N-acetyl-D-glucosamine substituted calix[4]arenes as stimulators of NK cell-mediated antitumor immune response

A series of calixarenes substituted with 2-acetamido-2-deoxy-beta-D-glucopyranose linked by a thiourea spacer was prepared and tested for binding activity to heterogeneously expressed activation receptors of the rat natural killer cells NKR-P1, and the receptor CD69 (human NK cells, macrophages). In the case of NKR-P1, the binding affinity of beta-D-GlcNAc-substituted calixarenes carrying two or four sugar units was in a good agreement with the inhibitory potencies of the linear chitooligomers (chitobiose to chitotetraose) reported previously. The influence of GlcNAc substitution of the calixarene skeleton on binding affinity for CD69 receptor was more profound and the 5,11,17,23-tetrakis[N-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-thioureido]-25,26,27,28-tetrapropoxycalix[4]arene (cone) (1) proved to be the best CD69 ligand identified to date. Lower GlcNAc substitution led to dramatic decrease of the binding activity (by about 1.5 order of magnitude per one GlcNAc unit). The immunostimulating activity results with the newly synthesized GlcNAc tetramers on calixarene scaffolds exhibited stimulation of natural cytotoxicity of human PBMC in concentrations 10(-4) and 10(-8)M. These calix-sugar compounds were superior to the previously tested PAMAM-GlcNAc(8)5.     

Binding of [H]methyltrienolone to androgen receptor in rat liver

The synthetic androgen methyltrienolone is superior to testosterone and androstenedione for the measurement of androgen receptor in tissues where the native ligands are metabolized into inactive derivatives. [³H]Methyltrienolone binds with a high affinity to androgen receptor in cytosol prepared from male rat livers, as the Scatchard analysis revealed that the K_d value was 3.3 · 10⁻⁸ M and the number of binding sites was 35.5 fmol/mg protein. Since methyltrienolone also binds glucocorticoid receptor which exists in rat liver, the apparent binding of androgen receptor is faulty when measured in the presence of glucocorticoid receptor. The binding of methyltrienolone to glucocorticoid receptor can be blocked by the presence of a 100-fold molar excess of unlabeled synthetic glucocorticoid, triamcinolone acetonide, without interfering in its binding to androgen receptor, because triamcinolone does not bind to androgen receptor. Triamcinolone-blocked cytosol exhibited that the K_d value was 2.5 · 10⁻⁸ M and the number of binding sites was 26.3 fmol/mg protein, indicating a reduction to of that in the untreated cytosol. The profile of glycerol gradient centrifiguration indicated that [³H]methyltriemolone-bound receptor migrated in the 8–9 S region in both untreated and triamcinolone-blocked cytosols, but the 8–9 S peak in triamcinolone-blocked cytosol was reduced to about of that of untreated cytosol.     

Phylogenetic and functional conservation of the NKR-P1F and NKR-P1G receptors in rat and mouse

Two clusters of rat Nkrp1 genes can be distinguished based on phylogenetic relationships and functional characteristics. The proximal (centromeric) cluster encodes the well-studied NKR-P1A and NKR-P1B receptors and the distal cluster, the largely uncharacterized, NKR-P1F and NKR-P1G receptors. The inhibitory NKR-P1G receptor is expressed only by the Ly49s3⁺ NK cell subset as detected by RT-PCR, while the activating NKR-P1F receptor is detected in both Ly49s3⁺ and NKR-P1B⁺ NK cells. The mouse NKR-P1G ortholog is expressed by both NKR-P1D⁻ and NKR-P1D⁺ NK cells in C57BL/6 mice. The rat and mouse NKR-P1F and NKR-P1G receptors demonstrate a striking, cross-species conservation of specificity for Clr ligands. NKR-P1F and NKR-P1G reporter cells reacted with overlapping panels of tumour cell lines and with cells transiently transfected with rat Clr2, Clr3, Clr4, Clr6 and Clr7 and mouse Clrc, Clrf, Clrg and Clrd/x, but not with Clr11 or Clrb, which serve as ligands for NKR-P1 from the proximal cluster. These data suggest that the conserved NKR-P1F and NKR-P1G receptors function as promiscuous receptors for a rapidly evolving family of Clr ligands in rodent NK cells.     

Purification and properties of a wheat leaf N-acetyl-β-d-hexosaminidase

An N-acetyl-β-d-hexosaminidase has been purified from primary wheat leaves (Triticum aestivum L.) by freeze-thawing, (NH₄)₂SO₄ precipitation, methanol precipitation, gel filtration, cation exchange chromatography and affinity chromatography on concanavalin A-Sepharose. The activity of the purified preparations could be stabilised by addition of Triton X-100 and the enzyme was stored at -20°C without significant loss of activity. The enzyme hydrolysed pNP-β-d-GlcNAc (optimum pH 5.2, K_m 0.29 mM, V_max 2.56 μkat mg⁻¹) and pNP-β-d-GalNAc (optimum pH 4.4, K_m 0.27 mM, V_max 2.50 μkat mg⁻¹). Five major isozymes were identified, with isoelectric points in the range 5.13–5.36. All five isozymes possessed both N-acety-β-d-glucosaminidase and N-acetyl-β-d-galactosaminidase activity. Inhibition studies and mixed substrate analysis suggested that both substrates are catalysed by the same active site. Both activities were inhibited by GlcNAc, 2-acetamido-2-deoxygluconolactone, GalNAc and the ions of mercury, silver and copper. The K_is for inhibition of N-acetyl-β-d-glucosaminidase activity were: GlcNAc (15.3 mM) and GalNAc (3.4mM). For inhibition of N-acety-β-d-galactosaminidase activity the corresponding values were: GlcNAc (18.2 mM) and GalNac (2.5 mM). The enzyme was considerably less active at releasing pNP from pNP-β-d-(GlcNAc)₂ and pNP-β-d-(GlcNAc)₃ than from pNP-β-d-GlcNAc. The ability of the N-acetyl-β-d-hexosaminidase to relase GlcNAc from chitin oligomers (GlcNAc)₂ (optimum pH 5.0) and (GlcNAc)₃₋₆ (optimum pH 4.4) was also low. Analysis of the reaction products revealed that the initial products from the hydrolysis of (GlcNAc)_n were predominantly (GlcNAc)_n−1 and GlcNAc.     

Glycan-binding profile of a D-galactose binding lectin purified from the annelid, Perinereis nuntia ver. vallata

A lectin recognizing D-galactose was purified from the pacific annelid Perinereis nuntia ver. vallata (Polychaeta) by affinity chromatography. Hemagglutinating activity, with a very low titer suggesting the presence of lectin appeared in the supernatant from the homogenization of body with Tris-buffered saline. However, dialyzed supernatant from the precipitate homogenized by galactose in the buffer revealed strong hemagglutinating activity against human erythrocytes. The crude supernatant was applied onto lactosyl–agarose column, and only the supernatant eluted from precipitate with galactose was obtained a galactose-binding lectin with 32 kDa polypeptide was obtained from the supernatant of the precipitate, extracted in presence of galactose. It suggests that the lectin tightly binds with glycoconjugate as endogenous ligand(s) in the tissue. Hemagglutinating activity against trypsinized and glutaraldehyde-fixed human erythrocytes was specifically inhibited by D-galactose, N-acetyl-D-galactosamine, lactose, melibiose, and asialofetuin. Glycan-binding profile of the lectin analyzed by frontal affinity chromatography shows that the lectin recognizes branched complex type N-linked oligosaccharides and both type 1 (Galβ1-3GlcNAc) and type 2 (Galβ1-4GlcNAc) lactosamine. The surface plasmon resonance study of the lectin against asialofetuin showed the k_ass and k_diss values are 5.14 × 10⁴ M^− 1 s^− 1 and 2.9 × 10⁻³ s^− 1, respectively. The partial primary structure of the lectin reveals 182 amino acids with novel sequence.     

Endogenous proteinases modulate the function of the β-adrenergic receptor-adenylate cyclase system

Photoaffinity labeling techniques have recently demonstrated that mammalian β₁- and β₂-adrenergic receptors reside on peptides of M_r 62 000–64 000. These receptor peptides are susceptible to endogenous metalloproteinases which produce peptides of M_r 30 000–55 000. Several proteinase inhibitors markedly attenuate this process, specifically EDTA and EGTA. In this study we investigated the functional significance of this proteolysis (and its inhibition) in the β₂-adrenergic receptor-adenylate cyclase system derived from rat lung membranes. Membrane preparations containing proteolytically derived fragments of the receptor of M_r 40000–55 000 are fully functional with respect to their ability to bind β-adrenergic antagonist radioligands such as [³H]dihydroalprenolol and β-adrenergic antagonist photoaffinity reagents such as p-azido-m-[^{125I]iodobenzylcarazolol}. They retain the ability to form a high-affinity, agonist-promoted, guanine nucleotide-sensitive complex thought to represent a ternary complex of agonist, receptor and guanine nucleotide regulatory protein. Nonetheless, after proteolysis, GTP is less able to revert this high-affinity receptor complex to one of lower affinity, and all aspects of adenylate cyclase stimulation are reduced. In addition, the functional integrity of the N protein in membranes prepared without proteinase inhibitors is reduced as assessed by reconstitution studies with the cyc[su− variant of S49 lymphoma cell membranes. These results suggest that endogenous proteolysis does not directly impair the ability of β-adrenergic receptors to either bind ligands or interact with the guanine nucleotide regulatory protein. However, they imply that endogenous proteolysis likely impairs the functionality of other components of the adenylate cyclase system, such as the nucleotide regulatory protein.     

Induction of β-N-acetylhexosaminidase in Aspergillus oryzae

Summary Extracellular -N-acetylhexosaminidase in basic specific activity 1.5 U/mg protein was induced 15 – 35 times (up to 50 U/mg protein) by mixture of chitooligomers (crude chitin hydrolysate), 10 – 20 times (20 – 30 U/mg protein) by N-acetylglucosamine, and 10 times (14 U/mg protein) by chitosan in Aspergillus oryzae. Addition of NaCl (15 – 23 g/l) to the cultivation medium enhanced the induction in 10 – 20 %.     

Androgen receptor in rat liver: Hormonal and developmental regulation of the cytoplasmic receptor and its correlation with the androgen-dependent synthesis of α2u-globulin

The cytoplasmic receptor for 5α-dihydrotestosterone has been identified in the rat liver and partially characterized. The receptor is a protein with a sedimentation coefficient of 3.5 S and binds both androgens (5α-dihydrotestosterone and testosterone) and estradiol-17β with high affinity. At saturating concentration, for every mole of estradiol there seem to be three moles of 5α-dihydrotestosterone bound to the receptor. Whereas estradiol stronly inhibits the uptake of 5α-dihydrotestosterone by the receptor, the presence of 5α-dihydrotestosterone only weakly interferes with estradiol binding.The level of the androgen receptor activity in the hepatic cytosol was found to follow closely the level of the urinary output of α-_2u-globulin, an androgen-dependent protein of hepatic origin. Immature and senile male as well as female rats, which do not normally produce α_2u-globulin, also lacked androgen receptor activity in their hepatic cytosol. Castration of the adult male rats results in a gradual drop of the urinary output of α_2u-globulin as well as of the hepatic androgen receptor activity. Androgen treatment of immature and senile male rats does not induce α_2u-globulin or any receptor activity. Administration of estradiol to adult male rates results in complete inhibition of both α_2u-synthesis as well as complete loss of the cytosol androgen receptor activity in these animals. These results strongly indicate that the hepatic the hepatic androgen receptor activity. Androgen treatment of immature and senile male rats does not induce α_2u-globulin or any receptor activity. Administration of estradiol to adult male rats results in complete inhibition of boty α_2u-synthesis as well as complete loss of the cytosol androgen receptor activity in these animals. These results strongly indicate that the hepatic androgen receptor is an inducible protein whose synthesis is regulated by its own ligands, the androgens acting as the positive and the estradiol as the negative signals.     

Enzymatic rearrangement of chitine hydrolysates with β-N-acetylhexosaminidase from Aspergillus oryzae

The role of higher chitooligomers in medical applications is increasing due to their interesting biological activities. Transglycosylation activity of β-N-acetylhexosaminidase from Aspergillus oryzae was employed to produce higher chitooligosaccharides (chitohexaose–chitooctaose) from a mixture of lower chitooligomers prepared by acid hydrolysis of chitin. Enzymatic rearrangement of the chitooligomer mixture was optimized in respect of substrate concentration, presence of inorganic salts, enzyme activity, and reaction time to achieve the highest production of longer chitooligomers.     

3-(1′,1′-Dimethylbutyl)-1-deoxy-Δ-THC and related compounds: synthesis of selective ligands for the CB2 receptor

The synthesis and pharmacology of 15 1-deoxy-Δ⁸-THC analogues, several of which have high affinity for the CB₂ receptor, are described. The deoxy cannabinoids include 1-deoxy-11-hydroxy-Δ⁸-THC (5), 1-deoxy-Δ⁸-THC (6), 1-deoxy-3-butyl-Δ⁸-THC (7), 1-deoxy-3-hexyl-Δ⁸-THC (8) and a series of 3-(1′,1′-dimethylalkyl)-1-deoxy-Δ⁸-THC analogues (2, n=0–4, 6, 7, where n=the number of carbon atoms in the side chain−2). Three derivatives (17–19) of deoxynabilone (16) were also prepared. The affinities of each compound for the CB₁ and CB₂ receptors were determined employing previously described procedures. Five of the 3-(1′,1′-dimethylalkyl)-1-deoxy-Δ⁸-THC analogues (2, n=1–5) have high affinity (K_i=<20 nM) for the CB₂ receptor. Four of them (2, n=1–4) also have little affinity for the CB₁ receptor (K_i=>295 nM). 3-(1′,1′-Dimethylbutyl)-1-deoxy-Δ⁸-THC (2, n=2) has very high affinity for the CB₂ receptor (K_i=3.4±1.0 nM) and little affinity for the CB₁ receptor (K_i=677±132 nM).

Scheme 3. (a) (C₆H₅)₃PCH₃⁺ Br⁻, n-BuLi/THF, 65°C; (b) LiAlH₄/THF, 25°C; (c) KBH(sec-Bu)₃/THF, −78 to 25°C then H₂O₂/NaOH.     

Enzymatic synthesis of dimeric glycomimetic ligands of NK cell activation receptors

This work reveals new structural relationships in the complex process of the interaction between activation receptors of natural killer cells (rat NKR-P1, human CD69) and novel bivalent carbohydrate glycomimetics. The length, glycosylation pattern and linker structure of receptor ligands were examined with respect to their ability to precipitate the receptor protein from solution, which simulates the in vivo process of receptor aggregation during NK cell activation. It was found that di-LacdiNAc triazole compounds show optimal performance, reaching up to 100% precipitation of the present protein receptors, and achieving high immunostimulatory activities without any tendency to trigger activation-induced apoptosis. In the synthesis of the compounds tested, two enzymatic approaches were applied. Whereas a β-N-acetylhexosaminidase could only glycosylate one of the two acceptor sites available with yields below 10%, the Y284L mutant of human placental β1,4-galactosyltransferase-1 worked as a perfect synthetic tool, accomplishing even quantitative glycosylation at both acceptor sites and with absolute regioselectivity for the C-4 position. This work insinuates new directions for further ligand structure optimisation and demonstrates the strong synthetic potential of the mutant human placental β1,4-galactosyltransferase-1 in the synthesis of multivalent glycomimetics and glycomaterials.     

Evolution of Bombesin Conjugates for Targeted PET Imaging of Tumors

Bombesin receptors are under intense investigation as molecular targets since they are overexpressed in several prevalent solid tumors. We rationally designed and synthesized a series of modified bombesin (BN) peptide analogs to study the influence of charge and spacers at the N-terminus, as well as amino acid substitutions, on both receptor binding affinity and pharmacokinetics. This enabled development of a novel ^64/67Cu-labeled BN peptide for PET imaging and targeted radiotherapy of BN receptor-positive tumors. Our results show that N-terminally positively charged peptide ligands had significantly higher affinity to human gastrin releasing peptide receptor (GRPr) than negatively charged or uncharged ligands (IC₅₀: 3.2±0.5 vs 26.3±3.5 vs 41.5±2.5 nM). The replacement of Nle¹⁴ by Met, and deletion of D-Tyr⁶, further resulted in 8-fold higher affinity. Contrary to significant changes to human GRPr binding, modifications at the N-terminal and at the 6^th, 11^th, and 14^th position of BN induced only slight influences on affinity to mouse GRPr. [Cu^II]-CPTA-[βAla¹¹] BN(7–14) ([Cu^II]-BZH7) showed the highest internalization rate into PC-3 cells with relatively slow efflux because of its subnanomolar affinity to GRPr. Interestingly, [^64/67Cu]-BZH7 also displayed similar affinities to the other 2 human BN receptor subtypes. In vivo studies showed that [^64/67Cu]-BZH7 had a high accumulation in PC-3 xenografts and allowed for clear-cut visualization of the tumor in PET imaging. In addition, a CPTA-glycine derivative, forming a hippurane-type spacer, enhanced kidney clearance of the radiotracer. These data indicate that the species variation of BN receptor plays an important role in screening radiolabeled BN. As well, the positive charge from the metallated complex at the N-terminal significantly increases affinity to human GRPr. Application of these observations enabled the novel ligand [^64/67Cu]-BZH7 to clearly visualize PC-3 tumors in vivo. This study provides a strong starting point for optimizing radiopeptides for targeting carcinomas that express any of the BN receptor subtypes.     

Characterization of chicken cystatin binding to rat renal brush-border membranes

Chicken cystatin, a homologue of human cystatin C, like other low-molecular-weight proteins is metabolized by renal proximal tubule cells. However, the precise mechanism(s) of this process has not been elucidated yet. To characterize chicken cystatin binding to renal brush-border membranes, the incubation of fluorescein labelled protein with rat cortical homogenate was performed. Saturation-dependent and reversible binding with low affinity (K_d = 3.67–4.07 μM) and high capacity (B_max = 2.32–2.79 nmol/mg) was observed. Bovine albumin was the most potent competitor (K_i = 0.7 μM) among other megalin/cubilin ligands tested. The presence of Ca^+ 2 ions was necessary to effective cystatin binding by brush-border membranes. Obtained data strongly support the hypothesis that chicken cystatin is a novel ligand for megalin/cubilin receptors tandem on proximal tubular cells.     

Fluorimetric Analysis of Phospholipase Activity in Tetrahymena pyriformis GL.

The unicellular Tetrahymena enzymatically split the synthetic phosphodiester, 4-methylum-belliferyl phosphocoline substrate. The enzyme activity was completely blocked in vitro and drastically inhibited in vivo by G-protein activating fluorides (NaF; AlF₄ ^– and BeF₃ ^–). The phospholipase A₂ inhibitor, quinacrine, and the protein phosphatase inhibitor, neomycin, inhibited the enzyme activity in vitro and activated it in vivo. Another phospholipase A₂ inhibitor 4-bromo phenacyl bromide was ineffective in vivo and in vitro alike, as well as the cyclooxygenase inhibitor indomethacin. Results of these experiments indicate that some treatments could be specific for a well defined activity (e.g., phospholipase A₂, G-protein) but subject to influence by other enzymes (e.g., phospholipase C, sphingomyelinase). The experiments call attention to the differences in the results of the in vivo and in vitro studies.     

Endocytosis of α1-acid glycoprotein variants and of neoglycoproteins containing mannose derivatives by a mouse hybridoma cell line (2C11–12). Comparison with mouse peritoneal macrophages

Macrophages from various origins are known to express membrane lectins that mediate the endocytosis of mannose-bearing glycoconjugates. Most macrophage tumor cell-lines lack such receptors. In this paper we show by flow cytometry analysis that a newly generated macrophage hybridoma (2C_11–12), which displays several macrophage characteristics, also expresses mannose membrane lectins, resulting in the internalization of fluoresceinylated neoglycoproteins into acidic compartments.Thioglycolate elicited mouse peritoneal macrophages and the 2C_11–12 hybridomas were compared by flow cytometry with regard to the binding and endocytosis of ₁-acid glycoprotein (AGP) variants separated by affinity chromatography on immobilized concanavalin A. AGP C eluted specifically with methyl -mannopyranoside, which contains two bi-antennary oligosaccharides, was endocytosed as mannosylated serum albumin (Man-BSA). In both types of macrophages, the fluoresceinylated ligands were internalized in acidic compartments as demonstrated by the fluorescence intensity increase upon monensin post-incubation. However the behaviour of the internalized ligands was found to be quite different. AGP C and Man-BSA were rapidly degraded by thioglycolate elicited peritoneal macrophages and excreted in the medium as small peptide fragments; conversely they remained a longer time in the 2C_11–12 hybridoma.     

Receptor,Ligand and Transducer Contributions to Dopamine D2 Receptor Functional Selectivity

Functional selectivity (or biased agonism) is a property exhibited by some G protein-coupled receptor (GPCR) ligands, which results in the modulation of a subset of a receptor’s signaling capabilities and more precise control over complex biological processes. The dopamine D2 receptor (D₂R) exhibits pleiotropic responses to the biogenic amine dopamine (DA) to mediate complex central nervous system functions through activation of G proteins and β-arrestins. D₂R is a prominent therapeutic target for psychological and neurological disorders in which DA biology is dysregulated and targeting D₂R with functionally selective drugs could provide a means by which pharmacotherapies could be developed. However, factors that determine GPCR functional selectivity in vivo may be multiple with receptors, ligands and transducers contributing to the process. We have recently described a mutagenesis approach to engineer biased D₂R mutants in which G protein-dependent (^[Gprot]D₂R) and β-arrestin-dependent signaling (^[βarr]D₂R) were successfully separated (Peterson, et al. PNAS, 2015). Here, permutations of these mutants were used to identify critical determinants of the D₂R signaling complex that impart signaling bias in response to the natural or synthetic ligands. Critical residues identified in generating ^[Gprot]D₂R and ^[βarr]D₂R conferred control of partial agonism at G protein and/or β-arrestin activity. Another set of mutations that result in G protein bias was identified that demonstrated that full agonists can impart unique activation patterns, and provided further credence to the concept of ligand texture. Finally, the contributions and interplay between different transducers indicated that G proteins are not aberrantly activated, and that receptor kinase and β-arrestin activities are inextricably linked. These data provide a thorough elucidation of the feasibility and malleability of D₂R functional selectivity and point to means by which novel in vivo therapies could be modeled.     

Binding and receptor-mediated endocytosis of pregnancy zone protein-proteinase complex in rat macrophages

¹²⁵I-labelled pregnancy zone protein complex was injected intravenously in rats and after 6 min uptake into cells of the liver and spleen was determined by electron microscopic autoradiography. The liver took up 68% of the injected radioactivity; 61% was in the hepatocytes and 7% was in the liver macrophages (Kupffer cells). The spleen took up 3–4% and nearly all the radioactivity was in the macrophages of the red pulp. The uptake per cell volume was several times higher in the macrophage than in the hepatocyte. The radioactivity associated with macrophages was largely in endocytotic vacuoles and lysosomes. Binding of labelled pregnancy zone protein complex to peritoneal macrophages at 4°C was 2–3-times higher than binding of the homologous α₂-macroglobulin complex. The two ligands competed for binding to the same receptors and the difference was due to a higher affinity of the pregnancy zone protein complex (K_d approx. 60 pM). After binding to the receptor, this ligand was internalised within 2–3 min at 37°C and radioactivity inside the cells largely represented intact pregnancy zone protein complex. Radioactivity was released from the cell as iodotyrosine after a lag time of about 10 min. It is concluded that pregnancy zone protein complex is bound with a high affinity to the α₂-macroglobulin receptors in rat macrophages followed by receptor-mediated endocytosis and degradation of the ligand in the lysosomes.     

Two isoforms of soluble auxin receptor in rice (Oryza sativa L.) plants: Binding property for auxin and interaction with plasma membrane H+-ATPase

An auxin receptor protein, isolated from the soluble fractionsof rice shoots and roots, was characterised in terms of the affinity andspecificity for IAA and the modulating effect onH⁺-ATPase of plant plasma membrane. The receptor proteingives a biphasic binding isotherm for IAA, indicating the existence ofthe primary and secondary binding sites. The predominant isoform of thereceptor in roots shows much higher affinity to IAA compared with thatin shoots. Being monomeric protein with about the same molecular mass(57–58 kDa) and showing a similar chromatographic behaviour, bothisoforms mediate IAA-induced modulation of the plasma membraneH⁺-ATPase in the respective IAA concentration rangesseparated by ca. 3 orders of magnitudes(10^-10–10^-7 M vs.10^-7–10^-4 M). Analysis of kinetic data ofthe H⁺-ATPase activity revealed that the receptor perse functions as an effector of the enzyme, causing a decrease inK_m and an increase in V_max through protein-proteininteraction at a 1:1 ratio. Further, it appeared that, while IAAdoes not affect by itself the kinetic parameters of theH⁺-ATPase, the auxin exerts its effect via thereceptor, biphasically regulating the efficiency of the effectormolecule probably by inducing two-phase conformational changes thatinvolve IAA binding to two separate binding sites. It was also foundthat other active auxins examined, such as indole-3-propionic acid,1-naphthalene acetic acid and 2,4-dichlorophenoxyacetic acid, do notwork together with the receptor to elicit the same response of theH⁺-ATPase as seen with IAA.