Mechanistic studies of the flavoprotein tryptophan 2-monooxygenase. 1. Kinetic mechanism

The flavoprotein tryptophan 2-monooxygenase catalyzes the oxidative decarboxylation of tryptophan to indole-3-acetamide, carbon dioxide, and water. The kinetic mechanism of the enzyme has been determined with tryptophan as substrate at pH 8.3. Initial velocity patterns, when both amino acid and oxygen concentrations are varied, are sequential with tryptophan and ping-pong with phenylalanine and methionine. Reduction by tryptophan in the absence of oxygen is biphasic. The rate of the rapid phase varies with the tryptophan concentration, with a limiting rate of 139 s-1 and an apparent Kd value of 0.11 mM. There is a primary deuterium kinetic isotope effect on the limiting rate of reduction of 2.4. The rapid phase is followed by a slow, concentration and isotope-independent phase that is much slower than turnover; this is ascribed to dissociation of a reduced enzyme-imino acid complex. In the absence of oxygen, tryptophan is converted to indolepyruvate imine. The rate of this reaction is the same as that of the rapid phase in the reduction. Reaction of the reduced enzyme-imino acid complex with oxygen to form oxidized flavin is monophasic, with a rate constant of 196 mM-1 s-1; no intermediates are detectable. The rate of formation of indole-3-acetamide agrees with the rate of reaction with oxygen. This is followed by slow product dissociation.

Purification and characterization of the flavoprotein tryptophan 2-monooxygenase expressed at high levels in Escherichia coli

Tryptophan 2-monooxygenase from Pseudomonas savastanoi is a flavoprotein which catalyzes the formation of indoleacetamide from tryptophan. This is the first step in a two-step pathway for the formation of indoleacetic acid during infection of plants and subsequent gall formation by this and other bacteria. The enzyme has been expressed in Escherichia coli at high levels, and a purification procedure has been developed which generates micromolar amounts of protein. The purified enzyme contains tightly bound indoleacetamide; a method involving dialysis against 20% methanol has been developed for removing the indoleacetamide without significant loss of enzyme activity. Amino acids with large hydrophobic side chains are the best substrates; N-substituted phenylalanines will also act as substrates. N-ethylmaleimide, methyl methanethiol-sulfonate, and diethylpyrocarbonate act as active site-directed reagents, consistent with a histidine and a cysteine at or near the enzyme active site. Vinylglycine partially inactivates the enzyme, while propargylglycine has no effect.

pH and kinetic isotope effects on the oxidative half-reaction of D-amino-acid oxidase

D-Amino-acid oxidase catalyzes the oxidation of D-amino acids to imino acids. In the oxidative half-reaction, oxygen reacts with the reduced enzyme-imino acid complex to reoxidize the bound FAD. This is then followed by dissociation of the imino acid. The effects of pH and D2O on the kinetics of the oxidative half-reaction of D-amino-acid oxidase have been determined with glycine, D-alanine, and D-serine as substrates. Reaction of the reduced enzyme with oxygen requires that a group with a pKa value of about 10.5 be protonated and a group with a pKa value of 8.5 be deprotonated. The former value is not seen with D-alanine as substrate; the latter is only seen with glycine. No solvent isotope effects are seen on the V/KO2 value with D-alanine, consistent with rate-limiting electron transfer. Product release involves a pH-dependent conformational change. This is rate-limiting at all pH values with D-alanine as substrate. Significant solvent isotope effects are seen on the Vmax value with D-alanine. The proton inventory at high pH is linear, consistent with release of a single proton in the slow step; at pH 6 the solvent inventory is bowl-shaped, consistent with a solvent isotope effect on the conformation of the protein. With glycine the DV value increases to the intrinsic value at pH 10.5; this establishes that CH bond cleavage becomes rate-limiting with this substrate above pH 10.

Bifunctional peptidylglcine alpha-amidating enzyme requires two copper atoms for maximum activity

The conversion of C-terminal glycine-extended peptides to C-terminal alpha-amidated peptides occurs in two distinct reactions, both of which are catalyzed by bifunctional peptidylglycine alpha-amidating enzyme. The first step is the alpha-hydroxylation of the C-terminal glycine residue and the second step is the dealkylation of the alpha-hydroxyglycine-extended peptide to the alpha-amidated peptide and glyoxylate. We show that the bifunctional enzyme requires 1.9 +/- 0.2 mol of copper/mol of enzyme for maximal dansyl-Tyr-Lys-Gly amidation activity under the conditions of high enzyme concentration (approximately 80 microM) required to measure initial rates for this poor substrate. The enzyme, as purified, contains a substoichiometric amount of copper and has only trace levels of amidation activity. Addition of exogenous Cu(II) ions stimulates amidation activity approximately 3000-fold at the optimum copper stoichiometry and the enzyme is then inhibited by excess Cu(II). No stimulation of amidation activity is observed upon the addition of the following divalent metal ions: Mn(II), Fe(II), Ni(II), Cd(II), and the oxovanadium cation, VO(II). The enzyme-catalyzed dealkylation of alpha-hydroxyhippuric acid to benzamide shows no dependence on copper, indicating that the copper dependence of the amidation reaction must be attributed to a copper dependence in peptide alpha-hydroxylation.

Intrinsic primary, secondary, and solvent kinetic isotope effects on the reductive half-reaction of D-amino acid oxidase: evidence against a concerted mechanism

D-Amino acid oxidase catalyzes the oxidation of D-amino acids to imino acids with subsequent transfer of the electrons to molecular oxygen. Proposed mechanisms for the mode of cleavage of the substrate CH bond include stepwise formation of a carbanion, followed by attack of the carbanion on the enzyme-bound FAD, direct hydride transfer of the substrate alpha-hydrogen to the FAD, and transfer of a hydride from the substrate amino group to the FAD. Conditions have previously been established under which large, limiting, primary deuterium kinetic isotope effects can be measured with D-alanine, D-serine, and glycine as substrates for D-amino acid oxidase [Denu, J. M., & Fitzpatrick, P. F. (1992) Biochemistry 31, 8207-8215]. To determine whether these values are the intrinsic isotope effects, primary tritium kinetic isotope effects have been determined with these three substrates. The values are 12.6, 8.6, and 6.4, respectively. These values are consistent with expression of the intrinsic isotope effects under these conditions, allowing for determination of the values of the intrinsic deuterium effects as 5.7, 4.5, and 3.6 for D-alanine, D-serine, and glycine, respectively. Under these conditions, the alpha-secondary tritium kinetic isotope effect with glycine, the beta-secondary deuterium kinetic isotope effect with D-alanine, and the solvent kinetic isotope effect with D-serine are all indistinguishable from unity. These results are not consistent with concerted mechanisms for CH bond cleavage with this enzyme, but are fully consistent with the involvement of a carbanion intermediate.

Identification of the intersubunit binding region in rat tyrosine hydroxylase

Limited proteolysis converts the 39200 molecular weight catalytic domain of rat tyrosine hydroxylase to a monomer with a molecular weight of 37600. The purified monomer is almost fully active, with minor changes in kinetic parameters at pH 7. Mass spectral analysis and N-terminal sequencing of the proteolytically generated species establish that 20 amino acids have been removed from the carboxyl terminus and five from the amino terminus. Based on these results, the carboxyl terminus is responsible for tetramer formation by tyrosine hydroxylase. The sequence of amino acids which is removed is consistent with a coiled coil structure in the intact tetramer.

Expression and characterization of catalytic and regulatory domains of rat tyrosine hydroxylase

Phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase constitute a family of tetrahydropterin-dependent aromatic amino acid hydroxylases. Comparison of the amino acid sequences of these three proteins shows that the C-terminal two-thirds are homologous, while the N-terminal thirds are not. This is consistent with a model in which the C-terminal two-thirds constitute a conserved catalytic domain to which has been appended discrete regulatory domains. To test such a model, two mutant proteins have been constructed, expressed in Escherichia coli, purified, and characterized. One protein contains the first 158 amino acids of rat tyrosine hydroxylase. The second lacks the first 155 amino acid residues of this enzyme. The spectral properties of the two domains suggest that their three-dimensional structures are changed only slightly from intact tyrosine hydroxylase. The N-terminal domain mutant binds to heparin and is phosphorylated by cAMP-dependent protein kinase at the same rate as the holoenzyme but lacks any catalytic activity. The C-terminal domain mutant is fully active, with Vmax and Km values identical to the holoenzyme; these results establish that all of the catalytic residues of tyrosine hydroxylase are located in the C-terminal 330 amino acids. The results with the two mutant proteins are consistent with these two segments of tyrosine hydroxylase being two separate domains, one regulatory and one catalytic.

Lysine241 of tyrosine hydroxylase is not required for binding of tetrahydrobiopterin substrate

The lysine residues at positions 194 and 198 in phenylalanine hydroxylase have been shown to react with a photoaffinity label which is an analog of phenyltetrahydropterin (Gibbs, B. S., and Benkovic, S. J. (1991) Biochemistry 30, 6795-6802), in a manner suggesting that these lysine residues are involved in tetrahydrobiopterin binding. The related enzyme tyrosine hydroxylase has a lysine at position 241 which, given the 75% identity between its C-terminal 330 amino acids and those of phenylalanine hydroxylase, corresponds to lysine194 of phenylalanine hydroxylase. Site-directed mutagenesis was used to alter lysine241 of tyrosine hydroxylase to alanine. Steady-state kinetic parameters were measured for wild-type and K241A tyrosine hydroxylase. No kinetic parameter differed between the wild-type and K241A enzymes, including Vmax values, Michaelis constants for tetrahydrobiopterin, 6-methyl-tetrahydropterin, and tyrosine, and the inhibition constants for norepinephrine. These results show that lysine241 is not required for tetrahydrobiopterin binding to tyrosine hydroxylase.

pH and kinetic isotope effects on the reductive half-reaction of D-amino acid oxidase

Primary deuterium kinetic isotope and pH effects on the reduction of D-amino acid oxidase by amino acid substrates were determined using steady-state and rapid reaction methods. With D-serine as substrate, reduction of the enzyme-bound FAD requires that a group with a pKa value of 8.7 be unprotonated and that a group with a pKa value of 10.7 be protonated. The DV/Kser value of 4.5 is pH-independent, establishing that these pKa values are intrinsic. The limiting rate of reduction of the enzyme shows a kinetic isotope effect of 4.75, consistent with this as the intrinsic value. At high enzyme concentration (approximately 15 microM) at pH 9,D-serine is slightly sticky (k3/k2 = 0.8), consistent with a decrease in the rate of substrate dissociation. With D-alanine as substrate, the pKa values are perturbed to 8.1 and 11.5. The DV/Kala value increases from 1.3 at pH 9.5 to 5.1 at pH 4, establishing that D-alanine is sticky with a forward commitment of approximately 10. The effect of pH on the DV/Kala value is consistent with a model in which exchange with solvent of the proton from the group with pKa 8.7 is hindered and is catalyzed by H2O and OH- above pH 7 and by H3O+ and H2O below pH 7. With glycine, the pH optimum is shifted to a more basic value, 10.3. The DV/Kgly value increases from 1.26 at pH 6.5 to 3.1 at pH 10.7, consistent with fully reversible CH bond cleavage followed by a pH-dependent step. At pH 10.5, the kinetic isotope effect on the limiting rate of reduction is 3.4.

The amino acid substrate of bovine tyrosine hydroxylase

Tyrosine hydroxylase catalyzes the tetrahydropterin-dependent hydroxylation of tyrosine to form 3,4-dihydroxyphenylalanine. Several nonphysiological aromatic amino acids have been examined as inhibitors and substrates for bovine adrenal tyrosine hydroxylase. The Ki values for para-substituted phenylalanines increase as the size of the substituent increases. For each A2 increase in surface area of the substituent, the free energy of binding becomes 50 cal more positive. Replacement of the phenyl ring with a pyridyl ring decreases the affinity about one order of magnitude. A number of these aromatic amino acids are also substrates for the enzyme. The KM values again increase in size with increasing size of the substituent, but the Vmax value is independent of the reactivity of the amino acid. The effect of size on binding is consistent with a tight interaction between the para position region of the substrate and the enzyme. The lack of a change in the Vmax value is consistent with the rate-limiting step in catalysis by bovine tyrosine hydroxylase being formation of the hydroxylating intermediate rather than hydroxylation of the amino acid. These results will be useful in designing mechanism-based inhibitors of catecholamine biosynthesis and establish that the mechanisms of rat and bovine tyrosine hydroxylase do not differ significantly.

Site-directed mutagenesis of serine 40 of rat tyrosine hydroxylase. Effects of dopamine and cAMP-dependent phosphorylation on enzyme activity

Rat tyrosine hydroxylase expressed with a baculovirus expression system contains covalent phosphate and has kinetic parameters consistent with those expected of phosphorylated enzyme (Fitzpatrick, P. F., Chlumsky, L. J., Daubner, S. C., and O'Malley, K. L. (1990) J. Biol. Chem. 265, 2042-2047). The phosphorylation site was identified as serine 40, by purifying the enzyme from cells grown in the presence of [32P]phosphate. Replacement of serine 40 with alanine by site-directed mutagenesis prevented phosphorylation but had little effect on the steady-state kinetic parameters at pH 7. Both wild type and S40A tyrosine hydroxylase were expressed in Escherichia coli; the kinetic parameters of the enzymes purified from bacteria were nearly identical to those of the enzymes expressed with the baculovirus system, although the bacterially expressed enzyme contained no covalent phosphate. Treatment of this wild type enzyme with cAMP-dependent protein kinase decreased the KBH4 value about 2-fold but had no effect on the Vmax value at pH 7. Treatment with a stoichiometric amount of dopamine decreased the Vmax value 15-fold and increased the KBH4 value 2-3-fold. Phosphorylation of the dopamine-bound enzyme increased the Vmax value 10-fold and decreased the KBH4 value 2-fold. The kinetic parameters of the dopamine-bound recombinant enzyme were identical to those of enzyme purified from PC12 cells. In contrast, the S40A enzyme was converted to a less active form by treatment with dopamine but was not affected by phosphorylating conditions. These results are consistent with a model in which the major effect of phosphorylation of serine 40 is to relieve tyrosine hydroxylase from the inhibitory effects of catecholamines.

Studies of the rate-limiting step in the tyrosine hydroxylase reaction: alternate substrates, solvent isotope effects, and transition-state analogues

Tyrosine hydroxylase catalyzes the formation of dihydroxyphenylalanine from tyrosine, utilizing a tetrahydropterin and molecular oxygen as cosubstrates. Several approaches were taken to examining the identity of the rate-limiting step in catalysis. Steady-state kinetic parameters were determined with a series of ring-substituted phenylalanines. The Vmax value was unchanged with substrates ranging in reactivity from tyrosine to 4-fluorophenylalanine. Neither 4-pyridylalanine N-oxide, a model of tyrosine phenoxide, nor 4-hydroxy-3-pyridylalanine N-oxide or alpha-amino-3-hydroxy-4-pyridone-1- propionic acid, models of a hydroxycyclohexadienone intermediate, was an effective inhibitor. There was no solvent isotope effect on either the Vmax or the V/KTyr value. These results establish that no chemistry occurs at the amino acid in the rate-limiting step and no exchangeable proton is in flight in the rate-limiting step. The results are consistent with a model in which the slow step in catalysis is formation of the hydroxylating intermediate.

Steady-state kinetic mechanism of rat tyrosine hydroxylase

The steady-state kinetic mechanism for rat tyrosine hydroxylase has been determined by using recombinant enzyme expressed in insect tissue culture cells. Variation of any two of the three substrates, tyrosine, 6-methyltetrahydropterin, and oxygen, together at nonsaturating concentrations of the third gives a pattern of intersecting lines in a double-reciprocal plot. Varying tyrosine and oxygen together results in a rapid equilibrium pattern, while the other substrate pairs both fit a sequential mechanism. When tyrosine and 6-methyltetrahydropterin are varied at a fixed ratio at different oxygen concentrations, the intercept replot is linear and the slope replot is nonlinear with a zero intercept, consistent with rapid equilibrium binding of oxygen. All the replots when oxygen is varied in a fixed ratio with either tyrosine or 6-methyltetrahydropterin are nonlinear with finite intercepts. 6-Methyl-7,8-dihydropterin and norepinephrine are competitive inhibitors versus 6-methyltetrahydropterin and noncompetitive inhibitors versus tyrosine. 3-Iodotyrosine, a competitive inhibitor versus tyrosine, shows uncompetitive inhibition versus 6-methyltetrahydropterin. At high concentrations, tyrosine is a competitive inhibitor versus 6-methyltetrahydropterin. These results are consistent with an ordered kinetic mechanism with the order of binding being 6-methyltetrahydropterin, oxygen, and tyrosine and with formation of a dead-end enzyme-tyrosine complex. There is no significant primary kinetic isotope effect on the V/K values or on the Vmax value with [3,5-2H2]tyrosine as substrate. No burst of dihydroxyphenylalanine production is seen during the first turnover. These results rule out product release and carbon-hydrogen bond cleavage as rate-limiting steps.

Expression of rat tyrosine hydroxylase in insect tissue culture cells and purification and characterization of the cloned enzyme

Rat tyrosine hydroxylase has been expressed at high levels in Spodoptera frugiperda cells using a baculovirus expression system. A cDNA containing the coding region for PC12 tyrosine hydroxylase was inserted into the unique EcoRI site of the transfer vector pLJC8 to yield the recombinant vector pLJC9. Spodoptera frugiperda cells were then co-infected with pLJC9 and wild type Autographa californica nuclear polyhedrosis virus. Recombinant virus particles containing the cDNA for tyrosine hydroxylase were selected by hybridization with authentic tyrosine hydroxylase cDNA. Three recombinant viruses were plaque-purified. All expressed a protein of Mr = 55,000 which reacted with antibodies to tyrosine hydroxylase. Forty-eight h after infection of cells with recombinant virus, the specific activity of tyrosine hydroxylase in the cell lysate was 30-100 nmol of dihydroxyphenylalanine produced/min/mg, consistent with 5-10% of the cell protein being tyrosine hydroxylase. Purification from 2.1 g of cells gave 5.8 mg of enzyme with a specific activity of 1.7 mumol of dihydroxyphenylalanine/min/mg. The purified enzyme is a tetramer of identical subunits, containing one covalently bound phosphoryl residue and 0.1 iron atom/subunit. No carbohydrate was detectable. Steady state kinetic results with tetrahydrobiopterin as substrate are consistent with a sequential mechanism for binding of tyrosine and tetrahydrobiopterin. Substrate inhibition occurs at tyrosine concentrations above 50 microM. Steady state kinetic parameters at pH 6.5 are Vmax = 74 min-1, KBH4 = 21 microM, KTyr = 9.4 microM, and Ko2 less than or equal to 6 microM. The Vmax value shows a broad pH optimum around pH 7. The KBH4 value is pH-dependent, increasing from about 20 microM below pH 7 to about 100 microM above pH 8. The KTyr value is independent of pH between pH 6 and pH 8.5.

Primary amino acid sequence of bovine dopamine beta-hydroxylase

Fifty-eight tryptic and Staphylococcus aureus V8 protease generated peptides from bovine dopamine beta-hydroxylase were isolated by reverse-phase high pressure liquid chromatography and sequenced. These peptide sequences were compared with the deduced amino acid sequences of bovine and human dopamine beta-hydroxylase obtained from the cloned cDNAs. Bovine peptide sequences had five differences with the sequence derived from the bovine cDNA, and four of the changes could be accounted for by a single base change in the DNA. N-terminal sequence analysis of the bovine enzyme indicated that it contained two N termini, one of which is 3 amino acids longer than the other and begins with the sequence Ser-Ala-Pro. The amino acid sequences deduced from the bovine and human cDNAs are 19 and 25 amino acids longer, respectively, and these additional amino acids represent leader peptide sequences. Two bovine peptide sequences contained glycosylation sites and gave positive tests for carbohydrate residues, and two others contained the consensus sequence for a glycosylation site but were negative in the carbohydrate test. The bovine enzyme contains 6 Trp, as compared with 7 in the bovine cDNA and 8 in the human cDNA. The protein and bovine cDNA contain 24 Tyr each, as compared with 26 in the human cDNA. These numbers indicate that the true epsilon 1% 280 = 8.95, and, therefore, that it is 28% lower than the previously determined value. The data also identify 5 His-containing regions that may be involved in Cu2+ coordination at the active site.

The metal requirement of rat tyrosine hydroxylase

The effect of added metals on purified rat tyrosine hydroxylase which is predominantly iron-free has been determined. The presence of 10 microM ferrous ammonium sulfate results in a ten-fold increase in the activity of enzyme containing 0.1 iron atom per subunit. The enzyme activity is half-maximal at a free ferrous iron concentration of 0.15 microM. Copper, zinc, silver, and nickel are unable to replace ferrous iron. Ferric iron is inactive unless ascorbate is included to reduce it.

Identification of the rate-limiting step in serine proteinases from the effect of temperature on steady-state kinetics

The effect of temperature on the steady-state kinetics of porcine pancreatic elastase can be used to determine whether acylation or deacylation is the rate-limiting step in catalysis. If acylation is rate-limiting, kcat and kcat/Km will show the same temperature dependence. If deacylation is rate-limiting, kcat will show a greater temperature dependence than kcat/Km. The temperature dependence of the steady-state kinetic parameters of t-Boc-Ala-Ala-Pro-Ala p-nitroanilide and N-acetyl-Ala-Ala-alpha-Aza-Ala p-nitrophenyl ester have been determined and are consistent with this prediction.

Apparatus for the electrical characterisation of conductive fluids

Non-invasive and fully automated conductimetric measurements of electrolyte and bacterial samples were achieved in a closed volume test cell, comprising a magnetic field coil and detector. By monitoring field induced currents in sample electrolytes the magnitude of the sample current was shown to vary as the inverse of the sample impedance. The impedance characteristic was shown to be that of an LCR resonant circuit. This characteristic is primarily a function of the applied frequency and the solution/cell properties being dependent on the solution conductivity and dielectric permittivity at any given concentration. Small changes in sample dielectric permittivity in the presence of a large background conductivity are shown to be significant. The apparatus described can provide fixed or swept frequency conductivity measurements in the range 1 kHz to 2.25 MHz with a lower conductivity sensitivity of 0.9 x 10(-3) Scm-1. Bulk impedimetric characteristics of cell suspensions are derived by a two stage measurement.

The pH dependence of binding of inhibitors to bovine adrenal tyrosine hydroxylase

The inhibition of purified bovine adrenal tyrosine hydroxylase by several product and substrate analogues has been studied to probe the kinetic mechanism. Norepinephrine, dopamine, and methylcatechol are competitive inhibitors versus tetrahydropterins and noncompetitive inhibitors versus tyrosine. 3-Iodotyrosine is an uncompetitive inhibitor versus tetrahydropterins and a competitive inhibitor versus tyrosine. The Ki value for 3-iodotyrosine depends on the tetrahydropterin used. These results are consistent with tetrahydropterin binding first to the free enzyme followed by binding of tyrosine. 5-Deaza-6-methyltetrahydropterin is a noncompetitive inhibitor versus tetrahydropterins and tyrosine. The effect of varying the concentration of tyrosine on the Ki value for 5-deaza-6-methyltetrahydropterin is consistent with the binding of this inhibitor to both the free enzyme and to an enzyme-dihydroxyphenylalanine complex. Dihydroxyphenylalanine also is a noncompetitive inhibitor versus tetrahydropterins and tyrosine; the effect of changing the fixed substrate is consistent with the binding of this inhibitor to both the free enzyme and to the enzyme-tetrahydropterin complex. The effect of pH on the Ki values was determined in order to measure the pKa values of amino acid residues involved in substrate binding. Tight binding of catechols requires that a group with a pKa value of 7.6 be deprotonated. Binding of 3-iodotyrosine involves two groups with pKa values of 7.5 and about 5.5, one of which must be protonated for binding. Binding of 5-deaza-6-methyltetrahydropterin requires that a group on the free enzyme with a pKa value of 6.1 be protonated. The Ki value for dihydroxyphenylalanine is relatively insensitive to pH, but the inhibition pattern changes from noncompetitive to competitive above pH 7.5, consistent with the measured pKa values for binding to the free enzyme and to the enzyme-tetrahydropterin complex.

Mechanism-based inhibitors of dopamine beta-hydroxylase

The copper-containing monooxygenase dopamine beta-hydroxylase catalyzes the hydroxylation of dopamine at the benzylic position to form norepinephrine. Mechanism-based inhibitors for dopamine beta-hydroxylase have been used as probes of the mechanism of catalysis. The variety of such inhibitors that have been developed for this enzyme can be divided into three groups: (i) those in which the inactivating species is formed by abstraction of a hydrogen atom to form a radical intermediate; (ii) those in which the inactivating species is formed by abstraction of an electron to form an epoxide-like intermediate; and (iii) those in which the product is the inactivating species. A mechanism consistent with inactivation by all three groups of inhibitors which proposes that hydroxylation of dopamine by dopamine beta-hydroxylase involves formation of a benzylic radical has been developed. The benzylic radical is formed by abstraction of a hydrogen atom from the substrate by a high-potential copper-oxygen species.

Use of alternate substrates to probe the order of substrate addition to dopamine beta-hydroxylase

In order to determine the order of substrate binding to dopamine beta-hydroxylase during catalysis, the effect of alternate substrates upon kinetic parameters was examined. The V/K value for ascorbate was unchanged when tyramine, phenylpropylamine, p-Cl-phenethylamine, p-CH3O-phenethylamine, or phenethylamine was the hydroxylated substrate. The V/K values for tyramine and oxygen were similarly unchanged when ferrocyanide was used as the reductant in place of ascorbate. In order to use ferrocyanide as reductant it was necessary to include copper to alleviate the substrate inhibition seen with this substrate. The pattern of substrate inhibition observed with ferrocyanide was consistent with a small amount of free cyanide present in the ferrocyanide. With ferrocyanide as reductant and [2,2-2H2]tyramine as substrate, there was a measurable isotope effect on the V/K value for oxygen, but none on the values of Vmax or V/K for tyramine. These results are consistent with a ping-pong mechanism in which tyramine binds to the enzyme after the release of oxidized ascorbate. Subsequently, oxygen binds to form a ternary complex.

The mechanism of inactivation of dopamine beta-hydroxylase by hydrazines

Dopamine beta-hydroxylase is inactivated by phenyl-, phenethyl-, benzyl-, and methylhydrazine, but not by hydrazine itself. With phenyl-, methyl-, and phenethylhydrazine, the rate of inactivation decreases in the presence of ascorbate and increases in the presence of tyramine. Reduction of the enzyme-bound copper occurs with all of the hydrazines tested. In the presence of the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone the carbon-centered radicals generated from each compound are trapped. This is consistent with reduction of the enzyme-bound copper by the hydrazine-containing compounds, resulting in formation of the hydrazine cation radical. Homolytic cleavage of the carbon-nitrogen bond then generates a carbon-centered radical which reacts with the enzyme, resulting in inactivation. Inactivation with [14C]phenylhydrazine results in the incorporation of 0.94 molecule of label per enzyme subunit. Benzylhydrazine behaves as a mechanism-based inhibitor of the enzyme. Both benzyl- and phenethylhydrazine are substrates for dopamine beta-hydroxylase. The second-order rate constant for inactivation of dopamine beta-hydroxylase by benzylhydrazine in the presence of ascorbate is increased about 4-fold when the benzylic hydrogens are replaced with deuterium. The apparent Vmax shows an observed deuterium kinetic isotope effect of 13 +/- 2. The partition ratio for product formation versus inactivation is 11-fold less for alpha,alpha-d2-benzylhydrazine. These results are interpreted in terms of a model where inactivation is due to abstraction of an electron from nitrogen instead of abstraction of a hydrogen atom from the benzylic carbon.

"Morbakka", another cubomedusan

There is a large member of the cubomedusan family, which has not yet been formally identified, that is very similar to Tamoya haplonema. It is present in Indo-Pacific tropical waters but is also present in colder Australian waters. A case of severe envenomation in South Queensland is discussed and its clinical and histological features are described. Vinegar inactivates the undischarged nematocysts of this species and is recommended as the initial first-aid treatment.

8-Azidoflavins as photoaffinity labels for flavoproteins

8-Azidoflavins have been synthesized and their potential as photoaffinity labels for flavoproteins has been explored. They are very photolabile, and in aqueous media they react with solvent to yield 8-aminoflavins and 8-hydroxlaminoflavins as the main products. They fulfill the criteria expected of a good photoaffinity label, since they bind stoichiometrically at the flavin-binding site of flavoproteins, thus minimizing problems of nonspecific labeling. Second, they absorb strongly in the visible, so that the reactive nitrene can be generated without short wavelength light, minimizing the possibility of light-induced damage of the protein. Third, in the absence of light, 8-N3-flavins are stable, permitting a study of their binding to apoproteins. 8-Azidoflavins have been bound to hen egg white riboflavin-binding protein, Megasphera elsdenii flavodoxin, yeast Old Yellow Enzyme, Aspergillus niger, glucose oxidase, and pig kidney D-amino acid oxidase, and the effect of exposure to visible light has been determined. Only small extents of covalent attachment of the flavin to the protein were found with flavodoxin, D-amino acid oxidase, and Old Yellow Enzyme; much more extensive labeling was obtained with glucose oxidase and riboflavin-binding protein. In addition to their photoreactivity, 8-azidoflavins have been found to be converted to 8-aminoflavins by reaction with sulfite or upon reduction. Similar reactions occur with 8-hydroxylamino-, 8-(O-methyl)hydroxylamino-, and 8-hydrazinoflavins, which serve as models for possible flavin-protein covalent linkages which could be formed in the photolabeling procedure. Some of the properties of these flavins, which were obtained by reaction of 8-F-flavin with the corresponding nucleophiles, are also described.

3-Phenylpropenes as mechanism-based inhibitors of dopamine beta-hydroxylase: evidence for a radical mechanism

A series of ring-substituted 3-phenylpropenes has been examined as mechanism-based inhibitors for the copper protein dopamine beta-hydroxylase. p-HO-, p-CH3O-, m-HO-, m-CH3O-, p-Br-, and p-CN-substituted phenylpropenes all inactivate the enzyme under turnover conditions, requiring ascorbate and oxygen. Replacement of the benzylic hydrogens in 3-(p-hydroxyphenyl)propene with deuterium results in a kinetic isotope effect of 2.0 on kinact/KO2 but in no effect on the partition ratio, Vmax/kinact, consistent with a stepwise mechanism for hydrogen abstraction and oxygen insertion. The partition ratio is unchanged in the pH range from 4.5 to 7.1. Determination of the kinetics of inactivation and the partition ratios for each of these ring-substituted phenylpropenes has allowed determination of the respective V/KO2 values. A linear free energy plot of these values as a function of sigma+ gives a rho value of -1.2, while the partition ratios show only a slight decrease upon going electron-withdrawing groups. The results are consistent with a mechanism for dopamine beta-hydroxylase in which a hydrogen atom is abstracted to form a benzylic radical, which then partitions between hydroxylation and enzyme inactivation.

Synthesis of several 2-substituted 3-(p-hydroxyphenyl)-1-propenes and their characterization as mechanism-based inhibitors of dopamine beta-hydroxylase

Three substrate analogs of dopamine beta-hydroxylase, viz. 2-X-3-(p-hydroxyphenyl)-1- propenes (where X = Br, Cl, H), have been synthesized, and all behave as substrates requiring O2 and ascorbate for the enzyme-catalyzed hydroxylation reaction. The products have been characterized by mass spectrometry as the respective 2-X-3-hydroxy-3-(p-hydroxyphenyl)-1- propenes . The relative kcat values for these compounds at pH 5.5, 0.25 mM O2 are 49 min-1 (2-H), 8.6 min-1 (2-Cl), and 7.0 min-1 (2-Br). All three compounds have the characteristics of mechanism-based inhibitors of dopamine beta-hydroxylase since incubation of enzyme with these compounds under turnover conditions leads to a time-dependent loss of activity. The kinact values at pH 5.5, 0.25 mM O2 are 0.08, 0.20, and 0.51 min-1, respectively, for the 2-Br-, 2-Cl-, and 2-H-substituted analogs. No reactivation was observed after exhaustive dialysis of enzyme inactivated by 2-Br-3-(p-hydroxyphenyl)-1-propene, suggesting irreversible inactivation of dopamine beta-hydroxylase.

The reaction of 8-mercaptoflavins and flavoproteins with sulfite. Evidence for the role of an active site arginine in D-amino acid oxidase

This work presents strong evidence that the role of the active site arginine in D-amino acid oxidase is to act as a positively charged group interacting with the flavin N(1)-C(2) = 0 locus. Modification with cyclohexanedione, which has been shown previously to modify specifically an active site arginine in D-amino acid oxidase (Ferti, C., Curti, B., Simonetta, M. P., Ronchi, S., Galliano, M., and Minchiotti, L. (1981) Eur. J. Biochem. 119, 553-557) destroys the ability of D-amino acid oxidase to stabilize the benzoquinoid type spectrum of 8-mercapto-FAD and destroys the ability to form a flavin N-5 adduct with sulfite. Both of these properties have been attributed to the presence of such a group. The active site lysine, histidine, and tyrosine have been ruled out as possibilities for such a group. In addition, the reactivity of flavoproteins containing 8-mercaptoflavin with sulfite has been examined and falls into the same two general classes as the reactivity of the native flavoproteins: oxidases form N-5 adducts while all of the other 8-mercaptoflavoproteins examined do not, forming instead the 8-sulfonate flavin.

The kinetic mechanism of D-amino acid oxidase with D-alpha-aminobutyrate as substrate. Effect of enzyme concentration on the kinetics

The kinetic mechanism of hog kidney D-amino acid oxidase with D-alpha-aminobutyrate as substrate has been examined in detail using a combination of steady state and rapid reaction methods. At concentrations of D-alpha-aminobutyrate below 0.5 mM, the rapid reaction and steady state results are consistent with the mechanism previously proposed for D-alanine (Massey, V., and Gibson, Q. H. (1964) Fed. Proc. 23, 18-29; Porter, D. J. T., Voet, J. G., and Bright, H. J. (1977) J. Biol. Chem. 252, 4464-4473). Both flavin reduction by D-alpha-aminobutyrate and reoxidation are quite rapid. Release of product from the oxidized enzyme has been measured directly and matches the turnover number at infinite concentrations of both substrates. Substitution of deuterium for the alpha-hydrogen decreases the rate of reduction 1.4-fold, without any effect on the apparent Kd. Computer simulations show that the kinetic isotope effects on the reductive half-reaction with D-alanine reported by Porter et al. (see above reference) can be explained using a two-step model with a kinetic isotope effect of 1.75 on the limiting rate of reduction. The effect of enzyme concentration on the kinetics has been examined in some detail. With D-alanine as substrate, increasing the enzyme concentration over the range 29 nM to 17 microM resulted in less than a 2-fold decrease in the turnover number. The Kd for benzoate binding also decreased marginally with increasing enzyme concentration. The effect of enzyme concentration is consistent with a decrease in the rate of release of ligands from the oxidized enzyme as the enzyme concentration is increased.

Proton release during the reductive half-reaction of D-amino acid oxidase

Changes in the net protonation of D-amino acid oxidase during binding of competitive inhibitors and during reduction by amino acids have been monitored using phenol red as a pH indicator. At pH 8.0, no uptake or release of protons from solution occurs upon binding the inhibitors benzoate, anthranilate, picolinate, or L-leucine. The Kd values for both picolinate and anthranilate were determined from pH 5.4 to 9.0. The results are consistent with a single group on the enzyme having a pK of 6.3 which must be unprotonated for tight binding, as is the case with benzoate binding (Quay, S., and Massey, V. (1977) Biochemistry 16, 3348-3354) and with tight binding of the inhibitor form with an unprotonated amino group. Upon reduction of the enzyme by amino acid substrates, two protons are released to solution. The first is released concomitantly with reduction to the reduced enzyme-imino acid charge transfer complex. The second is released only upon dissociation of the charge transfer complex to free reduced enzyme and imino acid. The first proton is assigned as arising from the amino acid group and the second from the amino acid alpha-hydrogen. These results are consistent with the flavin in reduced D-amino acid oxidase being anionic.

Thiazolidine-2-carboxylic acid, an adduct of cysteamine and glyoxylate, as a substrate for D-amino acid oxidase

A mixture of cysteamine and glyoxylate, proposed by Hamilton et al. to form the physiological substrate of hog kidney D-amino acid oxidase (Hamilton, G. A., Buckthal, D. J., Mortensen, R. M., and Zerby, K. W. (1979) Proc. Natl. Acad. Sci. U. S. A. 76, 2625-2629), was confirmed to act as a good substrate for the pure enzyme. As proposed by those workers, it was shown that the actual substrate is thiazolidine-2-carboxylic acid, formed from cysteamine and glyoxylate with a second order rate constant of 84 min-1 M-1 at 37 degrees C, pH 7.5. Steady state kinetic analyses reveal that thiazolidine-2-carboxylic acid is a better substrate at pH 8.5 than at pH 7.5. At both pH values, the catalytic turnover number is similar to that obtained with D-proline. D-Amino acid oxidase is rapidly reduced by thiazolidine-2-carboxylic acid to form a reduced enzyme-imino acid complex, as is typical with D-amino acid oxidase substrates. The product of oxidation was shown by NMR to be delta 2-thiazoline-2-carboxylic acid. Racemic thiazolidine-2-carboxylic acid is completely oxidized by the enzyme. The directly measured rate of isomerization of L-thiazolidine-2-carboxylic acid to the D-isomer was compared to the rate of oxidation of the L-isomer by D-amino acid oxidase. Their identity over the range of temperature from 2-30 degrees C established that the apparent activity with the L-amino acid can be explained quantitatively by the rapid, prior isomerization to D-thiazolidine-2-carboxylic acid.

Half-body radiotherapy of advanced cancer

Half-body radiotherapy to deliver doses up to 1000 rads in a single exposure evolved to meet a need in treating patients with advanced cancer. Large fields are required and a high radiation output. Treatments are well tolerated, although radiation sickness occurs in 80% of upper half and 33% of lower half-body treatments. Marrow depression is not a problem and the second half body can be treated when the peripheral blood count has returned to normal. Subjective improvement occurs in most patients and this group had a median survival of six months.

Bird-fancier's lung and jejunal villous atrophy

Sixteen patients with bird-fancier's lung were screened for evidence of coeliac disease by assessing their clinical features, red-bloodcell or serum folate levels, and serum for reticulin antibodies. Five of nine patients selected for jejunal biopsy showed villous atrophy, and in some this seemed to be a true gluten-sensitive enteropathy.