The inhibition of dihydrofolate reductase by folate analogues: structural requirements for slow- and tight-binding inhibition

Chemical space of Escherichia coli dihydrofolate reductase inhibitors: New approaches for discovering novel drugs for old bugs

Escherichia coli Dihydrofolate reductase is an important enzyme that is essential for the survival of the Gram-negative microorganism. Inhibitors designed against this enzyme have demonstrated application as antibiotics. However, either because of poor bioavailability of the small-molecules resulting from their inability to cross the double membrane in Gram-negative bacteria or because the microorganism develops resistance to the antibiotics by mutating the DHFR target, discovery of new antibiotics against the enzyme is mandatory to overcome drug-resistance. This review summarizes the field of DHFR inhibition with special focus on recent efforts to effectively interface computational and experimental efforts to discover novel classes of inhibitors that target allosteric and active-sites in drug-resistant variants of EcDHFR.

Poly(amidoamine) dendrimer-methotrexate conjugates: the mechanism of interaction with folate binding protein

Generation 5 poly(amidoamine) (G5 PAMAM) methotrexate (MTX) conjugates employing two small molecular linkers, G5-(COG-MTX)_n, G5-(MFCO-MTX)_n were prepared along with the conjugates of the G5-G5 (D) dimer, D-(COG-MTX)_n, D-(MFCO-MTX)_n. The monomer G5-(COG-MTX)_n conjugates exhibited only a weak, rapidly reversible binding to folate binding protein (FBP) consistent with monovalent MTX binding. The D-(COG-MTX)_n conjugates exhibited a slow onset, tight-binding mechanism in which the MTX first binds to the FBP, inducing protein structural rearrangement, followed by polymer–protein van der Waals interactions leading to tight-binding. The extent of irreversible binding is dependent on total MTX concentration and no evidence of multivalent MTX binding was observed.

Avidity mechanism of dendrimer-folic acid conjugates

Multivalent conjugation of folic acid has been employed to target cells overexpressing folate receptors. Such polymer conjugates have been previously demonstrated to have high avidity to folate binding protein. However, the lack of a monovalent folic acid–polymer material has prevented a full binding analysis of these conjugates, as multivalent binding mechanisms and polymer-mass mechanisms are convoluted in samples with broad distributions of folic acid-to-dendrimer ratios. In this work, the synthesis of a monovalent folic acid–dendrimer conjugate allowed the elucidation of the mechanism for increased binding between the folic acid–polymer conjugate and a folate binding protein surface. The increased avidity is due to a folate-keyed interaction between the dendrimer and protein surfaces that fits into the general framework of slow-onset, tight-binding mechanisms of ligand/protein interactions.

Biochemical characterization of an inhibitor of Escherichia coli UDP-N-acetylmuramyl-l-alanine ligase

UDP-N-acetylmuramyl-l-alanine ligase (MurC) is an essential bacterial enzyme involved in peptidoglycan biosynthesis and a target for the discovery of novel antibacterial agents. As a result of a high-throughput screen (HTS) against a chemical library for inhibitors of MurC, a series of benzofuran acyl-sulfonamides was identified as potential leads. One of these compounds, Compound A, inhibited Escherichia coli MurC with an IC(50) of 2.3 microM. Compound A exhibited time-dependent, partially reversible inhibition of E. coli MurC. Kinetic studies revealed a mode of inhibition consistent with the compound acting competitively with the MurC substrates ATP and UDP-N-acetyl-muramic acid (UNAM) with a K(i) of 4.5 microM against ATP and 6.3 microM against UNAM. Fluorescence binding experiments yielded a K(d) of 3.1 microM for the compound binding to MurC. Compound A also exhibited high-affinity binding to bovine serum albumin (BSA) as evidenced by a severe reduction in MurC inhibition upon addition of BSA. This finding is consistent with the high lipophilicity of the compound. Advancement of this compound series for further drug development will require reduction of albumin binding.

Inhibition of dihydrofolate reductase and cell growth activity by the phenanthroindolizidine alkaloids pergularinine and tylophorinidine: the in vitro cytotoxicity of these plant alkaloids and their potential as antimicrobial and anticancer agents

The phenanthroindolizidine plant alkaloids pergularinine (PGL) and tylophorinidine (TPD) isolated from the Indian medicinal herb Pergularia pallida have been evaluated for their biological activity and assessed for the first time employing dihydrofolate reductase (DHFR) (5,6,7,8-THF: NADP(+) oxidoreductase, EC 1.5.1.3) as the probe in the present investigations. The enzyme is a key target in cancer chemotherapy and has been purified from Lactobacillus leichmannii. Cytotoxicity studies showed that both PGL and TPD are potently toxic and inhibited the growth of L. leichmannii cells (IC(50)=45 and 40 microM, respectively). Both the alkaloids significantly inhibited DHFR activity (IC(50)=40 and 32 microM for PGL and TPD, respectively). Alkaloid concentrations greater than 75-95 microM resulted in a complete loss of DHFR activity. Our results are suggestive of the alkaloids as potential antimicrobial and antitumour compounds. Alkaloid binding to DHFR is slow and reversible. Inhibition kinetics revealed K(i) values of 9x10(-6) M and 7x10(-6) M for PGL and TPD, respectively for the enzyme and inhibition in both the cases was a simple linear 'non-competitive' type.

Dihydrofolate reductase and cell growth activity inhibition by the beta-carboline-benzoquinolizidine plant alkaloid deoxytubulosine from Alangium lamarckii: its potential as an antimicrobial and anticancer agent

Beta-carboline-benzoquinolizidine plant alkaloid deoxytubulosine (DTB) was evaluated and assessed for the first time for its biochemical and biological activity employing the biomarker dihydrofolate reductase (DHFR) (5,6,7,8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) as the probe enzyme, a key target in cancer chemotherapy. DHFR, employed in the present investigations was purified from Lactobacillus leichmannii. DTB, isolated from the Indian medicinal plant Alangium lamarckii was demonstrated to exhibit potent cytotoxicity. The alkaloid potently inhibited the cell growth of L. leichmannii and the cellular enzyme activity of DHFR (IC50=40 and 30 microM for the cell growth and enzyme inhibitions, respectively). DTB concentrations >75 microM resulted in a total loss of the DHFR activity, thus suggesting that the beta-carboline-benzoquinolizidine plant alkaloid is a promising potential antitumor agent. Our results are also suggestive of its potential antimicrobial activity. DTB binding to DHFR appears to be slow and reversible. Inhibition kinetics revealed that DHFR has a Ki value of 5x10(-6) M for DTB and that the enzyme inhibition is a simple linear 'non-competitive' type.

Slow-onset inhibition of ribosomal peptidyltransferase by lincomycin

In a system derived from Escherichia coli, we carried out a detailed kinetic analysis of the inhibition of the puromycin reaction by lincomycin. N-Acetylphenylalanyl-tRNA (Ac-Phe-tRNA; the donor) reacts with excess puromycin (S) according to reaction [1], C+S Ks <--> CS k3 --> C'+P, where C is the Ac-Phe-tRNA-poly(U)-ribosome ternary complex (complex C). The entire course of reaction [1] appears as a straight line when the reaction is analyzed as pseudo-first-order and the data are plotted in a logarithmic form (logarithmic time plot). The slope of this straight line gives the apparent ksobs = k3[S]/(Ks + [S]). In the presence of lincomycin the logarithmic time plot is not a straight line, but becomes biphasic, giving an early slope (ke = k3[S]/(Ks(1 + [I]/Ki) + [S])) and a late slope (k1 = k3[S]/(Ks(1 + [I]/K'i + [S])). Kinetic analysis of the early slopes at various concentrations of S and I shows competitive inhibition with Ki = 10.0 microM. The late slopes also give competitive inhibition with a distinct inhibition constant K'i = 2.0 microM. Excluding alternative models, the two phases of inhibition are compatible with a model in which reaction [1] is coupled with reaction [2], C+I k4 <--> k5 CI k6 <--> k7 C*I, where the isomerization step CI <--> CI* is slower than the first step C+I <--> CI, Ki = k5/k4 and K'i = Ki [k7/(k6 + k7)]. Corroborative evidence for this model comes from the examination of reaction [2] alone in the absence of S. This reaction is analyzed as pseudo-first-order going toward equilibrium with kIeq = k7 + (k6 [I]/(Ki + [I])). The plot of kIeq versus [I] is not linear. This plot supports the two-step mechanism of reaction [2] in which k6 = 5.2 min-1 and k7 = 1.3 min-1. This is the first example of slow-onset inhibition of ribosomal peptidyltransferase which follows a simple model leading to the determination of the isomerization constants k6 and k7. We suggest that lincomycin inhibits protein synthesis by binding initially to the ribosome in competition with aminoacyl-tRNA. Subsequently, as a result of a conformational change, an isomerization occurs (CI <--> C*I), after which lincomycin continues to interfere with the binding of aminoacyl-tRNA to the isomerized complex.

Mobility of the spin-labeled side chains of some novel antifolate inhibitors in their complexes with dihydrofolate reductase

Four spin-labeled inhibitors of dihydrofolate reductase (DHFR) have been synthesized, each of which has the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) reporting group at a different distance from the 2,4-diaminopyrimidine moiety by which the inhibitors are anchored and oriented in the active site. Inhibitors in which the TEMPO group is attached by a short side chain are weakly bound to DHFR from bacteria (Streptococcus faecium and Lactobacillus casei), to the bovine enzyme and to recombinant human DHFR. However, binding is sufficiently tight, especially in the ternary complexes with NADPH, for recording of the EPR spectra of the bound ligands. The spectra indicate that when these inhibitors are bound to the enzyme the TEMPO group is highly immobilized with correlation time, tau c, 4-20ns. Inhibitors that have the reporter group attached to the glutamate moiety of methotrexate bind to all four DHFRs more tightly than the inhibitors with shorter side chains by factors of up to 10(6). However, in most complexes formed by the inhibitors with longer side chains immobilization of the TEMPO group is slight (tau c 0.2-4 ns). These results are in general agreement with predictions from X-ray crystallographic results including thermal factors but there are some unanticipated differences between some results for bacterial and eukaryotic enzymes. Three of the splin-labeled inhibitors would provide good probes for distance measurements in and around the active site of mammalian DHFR.

Crystal structure of unliganded Escherichia coli dihydrofolate reductase. Ligand-induced conformational changes and cooperativity in binding

The crystal structure of unliganded dihydrofolate reductase (DHFR) from Escherichia coli has been solved and refined to an R factor of 19% at 2.3-A resolution in a crystal form that is nonisomorphous with each of the previously reported E. coli DHFR crystal structures [Bolin, J. T., Filman, D. J., Matthews, D. A., Hamlin, B. C., & Kraut, J. (1982) J. Biol. Chem. 257, 13650-13662; Bystroff, C., Oatley, S. J., & Kraut, J. (1990) Biochemistry 29, 3263-3277]. Significant conformational changes occur between the apoenzyme and each of the complexes: the NADP+ holoenzyme, the folate-NADP+ ternary complex, and the methotrexate (MTX) binary complex. The changes are small, with the largest about 3 A and most of them less than 1 A. For simplicity a two-domain description is adopted in which one domain contains the NADP+ 2'-phosphate binding site and the binding sites for the rest of the coenzyme and for the substrate lie between the two domains. Binding of either NADP+ or MTX induces a closing of the PABG-binding cleft and realignment of alpha-helices C and F which bind the pyrophosphate of the coenzyme. Formation of the ternary complex from the holoenzyme does not involve further relative domain shifts but does involve a shift of alpha-helix B and a floppy loop (the Met-20 loop) that precedes alpha B. These observations suggest a mechanism for cooperativity in binding between substrate and coenzyme wherein the greatest degree of cooperativity is expressed in the transition-state complex. We explore the idea that the MTX binary complex in some ways resembles the transition-state complex.

Progress-curve equations for reversible enzyme-catalysed reactions inhibited by tight-binding inhibitors

The rate equation for a tight-binding inhibitor of an enzyme-catalysed first-order reversible reaction was used to derive two integrated equations. One of them covers the situations in which competitive, uncompetitive or non-competitive inhibition occurs and the other refers to the special non-competitive case where the two inhibition constants are equal. For these equations, graphical and non-linear regression methods are proposed for distinguishing between types of inhibition and for calculating inhibition constants from progress-curve data. The application of the non-linear regression to the analysis of stimulated progress curves in the presence of a tight-binding inhibitor is also presented. The results obtained are valid for any type of 'dead-end'-complex-forming inhibitor and can be used to characterize an unknown inhibitor on the basis of progress curves.

Synthesis of lysine-containing sulphonium salts and their properties as proteinase inhibitors

Some sulphonium salts derived from lysine were synthesized with the general structure R-Lys-CH2S+-(alkyl)2. They were examined as inhibitors of the cysteine proteinase clostripain, which has a preference for cleaving peptide bonds at the carboxy group of basic amino acids, and of a number of trypsin-related serine proteinases. Clostripain was irreversibly inactivated by all reagents examined, but in the case of the serine proteinases, depending on the reagent structure, irreversible and reversible inhibitions were observed. These were kinetically characterized.

Nuclear magnetic resonance study of the state of protonation of inhibitors bound to mutant dihydrofolate reductase lacking the active-site carboxyl

13C nuclear magnetic resonance spectra have been obtained for complexes of [2-13C]methotrexate and [2-13C]trimethoprim with wild-type dihydrofolate reductase (DHFR) from Escherichia coli and with two mutant enzymes in which aspartic acid-27 is replaced by asparagine and by serine, respectively. In both the wild-type and mutated enzymes, exchange between the free inhibitor and the enzyme-complexed inhibitor is slow on the NMR time scale; hence, despite the considerably increased dissociation constants for binary complexes with the enzymes, the dissociation rate remains small relative to the frequency separation of the resonances. In all cases but one, the pKa of an inhibitor that is complexed to enzyme differs greatly from that of the free inhibitor. However, while the pKa of both inhibitors in complexes with the wild-type enzyme is elevated to above 10, the pKa of the inhibitors complexed with the Asn-27 and Ser-27 enzymes is lowered to a value below 4. Exact determinations of bound pKa values are limited by the solubility of the enzyme and the dissociation constants of the complexes. The single exception to these general conclusions is the ternary complex of the Ser-27 DHFR with trimethoprim and NADPH. In this complex, both free and enzyme-complexed trimethoprim exhibit similar pKa values (approximately equal to 7.6). However, both the exchange between free and enzyme-complexed inhibitor and the protonation of the enzyme-complexed inhibitor are slow in the NMR time scale, so that the spectra reveal three resonances corresponding to free inhibitor, to protonated enzyme-complexed inhibitor, and to unprotonated enzyme-complexed inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of inhibition of dihydrofolate reductases from bacterial and vertebrate sources by various classes of folate analogues

Different classes of folate analogues have been examined with respect to the mechanism of their inhibition of dihydrofolate reductases from Escherichia coli and chicken liver. In addition, the degree of synergism between the binding of these compounds and NADPH has been investigated. Methotrexate acts as a slow, tight-binding inhibitor of both enzymes whereas trimethoprim is a slow, tight-binding inhibitor of the enzyme from E. coli and a classical inhibitor of the chicken-liver enzyme. Pyrimethamine, 2,4-diamino-6,7-dimethylpteridine, a phenyltriazine, folate and folinate exhibit classical inhibition. The degree of synergism between the binding of NADPH and the inhibitor varied from low for pyrimethamine and folate to very large for the phenyltriazine which binds to the chicken-liver enzyme almost 50 000-times more tightly in the presence of NADPH. The degree of synergism is reflected in the type of inhibition that the folate analogues yield with respect to NADPH. Compounds which exhibit slight synergism give noncompetitive inhibition whereas those with a high degree of synergism yield uncompetitive inhibition. With the exception of folinate, all compounds that act as classical inhibitors give rise to competitive inhibition with respect to dihydrofolate. Folinate exhibits competitive inhibition against NADPH and noncompetitive inhibition against dihydrofolate. These results are consistent with the formation of an enzyme-dihydrofolate-folinate complex. The (6S, alphaS)-diastereoisomer of folinate was bound at least 1000-times more tightly than the (6R, alphaS)-diastereoisomer. Consideration has been given to the possible interactions that occur between residues on the enzyme and groups on the inhibitor that give rise to slow-binding inhibition.

Role of isomerization of initial complexes in the binding of inhibitors to dihydrofolate reductase

Stopped-flow measurements of protein fluorescence quenching when methotrexate (MTX) binds to dihydrofolate reductase (isoenzyme II) of Streptococcus faecium (SFDHFR II) analyze as the sum of two differentials: a rapid binding phase and a second phase for which the observed rate constant is independent of methotrexate concentration. Analysis of variation of the ratio of the amplitude of the fast and slow phases with methotrexate concentration indicates that the second phase is an isomerization of the initial binary complex. At pH 7.3, the equilibrium constant for this isomerization is 21.9, and the forward and reverse rate constants are 0.57 and 0.026 s-1, respectively. Similar results were obtained for binding of 3-deazamethotrexate to SFDHFR II, but the forward rate constant is greater (2.9 s-1 at pH 7.3). The equilibrium constants for these isomerizations are pH independent, but the rate constants decrease as the pH is raised, probably due to deprotonation of one or more groups on the enzyme. Analysis of progress curves obtained by the development of inhibition when SFDHFR II is added last to reaction mixtures containing dihydrofolate, NADPH, and MTX gives an association constant for initial reactions of 4.3 X 10(7) M-1. Since a preliminary estimate of the association constant for the binding reaction is 7.6 X 10(5) M-1, this suggests an isomerization of the ternary complex(es) with an equilibrium constant of about 56. In addition, analysis of the progress of development of inhibition indicates a further very slow isomerization with equilibrium constant 419 and forward rate constant 2.6 min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of methotrexate binding to dihydrofolate reductase from Neisseria gonorrhoeae

The kinetics of methotrexate inhibition of dihydrofolate reductase from Neisseria gonorrhoeae have been investigated. Methotrexate was shown to be a tight-binding inhibitor (Kt = 13 pM) competitive with dihydrofolate. However, "stoichiometric" or "pseudoirreversible" inhibition could not be demonstrated. Progress curves of inhibited assays quickly attained steady state regardless of the order of substrate addition, indicating that methotrexate association and dissociation processes were rapid. Kinetic techniques were used to measure the rate of methotrexate dissociation from the enzyme-NADPH-methotrexate ternary complex. At 30 degrees, the first-order off-rate constant (koff) was calculated to be 0.56 min-1. This value is approximately 40-fold greater than the dissociation rate constant of methotrexate for Escherichia coli dihydrofolate reductase. At lower temperatures, progress curves of methotrexate-inhibited gonococcal enzyme assays displayed marked increases in both curvature and the time to reach steady state. At 9 degrees, the methotrexate dissociation rate was slow enough (koff = 0.04 min-1) so that initial velocities of the reaction could be measured, and under these conditions methotrexate inhibition was shown to be "stoichiometric".

Inhibition of dihydrofolate reductase from bacterial and vertebrate sources by folate, aminopterin, methotrexate and their 5-deaza analogues

The inhibition of dihydrofolate reductases from Escherichia coli and chicken liver by folate, methotrexate, aminopterin and their 5-deaza analogues was investigated to examine the importance of the N-5 nitrogen in slow-binding inhibition. Methotrexate, aminopterin and their 5-deaza analogues acted as slow, tight-binding inhibitors of both enzymes. Inhibition by methotrexate and 5-deazamethotrexate conformed to a mechanism in which there is an initial rapid formation of an enzyme-NADPH-inhibitor complex followed by a slow isomerization of this complex (Mechanism B). Aminopterin exhibited the same type of inhibition with the enzyme from E. coli. With the chicken-liver enzyme, however, the inhibition by aminopterin conformed to another type of slow-binding mechanism which involves only the slow interaction of the inhibitor with the enzyme to form an enzyme-NADPH-inhibitor complex (Mechanism A). The inhibition of both enzymes by 5-deazaaminopterin was also described by Mechanism A. Folate behaved as a classical, steady-state inhibitor of both enzymes, whereas 5-deazafolate exhibited slow-binding inhibition (Mechanism B) with the enzyme from E. coli and classical, steady-state inhibition with the enzyme from chicken liver. The substitution of a carbon for a nitrogen at the 5-position of methotrexate and aminopterin did not affect the tightness of binding of these compounds. By contrast, 5-deazafolate was bound about 4000 times more tightly than folate to the enzyme from E. coli and about 30 times more tightly than folate to the chicken-liver enzyme. Reasons for the differences in the binding of folate and 5-deazafolate are discussed.

The pH-dependence of the binding of dihydrofolate and substrate analogues to dihydrofolate reductase from Escherichia coli

The interaction of dihydrofolate reductase (EC 1.5.1.3) from Escherichia coli with dihydrofolate and folate analogues has been studied by means of binding and spectroscopic experiments. The aim of the investigation was to determine the number and identity of the binary complexes that can form, as well as pKa values for groups on the ligand and enzyme that are involved with complex formation. The results obtained by ultraviolet difference spectroscopy indicate that, when bound to the enzyme, methotrexate and 2,4-diamino-6,7-dimethylpteridine exist in their protonated forms and exhibit pKa values for their N-1 nitrogens of above 10.0. These values are about five pH units higher than those for the compounds in free solution. The binding data suggest that both folate analogues interact with the enzyme to yield a protonated complex which may be formed by reaction of ionized enzyme with protonated ligand and/or protonated enzyme with unprotonated ligand. The protonated complex formed with 2,4-diamino-6,7-dimethylpteridine can undergo further protonation to form a protonated enzyme-protonated ligand complex, while that formed with methotrexate can ionize to give an unprotonated complex. A group on the enzyme with a pKa value of about 6.3 is involved with the interactions. However, the ionization state of this group has little effect on the binding of dihydrofolate to the enzyme. For the formation of an enzyme-dihydrofolate complex it is essential that the N-3/C-4 amide of the pteridine ring of the substrate be in its neutral form. It appears that dihydrofolate is not protonated in the binary complex.

Kinetic mechanism of the reaction catalyzed by dihydrofolate reductase from Escherichia coli

The kinetic mechanism of the reaction catalyzed by dihydrofolate reductase from Escherichia coli has been investigated by using progress curve, initial velocity, product inhibition, and dead-end inhibition studies as well as isotope effects. The results indicate that the reaction conforms to a random mechanism involving two dead-end complexes, viz., enzyme-DHF-THF and enzyme-NADP-DHF. At higher concentrations, DHF causes substrate inhibition by combining at the NADPH binding site on the enzyme. The steady-state velocity data can be analyzed adequately on the basis that rapid-equilibrium conditions apply. However, this can be only an approximate description of the reaction since the isotope effects observed with NADPD demonstrate clearly that catalysis cannot be rate limiting at pH 7.4. The choice of conditions for analysis of progress-curve data is discussed in the Appendix.

Chemical mechanism of the reaction catalyzed by dihydrofolate reductase from Streptococcus faecium: pH studies and chemical modification

The variation with pH of the kinetic parameters associated with dihydrofolate reductase from Streptococcus faecium has been used to gain information about the chemical mechanism of the reaction catalyzed by the enzyme. The pH dependence of log V/K for dihydrofolate showed that a group with a pK value of 4.7 must be ionized and that a group with a pK value of 6.6 must be protonated for activity. Temperature and solvent perturbation studies indicate that these groups are probably the carboxyls of the glutamate moiety of dihydrofolate and of an aspartate residue on the enzyme, respectively. The similarity of the pH profile and the magnitude of the pK value for the linear competitive inhibitor 2,4-diaminopteridine suggest that the carboxyl group is concerned with the binding of dihydrofolate and its analogues to the enzyme. This conclusion is confirmed by the result that a group with a pK value of 6.7 must be protonated for the binding of methotrexate. It is proposed that the binding involves the formation with N-5 of dihydrofolate or N-1 of methotrexate of a hydrogen bond which has considerable ionic character and which lies within a hydrophobic environment. Further, it is suggested that the same hydrogen acts as an auxiliary catalyst which facilitates hydride transfer from NADPH to dihydrofolate for its conversion to tetrahydrofolate. Evidence to support this suggestion comes from the finding that the V profile is similar to the V/K profile except that the pK of the group which must be protonated for maximum enzyme activity is shifted upward by about 2 pH units. Such an increase in a pK value is consistent with the formation of a hydrogen ionic bond in the ternary enzyme-NADPH-dihydrofolate complex. The results of inactivation experiments with trinitrobenzenesulfonate appear to indicate that a lysine residue is necessary to maintain the enzyme in its active conformation.

Characterization of the cloned Escherichia coli dihydrofolate reductase

Dihydrofolate reductase (5,6,7,8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) was purified from Escherichia coli strains that carried derivatives of the multicopy recombinant plasmid, pJFM8. The results of enzyme kinetic and two-dimensional gel electrophoresis experiments showed that the cloned enzyme is indistinguishable from the chromosomal enzyme. Therefore it can be concluded that these strains are ideal for use as a source of enzyme for further studies on the biochemistry and regulation of this important enzyme. The plasmid derivatives were constructed by recloning experiments that utilized several restriction endonucleases. From the analysis both of these plasmids and the purified dihydrofolate reductase enzymes it was possible to deduce the location and orientation of the dihydrofolate reductase structural gene on the parent plasmid, pJFM8.

Kinetic and e.p.r. studies on the inhibition of xanthine oxidase by alloxanthine (1 H-pyrazolo [3, 4-d] pyrimidine-4,6-diol)

The inhibition by alloxanthine of oxidation of xanthine by xanthine oxidase is characterized by a prolonged transient phase. Kinetic data accord with a mechanism that involves rapid formation of a reduced enzyme-alloxanthine complex that subsequently undergoes a relatively slow-reversible reaction. In this scheme the slowly formed complex cannot be fully reoxidized by oxygen. From the Ki value for the dissociation of alloxanthine from the rapidly formed complex (1.15 microM) and values of 0.37 min-1 and 0.011 min-1 for the forward and reverse rate constants of the slow reaction, an overall inhibition constant for alloxanthine of 35 nM was calculated. A molybdenum (V) e.p.r. signal from the slowly formed reduced enzyme-alloxanthine complex is described. The rate of appearance of this new signal is consistent with this assignment. The signal (the "Alloxanthine signal") was simulated with g1 2,0269, g2 1,9593, g3 11.9444 and shows indications of hyperfine coupling to nitrogen. Similarities between it and the Very Rapid signal are discussed. Close structural analogies between the catalytic intermediate represented by the Very Rapid signal and the inhibitor complex represented by the Alloxanthine signal are suggested.

Dihydrofolate reductase hysteresis and its effect of inhibitor binding analyses

Escherichia coli dihydrofolate reductase was shown to follow slow transient kinetics (hysteresis). Nonlinear reaction velocities were detected during the enzyme assay and required 10-15 min to reach a steady-state rate. The degree of hysteresis was influenced by the enzyme concentration and the order of substrate addition. Incubation of the enzyme with NADPH before addition of dihydrofolate resulted in slow initial velocities that increased up to 2-fold during the course of the assay. Increasing the enzyme concentration from 0.2 to 1 nM resulted in diminished hysteresis. NADPH-initiated reactions were linear at all enzyme concentrations tested. Certain drugs had profound effects on hysteresis. Pyrimethamine practically eliminated the hysteresis of dihydrofolate-started reactions, whereas trimethoprime augmented the non-linearities in the sense that hysteresis was detected in both enzyme- and NADPH-started reactions. The shape of these reaction tracings makes trimethoprim is not a slow-binding inhibitor when assayed under conditions that eliminate hysteresis. Contrary to this, sulfamethoxazole did not affect hysteresis or augment inhibition of the enzyme by trimethoprim. Sulfamethoxazole alone (at 6 mM) did not inhibit the hysteresis and allow reliable determinations of Ki values of both weak and tight binding inhibitors. For example, Ki values for pyrimethamine, trimethoprim, and methotrexate were found to be 214 nM, 1.3 nM, and 0.021 nM, respectively.