Catalytic properties of dihydroorotate dehydrogenase from Saccharomyces cerevisiae: studies on pH, alternate substrates, and inhibitors

Yeast dihydroorotate dehydrogenase (DHOD) was purified 2800-fold to homogeneity from its natural source. Its sequence is 70% identical to that of the Lactococcus lactis DHOD (family IA) and the two active sites are nearly the same. Incubations of the yeast DHOD with dideuterodihydroorotate (deuterated in the positions eliminated in the dehydrogenation) as the donor and [14C]orotate as the acceptor revealed that the C5 deuteron exchanged with H2O solvent at a rate equal to the 14C exchange rate, whereas the C6 deuteron was infrequently exchanged with H2O solvent, thus indicating that the C6 deuteron of the dihydroorotate is sticky on the flavin cofactor. The pH dependencies of the steady-state parameters (k(cat) and k(cat)/Km) are similar, indicating that k(cat)/Km reports the productive binding of substrate, and the parameters are dependent on the donor-acceptor pair. The lower pKa values for k(cat) and k(cat)/Km observed for substrate dihydroorotate (around 6) in comparison to the values determined for dihydrooxonate (around 8) suggest that the C5 pro S hydrogen atom of dihydroorotate (but not the analogous hydrogen of dihydrooxonate), which is removed in the dehydrogenation, assists in lowering the pKa of the active site base (Cys133). The pH dependencies of the kinetic isotope effects on steady-state parameters observed for the dideuterated dihydroorotate are consistent with the dehydrogenation of substrate being rate limiting at low pH values, with a pKa value approximating that assigned to Cys133. Electron acceptors with dihydroorotate as donor were preferred in the following order: ferricyanide (1), DCPIP (0.54), Qo (0.28), fumarate (0.15), and O2 (0.035). Orotate inhibition profiles versus varied concentrations of dihydroorotate with ferricyanide or O2 as acceptors suggest that both orotate and dihydroorotate have significant affinities for the reduced and oxidized forms of the enzyme.     

Acid-base catalysis in the chemical mechanism of inosine monophosphate dehydrogenase

Inosine-5'-monophosphate dehydrogenase (IMPDH) catalyzes the K+-dependent reaction IMP + NAD + H2O --> XMP + NADH + H+ which is the rate-limiting step in guanine nucleotide biosynthesis. The catalytic mechanism of the human type-II IMPDH isozyme has been studied by measurement of the pH dependencies of the normal reaction, of the hydrolysis of 2-chloro-IMP (which yields XMP and Cl- in the absence of NAD), and of inactivation by the affinity label 6-chloro-purine-ribotide (6-Cl-PRT). The pH dependence of the IMPDH reaction shows bell-shaped profiles for kcat and the kcat/Km values for both IMP and NAD, illustrating the involvement of both acidic and basic groups in catalysis. Half-maximal kcat values occur at pH values of 7.2 and 9.8; similar pK values of 6.9 and 9.4 are seen in the kcat/Km profile for NAD. The kcat/Km profile for IMP, which binds first in the predominantly ordered kinetic mechanism, shows pK values of 8.1 and 7.3 for acidic and basic groups, respectively. None of the kinetic pK values correspond to ionizations of the free substrates and thus reflect ionization of the enzyme or enzyme-substrate complexes. The rate of inactivation by 6-Cl-PRT, which modifies the active site sulfhydryl of cysteine-331, increases with pH; the pK of 7.5 reflects the ionization of the sulfhydryl in the E.6-Cl-PRT complex. The pKs of the acids observed in the IMPDH reaction likely also reflect ionization of the cysteine-331 sulfhydryl which adds to C-2 of IMP prior to NAD reduction. The kcat and kcat/Km values for hydrolysis of 2-Cl-IMP show a pK value of 9.9 for a basic group, similar to that seen in the overall reaction, but do not exhibit the ionization of an acidic group. Surprisingly, the rates of 2-Cl-IMP hydrolysis and of inactivation by 6-Cl-PRT are not stimulated by K+, in contrast to the >100-fold K+ activation of the IMPDH reaction. Apparently the enigmatic role of K+ lies in the NAD(H)-dependent segment of the IMPDH reaction. To evaluate the importance of hydrogen bonding in substrate binding, several deamino- and deoxy-analogues of IMP were tested as substrates and inhibitors. Only 2'-deoxy-IMP was a substrate; the other compounds tested were competitive inhibitors with Ki values at most 10-fold greater than the KD for IMP, illustrating the greater importance of hydrogen-bonding interactions in the chemistry of the IMPDH reaction than simply in nucleotide binding.     

Effects of external pH on substrate binding and on the inward chloride translocation rate constant of band 3

To test the hypothesis that amino acid residues in band 3 with titratable positive charges play a role in the binding of anions to the outside-facing transport site, we measured the effects of changing external pH (pH(O)) on the dissociation constant for binding of external iodide to the transport site, K(O)(I). K(O)(I) increased with increasing pH(O), and a significant increase was seen even at pH(O) values as low as 9.9. The dependence of K(O)(I) on pH(O) can be explained by a model with one titratable site with pK 9.5 +/- 0.2 (probably lysine), which increases anion affinity for the external transport site when it is in the positively charged form. A more complex model, analogous to one recently proposed by Bjerrum (1992), with two titratable sites, one with pK 9.3 +/- 0.3 (probably lysine) and another with pK > 11 (probably arginine), gives a slightly better fit to the data. Thus, titratable positively charged residues seem to be functionally important for the binding of substrate anions to the outward-facing anion transport site. In addition, analysis of Dixon plot slopes for L inhibition of Cl- exchange at different pH 0 values, coupled with the assumption that pH(O) has parallel effects on external I- and Cl- binding, indicates that k', the rate-constant for inward translocation of the complex of Cl- with the extracellular transport site, decreases with increasing pH(O). The data are compatible with a model in which titration of the pK 9.3 residue decreases k to 14 +/- 10% of its value at neutral pH(O). This result, however, together with Bjerrum's (1992) observation that the maximum flux J(M)) increases 1.6- fold when this residue is deprotonated, makes quantitative predictions that raise significant questions about the adequacy of the two titratable site ping-pong model or the assumptions used in analyzing the data.     

Phosphoglycerate mutase. Kinetics and effects of salts on the mutase and bisphosphoglycerate phosphatase activities of the enzyme from chicken breast muscle.

The steady state kinetics and effects of salts on chicken breast phosphoglycerate mutase have been examined. The enzyme can catalyze three phosphoryl transfer reactions: mutase, bisphosphoglycerate phosphatase, and bisphosphoglycerate synthase. The mutase rate was measured in the favorable direction (Keq = glycerate-3-P/glycerate-2-P approximately equal to 12) using [2T]glycerate-2-P as substrate. The bisphosphoglycerate phosphatase activity was studied in the presence of the activator, glycolate-2-P. The latter is an analog of the glycerate-P's and appears to act as an abortive mutase substrate. The kinetic pattern obtained with both activities is that of a ping-pong mechanism with inhibition by the second substrate occurring at a lower concentration than the Km value for that substrate. The kinetic parameters for the mutase determined in 50 mM N-[tris(hydroxymethyl)methyl-2-amino]ethanesulfonate (TES)/sodium buffer containing 0.1 M KCl, pH 7.5, 25 degrees C are: Km glycerate-2,3-P2, 0.069 micron; Km glycerate-2-P, 14 micron; Km glycerate-3-P approximately 200 micron; Ki glycerate-2-P, 4 micron. The kinetic parameters for the phosphatase reaction in 50 mM triethanolamine/Cl- buffer, pH 7.5, 25 degrees C are: Km glycerate-2,3-P2, 0.065 micron:Km glycolate-2P, 479 micron; Ki glycolate-2-P, 135 micron. The enzyme is sensitive to changes in the ionic environment. Increasing salt concentrations activate the phosphatase in the presence of glycolate-2-P by decreasing the apparent Km of glycerate-2,3-P2. The effects are due to the anionic component and Cl- greater than acetate greater than TES. The same salts are competitive inhibitors with respect to glycolate-2-P. With high levels of KCl that produce a 30-fold decrease in the apparent maximal velocity due to competition with glycolate-2-P, the Km of glycerate-2,3-P2 remains low. These observations lead us to postulate that each monophosphoglycerate substrate has a separate site on the enzyme and that glycerate-2,3-P2 can bind to either site. The binding of anions to one site of the nonphosphorylated enzyme allows an increase in the on and off rates of glycerate-2,3-P2 at the alternate site. Salts inhibit the mutase reaction. The Km of glycerate-2,3-P2 is increased as is that of glycerate-2-P. The effect on the Km of glycerate-2,3-P2 is attributed to an increase in the off rate/on rate ratio for glycerate-2,3-P2. The bisphosphoglycerate synthase reaction is shown to require added glycerate-3-P. The equilibrium between enzyme and glycerate-1,3-P2 is favorable (Kdiss less than or equal 7 X 10(-8) M) and suggests that in the absence of a separate synthase this reaction may have functional significance.     

Demonstration and chemical modification of a specific phosphate binding site in the phosphate-starvation-inducible outer membrane porin protein P of Pseudomonas aeruginosa

The interaction of phosphate ions with the Pseudomonas aeruginosa anion-specific protein P channel was probed. The single-channel conductance of protein P incorporated into planar lipid bilayer membranes in the presence of 0.3 M H2PO-4 was shown to be 6.0 pS, demonstrating that protein P channels allowed the permeation of phosphate. When large numbers of protein P channels were incorporated into lipid bilayer membranes in the presence of 40 mM Cl-, addition of small concentrations of phosphate resulted in reduction of macroscopic Cl- conductance in a dose- (and pH-) dependent fashion. This allowed calculation of an I50 value of e.g. 0.46 mM at pH 7.0, suggesting that the affinity of protein P for its normal substrate phosphate was at least 60-100-fold greater than the affinity of the channel for other ions such as chloride. Pyrophosphate and the phosphate analogue, arsenate, also inhibited macroscopic Cl- conductance through protein P with I50 values at pH 7.0 of 4.9 mM and 1.3 mM, respectively. To probe the nature of the phosphate binding site, the epsilon-amino groups of available lysine residues of protein P were chemically modified. Acetylation and carbamylation which produced uncharged, modified lysines destroyed both the anion (e.g. Cl-) binding site and the phosphate binding site as determined by single-channel experiments and macroscopic conductance inhibition experiments respectively. Nevertheless, the modified proteins still retained their trimeric configuration and their ability to reconstitute single channels in lipid bilayer membranes. Methylation, which allowed retention of the charge on the modified lysine residues, increased the Kd of the channel for Cl- 33-fold and the I50 for phosphate inhibition of macroscopic Cl- conductance 2.5-4-fold. A molecular model for the phosphate binding site of the protein P channel is presented.     

Interaction of Na+, K+, and Cl- with the binding of amphetamine, octopamine, and tyramine to the human dopamine transporter

Little information is available on the role of Na+, K+, and Cl- in the initial event of uptake of substrates by the dopamine transporter, i.e., the recognition step. In this study, substrate recognition was studied via the inhibition of binding of [3H]WIN 35,428 [2beta-carbomethoxy-3beta-(4-fluorophenyl)[3H]tropane], a cocaine analogue, to the human dopamine transporter in human embryonic kidney 293 cells. D-Amphetamine was the most potent inhibitor, followed by p-tyramine and, finally, dl-octopamine; respective affinities at 150 mM Na+ and 140 mM Cl- were 5.5, 26, and 220 microM. For each substrate, the decrease in the affinity with increasing [K+] could be fitted to a competitive model involving the same inhibitory cation site (site 1) overlapping with the substrate domain as reported by us previously for dopamine. K+ binds to this site with an apparent affinity, averaged across substrates, of 9, 24, 66, 99, and 134 mM at 2, 10, 60, 150, and 300 mM Na+, respectively. In general, increasing [Na+] attenuated the inhibitory effect of K+ in a manner that deviated from linearity, which could be modeled by a distal site for Na+, linked to site 1 by negative allosterism. The presence of Cl- did not affect the binding of K+ to site 1. Models assuming low binding of substrate in the absence of Na+ did not provide fits as good as models in which substrate binds in the absence of Na+ with appreciable affinity. The binding of dl-octopamine and p-tyramine was strongly inhibited by Na+, and stimulated by Cl- only at high [Na+] (300 mM), consonant with a stimulatory action of Cl- occurring through Na+ disinhibition.     

Myeloperoxidase inactivation in the course of catalysis of chlorination of taurine.

Myeloperoxidase (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7) was isolated from leukocytes of patients with chronic granulocyte leukemia. In the presence of H2O2 and Cl- at pH 4.0-6.6 the myeloperoxidase catalyses chlorination of taurine to monochloramine taurine and simultaneously undergoes inactivation. The myeloperoxidase inactivation rate depends on the concentration of H2O2 and Cl-: both the initial rate of chlorination and myeloperoxidase inactivation rate increase with increasing concentration of H2O2. However, an increase in concentration of Cl- results in a decrease in enzyme inactivation. At a given H2O2 concentration, myeloperoxidase inactivation is a first order reaction, which implied that the enzyme may react with a substrate a limited number of times.     

Kinetic properties of human placental aromatase. Application of an assay measuring 3H2O release from 1beta,2beta-3H-androgens.

The rapid and sensitive assay of 1beta,2beta-3H-androgen aromatization by measurement of 3H2O release (Thompson, E.A., Jr., and Siiteri, P.K. (1974) J. Biol. Chem. 249, 5364-5372) has been analyzed to determine its applicability to initial rate studies. It was found that aromatization is the sole reaction catalyzed by lyophilized placental microsomes that causes a loss of tritium from position 1 or 2 of androstenedione and testosterone. Tritium is, however, removed from position 2 of the estrogen products, presumably in 2-hydroxylation, but this does not invalidate use of the assay for initial rate measurements; it was therefore used to characterize the catalytic properties of aromatase. Aromatization by the freeze-dried preparation was stimulated by K+, EDTA, and dithiothreitol, and was maximally active at pH 7.5 TO 8.0. With incubation conditions optimized for these factors, the apparent Km for NADPH is approximately 1 muM. The maximum velocity of androstenedione aromatization exceeds that of testosterone, and the affinity of the substrate binding site is higher for the former substrate, the apparent Km values being 0.1 muM and 0.4 muM, respectively. Mutual competition experiments with the androgen substrates showed that each gives simple competitive inhibition of the other's aromatization; furthermore, the apparent Ki values for each are in close agreement with their respective Km values. Androst-1,4,6-triene-3,17-dione competitively inhibits the aromatization of both androstenedione and testosterone, the apparent Ki, in both cases being 0.2 muM. It is concluded that the two androgen substrates are aromatized at a single, identical site.     

Kinetics of the acid pump in the stomach. Proton transport and hydrolysis of ATP and p-nitrophenyl phosphate by the gastric H,K-ATPase

Hydrolysis of adenosine 5'-triphosphate (ATP) and p-nitrophenyl phosphate by the hydrogen ion-transporting potassium-stimulated adenosine triphosphatase (H,K-ATPase) was investigated. Hydrolysis of ATP was studied at pH 7.4 in vesicles treated with the ionophore nigericin. The kinetic analysis showed negative cooperativity with one high affinity (Km1 = 3 microM) and one low affinity (Km2 = 208 microM) site for ATP. The rate of hydrolysis decreased at 2000 microM ATP indicating a third site for ATP. When the pH was decreased to 6.5 the experimental results followed Michaelis-Menten enzyme kinetics with one low affinity site (Km = 116 microM). Higher concentrations than 750 microM ATP were inhibitory. Proton transport was measured as accumulation of acridine orange in vesicles equilibrated with 150 mM KCl. The transport at various concentrations of ATP in the pH interval from 6.0 to 8.0 correlated well with the Hill equation with a Hill coefficient between 1.5-1.9. The concentration of ATP resulting in half-maximal transport rate (S0.5) increased from 5 microM at pH 6.0 to 420 microM at pH 8.0. At acidic pH the rate of proton transport decreased at 1000 microM ATP. The K+-stimulated p-nitrophenylphosphatase (pNPPase) activity resulted in a Hill coefficient close to 2 indicating cooperative binding of substrate. The pNPPase was noncompetitively inhibited by ATP and ADP; half-maximal inhibition was obtained at 2 and 100 microM, respectively. Phospholipase C-treated vesicles lost 80% of the pNPPase activity, but the Hill coefficient did not change. These kinetic results are used for a further development of the reaction scheme of the H,K-ATPase.     

The catalytic mechanism of carbonic anhydrase. Hydrogen-isotope effects on the kinetic parameters of the human C isoenzyme.

1. The steady-state kinetics of the interconversion of CO2 and HCO3 catalyzed by human carbonic anhydrase C was studied using 1H2O and 2H2O as solvents. The pH-independent parts of the parameters k(cat) and Km are 3-4 times larger in 1H2O than in 2H2O for both directions of the reaction, while the ratios k(cat)/Km show much smaller isotope effects. With either CO2 or HCO3 as substrate the major pH dependence is observed in k(cat), while Km appears independent of pH. The pKa value characterizing the pH-rate profiles is approximately 0.5 unit larger in 2H2O than in 1H2O. 2. The hydrolysis of p-nitrophenyl acetate catalyzed by human carbonic anhudrase C is approximately 35% faster in 2H2O than in 1H2O. In both solvents the pKa values of the pH-rate profiles are similar to those observed for the CO2-HCO3 interconversion. 3. It is tentatively proposed that the rate-limiting step at saturating concentrations of CO2 or HCO3 is an intramolecular proton transfer between two ionizing groups in the active site. It cannot be decided whether the transformation between enzyme-bound CO2 and HCO3 involves a proton trnasfer or not.     

Kinetics of the hydrolysis of monodispersed dihexanoyllecithin catalyzed by a cobra (Naja naja atra) venom phospholipase A2

The hydrolysis of 1,2-dihexanoyl-sn-glycero-3-phosphorylcholine (diC6PC), catalyzed by a cobra (Naja naja atra) venom phospholipase A2, was studied at 25 degrees C ionic strength 0.1 in the presence of 3-10 mM Ca2+, which can saturate the Ca2+-binding site of the enzyme. The initial velocity data, obtained at various concentrations of the substrate below the critical micellar concentration (cmc), were analyzed according to the Michaelis-Menten equation. The Km value was practically independent of pH (between pH 6.75 and 10.30). This finding was consistent with the result of a direct binding study on monodispersed n-alkylphosphorylcholines (Teshima et al. (1981) J. Biochem. 89, 1163-1174). The hydrolysis of the substrate was competitively inhibited by the presence of monodispersed n-dodecylphosphorylcholine (n-C12PC). These results indicated that the substrate and n-C12PC compete for the same site on the enzyme molecule. The pH dependence curve of the kinetic parameter, kcat/Km, exhibited three transitions, below pH 8, between pH 8 and 9.5, and above pH 10. The analysis indicated the participation of three ionizable groups with pK values of 7.25, 8.50, and 10.4. The deprotonation of the first group and the protonation of the third group were found to be essential for the catalysis. The first group was assigned as His 48 in the active site on the basis of its pK value, which had been determined from the pH dependence of the binding constant of Ca2+ (Teshima et al. (1981) J. Biochem. 89, 13-20).(ABSTRACT TRUNCATED AT 250 WORDS)     

Location of the binding site for chloride ion activation of cathepsin C.

Cathepsin C, a tetrameric lysosomal dipeptidyl-peptide hydrolase, is activated by chloride ion. The activation is shown here to be specific and pH-dependent, dissociation constants for chloride being lower at low pH. Bound chloride decreases the Km for the hydrolysis of chromophore labelled substrates without any significant change in Vmax, confirming its involvement in substrate binding. Determination of the kinetic parameters of chloride activation, using unlabelled substrates, has enabled its site of action to be located. The lower Km for the hydrolysis of simple amide substrates in the presence of Cl- shows that the S sites are involved. Possible involvement of the S' sites is excluded by the finding that the Km for the nucleophile in the transferase reaction is unaffected by chloride. The rates of inhibition by E-64 and iodoacetate are both chloride-dependent and, from the structure of the papain-E-64 complex, it is concluded that chloride binds close to the S2 site. The binding of guanidinium ion, a positively charged inhibitor, to the S site is dependent on chloride. Based on these results, a model is proposed to explain the chloride activation of cathepsin C. The possible physiological role of chloride in the regulation of proteolysis in the lysosome is discussed.     

Inhibition of inorganic anion transport across the human red blood cell membrane by chloride-dependent association of dipyridamole with a stilbene disulfonate binding site on the band 3 protein

The inhibition of inorganic anion transport by dipyridamole (2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido[5,4-d] pyrimidine) takes place only in the presence of Cl-, other halides, nitrate or bicarbonate. At any given dipyridamole concentration, the anion flux relative to the flux in the absence of dipyridamole follows the equation: Jrel = (1 + alpha 2[Cl-])/(1 + alpha 4[Cl-]) where alpha 2 and alpha 4 are independent of [Cl-] but dependent on dipyridamole concentration. At high [Cl-] the flux approaches alpha 2/alpha 4, which decreases with increasing dipyridamole concentration. Even when both [Cl-] and dipyridamole concentration assume large values, a small residual flux remains. The equation can be deduced on the assumption that Cl- binding allosterically increases the affinity for dipyridamole binding to band 3 and that the bound dipyridamole produces a non-competitive inhibition of sulfate transport. The mass-law constants for the binding of Cl- and dipyridamole to their respective-binding sites are about 24 mM and 1.5 microM, respectively (pH 6.9, 26 degrees C). Dipyridamole binding leads to a displacement of 4,4'-dibenzoylstilbene-2,2'-disulfonate (DBDS) from the stilbenedisulfonate binding site of band 3. The effect can be predicted quantitatively on the assumption that the Cl- -promoted dipyridamole binding leads to a competitive replacement of the stilbenedisulfonates. For the calculations, the same mass-law constants for binding of Cl- and dipyridamole can be used that were derived from the kinetic studies on Cl- -promoted anion transport inhibition. The newly described Cl- binding site is highly selective with respect to Cl- and other monovalent anion species. There is little competition with SO4(2-), indicating that Cl- binding involves other than purely electrostative forces. The affinity of the binding site to Cl- does not change over the pH range 6.0-7.5. Dipyridamole binds only in its deprotonated state. Binding of the deprotonated dipyridamole is pH-independent over the same range as Cl- binding.     

Nucleoside hydrolase from Crithidia fasciculata. Metabolic role, purification, specificity, and kinetic mechanism

Crithidia fasciculata cells grown on complex medium with added [8-14C, 5'-3H]inosine or [8-14C,5'-3H]adenosine metabolize greater than 50% of the salvaged nucleosides through a pathway involving N-glycoside bond cleavage. Cell extracts contain a substantial nucleoside hydrolase activity but an insignificant purine nucleoside phosphorylase. The nucleoside hydrolase has been purified 1000-fold to greater than 99% homogeneity from kilogram quantities of C. fasciculata. The enzyme is a tetramer of Mr 34,000 subunits to give an apparent holoenzyme Mr of 143,000 by gel filtration. All of the commonly occurring nucleosides are substrates. The Km values vary from 0.38 to 4.7 mM with purine nucleosides binding more tightly than the pyrimidines. Values of Vmax/Km vary from 3.4 x 10(3) M-1 s-1 to 1.7 x 10(5) M-1 s-1 with the pyrimidine nucleosides giving the larger values. The turnover rate for inosine is 32 s-1 at 30 degrees C. The kinetic mechanism with inosine as substrate is rapid equilibrium with random product release. The hydrolytic reaction can be reversed to give an experimental Keq of 106 M with H2O taken as unity. The product dissociation constants for ribose and hypoxanthine are 0.7 and 6.2 mM, respectively. Deoxynucleosides or 5'-substituted nucleosides are poor substrates or do not react, and are poor inhibitors of the enzyme. The enzyme discriminates against methanol attack from solvent during steady-state catalysis, indicating the participation of an enzyme-directed water nucleophile. The pH profile for inosine hydrolysis gives two apparent pKa values of 6.1 with decreasing Vmax/Km values below the pKa and a plateau at higher pH values. These effects are due to the pH sensitivity of the Vmax values, since Km is independent of pH. The pH profile implicates two negatively charged groups which stabilize a transition state with oxycarbonium character.     

Secretion of ligninperoxidase by Penicillium citrinum,Fusarium oxysporum and Aspergillus terreus

Secretion of ligninperoxidase [E.C.1.11.1.7] by Penicillium citrinum, Fusarium oxysporum and Aspergillus terreus in liquid culture growth medium has been demonstrated. Enzymatic characteristics like Km, pH and temperature optima using veratryl alcohol as the organic substrate of ligninperoxidases from above sources have been determined. Km values using veratryl alcohol as substrate for enzymes from P. citrinum, F. oxysporum and A. terreus were 69, 64 and 60 microM respectively. Km values using H2O2 as the variable substrate were 64, 72 and 80 microM.The pH optima were 4.0, 2.3 and 2.0 respectively. The values of temperature optima were 30 degrees, 25 degrees and 22 degrees C for the enzymes from P. citrinum, F. oxysporum and A. terreus respectively.     

ESR studies on the oxidation of N,N-dimethyl-p-anisidine and its analogues catalyzed by myeloperoxidase

N,N-Dimethyl-p-anisidine (DMA) was used as a substrate to differentiate between the direct, or chloride-independent, and the indirect, or chloride-dependent, pathways characteristic of myeloperoxidase (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7). The chemical oxidation by sodium hypochlorite and the horseradish peroxidase-catalyzed oxidation by H2O2 were also investigated for a comparison. The chemical oxidation of DMA by NaOCl (DMA/NaOCl = 1) gave the p-N,N-dimethylaminophenoxy radical at pH 5 and 7. p-Benzoquinone and formaldehyde were determined as stable end-products. On the other hand, the cation radical of DMA was detected and p-benzoquinone was not obtained in the horseradish peroxidase-H2O2-Cl- system. In the presence of Cl- the myeloperoxidase-catalyzed oxidation at pH 5 gave nearly the same result as did the oxidation by NaOCl, whereas in the absence of Cl- the result of the oxidation was similar to that of the horseradish peroxidase-catalyzed oxidation, except for a low yield of formaldehyde formation, which was ascribed to the decomposition of H2O2 by the catalase activity of myeloperoxidase. Although the myeloperoxidase-catalyzed oxidation of DMA at pH 7 in the presence of Cl- gave only the cation radical of DMA, a fairly large amount of p-benzoquinone was obtained as a product. This result indicates that the indirect chloride-dependent oxidation is also operating at pH 7. The reaction mechanism for the myeloperoxidase-catalyzed oxidation of DMA is proposed.     

A functional arginine residue in the vacuolar H(+)-ATPase of higher plants.

The arginine-specific reagent phenylglyoxal inactivated the vacuolar H(+)-ATPase of red beet. Inactivation by phenylglyoxal followed pseudo-first-order kinetics and a double log plot of the t1/2 of inactivation versus phenylglyoxal concentration yielded a slope of 1.18. Neither inorganic anions nor DIDS protected from phenylglyoxal-mediated inactivation of the H(+)-ATPase. Indeed, Cl- stimulated the rate of phenylglyoxal-mediated H(+)-ATPase inactivation relative to SO4(2-). ATP, but not MgATP or ADP, protected from phenylglyoxal-mediated inactivation and inactivation resulted in a decrease in the Vmax of the H(+)-ATPase with little effect on the Km. Collectively, these results are consistent with phenylglyoxal-mediated inactivation of the vacuolar H(+)-ATPase resulting from modification of a single arginine residue in the catalytic nucleotide binding site of the vacuolar H(+)-ATPase. Stimulation of phenylglyoxal-mediated inactivation by Cl- indicates that exposure of the phenylglyoxal-sensitive functional arginine residue is enhanced in the presence of Cl-. The failure of MgATP to protect from phenylglyoxal inactivation suggests that ATP, rather than MgATP, binds directly to the catalytic site and that Mg2+ may act to promote catalysis subsequent to ATP binding.     

Similarity of kinetics of three types of myeloperoxidase from human leukocytes and four types from HL-60 cells

Km values for H2O2 and Vmax values for three types of myeloperoxidase (MPO) from human leukocytes (MPO-I, -II, and -III) and four types from human myeloid leukemia HL-60 cells (MPO-IA, -IB, -II, and -III) were determined. Km values of human leukocyte MPOs decreased with increasing pH from 4.4 to 6.2 and increased with increasing NaCl concentration from 0.025 to 0.14 M. There was no significant difference among Km values of leukocyte MPO-I, -II, and -III. NaBr also showed a tendency similar to that of NaCl with regard to the effects of pH and halide concentration on Km values. However, Km values in the presence of NaBr were lower than those in the presence of NaCl. Effects of pH and NaCl concentration on Vmax values of MPO-I, -II, and -III were also examined. Vmax values of MPO-I, -II, and -III were higher at pH 4.9 and 5.4 and increased with increasing NaCl concentration. In addition, no difference was observed between Km values of leukocyte and those of HL-60 cells. MPO-IB, the half-molecular-weight enzyme of HL-60 cells, also had the same Km values as the others. Furthermore, inhibition of the activities of seven MPOs of leukocytes and HL-60 cells by H2O2 was similarly observed at concentrations above 1 mM at pH 5.4 but not at pH 4.4. These results indicate that there is no difference in the affinity to H2O2 among leukocyte MPO-I, -II, and -III and HL-60 cell MPO-IA, -IB, -II, and -III.     

Electronic effects in the acylation of alpha-chymotrypsin by substituted N-benzoylimidazoles

Rate constants for the acylation of alpha-chymotrypsin by a series of acyl-substituted N-benzoylimidazoles have been determined by proflavin displacement from the active site. The second-order acylation rate constants k2/Km are large [e.g., that for N-(m-nitrobenzoyl)imidazole is 1.7 X 10(4) M-1 s-1 at pH 7.5], even though Km must be quite large (plots of k vs. k/[S]0 have infinite slopes). The values of k2/Km are nearly independent of pH in the range 5.0-9.0 when the substituent group is electron donating. Electron-withdrawing substituents produce an increase in k2/Km with increasing pH until a maximum is reached near pH 7. This is also the case in acylation by the N-[p-(dimethylamino)benzoyl]-N'-methylimidazolium ion (pKapp = 6.5). While the reaction of the N'-methylated derivative is via a positively charged species at all pH values, the unmethylated compounds react through both the neutral species and the conjugate acids, with the observed pH dependence depending on the relative values of the rate constants. The limiting value of k2/Km for the N-[p-(dimethylamino)benzoyl]-N'-methylimidazolium ion is 2.1 times less in D2O than in H2O. Thus, His-57 must be participating in the acylation reaction as a general base. The limiting values of k2/Km for the corresponding N'-methylated and unmethylated derivatives differ by a factor of only 150, which is similar to the difference in the second-order rate constants for nonenzymatic OH- -catalyzed hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)     

The oxygen-evolving complex requires chloride to prevent hydrogen peroxide formation.

Illumination of PSII core preparations can cause the production of H2O2 at rates which approach 60 mumol of H2O2 (mg of Chl.h)-1. The rate of peroxide production is maximal at pH 7.2 at low sucrose concentrations and at concentrations of Cl- (1.5-3.0 mM) that limit the rate of the oxidation of water to O2. The rate of H2O2 production increased with pH from pH 6.8 to 7.2 and was inversely proportional to the oxidation of water to O2 from pH 6.8 to 7.5. While EDTA does not inhibit H2O2 production, this reaction is abolished by 5 mM NH2OH and inhibited by the same concentrations of NH3 that affect water oxidation which indicates that the oxygen-evolving complex is responsible for the production of peroxide generated upon illumination of PSII core preparations. These results support a mechanism in which bound Cl- in the S2 state is displaced by OH- ions which are then oxidized by the OEC to form H2O2. Thus, the OEC requires Cl- to prevent access to the active site of the OEC until four oxidizing equivalents can be generated to allow the oxidation of water to O2.