity site with a KD of approximately 600 nM and a Bmax of 7.0 pmol/mg of protein. It is argued that, in the light of current literature evidence, the low-affinity binding site may represent an agonist-dependent receptor, linked to physiological processes such as neurotransmitter release and channel regulation, whereas the high-affinity binding site may be linked to an antagonist-preferred receptor, for which no function has yet been reported.

Requirements for reliable determination of binding affinity constants by saturation analysis approach

Accurate calculation of the equilibrium association constant (K) and binding site concentration (N) related to a receptor (R)/ligand (L) interaction, via R saturation analysis, requires exact determination of the specifically bound L concentration (B(S)) and the unbound L concentration (U) at equilibrium. However, most binding determinations involve a procedure for separation of bound and unbound L. In such situations, it was previously shown that correct calculation of B(S) and U from binding data requires prior determination of alpha, i.e. the procedure parameter representing the proportion of equilibrium B(S) recovered after running the separation process, and of kn, i.e. the equilibrium nonspecific binding coefficient. For the simplest model of R/L interaction, the consequences of alpha neglect and/or kn neglect on determination of K and N, via R saturation analysis, are investigated. When alpha but not kn has been determined, B(S) can be accurately calculated, whereas U is overestimated by factor (kn + 1). Consequently the type (linear or hyperbolic) of theoretic curves obtained by usual representations (such as the Scatchard, the Lineweaver-Burk or the Michaelis-Menten plot) of the R/L binding is unchanged; these curves afford correct N and underestimation of K by factor (kn + 1). When alpha (alpha < 1) has not been determined B(S) and U are underestimated and overestimated, respectively. Then erroneous representations of the R/L binding result (e.g. instead of regular straight line segments, Scatchard plot and Lineweaver-Burk plot involve convex-upward and convex-downward hyperbola portions, respectively, suggestive of positive cooperativity of L binding), which leads to incorrect N and K. Errors in N and K would depend on (i) the binding (K, N and kn) and method (alpha) parameters and (ii) the expressions used to calculate approximate B(S) and U values. Simulations involving variable alpha, KN and kn values indicate that: (1) the magnitude of error in N determination (mainly involving moderate underestimation) directly depends on the alpha value; (2) the magnitude of K underestimation mainly depends on the KN value; it is moderate (usually < two-fold) with KN values < 1, but could become very high (e.g. > 100-fold), when KN > 10(2). In this case, the K underestimation is modulated by the alpha and kn values. Practical situations which afford high KN and thus might result in very marked underestimation of K are discussed. A single R dilution method is proposed to assess the validity of K determinations using the R saturation analysis approach.

A Novel Arabidopsis thaliana Protein is a Functional Peripheral-Type Benzodiazepine Receptor

A key element in the regulation of mammalian steroid biosynthesis is the 18 kDa peripheral-type benzodiazepine receptor (PBR), which mediates mitochondrial cholesterol import. PBR also possess an affinity to the tetrapyrrole metabolite protoporphyrin. The bacterial homolog to the mammalian PBR, the Rhodobacter TspO (CrtK) protein, was shown to be involved in the bacterial tetrapyrrole metabolism. Looking for a similar mitochondrial import mechanism in plants, protein sequences from Arabidopsis and several other plants were found with significant similarities to the mammalian PBR and to the Rhodobacter TspO protein. A PBR-homologous Arabidopsis sequence was cloned and expressed in E. coli. The recombinant gene product showed specific high affinity benzodiazepine ligand binding. Moreover, the protein applied to E. coli protoplasts caused an equal benzodiazepine-stimulated uptake of cholesterol and protoporphyrin IX. These results suggest that the PBR like protein is involved in steroid import and is directing protoporphyrinogen IX to the mitochondrial site of protoheme formation.

Estimation of drug-protein binding parameters on assuming the validity of thermodynamic equilibrium

This contribution focuses the reader's attention on the pitfalls usually emerging during the phase of evaluation of experimental data of drug-protein binding studies. To overcome the occurrence of problem(s) apparently defying solution, the concept of "affinity spectra" is recommended to be implemented for data evaluation. A (general) "binding study protocol" is also suggested, which can prevent the formation of inadequate conclusions and the generation of unrealistic drug-protein binding parameters.

Mutations that affect ligand binding to the Escherichia coli aspartate receptor: implications for transmembrane signaling

Three arginine residues of the binding site of the Escherichia coli aspartate receptor contribute to its high affinity for aspartate (K(d) approximately 3 microm). Site-directed mutations at residue 64 had the greatest effect on aspartate binding. No residue could substitute for the native arginine; all changes resulted in an apparent K(d) of approximately 35 mm. These mutations had little impact on maltose responses. At residue Arg-69, a lysine substitution was least disruptive, conferring an apparent K(d) of 0.3 mm for aspartate. Results obtained for an alanine mutant were similar to those with cysteine and histidine mutants (K(d) approximately 5 mm) indicating that side chain size was not an important factor here. Proline and aspartate caused more severe defects, presumably for reasons related to conformation and charge. The impact of residue 69 mutations on the maltose response was small. Mutations at Arg-73 had similar effects on aspartate binding (K(d) 0.3-7 mm) but more severe consequences for maltose responses. Larger side chains resulted in the best aspartate binding, implying steric considerations are important here. Signaling in the mutant proteins was surprisingly robust. Given aspartate binding, signaling occurred with essentially wild-type efficiency. These results were evaluated in the context of available structural data.

Vascular endothelial growth factor binds to fibrinogen and fibrin and stimulates endothelial cell proliferation

Vascular development and response to injury are regulated by several cytokines and growth factors including the members of the fibroblast growth factor and vascular endothelial cell growth factor (VEGF) families. Fibrinogen and fibrin are also important in these processes and affect many endothelial cell properties. Possible specific interactions between VEGF and fibrinogen that could play a role in coordinating vascular responses to injury are investigated. Binding studies using the 165 amino acid form of VEGF immobilized on Sepharose beads and soluble iodine 125 (125I)–labeled fibrinogen demonstrated saturable and specific binding. Scatchard analysis indicated 2 classes of binding sites with dissociation constants (Kds) of 5.9 and 462 nmol/L. The maximum molar binding ratio of VEGF:fibrinogen was 3.8:1. Further studies characterized binding to fibrin using 125I-labeled VEGF- and Sepharose-immobilized fibrin monomer. These also demonstrated specific and saturable binding with 2 classes of sites havingKds of 0.13 and 97 nmol/L and a molar binding ratio of 3.6:1. Binding to polymerized fibrin demonstrated one binding site with a Kd of 9.3 nmol/L. Binding of VEGF to fibrin(ogen) was independent of FGF-2, indicating that there are distinct binding sites for each angiogenic peptide. VEGF bound to soluble fibrinogen in medium and to surface immobilized fibrinogen or fibrin retained its capacity to support endothelial cell proliferation. VEGF binds specifically and saturably to fibrinogen and fibrin with high affinity, and this may affect the localization and activity of VEGF at sites of tissue injury.

Vascular endothelial growth factor binds to fibrinogen and fibrin and stimulates endothelial cell proliferation

Vascular development and response to injury are regulated by several cytokines and growth factors including the members of the fibroblast growth factor and vascular endothelial cell growth factor (VEGF) families. Fibrinogen and fibrin are also important in these processes and affect many endothelial cell properties. Possible specific interactions between VEGF and fibrinogen that could play a role in coordinating vascular responses to injury are investigated. Binding studies using the 165 amino acid form of VEGF immobilized on Sepharose beads and soluble iodine 125 (125I)–labeled fibrinogen demonstrated saturable and specific binding. Scatchard analysis indicated 2 classes of binding sites with dissociation constants (Kds) of 5.9 and 462 nmol/L. The maximum molar binding ratio of VEGF:fibrinogen was 3.8:1. Further studies characterized binding to fibrin using 125I-labeled VEGF- and Sepharose-immobilized fibrin monomer. These also demonstrated specific and saturable binding with 2 classes of sites havingKds of 0.13 and 97 nmol/L and a molar binding ratio of 3.6:1. Binding to polymerized fibrin demonstrated one binding site with a Kd of 9.3 nmol/L. Binding of VEGF to fibrin(ogen) was independent of FGF-2, indicating that there are distinct binding sites for each angiogenic peptide. VEGF bound to soluble fibrinogen in medium and to surface immobilized fibrinogen or fibrin retained its capacity to support endothelial cell proliferation. VEGF binds specifically and saturably to fibrinogen and fibrin with high affinity, and this may affect the localization and activity of VEGF at sites of tissue injury.

Characterization of the binding of FK633 to the platelet fibrinogen receptor

125I-fibrinogen bound to ADP-activated fixed platelets in a saturable manner. The Scatchard plot was curvilinear but nonlinear model fitting of the data suggested that there was only one binding site with KA=4.19+/-1.3 x 10(6) M(-1) and a maximum number of binding sites of 3.9+/-1.1 x 10(4) molecules/platelet. The GPIIb/IIIa antagonists RGDS and FK633 both inhibited 125I-fibrinogen binding (50 microg/ml) in a dose-dependent manner. FK633 had a KB value of 2.5+/-0.48 x 10(7) M(-1) (Kb=39.9 nM) and an IC50, value of 64 nM, while RGDS had an KB of 2.55+/-0.76 x 10(4) M(-1) (Kb=39.2 microM) with an IC50 value of 63 microM. At concentrations below its IC50 value FK633 was a competitive antagonist of fibrinogen binding. However, at concentrations above its IC50 value it was a noncompetitive antagonist. The IC50 values of FK633 remained constant over a wide range of fibrinogen concentrations while its KB value changed-increasing from 1.8+/-0.6 x 10(7) M(-1) at 10 microg/ml 125I-fibrinogen to 4.5+/-1.6 x 10(7) M(-1) at 300 microg/ml and decreasing to 0.3+/-0.2 x 10(7) M(-1) at 2.4 mg/ml. Thus, FK633 is a reversible, noncompetitive antagonist of fibrinogen binding to the platelet GPIIb/IIIa receptor.

Characterization of the initial alpha-thrombin interaction with glycoprotein Ib alpha in relation to platelet activation

We have evaluated the properties of alpha-thrombin interaction with platelets within 1 min from exposure to the agonist, a time frame during which most induced activation responses are initiated and completed. Binding at 37 degrees C was rapidly reversible and completely blocked by a monoclonal antibody, LJ-Ib10, previously shown to be directed against the alpha-thrombin interaction site on glycoprotein (GP) Ib alpha. By 2-5 min, however, binding was no longer fully reversible and was only partially inhibited by the anti-GP Ib alpha antibody. Results were similar at room temperature (22-25 degrees C), whereas the initial characteristics of alpha-thrombin interaction with platelets were preserved for at least 20 min at 4 degrees C. Equilibrium binding isotherms obtained at the latter temperature were compatible with a two-site model, but the component ascribed to GP Ib alpha, completely inhibited by LJ-Ib10, had "moderate" affinity (kd on the order of 10(-8) M) and relatively high capacity, rather than "high" affinity (kd on the order of 10(-10) M) and low capacity as currently thought. The parameters of alpha-thrombin binding to intact GP Ib alpha on platelets at 4 degrees C corresponded closely to those measured with isolated GP Ib alpha fragments regardless of temperature. Blocking the alpha-thrombin-GP Ib alpha interaction caused partial inhibition of ATP release and prevented the association with platelets of measurable proteolytic activity. These results support the concept that GP Ib alpha contributes to the thrombogenic potential of alpha-thrombin.

Synthesis of key sandramycin analogs: systematic examination of the intercalation chromophore

The preparation and examination of 2-22 constituting a systematic study of the chromophore of sandramycin (1) are detailed. Fluorescence quenching studies were used to establish binding constants for 1-24 within calf thymus DNA, within a single high affinity bis-intercalation binding site 5'-d(GCATGC)2, and to establish the preference for sandramycin binding to 5'-d(GCXXGC)2 where XX = AT, TA, GC, and CG. From the latter studies, sandramycin was found to exhibit a preference that follows the order: 5'-d(GCATGC)2 > 5'-d(GCGCGC)2, deltadeltaGo = 0.3kcal/mol > 5'-d(GCTAGC)2, 5'-d(GCCGGC)2, deltadeltaGo = 0.6 kcal/mol although it binds with high affinity to all four deoxyoligonucleotides. The two highest affinity sequences constitute repeating 5'-PuPy motifs with each intercalation event occurring at a 5'-PyPu step. The most effective sequence constitutes the less stable duplex, contains the sterically most accessible minor groove central to the bis-intercalation site, and the ability to accept two gly-NH/TC2 carbonyl H-bonds identified in prior NMR studies. Similarly, the contribution of the individual structural features of the chromophore were assessed with the high affinity duplex sequence 5'-d(GCATGC)2. To a first approximation, the cytotoxic properties were found to parallel trends established in the DNA binding affinities. The exception to this generalization was 4 which lacks the sandramycin chromophore phenol. Although typically 4-10x less potent than sandramycin against leukemia cell lines, it proved to be 1-10,000x more potent against melanomas, carcinomas, and adenocarcinomas exhibiting IC50 values of 1 pM-10 nM placing it among the most potent agents identified to date. Additionally, the first disclosure of the HIV-1 reverse transcriptase inhibitory activity of sandramycin (1) as well as that of its key analogs are described and define the chromophore structural features required for their exceptional potency. Two analogs, 18 and 3, roughly maintain the HIV-1 reverse transcriptase inhibitory potency of 1 but exhibit substantially diminished cytotoxic activity (10(2)-10(3)x).

Chronic imipramine treatment upregulates IR2-imidazoline receptive sites in rat brain

A low density of brain IR2-imidazoline receptive sites has previously been linked to depression. In this study we evaluated brain IR2-binding sites in a rat model of depression, olfactory bulbectomy, and determined the effects of chronic imipramine treatment in vivo on these sites. Compared with sham-operated controls, adaptation to olfactory bulbectomy had no effect on either the density (Bmax) or affinity (KD) of [3H]-idazoxan binding to brain IR2 sites. However, 25 days of imipramine treatment (i.p., 20 mg/kg/day) enhanced significantly the density of IR2 binding sites, with no change in affinity in both the model and the control group. These results indicate that the brain IR2-imidazoline receptive sites might be a target for antidepressants.

Relationship between receptors for epidermal growth factor and steroid hormones in normal, dysplastic and neoplastic canine mammary tissues

The concentrations of receptors for epidermal growth factor (EGF-R), oestrogen (ER) and progesterone (PR) were measured in 108 samples from canine mammary tumours and 132 samples of normal mammary tissue removed surgically from 84 bitches. The history and clinical signs were also recorded. Binding sites of high affinity were detected in 70 per cent of both types of tissue and no significant variations in EGF-R concentrations or positivity were observed with the histology, location, size or number of mammary tumours or the age of the animal. A significant direct correlation (P = 0.002) was observed between the concentrations of ER and EGF-R only in malignant tumours. The concentrations of EGF-R were significantly correlated (P = 0.04) in normal mammary tissues adjacent to and distant from the lesions, but not between normal tissue and tumour tissue. No significant differences were observed in the expression of EGF-R in normal and neoplastic tissues from the same bitches. The direct correlation between the concentrations of EGF-R and ER in malignant tumours could be related to an oestrogen-dependent expression of EGF-R or to a similar pattern of regulation of the receptors.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.

Chromatographic modelling of interactions between melanin and phenothiazine and dibenzazepine drugs

Differences in drug-melanin interactions were determined for 13 phenothiazine neuroleptics and 2 dibenzazepine thymoleptics by means of high-performance liquid chromatography. The chromatographic column was packed with a stationary phase obtained by chemical immobilization of synthetic L-dopa melanin on silica particles. For six phenothiazines the melanin-binding parameters were also determined by an ultrafiltration method. Correlation between measures of drug-melanin interaction determined chromatographically and by the standard slow-equilibrium method was significant, however moderate. The chromatographic method of assessing interactions between drugs and melanin permitted reliable and quantitatively comparable data for representative series of solutes to be readily obtained. Such data were subjected to the analysis of quantitative structure-retention relationships (QSRR). It was found that retention of the agents on the immobilized melanin column could be described by two-parameter regression equations comprising the energy of the lowest unoccupied molecular orbital and either the water-accessible surface area of a drug molecule or its hydrophobicity parameter, determined chromatographically on an immobilized artificial membrane column. The QSRR equation derived allows for the estimation of melanin binding based on the structure of a compound candidate, and thus rationalizes predictions of potential toxicity of drugs or drug candidates.

Stereoselective binding of isradipine to human plasma proteins

Isradipine (PN 200-110) is a highly potent calcium entry blocker with an asymmetrically substituted dihydropyridine ring (methyl- and isopropylester, respectively). The binding of the (+)-(S)-isradipine and (-)-(R)-isradipine to isolated human serum albumin (HSA, 30 mumol/l) and alpha 1-acid glycoprotein (AAG, 10 mumol/l) has been studied in vitro over a wide range of isradipine concentrations (0.06-20 mumol/l) using high-performance liquid chromatography (HPLC). HPLC experiments revealed that both isradipine enantiomers were bound to one class of high-affinity binding sites on the AAG molecule (n(S) = 0.83 +/- 0.05, Ka(S) = (1.33 +/- 0.25) x 10(6) l/mol, n(R) = 0.85 +/- 0.07, Ka(R) = (1.17 +/- 0.44) x 10(7) l/mol). The (R)-enantiomer also exhibited an interaction with the secondary low-affinity binding sites (n'Ka'(R) = (2.66 +/- 0.65) x 10(4) l/mol). In contrast, the pharmacologically more potent (+)-(S)-enantiomer was more strongly bound to HSA than its optical antipode (n(S) = 1.07 +/- 0.07, Ka(S) = (1.76 +/- 0.26) x 10(5) l/mol, nKa(R) = (3.62 +/- 0.06) x 10(4) l/mol). In general, the resulting binding characteristics of individual isradipine enantiomers showed stereoselectivity, but this was opposite for the two most important plasma binding proteins. The process of accumulation of isradipine by human platelets in the therapeutically relevant range (10-80 ng/ml) at 37 degrees C was devoid of stereoselectivity.

Evaluation of [O-methyl-3H]WAY-100635 as an in vivo radioligand for 5-HT1A receptors in rat brain

N-(2-(4-(2-Methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridyl)cyclohexanecarboxamide trihydrochloride (WAY-100635) is a new, potent and selective 5-HT1A receptor antagonist. We have evaluated radiolabelled WAY-100635 as a prospective radioligand for positron emission tomography (PET) by studying biodistribution in rat ex vivo. After intravenous injection, [O-methyl-3H]WAY-100635 cleared rapidly from plasma but was retained in brain. Specific binding was quantified from kinetic studies, using a reference-tissue compartment model, fitting for binding potential (k3/k4). The regional variation in binding potential correlated with the known distribution of 5-HT1A receptors. Saturation studies gave Bmax values in vivo that were consistent with those reported in vitro. At 60 min after injection, the ratio of radioactivity in 5-HT1A receptor-rich regions (e.g. septum, entorhinal cortex and hippocampus) to that in cerebellum reached approximately 16. Pre-dosing the rats with WAY-100635 (2 mg/kg) reduced this ratio to one, whereas similar pre-dosing with citalopram (5-HT uptake site inhibitor), prazosin (alpha 1A-adrenoceptor antagonist) or idazoxan (alpha 2-adrenoceptor antagonist) caused little or no reduction. Substantial (77%) blockade of [3H]WAY-100635 binding was achieved with the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and the partial agonists, ipsapirone and buspirone. Thus, the properties of WAY-100635 are such that, when labelled with carbon-11, it could provide a radioligand suitable for clinical and pharmacological investigations of central 5-HT1A receptors in man using PET.

The relationship between receptor-effector unit heterogeneity and the shape of the concentration-effect profile: pharmacodynamic implications

The apparent concentration-effect relationship is the ensemble of many effector units (such as individual cells or channels) that do not always exhibit a uniform stimulus-effect relationship. This concept is substantiated by many observations of heterogeneity in receptor-effector populations including hormone secreting cells, response to hormonal stimuli, activity pattern of second messengers, stimulus-evoked synaptic currents, and single ion channels. The relationship between drug concentration and magnitude of pharmacologic response is commonly described by the sigmoidal Emax model which was derived from the Hill equation. The sigmoidicity factor (N) in this model is assumed to be a pure mathematical parameter without physiological connotations. This work demonstrates that the numerical value of N (measured empirically) is the product of two factors: (i) the degree of heterogeneity of the effector subunits, i.e., the elemental component that upon drug stimulus contributes its pharmacological effect independently and does not interact with other subunits (it could range from a single receptor up to a whole tissue), and (ii) value of N*--the shape factor of the subunits' concentration-effect relationship. A special case of this approach occurs when N* > 5, which is an on-off case. Here N is determined by the distribution (density equation) of the subunit values. In case of heterogeneity of the microparameters of the effector subunits the apparent N will always have a lower value than N*. According to this theory it can be concluded that without knowledge of the distribution of the microparameters no mechanistic interpretation can be deduced from the apparent N value. If in the future N* can be determined by theoretical or experimental methods, the distribution function relating N* to N can be calculated. The relevance of this theory is increased in view of the progress being made in advanced research techniques which may enable us to determine the concentration-effect relationship at the level of the individual effector unit.

Propafenone binding interaction with human alpha 1-acid glycoprotein: assessing experimental design and data evaluation

Binding data on racemic RS-propafenone as well as individual R- and S-drug enantiomers interacting reversibly with human alpha 1-acid glycoprotein, as obtained by a high-performance liquid chromatographic method, are evaluated according to three different approaches introduced, respectively, by Scatchard, Bjerrum, and by Tobler and Engel. A non-linear curve-fitting procedure was applied to compute the binding parameters exclusively for the binary system comprising the examined protein and R- and S-propafenone, individually. The exactness of the study design rather than the numerical values were the focus of attention in the evaluation of the data found.

Voltage-sensitive calcium channel development in epileptic DBA/2J mice suggests altered presynaptic function

Aberrant synapse formation has been implicated in development and propagation of epileptic potential. Litzinger et al. (1993a) showed that omega-GVIA conotoxin may be used as a marker for synapse formation in nonepileptic mice. We conducted omega-GVIA binding in synaptosomal preparations from epileptic DBA/2J mice at different developmental ages. Binding in DBA/2J mice was compared with omega-GVIA binding in synaptosomal preparations from nonepileptic C57/B1, Swiss Webster, and AJ mice. Striking differences between these strains of mice are evident in the developmental sequence and pattern of N-type voltage-sensitive calcium channels (VSCC). In contrast to nonepileptic mice, the DBA/2J mice show a slow increase in omega-GVIA binding between postnatal days 2 and 8. This increase corresponds to onset of susceptibility to seizure in this strain. In addition to the difference in developmental sequence, DBA/2J mice have fewer binding sites for omega-GVIA throughout development, suggesting changes in channel structure or number. These data show that in DBA/2J mice development of the VSCC in brain is different from that in nonepileptic mice. This difference in development in presynaptic membranes responsible for neurotransmitter release may represent a change in synaptic activity that plays a role in epileptogenesis.

Multiphasic modelling of ligand/acceptor interactions. The hydrophobicity-dependent binding of relatively small amphiphilic substances to acceptor proteins and the nature and facedness of acceptor sites

The modelling of multiphasic ligand/acceptor equilibrium binding systems proceeds at three logically distinct levels: (1) A suitable response quantity, e.g. the amount of acceptor-bound ligand nEL, is expressed as a function of the ligand concentrations [Li] (L = A,B,...) in the compartment i that contains the acceptor sites. One thus obtains a response function nEL = f1([Li]). In general, the equilibrium constants KL contained in such mathematical models are physically ill-defined. (2) Each local concentration [Li] is further expressed as a function of [Laq], the corresponding concentration in the aqueous phase, leading to nEL = f2([Laq]). In this way, the constants KL are transformed into effective constants K'L which (i) can be assessed experimentally and (ii) depend on ligand hydrophobicity in a way that is characteristic of the binding site. Formulation of the functions f1 and F2 only requires knowledge of the reactions in which the acceptor sites participate directly. (3) For each ligand, the experimentally accessible total ligand concentration Lt is expressed as a function of [Laq], leading to concentration balance equations Lt = Lt([Laq]). The latter transformation takes account of any reactions, distinct from ligand/acceptor interaction, in which the ligands are involved, e.g. binding to additional protein sites. As a result of steps 2 and 3, each binding system is described by a set of simultaneous equations dependent on the auxiliary variable [Laq]: (i) the response function f2([Laq]) and (ii) a concentration balance for each ligand Lt = Lt([Laq]). The formulae are rendered more conscise and their discussion and application to data fitting are simplified by introducing, for each ligand L, a function FL characterising the distribution of unbound monomeric ligand over the various partition compartments. When the acceptor acts on unbound ligand, the formulae are further expressed in terms of a new auxiliary variable i.e. the total concentration of unbound monomeric ligand microL. In contrast to data analysis as a function of local concentrations, analysis in terms of total ligand concentrations avoids losing sight of alternate hypotheses about the nature of the binding sites. The present formulation has also permitted clarification of several consequences of the multiphasic nature of the binding systems that, as yet, have been poorly recognised.(ABSTRACT TRUNCATED AT 400 WORDS)

Some approaches to the pharmacology of multisubstrate enzyme systems

Analytical and exploratory in vitro, in situ and in vivo, physio-pharmacotoxicology, from enzymology to population epidemiology, now embraces those approaches that correlate complex dynamic multisubstrate kinetics through conventional and more recent non-invasive quantitative methodologies. Basically, substrates may be classed as pertaining to fundamental energy turnovers (first-order cellular metabolic pathways or networks) and to iso- vs allosteric modulator systems (second-order metabolic control network). Pairs of substrates and cofactors set-up the third-order multienzyme-receptor patterns, which in intact, native in vivo structures establish and maintain the compartmentalized, dynamically superimposed overall coordination of local redox and phosphate potentials. Perturbations of the various levels of the metabolic hierarchy induced by drugs, as well their relaxations, can be readily submitted to non-invasive kinetic analysis. Both indirect and direct titrations of substrate levels, their modelling and statistical ad hoc evaluations of their interrelations can lead to the identification of the multiple sites involved in drug effects as structured at the different orders/levels of concomitant functional variations. Fractal geometries contribute towards defining the space- and time-related events.

The ontogeny of tolerance to the 5-HT1A agonist 8-OH-DPAT: a study in the rat

The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) induces hypothermia and a flat body posture in rats. Tolerance to 8-OH-DPAT develops in adult rats with regard to these responses. The ontogeny of the ability to induce tolerance to the hypothermia and the flat body posture elicited by 8-OH-DPAT was studied. Rat pups of both sexes were given 8-OH-DPAT, 100 micrograms/kg (0.352 mumol/kg) or saline for 1, 4 or 12 days between 22 and 34 days of age. At 26 days of age no attenuation was induced by treatment during the previous 4 days. In contrast, at 34 days of age there was a clear attenuation of both responses induced by 8-OH-DPAT after an analogous 4 day treatment. The data indicate that the ability to induce tolerance to 8-OH-DPAT is not developed before 26 days of age in the rat.

Interpretation and analysis of receptor binding experiments which yield non-linear Scatchard plots and binding constants dependent upon receptor concentration

Receptor-binding assays with radiolabelled ligands are widely used to evaluate the biological activity of drugs and hormones. The affinity, usually expressed as the dissociation constant (Kd), and the capacity (Bmax) of the receptor preparation for various ligands are determined in order to compare quantitatively various agonists and antagonists. Experiments with the same ligand and receptor, however, often yield rather disparate values for these binding parameters. One obvious reason for variability in Kd is that straight lines are fitted to data that are clearly curved. Another and more serious reason is that a ligand's apparent dissociation constant decreases when the receptor preparation is diluted and so experiments done at different receptor concentrations do not give identical results. We demonstrate that both of these observations, i.e. the effect of receptor concentration and the curvature of Scatchard plots, can be explained by the presence of a competitive inhibitor in the receptor preparation, a possibility which is not normally considered in the analysis and interpretation of receptor binding assays. We show that the apparent Kd obtained by the conventional one- or two-site analysis may be several orders of magnitude larger than the true dissociation constant and the affinity is therefore seriously underestimated. Application of a model, which assumes that an inhibitor is present in the receptor preparation, will improve the quantitative determination of Kd and Bmax significantly. As a simple alternative method we explain how the apparent binding parameters obtained by the conventional method should be interpreted and how they can be used to estimate the true affinity, provided sufficiently low concentration data are available.

The substrate-depletion error on the multiple Michaelis-Menten equation with and without an added term linearly dependent on the substrate concentration

Saturation curves for substrate interaction with an acceptor (enzyme, binding or carrier protein) are often analysed on the assumption that the amount of acceptor-bound substrate is negligible compared to its total amount. Analytical criteria permitting one to decide whether the assumption is justified or not for systems described by a single Michaelis-Menten equation have been derived previously. For more complicated systems, error formulae often cannot be obtained in closed form, and, if obtainable, are unwieldy. How this more complicated problem can be tackled is shown for mixtures of acceptor sites described by a sum of several Michaelis-Menten terms, without or with an additional term for 'non-specific' uptake or binding. In particular, it is shown that the maximum error, for which simple analytical expressions are obtained, provides a valid criterion for assessing whether substrate depletion is negligible or not.

The existence of conformationally labile (preformed) drug binding sites in human serum albumin as evidenced by optical rotation measurements

The ability of certain drugs to induce conformational changes in human serum albumin has been examined by differential optical rotation measurements at 233 nm. At drug:protein molar ratios ([D]/[P]) of unity, the optical rotation increased, decreased or remained the same depending on the drug used. The change in the optical rotatory dispersion (ORD) signal was investigated as a function of the drug concentration. Drug-protein interactions were relatively specific. There exists at least one, and possibly more, stable preformed high affinity sites for the binding of drugs to albumin. At low [D]/[P] ratios, the ORD titration curves suggest that the high affinity sites are conformationally labile and that the albumin molecule is flexible.

EGF-R and steroid receptors in breast cancer: a comparison with tumor grading, tumor size, lymph node involvement, and age

In this study epidermal growth factor receptor (EGF-R), estrogen receptor (ER), and progesterone receptor (PR) status was evaluated in 326 primary breast carcinomas. Nineteen percent of samples were EGF-R positive, 63% were positive for ER, and 54% for PR. In 46% of the tumors both ER and PR were positive. These data are presented together, with grading, size of tumor, lymph node involvement, histological subtype, and age. Sixty-nine percent of EGF-R negative tumors were ER-positive and 51% were positive for ER as well as PR. In particular, negative correlation between EGF-R and steroid receptor status was found. A quantitative correlation was also shown. A combination of negative steroid receptor and positive EGF-R was found more often in the population of poorly differentiated tumors. Tumors bigger than 5 cm were related to a positive EGF-R status. No correlation between nodal status and any receptor status was found. Intraductal carcinomas were more often EGF-R positive than infiltrating ductal (NOS) or infiltrating lobular lesions. The age of patients correlated with the concentration of ER only. In our study we reaffirmed the negative correlation between steroid receptor status and the overexpression of EGF-R; furthermore the combination of EGF-R+ and ER- tumors was observed more often in histological high-risk tumors. Patient outcome did not show statistically significant differences concerning the EGF-R status, but was associated with the steroid receptor status.

The influence of pH on the interaction of lipophilic anthracyclines with bovine serum albumin. Quantitative characterization by measurement of fluorescence quenching

We have investigated the interaction of the lipophilic anthracyclines 4'-iodo-4'-deoxydoxorubicin (IDX) and 4-demethoxy-daunorubicin (DDN) with bovine serum albumin by the quantitation of fluorescence quenching. The protein binding of IDX was extremely sensitive to the pH of the solution in which the complex was formed and paralleled the effect of pH on dimerization of the drug. The effect of pH on the protein binding and self-association of DDN was less extensive. Both compounds exhibited curvilinear Scatchard plots indicating apparent cooperativity in the binding process. Because of the self-association of the drugs in aqueous solution, we attempted to resolve this cooperativity in terms of the preferential binding of the dimer to the acceptor. However, we found that similar Scatchard plots could be simulated by using slightly erroneous estimates of the fluorescence yield of the complex, rendering any such analysis inconclusive. Consequently, the relationship between acceptor concentration and the fraction of ligand bound was considered to be fitted adequately in terms of a single acceptor site per albumin molecule. The pH dependence of the association constants for bovine serum albumin was described best by the hydrophobic interaction of neutral drug monomer with a binding site with titratable affinity. We postulate that the pH-dependent binding of some anthracyclines with albumin may lead to their enhanced uptake, relative to that of non-target organs, into tumours with an acidotic extracellular milieu.

Analysis of membrane-bound acceptors. A correction function for non-specific accumulation of poorly water-soluble hydrophobic or amphipathic ligands based on the ligand partition concept

Non-specific ligand accumulation into membrane material, which may contribute considerably to the experimental signal obtained in binding studies with labelled amphipathic and hydrophobic ligands, may be accounted for by linear partition of the ligands into the membrane phase. For application to binding data obtained at a single membrane-lipid concentration, a fitting procedure is proposed which allows one to correct for non-specific ligand partition. If the assumption is met that the amount of acceptor-bound ligand is small compared to the total amount present in the system, one can validly interpret the data in terms of total ligand concentrations. The apparent dissociation constants Kd(app) thus obtained should be corrected for the often large effects of the size of the partition compartment(s), by performing assays at several membrane-lipid concentrations. The importance of the latter correction is stressed and an approach for obtaining the characteristic effective dissociation constants Kd' is indicated. The procedure also yields estimates of the ligand/membrane partition coefficients.

Transferrin binding of bone marrow cells and metabolic activity of erythrocytes after 5 Gy irradiation

The effect of total body irradiation (5 Gy) on functional mouse erythroid lineage has been studied. The transferrin binding capacity by bone marrow cells and the activity of glycolytic regulatory enzymes and intracellular levels of 2,3 bisphosphoglycerate in peripheral blood erythrocytes have been determined. Results obtained along one year post-irradiation period suggest a complete recovery in the erythroid cell lineage with respect to the biological endpoints investigated.

Intermolecular cystine-bonding of murine interleukin 2 indicates that ligand dimerization is important for the formation of the high-affinity receptor complex

Interleukin 2 is thought to be active as a monomeric protein. As the nonessential Cys-140 of murine interleukin 2 (mIL2) is located in the hydrophobic interface of the amphiphilic F domain it was successfully used to stabilize hydrophobic amino acid contacts between two mIL2 cores yielding biologically active cystine-bonded dimeric mIL2. (3H) thymidine incorporation assays with intermolecular cystine-bonded or monomeric mIL2 revealed almost identical median effective concentrations (EC50) and high-affinity dissociation constants (Kdh), respectively. Comparative binding and internalization assays suggest that one cystine-bonded dimeric or two monomeric mIL2 molecules bind to the high-affinity receptor complex. Furthermore, DSS concentration-dependent crosslinking studies using monomeric mIL2 revealed four membrane-derived protein-complexes with apparent molecular weights of about 70 kDa, 85 kDa, 95 kDa and 100 kDa, respectively, showing that both mIL2 receptor chains may be crosslinked to a monomeric or dimeric ligand molecule, respectively. We therefore propose that dimerization of murine interleukin 2 occurring either in solution at concentrations above the low-affinity dissociation constant or at the low-affinity receptor is important for regulation of high-affinity complex formation and signal transduction.

Evidence for two classes of 1,25-dihydroxyvitamin D3 binding sites in classical vs. nonclassical target tissues

Possible differences in 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] binding sites in classical and nonclassical target tissues were tested by Scatchard analysis of [3H]1,25(OH)2D3 binding in parallel chromatin preparations of rat kidney vs. testis. Two distinct binding components were resolved in kidney (p less than 0.005). Moreover, the single binding site in testis exhibited a 10-fold lower Kd (p less than 0.05) than did the principal binding site in kidney (50 +/- 4 vs. 405 +/- 142 pM). Secondly, regulation of [3H]1,25(OH)2D3 binding sites also differed. 1,25(OH)2D3 injection resulted in increased 1,25(OH)2D3 binding (p less than 0.05) in kidney (92%) and intestine (415%), but not in testis, lung or heart. These results suggest that the principal 1,25(OH)2D3 binding sites in classical targets kidney and intestine may be intrinsically different from those in at least some nonclassical targets.

Application of a sol particle immunoassay to the determination of affinity constants of monoclonal antibodies

The affinity constants (Ka) of monoclonal antibodies (MAb) for binding to their corresponding antigens (Ag), unlabelled and in buffered solution were determined by the following procedure: 1. Incubation of MAb (fixed concentration) with Ag (concentration dilution series). 2. Rapid bound/free separation by adding immobilized second antibody, followed by centrifugation. 3. Determination of free Ag in the supernatant using a gold sol particle agglutination immunoassay (SPIA) in a microtitration plate format. 4. Calculations and interpretation were based on Scatchard and Sips plots. Ka values found by this procedure were found to be similar to those obtained by a radio-immunoassay (RIA) procedure. The present method avoids possible artefacts in Ka values introduced by the procedure or chemical modification due to labelling of MAb or Ag. It enables rapid, simultaneous screening of a considerable number of different MAbs under non-specialized (i.e. RIA) laboratory conditions.

Binding of [3H]-p-aminoclonidine to alpha 2-adrenoceptor states plus a non-adrenergic site on human platelet plasma membranes

Characterization of the binding of [3H]p-aminoclonidine ([3H]PAC) to purified plasma membranes from human platelets has revealed multiple binding sites. [3H]PAC identified site-1 in the picomolar affinity range (site-1 KD estimates ranged from 13 to 94 pM). Site-1 displayed a rank order of competition by various compounds for [3H]PAC, indicative of an alpha 2-adrenoceptor, and was sensitive to 0.1 mM GTP. [3H]PAC also identified a second site with nanomolar affinity (site-2 KD estimates ranged from 0.7 to 1.7 nM). In the presence of 0.1 mM GTP, site-2 was not diminished significantly. Also in contrast to site-1, site-2 displayed low affinity for yohimbine (YOH), (-)-epinephrine and (-)-norepinephrine (NE). Therefore, site-2 could not be an active alpha 2-adrenoceptor; instead it had properties similar to a previously reported imidazoline-preferring binding site. A third site (site-3) bound [3H]PAC with a KD for site-3 of 26.6 +/- 10.0 nM (SD). Site-3 had a rank order of competition by various compounds for 5 nM [3H]yohimbine ([3H]YOH) binding which was indicative of an alpha 2-adrenoceptor. (-)-NE competed for 5 nM [3H]YOH binding at two sites: site-1 Ki = 32 pM, site-3 Ki = 239 nM. Treatment with 0.1 mM GTP completely removed site-1 and transferred the competitive binding of (-)-NE to low affinity (Ki = 437 nM). Thus, site-3 appears to be a free alpha 2-adrenoceptor. Bmax estimates for untreated membranes, derived from simultaneous multi-experiment curve-fitting analyses, were site-1 = 36 +/- 29 fmol/mg plasma membrane protein, site-2 = 95 +/- 34 fmol/mg and site-3 = 154 +/- 35 fmol/mg. We are the first to report a site for [3H]PAC binding on platelets (site-2) with properties uncharacteristic of an adrenoceptor. This observation appears to be due to our use of purified plasma membrane and low ionic strength buffer. These studies relate to reports of increased binding of [3H]PAC to platelets from depressed patients.

The significance of functional receptor heterogeneity in the biological responses of the rabbit neutrophil to stimulation by chemotactic formyl peptides

The characteristics of binding to the chemotactic receptors on rabbit peritoneal neutrophils were examined for seven formyl peptide analogues. These receptor-binding characteristics were compared with the abilities of the analogues to induce the biological responses of degranulation and chemotaxis. Five of the analogues showed distinct functional heterogeneity in their receptor-binding patterns, whereas the two most potent compounds displayed homogeneous binding patterns. The relative potencies of the formyl peptide analogues for stimulation of degranulation correlated well with their relative potencies for high-affinity, but not low-affinity, binding. The biphasic patterns for stimulation of chemotactic migration were similar for the less potent analogues, and their potencies paralleled those for both degranulation and receptor binding. In contrast, the most potent analogues induced a greater maximal extent of chemotactic migration than the other compounds, but displayed a lower than expected potency (i.e. they required higher than expected concentrations). These anomalies in the patterns of the chemotactic response cannot be reconciled with a simple receptor model comprising two independent classes of receptors. Instead, a model comprising interconvertible states of different affinities is proposed. The state of higher affinity appears to play a central role in initiation of both degranulation and chemotaxis. The more potent formyl peptide analogues are thought to stabilize an activated, higher-affinity, state of the receptor; this can explain their greater efficacy in stimulating chemotaxis. The proposed model may also be applicable to other receptors that are coupled by a guanine-nucleotide-binding regulatory protein to their associated effector.

Heparin inhibition of von Willebrand factor-dependent platelet function in vitro and in vivo

The intravenous administration of heparin to patients before open heart surgery reduced ristocetin cofactor activity by 58% (P less than 0.01, t test), and this impairment of von Willebrand factor-dependent platelet function was closely related to plasma heparin levels (r2 = 0.9), but not to plasma von Willebrand factor (vWF) levels. We hypothesized that heparin may inhibit vWF-dependent platelet hemostatic functions by directly binding vWF in solution and interfering with vWF-GpIb binding. Using the in vitro techniques of ristocetin-induced platelet agglutination, fluorescent flow cytometric measurement of vWF-platelet binding, and conventional radioligand binding assays we observed that heparin inhibited both vWF-dependent platelet function and vWF-platelet binding in a parallel and dose-dependent manner. Heparin also inhibited platelet agglutination induced by bovine vWF and inhibited the binding of human asialo-vWF to platelets in ristocetin-free systems. The inhibitory potency of heparin was not dependent upon its affinity for antithrombin III, but was molecular weight dependent: homogeneous preparations of lower molecular weight were less inhibitory. Heparin impairment of vWF function may explain why some hemorrhagic complications of heparin therapy are not predictable based on techniques for monitoring the conventional anticoagulant effects of heparin.