Thymidine Phosphorylase is Noncompetitively Inhibited by 5′-O-Trityl-Inosine (KIN59) and Related Compounds

We found that 5′-O-trityl-inosine (KIN59) inhibits recombinant bacterial (E. coli) and human thymidine phosphorylase (TPase) with an IC₅₀ of 44 μM and 67 μM, respectively. In contrast to previously described TPase inhibitors, KIN59 does not compete with thymidine (dThd) at the pyrimidine nucleoside-binding site or with inorganic phosphate (Pi) at the phosphate-binding site of the enzyme. These findings are strongly suggestive for the presence of an allosteric binding site at the enzyme. TPase is identical to the angiogenic protein platelet-derived endothelial cell growth factor (PD-ECGF). As such, PD-ECGF stimulates angiogenesis in the chick chorioallantoic membrane (CAM) assay. This angiogenic response was completely inhibited by KIN59. Inosine did not inhibit the enzyme or the angiogenic effect of TPase, confirming that the 5′-O-trityl group in KIN59 is essential for the observed effect. Our observations indicate that allosteric sites in TPase may regulate its biological activity.     

The nucleoside derivative 5'-O-trityl-inosine (KIN59) suppresses thymidine phosphorylase-triggered angiogenesis via a noncompetitive mechanism of action

Thymidine phosphorylase (TPase) catalyzes the reversible phosphorolysis of pyrimidine deoxynucleosides to 2-deoxy-d-ribose-1-phosphate and their respective pyrimidine bases. The enzymatic activity of TPase was found to be essential for its angiogenesis-stimulating properties. All of the previously described TPase inhibitors are either pyrimidine analogues that interact with the nucleoside-binding site of the enzyme or modified purine derivatives that mimic the pyrimidine structure and either compete with thymidine or act as a multisubstrate (competitive) inhibitor. We now describe the inhibitory activity of the purine riboside derivative KIN59 (5'-O-tritylinosine) against human and bacterial recombinant TPase and TPase-induced angiogenesis. In contrast to previously described TPase inhibitors, KIN59 does not compete with the pyrimidine nucleoside or the phosphate-binding site of the enzyme but noncompetitively inhibits TPase when thymidine or phosphate is used as the variable substrate. In addition, KIN59 was far more active than other TPase inhibitors, previously tested by us, against TPase-induced angiogenesis in the chorioallantoic membrane assay. The observed anti-angiogenic effect of KIN59 was not accompanied by inflammation or any visible toxicity. Inosine did not inhibit the enzymatic or angiogenic activity of the enzyme, indicating that the 5'-O-trityl group in KIN59 is essential for the observed effects. In contrast with current concepts, our data indicate that the angiogenic activity of TPase is not solely directed through its functional nucleoside and phosphate-binding sites. Other regulatory (allosteric) site(s) in TPase may play an important role in the mechanism of TPase-triggered angiogenesis stimulation and apoptosis inhibition. Identification of these site(s) is important to obtain a better insight into the molecular role of TPase in the progression of cancer and angiogenic diseases.     

Role of histidine-85 in the catalytic mechanism of thymidine phosphorylase as assessed by targeted molecular dynamics simulations and quantum mechanical calculations

The structural changes taking place in the enzyme thymidine phosphorylase (TPase, also known as PD-ECGF) that are required to achieve catalytic competence upon binding thymidine and phosphate have been simulated by means of targeted molecular dynamics (tMD). The hinge regions were characterized by structural homology comparisons with pyrimidine nucleoside phosphorylase, whose X-ray structure has been solved both in a closed and in an open form. The rearrangement of residues around the substrate that was observed during the tMD trajectory suggested that His-85 could be playing an important role in the catalytic mechanism. A quantum mechanical study of the reaction in the presence of the most relevant active site residues was then performed at the semiempirical level. The results revealed that His-85 could be involved in the protonation of the pyrimidine base at the O2 position to yield the enol tautomer of the base. To establish the role of this oxygen atom in the reaction, ground states, transition states, and final products were studied using higher level ab initio methods starting from both thymidine and 2-thiothymidine as alternative substrates. Comparison of both transition states showed that replacing the oxygen at position 2 of the pyrimidine base by sulfur should accelerate the reaction rate. Consistent with this result, 2-thiothymidine was shown to be a better substrate for TPase than the natural substrate, thymidine. For simulating the final step of the reaction, tMD simulations were used to study domain opening upon product formation considering both the enol and keto tautomers of thymine. Product release from the enzyme was easiest in the simulation that incorporated the keto tautomer of thymine, suggesting that the enol intermediate spontaneously tautomerizes back to the more energetically stable keto form. These results highlight a previously unreported role for His-85 in the catalytic mechanism of TPase and can have important implications for the design of novel TPase inhibitors.     

Kinetic analysis of novel multisubstrate analogue inhibitors of thymidine phosphorylase

A kinetic analysis was performed for the novel 1-(8-phosphonooctyl)-6-amino-5-bromouracil and 1-(8-phosphonooctyl)-7-deazaxanthine inhibitors of Escherichia coli thymidine (dThd) phosphorylase (TPase). The structure of the compounds was rationally designed based on the available crystal structure coordinates of bacterial TPase. These inhibitors reversibly inhibited TPase. Kinetic analysis revealed that the compounds inhibited TPase in a purely competitive or mixed fashion not only when dThd, but also when inorganic phosphate (Pi), was used as the variable substrate. In contrast, the free bases 6-amino-5-bromouracil and 7-deazaxanthine behaved as non-competitive inhibitors of the enzyme in the presence of variable Pi concentrations while being competitive or mixed with respect to thymine as the natural substrate. Our kinetic data thus revealed that the novel 1-(8-phosphonooctyl)pyrimidine/purine derivatives are able to function as multisubstrate inhibitors of TPase, interfering at two different sites (dThd(Thy)- and phosphate-binding site) of the enzyme. To our knowledge, the described compounds represent the first type of such multisubstrate analogue inhibitors of TPase; they should be considered as lead compounds for the development of mechanistically novel type of TPase inhibitors.     

Kinetic studies of thymidine phosphorylase from mouse liver

Initial velocity and product inhibition studies of thymidine phosphorylase from mouse liver revealed that the basic reaction mechanism of this enzyme is a rapid equilibrium random bi-bi mechanism with an enzyme-phosphate-thymine dead-end complex. Thymine displayed both substrate inhibition and nonlinear product inhibition, i.e., slope and intercept replots vs. 1/[thymine] were nonlinear, indicating that there is more than one binding site on the enzyme for thymine and that when thymine is bound to one of these sites, the enzyme is inhibited. Furthermore, both thymidine and phosphate showed "cooperative effects" in the presence of thymine at concentrations above 60 microM, suggesting that the enzyme may have multiple interacting allosteric and/or catalytic sites. The deoxyribosyl transferase reaction catalyzed by this enzyme is phosphate-dependent, requires nonstoichiometric amounts of phosphate, and can proceed by an "enzyme-bound" 2-deoxyribose 1-phosphate intermediate. These findings are in accord with the rapid equilibrium random bi-bi mechanism and demonstrate that deoxyribosyl transfer by this enzyme involves an indirect-transfer mechanism. These results strongly suggest that phosphorolysis and deoxyribosyl transfer are catalyzed by the same site on thymidine phosphorylase.     

Platelet-derived endothelial cell growth factor has thymidine phosphorylase activity.

Platelet-derived endothelial cell growth factor (PD-ECGF), a protein which stimulates angiogenesis in vivo, is shown to have a 39.2% amino acid sequence similarity over a 439 amino acid region with the thymidine phosphorylase of Escherichia coli (E. coli). Using recombinant human PD-ECGF, we show that PD-ECGF has thymidine phosphorylase activity. Analysis by gel chromatography revealed that recombinant human PD-ECGF occurs as a 90 kDa homodimer, similar to other thymidine phosphorylases. In addition to a possible effect on DNA synthesis, PD-ECGF was shown to affect [3H]thymidine assays in a manner which is not related to cell proliferation. The in vitro and in vivo effects of PD-ECGF may thus occur by an indirect mechanism through its enzymatic activity.     

Mutant R1 proteins from Escherichia coli class Ia ribonucleotide reductase with altered responses to dATP inhibition

Aerobic ribonucleotide reductase from Escherichia coli regulates its level of activity by binding of effectors to an allosteric site in R1, located to the proposed interaction area of the two proteins that comprise the class I enzyme. Activity is increased by ATP binding and decreased by dATP binding. To study the mechanism governing this regulation, we have constructed three R1 proteins with mutations at His-59 in the activity site and one R1 protein with a mutation at His-88 close to the activity site and compared their allosteric behavior to that of the wild type R1 protein. All mutant proteins retained about 70% of wild type enzymatic activity. We found that if residue His-59 was replaced with alanine or asparagine, the enzyme lost its normal response to the inhibitory effect of dATP, whereas the enzyme with a glutamine still managed to elicit a normal response. We saw a similar result if residue His-88, which is proposed to hydrogen-bond to His-59, was replaced with alanine. Nucleotide binding experiments ruled out the possibility that the effect is due to an inability of the mutant proteins to bind effector since little difference in binding constants was observed for wild type and mutant proteins. Instead, the interaction between proteins R1 and R2 was perturbed in the mutant proteins. We propose that His-59 is important in the allosteric effect triggered by dATP binding, that the conserved hydrogen bond between His-59 and His-88 is important for the communication of the allosteric effect, and that this effect is exerted on the R1/R2 interaction.     

Cytokinin inhibits lateral root initiation but stimulates lateral root elongation in rice (Oryza sativa)

Research in lateral root (LR) development mainly focuses on the role of auxin. This article reports the effect of cytokinins (kinetin and trans-zeatin) on LR formation in rice (Oryza sativa L.). Our results showed that cytokinin has an inhibitory effect on LR initiation and stimulatory effect on LR elongation. Both KIN and ZEA at a concentration of 1 microM and above completely inhibited lateral root primordium (LRP) formation. The inhibitory effect of cytokinin on LR initiation required a continuous presence of KIN or ZEA in the growth solution. Cytokinin did not show any inhibitory effect on LR emergence from the seminal root once LRPs had been formed. The LRPs that developed in cytokinin-free solution can emerge normally in the solution containing inhibitory concentration (1 microM) of KIN and ZEA. The KIN and ZEA treatment dramatically stimulated LR elongation at all the concentrations tested. Maximum LR elongation was observed at a concentration of 0.01 microM KIN and 0.001 microM ZEA. The epidermal cell length increased significantly in LRs of cytokinin treated seedlings compared to those of untreated control. This result indicates that the stimulation of LR elongation by cytokinin is due to increased cell length. Exogenously applied auxin counteracted the effect of cytokinin on LR initiation and LR elongation, suggesting that cytokinin acts on LR elongation through an auxin dependent pathway.     

Binding of the substrates and the allosteric inhibitor adenosine 5'-diphosphate to phosphoribosylpyrophosphate synthetase from Salmonella typhimurium

The binding of the substrates, ATP and ribose-5-P, and the most effective inhibitor, ADP, to phosphoribosylpyrophosphate synthetase from Salmonella typhimurium was characterized using equilibrium dialysis of these compounds labeled with 32P. In the absence of ribose-5-P, ATP, ADP, and the ATP analogue alpha,beta-methylene ATP each bind cooperatively with half-saturation at 50 to 90 microM and Hill coefficients of 1.5 to 2. We propose that all three compounds bind at the same set of sites, which are presumably the active sites. When ribose-5-P was added, methylene ATP and ADP binding at these sites became tighter (Kd approximately 3 to 6 microM at 10 mM ribose-5-P) and lost its cooperativity. In the presence of ribose-5-P, ADP, but not methylene ATP, bound to a second site with half-saturation at approximately 150 microM and a Hill coefficient greater than 3. This result confirms the existence of an allosteric ADP site, which was previously postulated from kinetic studies (Switzer, R. L., and Sogin, D. C. (1973) J. Biol. Chem. 248, 1063-1073). Binding of ribose-5-P could not be detected in the absence of nucleotides, but it was readily measured in their presence. The apparent Kd of ribose-5-P varied from greater than 1 mM to approximately 5 microM as the concentration of either ADP or methylene ATP was increased from 0 to 2 mM. Inhibition of the enzyme by action of ADP at both active and allosteric sites could be observed kinetically.     

Antagonistic effect of cAMP and cGMP on the kinetic behaviour of phosphofructokinase from rat erythrocytes and reticulocytes

Fructose-6-phosphate (F6P)-saturation curves (up to 5 mM F6P) for phosphofructokinase (PFK) have been studied at physiological pH (7.1) and inhibitory (1.5 mM) or non-inhibitory (0.25 mM) ATP levels, in rat erythrocytes and reticulocytes. The addition of 300 microM cAMP to control samples activates the enzyme and displaces F6P-saturation curve towards the left, while the addition of cGMP inhibits the enzyme and shifts the curve to the right. The cAMP positive allosteric effect is more evident at inhibitory ATP levels, while the inhibitory effect of cGMP is very similar at both ATP levels. This antagonistic effect is exerted at the same regulatory site, since cAMP also activates the enzyme when cGMP is previously present in the reaction mixture. The physiological significance of this antagonism is not yet clear.     

Affinity labeling of the catalytic and AMP allosteric sites of 3-hydroxy-3-methylglutaryl-coenzyme A reductase kinase by 5'-p-fluorosulfonylbenzoyladenosine

The nucleotide analogue 5'-p-fluorosulfonylbenzoyladenosine (FSBA) reacts irreversibly with rat liver cytosolic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase kinase, causing a rapid loss of the AMP activation capacity and a slower inactivation of the catalytic activity. The rate constant for loss of AMP activation is about 10 times higher (kappa 1 = 0.112 min-1) than the rate constant of inactivation (kappa 2 = 0.0106 min-1). There is a good correspondence between the time-dependent inactivation of reductase kinase and the time-dependent incorporation of 5'-p-sulfonylbenzoyl[14C]adenosine ([14C]SBA). An average of 1.65 mol of reagent/mol of enzyme subunit is bound when reductase kinase is completely inactivated. The time-dependent incorporation is consistent with the postulate that covalent reaction of 1 mol of SBA/mol of subunit causes complete loss of AMP activation, whereas reaction of another mole of SBA/mol of subunit would lead to total inactivation. Protection against inactivation by the reagent is provided by the addition of Mg2+, AMP, Mg-ATP, or Mg-AMP to the incubation mixtures. In contrast, addition of ATP, 2'-AMP, or 3'-AMP has no effect on the rate constants. Mg-ATP protects preferentially the catalytic site against inactivation, whereas Mg-AMP at low concentration protects preferentially the allosteric site. Mg-ADP affords less protection than Mg-AMP to the allosteric site when both nucleotides are present at a concentration of 50 microM with 7.5 mM Mg2+. Experiments done with [14C]FSBA in the presence of some protectants have shown that a close correlation exists between the pattern of protection observed and the binding of [14C]SBA. The postulate is that there exists a catalytic site and an allosteric site in the reductase kinase subunit and that Mg-AMP is the main allosteric activator of the enzyme.     

Expression of platelet-derived endothelial cell growth factor in Escherichia coli and confirmation of its thymidine phosphorylase activity.

Platelet-derived endothelial cell growth factor (PD-ECGF) has been expressed in Escherichia coli as a fusion protein with glutathione S-transferase (GST). The fusion protein was purified by one-step affinity chromatography on glutathione-agarose beads, and recombinant PD-ECGF was proteolytically cleaved with thrombin from its GST leader peptide to yield pure protein. Recombinant PD-ECGF stimulated [3H]methylthymidine uptake by endothelial cells in vitro; however, we were unable to detect stimulation of cell proliferation under a wide variety of conditions. We confirm that in accord with the recent report that PD-ECGF and human thymidine phosphorylase are products of the same gene [Furukawa, T., Yoshimura, A., Sumizawa, T., Haraguchi, M., & Akiyama, S. I. (1992) Nature 356, 668] recombinant PD-ECGF has thymidine phosphorylase activity comparable to that of E. coli thymidine phosphorylase. Further, E. coli thymidine phosphorylase was able to mimic the activity of recombinant PD-ECGF in the [3H]methylthymidine uptake assay, and it appears that recombinant PD-ECGF's effect on the uptake of thymidine by endothelial cells may be due to modulation of cellular thymidine pools. The mechanism by which PD-ECGF stimulates angiogenesis remains to be elucidated.     

Deoxyribose protects against rapamycin-induced cytotoxicity in colorectal cancer cells in vitro

Thymidine phosphorylase (TPase) is also known as the platelet-derived endothelial cell growth factor (PD-ECGF) and plays a role in angiogenesis. Deoxyribose (dR; a downstream TPase-product) addition to endothelial cells may stimulate FAK and p70/S6k signaling, which can be inhibited by rapamycin. Rapamycin is a specific mammalian target of the rapamycin (mTOR) inhibitor, a kinase that lies directly upstream of p70/S6k. This suggests a role for TPase in the mTOR/p70/S6k pathway. In order to study this in more detail, we exposed cells with and without TPase expression to dR and rapamycin and determined the effect on cell growth. We observed protection in cytotoxicity in Colo320 cells, but not Colo320 TP1 cells. This was in part mediated by activation of p70/S6k and inhibition of autophagy. Further studies are recommended to elucidate the mechanism behind the protective effect of dR.     

Effects of various steroids on platelet-derived endothelial cell growth factor (PD-ECGF) and its mRNA expression in uterine endometrial cancer cells

Progestins diminish the estrogen-induced angiogenic potential related to basic fibroblast growth factor (FGF) and vascular endothelial growth factor (VEGF) in uterine endometrial cancer cells. This led us to study the effect of various steroids on the expression of platelet-derived endothelial cell growth factor (PD-ECGF) as the other pertinent angiogenic factor in well-differentiated uterine endometrial cancer cell line Ishikawa.In Ishikawa cells, estradiol induced the expression of PD-ECGF and its mRNA. The estrogen-induced expression was increased approximately two-fold by progesterone and by its metabolite, 17alpha-hydroxyprogesterone, but not by medroxyprogesterone acetate (MPA). Therefore, progesterone and 17alpha-hydroxyprogesterone as endogenous steroids might induce PD-ECGF-related angiogenic potential in uterine endometrial cancer cells, but not MPA as a synthetic steroid. In conclusion, the failure of PD-ECGF induction by MPA might be the great merit of anti-angiogenic treatment with MPA for uterine endometrial cancers.     

Thymidine phosphorylase inhibits apoptosis induced by cisplatin

An angiogenic factor, platelet-derived endothelial cell growth factor/thymidine phosphorylase (PD-ECGF/TP), stimulates the chemotaxis of endothelial cells and confers resistance to apoptosis induced by hypoxia. 2-Deoxy-D-ribose, a degradation product of thymidine generated by TP, partially prevents hypoxia-induced apoptosis. TP is expressed at higher levels in tumor tissues compared to the adjacent non-neoplastic tissues in a variety of human carcinomas. High expression of TP is associated with an unfavorable prognosis. To investigate the effect of TP on cisplatin-induced apoptosis, human leukemia Jurkat cells were transfected with wild-type or mutant (L148R) TP cDNA. TP inhibited a number of steps in the cisplatin-induced apoptotic pathway, activation of caspases 3 and 9 and mitochondrial cytochrome c release. These findings suggest a mechanism by which TP confers resistance to apoptosis induced by cisplatin. Moreover, mutant TP that has no enzymatic activity also suppressed cisplatin-induced apoptosis. These findings indicate that TP has cytoprotective functions against cytotoxic agents which are independent of its enzymatic activity.     

Effect of pH on conformation and kinetic properties of phosphorylase from bovine skeletal muscles

Interaction of phosphorylase with 8-anilino-1-naphthalene-sulfonate (ANS) results in the formation of an ANS-protein complex. The microenvironment of the protein-bound dye changes depending on pH. Using fluorimetric titration, the dissociation constants for the complex (Kd = 23 and 57 microM for pH 6.2 and 6.8, respectively) were determined. The mode of the enzyme inhibition by ANS also changes depending on pH. At pH 6.8, ANS competitively inhibits the enzyme with respect to AMP, but does not compete with the nucleotide at pH 6.2; the corresponding Ki values are equal to 160 and 26 microM. The protective effect of ligands from the inhibiting effect of ANS was studied. It was shown that at pH 6.2, the enzyme is protected from the inhibition only by the substrate, glucose-1-phosphate, whereas at pH 6.8--by the allosteric inhibitor, glucose-6-phosphate. These findings suggest that at pH 6.2 the conformation of the enzyme molecule is induced by the substrate, while at pH 6.8--by the allosteric inhibitor. ANS binding in the vicinity of the active or allosteric centers is due to the pH-dependent conformational transition. The data obtained suggest that the pH changes within the range of 6.2-6.8 are essential for the regulation of enzyme activity.     

Gene therapy for chronic myocardial ischemia using platelet-derived endothelial cell growth factor in dogs

Platelet-derived endothelial cell growth factor (PD-ECGF), also known as thymidine phosphorylase (TP), has been reported to possess angiogenic activity and to inhibit apoptosis. This study was performed to determine whether PD-ECGF/TP can be used to ameliorate chronic myocardial ischemia. Myocardial ischemia was created in 40 mongrel dogs by placement of an ameroid constrictor on the proximal left anterior descending coronary artery (LAD). Plasmid vector encoding human PD-ECGF/TP cDNA (pCIhTP group; n = 12), empty vector pCI (pCI group; n = 12), or saline (Saline group; n = 12) was directly injected into the LAD territory 3 wk after ameroid constrictor implantation. Myocardial blood flow was detected using PET at baseline, 3 wk after ameroid constrictor implantation, and 2 wk after therapeutic treatment. At the end of the experiment, the hearts were isolated for biological and histological analysis. In the pCIhTP group, the transfected heart strongly expressed PD-ECGF/TP. The size of the infarct was smaller in the pCIhTP group than in the pCI or Saline group. The number of apoptotic myocardial cells was decreased in the pCIhTP group compared with the control groups based on triple immunohistochemical staining for von Willebrand factor, alpha-actin smooth muscle cells, and single-strand DNA. The level of proapoptotic protein Bax markedly decreased in the pCIhTP group compared with the other groups. Double immunohistochemical staining for von Willebrand factor and alpha-actin smooth muscle cells demonstrated that angiogenesis and arteriogenesis occurred, and paralleled the changes in myocardial blood flow and myocardial function in the pCIhTP group. We conclude that genetic approaches using PD-ECGF/TP to target the myocardium are effective for alleviating chronic myocardial ischemia.     

The activity of thymidine phosphorylase correlates with tumor size and lymph nodes status in breast carcinoma

The platelet-derived endothelial cell growth factor (PD-ECGF) is one of the potent angiogenic factors. Recently, its homology with thymidine phosphorylase (dThdPase), an enzyme involved in pyrimidine nucleoside metabolism, has been shown. In the present study, dThdPase activity was evaluated spectrophotometrically in 43 breast carcinomas and in 19 cases of non-neoplastic breast tissues. The mean dThdPase activity in breast cancer was almost six fold higher than in normal, non-neoplastic breast tissues (1.92 and 0.29 mumol thymine (T) x mg prot.-1 x h-1 respectively). The enzyme activity significantly correlated with axillary lymph node status (p = 0.0076) and with tumor size (p = 0.0099). Besides, the intratumoral microvessel density (MD) was evaluated using the CD 31 mouse anti-human monoclonal antibody, and there was no correlation between the level of enzymatic activity and a number of microvessels. The positive significant correlation of thymidine phosphorylase activity with prognostic factors in breast cancer patients with no relation to the number of microvessels needs further examination to confirm the prognostic significance of the level of dThdPase.     

ADP, chloride ion, and metal ion binding to bovine brain glutamine synthetase

The binding of divalent cations and nucleotide to bovine brain glutamine synthetase and their effects on the activity of the enzyme were investigated. In ADP-supported gamma-glutamyl transfer at pH 7.2, kinetic analyses of saturation functions gave [S]0.5 values of approximately 1 microM for Mn2+, approximately 2 mM for Mg2+, 19 nM for ADP.Mn, and 7.2 microM for ADP.Mg. The method of continuous variation applied to the Mn2+-supported reaction indicated that all subunits of the purified enzyme express activity when 1.0 equiv of ADP is bound per subunit. Measurements of equilibrium binding of Mn2+ to the enzyme in the absence and presence of ADP were consistent with each subunit binding free Mn2+ (KA approximately equal to 1.5 X 10(5) M-1) before binding the Mn.ADP complex (KA' approximately equal to 1.1 X 10(6) M-1). The binding of the first Mn2+ or Mg2+ to each subunit produces structural perturbations in the octameric enzyme, as evidenced by UV spectral and tryptophanyl residue fluorescence changes. The enzyme, therefore, has one structural site per subunit for Mn2+ or Mg2+ and a second site per subunit for the metal ion-nucleotide complex, both of which must be filled for activity expression. Chloride binding (KA' approximately equal to 10(4) M-1) to the enzyme was found to have a specific effect on the protein conformation, producing a substantial (30%) quench of tryptophanyl fluorescence and increasing the affinity of the enzyme 2-4-fold for Mg2+ or Mn2+. Arsenate, which activates the gamma-glutamyl transfer activity by binding to an allosteric site, and L-glutamate also cause conformational changes similar to those produced by Cl- binding. Anion binding to allosteric sites and divalent metal ion binding at active sites both produce tryptophanyl residue exposure and tyrosyl residue burial without changing the quaternary enzyme structure.     

Selective thiol group modification renders fructose-1,6-bisphosphatase insensitive to fructose 2,6-bisphosphate inhibition

Limited treatment of native pig kidney fructose-1,6-bisphosphatase (50 microM enzyme subunit) with [14C]N-ethylmaleimide (100 microM) at 30 degrees C, pH 7.5, in the presence of AMP (200 microM) results in the modification of 1 reactive cysteine residue/enzyme subunit. The N-ethylmaleimide-modified fructose-1,6-bisphosphatase has a functional catalytic site but is no longer inhibited by fructose 2,6-bisphosphate. The enzyme derivative also exhibits decreased affinity toward Mg2+. The presence of fructose 2,6-bisphosphate during the modification protects the enzyme against the loss of fructose 2,6-bisphosphate inhibition. Moreover, the modified enzyme is inhibited by monovalent cations, as previously reported (Reyes, A., Hubert, E., and Slebe, J.C. (1985) Biochem. Biophys. Res. Commun. 127, 373-379), and does not show inhibition by high substrate concentrations. A comparison of the kinetic properties of native and N-ethylmaleimide-modified fructose-1,6-bisphosphatase reveals differences in some properties but none is so striking as the complete loss of fructose 2,6-bisphosphate sensitivity. The results demonstrate that fructose 2,6-bisphosphate interacts with a specific allosteric site on fructose-1,6-bisphosphatase, and they also indicate that high levels of fructose 1,6-bisphosphate inhibit the enzyme by binding to this fructose 2,6-bisphosphate allosteric site.