Peroxidase self-inactivation in prostaglandin H synthase-1 pretreated with cyclooxygenase inhibitors or substituted with mangano protoporphyrin IX

Self-inactivation imposes an upper limit on bioactive prostanoid synthesis by prostaglandin H synthase (PGHS). Inactivation of PGHS peroxidase activity has been found to begin with Intermediate II, which contains a tyrosyl radical. The structure of this radical is altered by cyclooxygenase inhibitors, such as indomethacin and flurbiprofen, and by replacement of heme by manganese protoporphyrin IX (forming MnPGHS-1). Peroxidase self-inactivation in inhibitor-treated PGHS-1 and MnPGHS-1 was characterized by stopped-flow spectroscopic techniques and by chromatographic and mass spectrometric analysis of the metalloporphyrin. The rate of peroxidase inactivation was about 0.3 s(-)1 in inhibitor-treated PGHS-1 and much slower in MnPGHS-1 (0.05 s(-)1); as with PGHS-1 itself, the peroxidase inactivation rates were independent of peroxide concentration and structure, consistent with an inactivation process beginning with Intermediate II. The changes in metalloporphyrin absorbance spectra during inactivation of inhibitor-treated PGHS-1 were similar to those observed with PGHS-1 but were rather distinct in MnPGHS-1; the kinetics of the spectral transition from Intermediate II to the next species were comparable to the inactivation kinetics in each case. In contrast to the situation with PGHS-1 itself, significant amounts of heme degradation occurred during inactivation of inhibitor-treated PGHS-1, producing iron chlorin and heme-protein adduct species. Structural perturbations at the peroxidase site (MnPGHS-1) or at the cyclooxygenase site (inhibitor-treated PGHS-1) thus can influence markedly the kinetics and the chemistry of PGHS-1 peroxidase inactivation.

Substrate binding is the rate-limiting step in thromboxane synthase catalysis

Thromboxane synthase (TXAS) is a "non-classical" cytochrome P450. Without any need for an external electron donor, or for a reductase or molecular oxygen, it uses prostaglandin H2 (PGH2) to catalyze either an isomerization reaction to form thromboxane A2 (TXA2) or a fragmentation reaction to form 12-l-hydroxy-5,8,10-heptadecatrienoic acid and malondialdehyde (MDA) at a ratio of 1:1:1 (TXA2:heptadecatrienoic acid:MDA). We report here kinetics of TXAS with heme ligands in binding study and with PGH2 in enzymatic study. We determined that 1) binding of U44069, an oxygen-based ligand, is a two-step process; U44069 first binds TXAS, then ligates the heme-iron with a maximal rate constant of 105-130 s(-1); 2) binding of cyanide, a carbon-based ligand, is a one-step process with k(on) of 2.4 M(-1) s(-1) and k(off) of 0.112 s(-1); and 3) both imidazole and clotrimazole (nitrogen-based ligands) bind TXAS in a two-step process; an initial binding to the heme-iron with on-rate constants of 8.4 x 10(4) M(-1) s(-1) and 1.5 x 10(5) M(-1) s(-1) for imidazole and clotrimazole, respectively, followed by a slow conformational change with off-rate constants of 8.8 s(-1) and 0.53 s(-1), respectively. The results of our binding study indicate that the TXAS active site is hydrophobic and spacious. In addition, steady-state kinetic study revealed that TXAS consumed PGH2 at a rate of 3,800 min(-1) and that the k(cat)/K(m) for PGH2 consumption was 3 x 10(6) M(-1) s(-1). Based on these data, TXAS appears to be a very efficient catalyst. Surprisingly, rapid-scan stopped-flow experiments revealed marginal absorbance changes upon mixing TXAS with PGH2, indicating minimal accumulation of any heme-derived intermediates. Freeze-quench EPR measurements for the same reaction showed minimal change of heme redox state. Further kinetic analysis using a combination of rapid-mixing chemical quench and computer simulation showed that the kinetic parameters of TXAS-catalyzed reaction are: PGH2 bound TXAS at a rate of 1.2-2.0 x 10(7) M(-1) s(-1); the rate of catalytic conversion of PGH2 to TXA2 or MDA was at least 15,000 s(-1) and the lower limit of the rates for products release was 4,000-6,000 s(-1). Given that the cellular PGH2 concentration is quite low, we concluded that under physiological conditions, the substrate-binding step is the rate-limiting step of the TXAS-catalyzed reaction, in sharp contrast with "classical" P450 enzymes.

Identification of thromboxane synthase amino acid residues involved in heme-propionate binding

Thromboxane A2 synthase (TXAS) is a member of the cytochrome P450 superfamily and catalyzes an isomerization reaction that converts prostaglandin H2 to thromboxane A2. As a step toward understanding the structure/function relationships of TXAS, we mutated amino acid residues predicted to bind the propionate groups of A- and D-pyrrole rings of the heme. These mutations at each of these residues (Asn-110, Trp-133, Arg-137, Arg-413, and Arg-478) resulted in altered heme binding, as evidenced by perturbation of the absorption spectra and EPR. The mutations, although causing no significant changes in the secondary structure of the proteins, induced tertiary structural changes that led to increased susceptibility to trypsin digestion and alteration of the intrinsic protein fluorescence. Moreover, these mutant proteins lost their binding affinity to the substrate analog, had a lower heme content and retained less than 5% of the wild-type catalytic activity. However, mutations at the neighboring amino acid of the aforementioned residues yielded mutant proteins retaining the biochemical and biophysical properties of the wild type TXAS. Aligning the TXAS sequence with the structurally known P450s, we proposed that in TXAS the A-ring propionate of the heme is hydrogen bonded to Asn-110, Arg-413, and Arg-478, whereas D-ring propionate is hydrogen bonded to Trp-133 and Arg-137. Furthermore, both A- and D-ring propionates bulge away from the heme plane and both lie on the proximal face of heme plane, a structure similar to P450terp.

Resonance Raman studies indicate a unique heme active site in prostaglandin H synthase

Prostaglandin H synthase isoforms 1 and 2 (PGHS-1 and -2) catalyze the first two steps in the biosynthesis of prostaglandins. Resonance Raman spectroscopy was used to characterize the PGHS heme active site and its immediate environment. Ferric PGHS-1 has a predominant six-coordinate high-spin heme at room temperature, with water as the sixth ligand. The proximal histidine ligand (or the distal water ligand) of this hexacoordinate high-spin heme species was reversibly photolabile, leading to a pentacoordinate high-spin ferric heme iron. Ferrous PGHS-1 has a single species of five-coordinate high-spin heme, as evident from nu(2) at 1558 cm(-1) and nu(3) at 1471 cm(-1). nu(4) at 1359 cm(-1) indicates that histidine is the proximal ligand. A weak band at 226-228 cm(-1) was tentatively assigned as the Fe-His stretching vibration. Cyanoferric PGHS-1 exhibited a nu(Fe)(-)(CN) line at 446 cm(-1) and delta(Fe)(-)(C)(-)(N) at 410 cm(-1), indicating a "linear" Fe-C-N binding conformation with the proximal histidine. This linkage agrees well with the open distal heme pocket in PGHS-1. The ferrous PGHS-1 CO complex exhibited three important marker lines: nu(Fe)(-)(CO) (531 cm(-1)), delta(Fe)(-)(C)(-)(O) (567 cm(-1)), and nu(C)(-)(O) (1954 cm(-1)). No hydrogen bonding was detected for the heme-bound CO in PGHS-1. These frequencies markedly deviated from the nu(Fe)(-)(CO)/nu(C)(-)(O) correlation curve for heme proteins and porphyrins with a proximal histidine or imidazolate, suggesting an extremely weak bond between the heme iron and the proximal histidine in PGHS-1. At alkaline pH, PGHS-1 is converted to a second CO binding conformation (nu(Fe)(-)(CO): 496 cm(-1)) where disruption of the hydrogen bonding interactions to the proximal histidine may occur.

Characterization of interactions among the heme center, tetrahydrobiopterin, and L-arginine binding sites of ferric eNOS using imidazole, cyanide, and nitric oxide as probes

Endothelial nitric oxide synthase (eNOS) is a self-sufficient P450-like enzyme. A P450 reductase domain is tethered to an oxygenase domain containing the heme, the substrate (L-arginine) binding site, and a cofactor, tetrahydrobiopterin (BH(4)). This "triad", located at the distal heme pocket, is the center of oxygen activation and enzyme catalysis. To probe the relationships among these three components, we examined the binding kinetics of three different small heme ligands in the presence and absence of either L-arginine, BH(4), or both. Imidazole binding was strictly competitive with L-arginine, indicating a domain overlap. BH(4) had no obvious effect on imidazole binding but slightly increased the k(on) for L-arginine. L-Arginine decreased the k(on) and k(off) for cyanide by two orders, indicating a "kinetic obstruction" mechanism. BH(4) slightly enhanced cyanide binding. Nitric oxide (NO) binding kinetics were more complex. Increasing the L-arginine concentration decreased the NO binding affinity at equilibrium. In both BH(4)-abundant and BH(4)-deficient eNOS, half of the NO binding sites showed a sizable decrease of the binding rate by L-arginine, with the rate of NO binding at the other half of the sites remaining essentially unaltered by L-arginine, implying that the two heme centers in the eNOS dimer are functionally distinct.

Geminate recombination of nitric oxide to endothelial nitric-oxide synthase and mechanistic implications

The nitric-oxide synthase (NOS) catalyzes the oxidation of L-arginine to L-citrulline and NO through consumption of oxygen bound to the heme. Because NO is produced close to the heme and may bind to it, its subsequent role in a regulatory mechanism should be scrutinized. We therefore examined the kinetics of NO rebinding after photodissociation in the heme pocket of human endothelial NOS by means of time-resolved absorption spectroscopy. We show that geminate recombination of NO indeed occurs and that this process is strongly modulated by L-Arg. This NO rebinding occurs in a multiphasic fashion and spans over 3 orders of magnitude. In both ferric and ferrous states of the heme, a fast nonexponential picosecond geminate rebinding first takes place followed by a slower nanosecond phase. The rates of both phases decreased, whereas their relative amplitudes are changed by the presence of L-Arg; the overall effect is a slow down of NO rebinding. For the isolated oxygenase domain, the picosecond rate is unchanged, but the relative amplitude of the nanosecond binding decreased. We assigned the nanosecond kinetic component to the rebinding of NO that is still located in the protein core but not in the heme pocket. The implications for a mechanism of regulation involving NO binding are discussed.

Comparison of the peroxidase reaction kinetics of prostaglandin H synthase-1 and -2

Prostaglandin H synthase isoforms 1 and 2 (PGHS-1 and -2) each have a peroxidase activity and also a cyclooxygenase activity that requires initiation by hydroperoxide. The hydroperoxide initiator requirement for PGHS-2 cyclooxygenase is about 10-fold lower than for PGHS-1 cyclooxygenase, and this difference may contribute to the distinct control of cellular prostanoid synthesis by the two isoforms. We compared the kinetics of the initial peroxidase steps in PGHS-1 and -2 to quantify mechanistic differences between the isoforms that might contribute to the difference in cyclooxygenase initiation efficiency. The kinetics of formation of Intermediate I (an Fe(IV) species with a porphyrin free radical) and Intermediate II (an Fe(IV) species with a tyrosyl free radical, thought to be the crucial oxidant in cyclooxygenase catalysis) were monitored at 4 degrees c by stopped flow spectrophotometry with several hydroperoxides as substrate. With 15-hydroperoxyeicosatetraenoic acid, the rate constant for Intermediate I formation (k1) was 2.3 x 10(7) M-1 s-1 for PGHS-1 and 2.5 x 10(7) M-1 s-1 for PGHS-2, indicating that the isoforms have similar initial reactivity with this lipid hydroperoxide. For PGHS-1, the rate of conversion of Intermediate I to Intermediate II (k2) became the limiting factor when the hydroperoxide level was increased, indicating a rate constant of 10(2)-10(3) s-1 for the generation of the active cyclooxygenase species. For PGHS-2, however, the transition between Intermediates I and II was not rate-limiting even at the highest hydroperoxide concentrations tested, indicating that the k2 value for PGHS-2 was much greater than that for PGHS-1. Computer modelling predicted that faster formation of the active cyclooxygenase species (Intermediate II) or increased stability of the active species increases the resistance of the cyclooxygenase to inhibition by the intracellular hydroperoxide scavenger, glutathione peroxidase. Kinetic differences between the PGHS isoforms in forming or stabilizing the active cyclooxygenase species can thus contribute to the difference in the regulation of their cellular activities.

A mechanistic study of self-inactivation of the peroxidase activity in prostaglandin H synthase-1

Prostaglandin H synthase (PGHS) is a self-activating and self-inactivating enzyme. Both the peroxidase and cyclooxygenase activities have a limited number of catalytic turnovers. Sequential stopped-flow measurements were used to analyze the kinetics of PGHS-1 peroxidase self-inactivation during reaction with several different hydroperoxides. The inactivation followed single exponential kinetics, with a first-order rate constant of 0.2-0.5 s-1 at 24 degrees C. This rate was independent of the peroxide species and concentration used, strongly suggesting that the self-inactivation process originates after formation of Compound I and probably with Intermediate II, which contains an oxyferryl heme and a tyrosyl radical. Kinetic scan and rapid scan experiments were used to monitor the heme changes during the inactivation process. The results from both experiments converged to a simple, linear, two-step mechanism in which Intermediate II is first converted in a faster step (0.5-2 s-1) to a new compound, Intermediate III, which undergoes a subsequent slower (0.01-0.05 s-1) transition to a terminal species. Rapid-quench and high pressure liquid chromatography analysis indicated that Intermediate III likely retains an intact heme group that is not covalently linked with the PGHS-1 protein.

Expression, purification, and spectroscopic characterization of human thromboxane synthase

Thromboxane A2 (TXA2) is a potent inducer of vasoconstriction and platelet aggregation. Large scale expression of TXA2 synthase (TXAS) is very useful for studies of the reaction mechanism, structural/functional relationships, and drug interactions. We report here a heterologous system for overexpression of human TXAS. The TXAS cDNA was modified by replacing the sequence encoding the first 28 amino acid residues with a CYP17 amino-terminal sequence and by adding a polyhistidine tag sequence prior to the stop codon; the cDNA was inserted into the pCW vector and co-expressed with chaperonins groES and groEL in Escherichia coli. The resulting recombinant protein was purified to electrophoretic homogeneity by affinity, ion exchange, and hydrophobic chromatography. UV-visible absorbance (UV-Vis), magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectra indicate that TXAS has a typical low spin cytochrome P450 heme with an oxygen-based distal ligand. The UV-Vis and EPR spectra of recombinant TXAS were essentially identical to those of TXAS isolated from human platelets, except that a more homogenous EPR spectrum was observed for the recombinant TXAS. The recombinant protein had a heme:protein molar ratio of 0.7:1 and a specific activity of 12 micromol of TXA2/min/mg of protein at 23 degreesC. Furthermore, it catalyzed formation of TXA2, 12-hydroxy-5,8,10-heptadecatrienoic acid, and malondialdehyde in a molar ratio of 0.94:1.0:0.93. Spectral binding titrations showed that bulky heme ligands such as clotrimazole bound strongly to TXAS (Kd approximately 0.5 microM), indicating ample space at the distal face of the heme iron. Analysis of MCD and EPR spectra showed that TXAS was a typical low spin hemoprotein with a proximal thiolate ligand and had a very hydrophobic distal ligand binding domain.

Effects of Asp-369 and Arg-372 mutations on heme environment and function in human endothelial nitric-oxide synthase

Eight polar amino acid residues in the putative substrate-binding region from Thr-360 to Val-379 in human endothelial nitric-oxide synthase (eNOS) (Thr-360, Arg-365, Cys-368, Asp-369, Arg-372, Tyr-373, Glu-377, and Asp-378) were individually mutated. Only two of these residues, Asp-369 and Arg-372, were found to be essential for enzyme activity. A further series of mutants was generated by replacing these two residues with various amino acids and the mutant proteins were expressed in a baculovirus system. Mutant eNOS had a very low L-citrulline formation activity with the exception of D369E and R372K, which retained 27% and 44% of the wild-type enzyme activity, respectively. Unlike the wild-type enzyme, all mutants except D369E, R372K, and R372M had a low spin heme (Soret peak at 416 nm). All the Asp-369 mutants had higher Kd values for L-arginine (1-10 mM) than wild-type eNOS (0.4 microM) and an unstable heme-CO complex, and except for D369E, had a very low (6R)-5,6,7, 8-tetrahydro-L-biopterin (BH4) content. In contrast, each of Arg-372 mutants retained a considerable amount of BH4, had a moderate reduction in L-arginine affinity, and had a more stable heme-CO complex. 1-Phenylimidazole did not bind to wild-type eNOS heme, but bound to all Asp-369 and Arg-372 mutants (Kd ranged from 10 to 65 microM) except R372K. Heme spin-state changes caused by binding of 3, 5-lutidine appeared to depend on both charge and size of the side chains of residues 369 and 372. Furthermore, all Asp-369 and Arg-372 mutants were defective in dimer formation. These results suggest that residues Asp-369 and Arg-372 in eNOS play a critical role in oxygenase domain active-site structure and activity.

Dominant role of local dipolar interactions in phosphate binding to a receptor cleft with an electronegative charge surface: equilibrium, kinetic, and crystallographic studies

Stringent specificity and complementarity between the receptor, a periplasmic phosphate-binding protein (PBP) with a two-domain structure, and the completely buried and dehydrated phosphate are achieved by hydrogen bonding or dipolar interactions. We recently found that the surface charge potential of the cleft between the two domains that contains the anion binding site is intensely electronegative. This novel finding prompted the study reported here of the effect of ionic strength on the equilibrium and rapid kinetics of phosphate binding. To facilitate this study, Ala197, located on the edge of the cleft, was replaced by a Trp residue (A197W PBP) to generate a fluorescence reporter group. The A197W PBP-phosphate complex retains wild-type Kd and X-ray structure beyond the replacement residue. The Kd (0.18 microM) at no salt is increased by 20-fold at greater than 0.30 M NaCl. Stopped-flow fluorescence kinetic studies indicate a two-step binding process: (1) The phosphate (L) binds, at near diffusion-controlled rate, to the open cleft form (Po) of PBP to produce an intermediate, PoL. This rate decreases with increasing ionic strength. (2) The intermediate isomerizes to the closed-conformation form, PcL. The results indicate that the high specificity, affinity, and rate of phosphate binding are not influenced by the noncomplementary electronegative surface potential of the cleft. That binding depends almost entirely on local dipolar interactions with the receptor has important ramification in electrostatic interactions in protein structures and in ligand recognition.

An improved sample packing device for rapid freeze-trap electron paramagnetic resonance spectroscopy kinetic measurements

A new method has been developed for sample packing in rapid freeze-quench electron paramagnetic resonance spectroscopy (EPR) kinetic experiments. Sample particles freeze-quenched in chilled isopentane are filtered under pressure through a stainless steel funnel attached to an EPR tube fitted with a porous disk at its bottom. Isopentane exits through the porous disk and the sample particles can be transferred essentially quantitatively into the receiving EPR tube. This device provides a more predictable, reproducible, and time-saving method for sample packing, enables use of a wider range of flow velocity, and allows efficient use of valuable reactants.

Superoxide generation from endothelial nitric-oxide synthase. A Ca2+/calmodulin-dependent and tetrahydrobiopterin regulatory process

It has been previously shown that besides synthesizing nitric oxide (NO), neuronal and inducible NO synthase (NOS) generates superoxide (O-2) under conditions of L-arginine depletion. However, there is controversy regarding whether endothelial NOS (eNOS) can also produce O-2. Moreover, the mechanism and control of this process are not fully understood. Therefore, we performed electron paramagnetic resonance spin-trapping experiments to directly measure and characterize the O-2 generation from purified eNOS. With the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), prominent signals of O-2 adduct, DMPO-OOH, were detected from eNOS in the absence of added tetrahydrobiopterin (BH4), and these were quenched by superoxide dismutase. This O-2 formation required Ca2+/calmodulin and was blocked by the specific NOS inhibitor N-nitro-L-arginine methyl ester (L-NAME) but not its non-inhibitory enantiomer D-NAME. A parallel process of Ca2+/calmodulin-dependent NADPH oxidation was observed which was also inhibited by L-NAME but not D-NAME. Pretreatment of the enzyme with the heme blockers cyanide or imidazole also prevented O-2 generation. BH4 exerted dose-dependent inhibition of the O-2 signals generated by eNOS. Conversely, in the absence of BH4 L-arginine did not decrease this O-2 generation. Thus, eNOS can also catalyze O-2 formation, and this appears to occur primarily at the heme center of its oxygenase domain. O-2 synthesis from eNOS requires Ca2+/calmodulin and is primarily regulated by BH4 rather than L-arginine.

A mechanism for calmodulin (CaM) trapping by CaM-kinase II defined by a family of CaM-binding peptides

Autophosphorylation of Ca2+/calmodulin (CaM)-dependent protein kinase II (CaM-kinase II) induces a striking >1,000-fold increase in its affinity for CaM, which has been called CaM trapping. Two peptides modeled after the CaM binding domain of CaM-kinase II were previously shown to kinetically resemble CaM binding to phosphorylated and dephosphorylated forms of the enzyme, thus providing a model system with which to define the molecular basis of CaM trapping. In this report, the specific contribution of each amino acid to the rates of association and dissociation, and the overall Kd of CaM binding to CaM-kinase II was determined using an overlapping peptide family, and a fluorescently labeled CaM. The association rate constants were similar for the entire family of peptides and ranged from 8 x 10(7) to 32 x 10(7) M-1 s-1. In contrast, the dissociation rate constants for the peptides varied by >3500-fold and ranged from 0.26 to 7 x 10(-5) s-1. These rate constants yield overall Kd values for binding CaM to the peptides that range from 2 x 10(-9) M to 2 x 10(-13) M. Extending the low affinity CaM-binding peptide, CKII(296-312), to include 293Phe-Asn-Ala295 provided the single largest contribution to the decreased dissociation rate constant, 1,300-fold. It was further shown using Ala-substituted peptides that the basic residues 296Arg-Arg-Lys299 were also essential for slow CaM dissociation; however, their contribution was realized only when 293Phe-Asn-Ala295 were present. These results suggest a plausible model in which autophosphorylation of CaM-kinase II leads to a conformational change in the region of 293Phe-Asn-Ala295 which makes these residues accessible for binding to CaM. As a consequence of these changes, further CaM contacts with 296Arg-Arg-Lys299 are established leading to high affinity CaM binding or "CaM trapping."

Effects of various imidazole ligands on heme conformation in endothelial nitric oxide synthase

We have evaluated the influence of a series of substituted imidazoles on the heme structure of endothelial nitric oxide synthase (eNOS). Optical, MCD, and EPR spectra reveal widely differing effects on heme spin state and geometry. 1-Substituted imidazoles always yield low-spin heme complexes, but the size of the 2- and 4-substituent influences their structural effects on the heme. Methyl substituents lead to low-spin complexes while the bulky phenyl group yields high-spin complexes. The only exception to this behavior is provided by 2-aminoimidazole. Although this compound has three functional groups which can serve as an axial ligand to the heme, its binding to eNOS leads to a pure high-spin complex. This result can only be interpreted as due to a direct binding of 2-aminoimidazole to the guanidine binding subdomain of L-arginine. MCD spectra also imply that an O-ligand is present in the low-spin resting eNOS, while EPR data reveal the presence of two low-spin heme complexes in resting eNOS and its imidazole complexes. EPR also distinguishes four different high-spin forms of eNOS generated by different imidazole analogues. This series of ligands promises to be useful in probing the subtle structural difference among the active sites of three NOS isozymes and in developing selective inhibitors to these important enzymes.

Stoichiometry of the interaction of prostaglandin H synthase with substrates

Prostaglandin H synthase (PGHS) catalyzes both peroxidase and cyclooxygenase reactions. Resolution of several current issues regarding the PGHS catalytic mechanism hinges on the stoichiometry of the reaction of PGHS with hydroperoxide, fatty acid, and oxygen. The dependence of wide-doublet tyrosyl radical accumulation in PGHS isoform 1 on hydroperoxide stoichiometry, has been determined; this catalytically active radical is formed efficiently at stoichiometries </=1 after only 300 ms of reaction. This is consistent with intramolecular formation of the radical from PGHS Compound I but inconsistent with an alternative pathway involving reduction of Compound I to Compound II by a second hydroperoxide molecule. Results from stopped-flow studies indicate that the hydroperoxide level influences the rate of Compound II formation indirectly, via changes in the transient accumulation of Compound I, rather than by reducing Compound I. PGHS and soybean lipoxygenase reactions with 11,14-eicosadienoic acid (20:2) were also analyzed using a spectrophotometer cuvette fitted with an oxygen electrode to monitor lipid product formation and oxygen consumption simultaneously. The results show that the oxygen electrode signal is inherently dampened and thus underestimates the oxygen consumption rate; the discrepancy is much larger for the more rapidly accelerating PGHS reaction than for the lipoxygenase reaction. When correction is made for the electrode dampening, the ratio between the peak rates of oxygen consumption and lipid product formation was near unity for both PGHS and lipoxygenase, indicating a reaction stoichiometry of about 1 mol of O2 consumed/mol of 20:2 oxygenated for both enzymes. Separately, a stoichiometry of 0.9 mol of O2 consumed / mol oxygenated fatty acid was obtained when limiting amounts of 20:2 were reacted to completion with excess PGHS; the corresponding stoichiometry with arachidonic acid was 1.9. These O2/fatty acid stoichiometries are consistent with a dioxygenase mechanism for reaction of PGHS with both fatty acids and inconsistent with a mixed dioxygenase/monooxygenase mechanism proposed for the reaction with 20:2. The present conclusions reduce the complexity of the mechanisms that need to be considered for PGHS catalysis.

Prediction of bleeding diathesis in patients undergoing cardiopulmonary bypass during cardiac surgery: viscoelastic measures versus routine coagulation test

BACKGROUND

Severe hemorrhagic tendency often complicates cardiopulmonary bypass (CPB) in cardiac surgery. In this study, we compared the effectiveness of thromboelastography (TEG), Sonoclot (SCT), and routine coagulation test (RCT) in the prediction of coagulation defects.

METHODS

Forty-three patients undergoing cardiac surgery with CPB were included. Blood for RCT, TEG, and SCT profiles was sampled before systemic heparinization and after protamine administration. Clinically significant bleeding was defined as chest tube drainage in excess of 100 ml/h for 3 consecutive hours or 300 ml/h in 1 h. All coagulation parameters obtained before and after CPB were compared. The sensitivity, specificity, accuracy, false positive, and false negative rate were also calculated and compared.

RESULTS

All coagulation tests were within normal range except higher partial thromboplastin time. Variables which were significantly different from those before CPB included platelet count, fibrinogen level, prothrombin time, and thrombin time in RCT, alpha angle and maximum amplitude in TEG, and R2 and peak time in SCT. In the TEG tracing, all variables had high sensitivity, specificity, and accuracy (average 85.4%, 83%, and 83.5% respectively) and low false positive and negative rate (12.5% and 5% respectively). Although SCT had high sensitivity (76.3%) and low false negative rate (6.5%), its specificity and accuracy were all under 50%.

CONCLUSIONS

Our data demonstrated that the TEG monitoring is a useful tool for detecting post-CPB bleeding diathesis and can provide much predictive information. RCT and SCT are of limited value because of higher rate of unreliable results.

Mutation of Glu-361 in human endothelial nitric-oxide synthase selectively abolishes L-arginine binding without perturbing the behavior of heme and other redox centers

Nitric oxide (NO) and L-citrulline are formed from the oxidation of L-arginine by three different isoforms of NO synthase (NOS). Defining amino acid residues responsible for L-arginine binding and oxidation is a primary step toward a detailed understanding of the NOS reaction mechanisms and designing strategies for the selective inhibition of the individual isoform. We have altered Glu-361 in human endothelial NOS to Gln or Leu by site-directed mutagenesis and found that these mutations resulted in a complete loss of L-citrulline formation without disruption of the cytochrome c reductase and NADPH oxidase activities. Optical and EPR spectroscopic studies demonstrated that the Glu-361 mutants had similar spectra either in resting state or reduced CO-complex as the wild type. The heme ligand, imidazole, could induce a low spin state in both wild-type and Glu-361 mutants. However, unlike the wild-type enzyme, the low spin imidazole complex of Glu-361 mutants was not reversed to a high spin state by addition of either L-arginine, acetylguanidine, or 2-aminothiazole. Direct L-arginine binding could not be detected in the mutants either. These results strongly indicate that Glu-361 in human endothelial NOS is specifically involved in the interaction with L-arginine. Mutation of this residue abolished the L-arginine binding without disruption of other functional characteristics.

Analysis of hydroperoxide-induced tyrosyl radicals and lipoxygenase activity in aspirin-treated human prostaglandin H synthase-2

A hydroperoxide-induced tyrosyl radical has been proposed as a key cyclooxygenase intermediate for the "basal" isoform of prostaglandin H synthase (PGHS-1). In the present study with the "inducible" isoform (PGHS-2), hydroperoxide was also found to generate a radical in high yield, a wide singlet at g = 2.0058 (29 G peak to trough). Reaction of PGHS-2 with a tyrosine-modifying reagent, tetranitromethane (TNM), resulted in cyclooxygenase inactivation and a much narrower radical EPR signal (22 G peak to trough). Addition of a cyclooxygenase inhibitor, nimesulide, similarly resulted in a narrow PGHS-2 radical. In PGHS-1, cyclooxygenase inhibition by tyrosine nitration with TNM or by active site ligands leads to generation of a narrow EPR instead of a wide EPR, with both signals originating from authentic tyrosyl radicals, indicating that the hydroperoxide-induced radicals in PGHS-2 are also tyrosyl radicals. Treatment of PGHS-2 with aspirin (acetyl salicylic acid, ASA) was previously shown to result in acetylation of a specific serine residue, cyclooxygenase inhibition, and increased lipoxygenase activity. Acetylation of PGHS-1 by ASA, in contrast, inhibited both lipoxygenase and cyclooxygenase activity. We now have found the ASA-treated PGHS-2 radical to be indistinguishable from that in control PGHS-2. Addition of nimesulide to ASA-treated PGHS-2 inhibited the lipoxygenase and resulted in a narrow radical EPR like that seen in PGHS-2 treated with TNM or nimesulide alone. Retention of PGHS-2 oxygenase activity was thus associated with retention of the native radical, and loss of activity was associated with alteration of the radical. Both native and ASA-treated PGHS-2 produced only the R stereoisomer of 11- and 15-HETE, demonstrating that the lipoxygenase stereochemistry was not changed by ASA. Native and ASA-treated PGHS-2 had lipoxygenase K(m) values considerably higher than that of the control PGHS-2 cyclooxygenase. Taken together, these results suggest that the same PGHS-2 tyrosyl radical serves as the oxidant for both cyclooxygenase and lipoxygenase catalysis and that acetylation of PGHS-2 by ASA favors arachidonate binding in an altered conformation which results in abstraction of the pro-R hydrogen from C13 and formation of 11(R)- and 15(R)-HETE.

Spatial relationship between L-arginine and heme binding sites of endothelial nitric-oxide synthase

Binding of L-arginine and imidazole to the endothelial nitric-oxide synthase (eNOS) was characterized by direct heme spectral perturbation. L-Arginine is competitive with imidazole for binding to eNOS. Both equilibrium binding and kinetic binding were measured at 4 and 23 degrees C for these two ligands. Kd (imidazole) is 60 microM and 110 microM, kon (imidazole) is 2.5 x 10(5) M-1 s-1 and 1. 2 x 10(6) M-1 s-1, koff (imidazole) is 11.8 s-1 and 116 s-1 at 4 and 23 degrees C, respectively. Corresponding values for L-arginine are calculated from the data of binding competition with imidazole and computer modeling. Kd (L-arginine) is 0.5 microM and 2.0 microM, kon (L-arginine) is 2 x 10(5) M-1 s-1 and 8 x 10(5) M-1 s-1, koff (L-arginine) is 0.08 s-1 and 1.6 s-1 at 4 and 23 degrees C, respectively. It is suggested that binding of both ligands occurs through the same access channel to the heme site based on their similarly slow association rate constants. A series of potential heme ligands and amino acid analogs of L-arginine were evaluated for their binding and their effect on the heme structure. All ligands besides cyanide tested for binding inhibition are competitive with either L-arginine or imidazole. The space for the distal heme ligand was estimated to be approximately 6.3 x 6.7 A by three groups of rigid planar ligands: imidazole, pyridine, and pyrimidine. Results of the thiazole and amino acid ligand series permitted the conclusion that the guanidine group of L-arginine is critical for its binding affinity and its specific orientation relative to the heme. Such a specific conformation is essential for the oxygenase mechanism of eNOS.

Characterization of endothelial nitric-oxide synthase and its reaction with ligand by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance was used to characterize the heme structure of resting endothelial nitric-oxide synthase (eNOS), eNOS devoid of its myristoylation site (G2A mutant), and their heme complexes formed with 16 different ligands. Resting eNOS and the G2A mutant have a mixture of low spin and high spin P450-heme with widely different relaxation behavior and a stable flavin semiquinone radical identified by EPR as a neutral radical. This flavin radical showed efficient electron spin relaxation as a consequence of dipolar interaction with the heme center; P1/2 is independent of Ca2+-calmodulin and tetrahydrobiopterin. Seven of the 16 ligands led to the formation of low spin heme complexes. In order of increasing rhombicity they are pyrimidine, pyridine, thiazole, L-lysine, cyanide, imidazole, and 4-methylimidazole. These seven low spin eNOS complexes fell in a region between the P and O zones on the "truth diagram" originally derived by Blumberg and Peisach (Blumberg, W. E., and Peisach, J. (1971) in Probes and Structure and Function of Macromolecules and Membranes (Chance, B., Yonetani, T., and Mildvan, A. S., eds) Vol. 2, pp. 215-229, Academic Press, New York) and had significant overlap with complexes of chloroperoxidase. A re-definition of the P and O zones is proposed. As eNOS and chloroperoxidase lie closer than do eNOS and P450cam on the truth diagram, it implies that the distal heme environment in eNOS resembles chloroperoxidase more than P450cam. In contrast, 4-ethylpyridine, 4-methylpyrimidine, acetylguanidine, ethylguanidine, 2-aminothiazole, 2amino-4,5-dimethylthiazole, L-histidine, and 7-nitroindazole resulted in high spin heme complexes of eNOS, similar to that observed with L-arginine. This contrasting EPR behavior caused by families of ligands such as imidazole/L-histidine or thiazole/2-aminothiazole confirms the conclusion derived from parallel optical and kinetic studies. The ligands resulting in the low spin complexes bind directly to the heme iron, while their cognate ligands induce the formation of high spin complexes by indirectly perturbing the heme structure and excluding the original axial heme ligand in the resting eNOS (V. Berka, P.-F. Chen, and A. -L. Tsai (1997) J. Biol. Chem. 272, in press). The difference in EPR spectra of these high spin eNOS complexes, although subtle, are different for different homologs.

Endothelial nitric-oxide synthase. Evidence for bidomain structure and successful reconstitution of catalytic activity from two separate domains generated by a baculovirus expression system

A baculovirus system was used to express the oxygenase and reductase domains of human endothelial nitric-oxide synthase (ecNOS) as distinct proteins. The oxygenase domain (residues 1-491) was expressed using a vector containing a His6 tag at the N terminus. The purified oxygenase domain had an apparent molecular mass of approximately 54 kDa, and retained the ability to bind L-arginine and form the ferrous CO complex. The purified reductase domain (residues 492-1244) had an apparent molecular mass of approximately 82 kDa and retained the ability to catalyze NADPH-dependent cytochrome c reduction, which was enhanced 10-fold by the presence of Ca2+/calmodulin. Both purified domains exhibited immunoreactivity to rabbit anti-ecNOS IgG. The NOS activity was successfully reconstituted by mixing the two domains. These results demonstrate for the first time that the two domains of ecNOS are catalytically intact and can be reconstituted in vitro.

Cysteine 99 of endothelial nitric oxide synthase (NOS-III) is critical for tetrahydrobiopterin-dependent NOS-III stability and activity

Tetrahydrobiopterin (BH4) is an essential cofactor for all three isoforms of nitric oxide synthase (NOS). However, its binding sites and functional roles remain elusive. Here, we demonstrated that cys-99 of human endothelial NOS (ecNOS) is critical for BH4 involvement in NOS catalytic activity and stability. Mutation of cys-99 to alanine in ecNOS resulted in loss of catalytic activity which could be restored to the level of wild type by adding a high concentration of exogenous BH4 to the crude extract. Purified C99A mutant was unstable and its maximal activity was only about 20% of the purified wild type activity. Comparison of BH4 concentration-dependent citrulline formation between C99A and the wild type revealed that the BH4 concentrations required for generating half-maximal citrulline were 10-fold higher for C99A. Purified C99A had no detectable BH4 and had a reduced heme content when compared to the purified wild type, but retained the ability of forming CO-ferrous heme complex and had the same Km value for L-arginine (approximately 4 microM) as the wild type. These findings indicate that Cys-99 is critically involved in BH4 binding. Mutation of this residue leads to reduced affinity for BH4 and the resultant enzyme instability and irreversible heme loss.

Comparison of branched-chain and tightly coupled reaction mechanisms for prostaglandin H synthase

Two types of mechanisms have been proposed to account for the combination of peroxidase and cyclooxygenase activities in prostaglandin H synthase (PGHS). One, a branched-chain mechanism [Dietz, R., et al. (1988) Eur. J. Biochem. 171, 321-328], postulates that the cyclooxygenase reaction propagates essentially independently of peroxidase catalysis. The second, a tightly coupled mechanism [Bakovic, M., & Dunford, H. B. (1994) Biochemistry 33, 6475-6482], postulates that peroxidase catalysis is an integral part of cyclooxygenase propagation. Qualitative and quantitative predictions from the two mechanisms have been compared with several observed characteristics of the PGHS reaction with arachidonate, including the ability to accumulate PGG2 and oxidized enzyme intermediates, the stoichiometry between cosubstrate and fatty acid consumption, and the hydroperoxide activator requirement. The observed characteristics, particularly the accumulation of micromolar levels of PGG2 even in the presence of cosubstrate and the stoichiometry between cosubstrate oxidation and fatty acid oxygenation of less than 1.3 (compared to a theoretical maximum of 2.0), were largely consistent with predictions from the branched-chain mechanism, but contradicted important predictions of the tightly coupled mechanism. These results indicate that PGHS catalysis is more accurately described by the branched-chain mechanism than by the tightly coupled mechanism.

Kinetics of prostacyclin synthesis in PGHS-1-overexpressed endothelial cells

The availability of a human endothelial cell overexpressed with prostaglandin H synthase-1 (PGHS-1) by retrovirus-mediated gene transfer made it possible to quantify the kinetics of prostacyclin [prostaglandin (PG)I2] synthesis and PGHS-1 turnover. Prostacyclin synthesis in response to arachidonate (AA) and ionophore A-23187 fit a single exponential kinetics. The rate constants for AA- and ionophore-treated cells were 0.064 min-1 [half-life (t1/2) of 11 min] and 0.032 min-1 (t1/2 = 22 min), respectively. The rate constant of PGI2 synthesis from PGH2 was 0.13 min-1. Using kinetic analysis coupled with computer modeling, the PGHS-1 half-life was determined to be 10.8 min. PGI2 production under successive treatments with AA or ionophore was reduced by only approximately 30% after each treatment. The decline of PGI2 synthesis corresponded to the reduction of PGHS-1 mass. The half-life of PGI2 synthesis from this analysis was at least an order of magnitude higher than that estimated from the single-dose experiment. These findings indicate that approximately 30% of PGHS-1 was degraded during each catalysis-induced autoinactivation and that the extent and duration of PGI2 synthesis are governed by the level of PGHS-1 mass.

Cysteine 184 of endothelial nitric oxide synthase is involved in heme coordination and catalytic activity

Nitric oxide synthase catalyzes the formation of an important messenger molecule, nitric oxide (NO). It is a P450-type hemoprotein, containing a cysteine thiolate as its proximal heme ligand, but the exact cysteine residue involved in heme coordination has not been identified. To locate this specific cysteine, we altered three potential cysteine residues (Cys-99, Cys-184, and Cys-441) to alanine residues in human endothelial nitric oxide synthase (eNOS) by oligonucleotide-directed mutagenesis and expressed the wild-type and mutant eNOSs in COS-1 and the baculovirus expression system. Mutation of Cys-235 to alanine was included to serve as a control. Mutation of Cys-184 resulted in a complete loss of NOS catalytic activity and abrogation of the formation of carbon monoxide (CO)-heme ferrous complex, which was detected on CO difference spectra as a distinct peak centered on 444-446 nm, without reduction in the quantity of eNOS protein. Mutation of Cys-99 also resulted in a loss of catalytic activity but did not eliminate the 444-446 nm peak. C441A and C235A mutants displayed considerable NOS activity and retained the CO-heme peak on CO-ferrous difference spectra. These results indicate that the cysteine 184 of human eNOS is most likely the proximal heme ligand.

Interaction between nitric oxide and prostaglandin H synthase

Prostaglandin H synthase (PGHS) is a hemeprotein, and thus its catalytic activity potentially could be modulated by direct interaction with nitric oxide (NO). We have monitored spectroscopic and activity changes in pure ovine PGHS isoform-1 to investigate its interaction with NO in more detail. The binding kinetics for NO and the ferric heme in resting PGHS were analyzed by stopped-flow spectrophotometry at 21 degrees C. The rate constants for association and dissociation were estimated to be 6.5 x 10(4) M-1 s-1 and 60 s-1, respectively, leading to an equilibrium dissociation constant (Kd) of 0.92 mM. NO thus has a relatively weak affinity for heme in ferric PGHS, the resting oxidation state of this hemeprotein. NO did react strongly and completely with ferrous PGHS under anaerobic conditions, displacing the proximal histidine ligand to the prosthetic group. Dissolved NO at up to 2 mM produced only slight decreases in the cyclooxygenase activity of microsomal, detergent-extracted, or homogeneous preparations of ovine PGHS. The NO donors sodium nitroprusside and glyceryl trinitrate at levels of up to 1 mM also had little effect on the activity of the PGHS preparations. Thus, there was no evidence for significant direct interaction of PGHS with NO at concentrations likely to be encountered in vivo.

Reaction and free radical kinetics of prostaglandin H synthase with manganese protoporphyrin IX as the prosthetic group

Prostaglandin H synthase (PGHS) is a hemoprotein with both cyclooxygenase and peroxidase activities. Several aspects of the peroxidase and cyclooxygenase activities and the formation of substrate-induced free radical species were characterized with ovine seminal vesicle PGHS reconstituted with manganese protoporphyrin IX (Mn-PGHS) for comparison with the enzyme-containing heme (Fe-PGHS). Compared to Fe-PGHS, the Km of Mn-PGHS peroxidase for ethyl hydroperoxide was much higher, but that for 15-hydroperoxyeicosatetraenoic acid (15-HPETE) was little changed. The Mn-PGHS peroxidase Vmax value with 15-HPETE was about 4% that with Fe-PGHS. Mn-PGHS oxidized 0.95 +/- 0.05 and Fe-PGHS oxidized 2.06 +/- 0.09 mol of TMPD/mol of 15-HPETE. Reaction of 15-HPETE with Mn-PGHS resulted in approximately equal proportions of two lipid products: 15-hydroxyeicosatetraenoic acid (15-HETE) and a compound identified as 15-ketoeicosatetraenoic acid. Fe-PGHS produced only 15-HETE. Thus, 15-HETE can serve as an efficient two-electron reductant of oxidized Mn-PGHS intermediates. The rate of accumulation of oxidized Mn-PGHS intermediate was dependent on the substrate, decreasing in the following order: 15-HPETE > 11,14-eicosadienoate > arachidonate > EtOOH. The cyclooxygenase specific activity increased in a saturable fashion as the concentration of Mn-PGHS was increased, reaching a value higher than that for Fe-PGHS. Computer simulations of the reaction kinetics indicated that this dependence on the Mn-PGHS level was a consequence of the low rate of formation of oxidized peroxidase intermediate. Incubation of Mn-PGHS with either 15-HPETE or arachidonate resulted in the rapid production of a free radical species. The initial accumulation of radical coincided with the synthesis of PGG2 and PGH2, indicating that the radical was kinetically competent to participate in cyclooxygenase catalysis. Reaction with tetranitromethane, a reagent that selectively nitrates tyrosyl residues, destroyed the cyclooxygenase activity of Mn-PGHS, resulting in a much narrower free radical EPR signal. These effects indicate that tyrosine residues are essential for cyclooxygenase activity and that they influence the radical structure in Mn-PGHS.

Heme coordination of prostaglandin H synthase

The heme coordination of ovine prostaglandin H synthase (PGHS) has been characterized by EPR, magnetic circular dichroism, resonance Raman, and optical spectroscopies. The EPR spectrum of ferric PGHS is consistent with an equilibrium mixture of high-spin and low-spin heme species. Both species disappear on reaction of the synthase with hydroperoxides. The high-spin to low-spin interconversion is temperature- and concentration-dependent. Correlation between the axial and rhombic ligand fields of the low-spin heme species suggests that it has bishistidine axial ligation. Magnetic circular dichroism spectra of PGHS also show a temperature-dependent spin transition. Resonance Raman spectra indicate that the enzyme exists as a mixture of six-coordinate low-spin and six-coordinate high-spin ferric heme species. No Raman bands attributable to five-coordinate high-spin heme species are detectable. The magnetic circular dichroism spectra of the fluoride, azide, cyanide, and imidazole derivatives of PGHS resemble those of the corresponding metmyoglobin derivatives and are very different from those of the catalase derivatives. EPR spectra of the imidazole derivative of these three proteins provide additional evidence that the heme coordination structure of PGHS is similar to that of metmyoglobin rather than that of catalase. The midpoint potential of the PGHS Fe(III)/Fe(II) pair is in the range observed for hemeproteins with mono- or bishistidine coordination. These data provide a convincing case that the axial heme ligands of PGHS-1 are a pair of histidine residues, with the distal histidine weakly associated and possibly exchangeable with a weak-field ligand.

Trp387 and the putative leucine zippers of PGH synthases-1 and -2

The active site sequence 385-YHWH-388 of ovine prostaglandin endoperoxide synthase-1 (PGHS-1) has residues critical for cyclooxygenase and peroxidase catalysis. Tyr385 is essential for cyclooxygenase activity, His386, for peroxidase activity, and His388, for both activities. To determine the importance of Trp387, we used site-directed mutagenesis to replace Trp387 of PGHS-1 with arginine, phenylalanine, and serine. W387R and W387S lacked significant activity. W387F retained both cyclooxygenase and peroxidase activities. Thus, we conclude that Trp387 is not essential for catalysis by PGHS-1. Purified PGHS-1 is a homodimer. There are two putative leucine zipper regions in ovine PGHS-1 involving residues 345-366 and 487-508. We tested for a role of these leucine zippers as determinants of dimer formation. Helix-breaking proline mutations were introduced at Leu359 or Leu501. Neither of these residues proved to be essential for peroxidase activity; but, mutations at each residue greatly reduced or eliminated cyclooxygenase activity. Both mutant proteins chromatographed as dimers on Sephacryl G-200. Thus, neither of these putative leucine zipper regions alone is responsible for PGHS-1 dimer formation.

Substrate-induced free radicals in prostaglandin H synthase

Reaction of ovine PGH synthase with arachidonic acid or hydroperoxides produces several tyrosine radical species that can be distinguished by electron paramagnetic resonance (EPR) spectroscopy. We have correlated the temporal sequence of the EPR signals with optical changes of the heme center, and with product formation. The synthase was reconstituted with either heme (Fe-PGHS) or Mn protoporphyrin IX (Mn-PGHS). Incubation of Fe-PGHS with equimolar arachidonate resulted in rapid appearance of a wide doublet tyrosyl radical EPR signal (34 G peak-to-trough); the intensity was near maximal by 7 s. The doublet gave way over the next 10 s to a wide singlet (32 G peak-to-trough) which peaked at 46 s and then decayed slowly. Electronic absorbance spectra indicated that formation of peroxidase Compound I was complete within 1 s; accumulation of peroxidase Compound II paralleled accumulation of the wide doublet tyrosyl radical. PGG2 and PGH2 accumulated rapidly during the first 5 s of reaction; little arachidonate remained after 12 s. The tyrosyl radical giving the wide doublet EPR signal is thus the best candidate for the oxidizing species postulated to abstract the 13S hydrogen atom from arachidonate during cyclooxygenase catalysis by Fe-PGHS. Incubation of Mn-PGHS with arachidonate also led to rapid generation of an oxidized peroxidase cycle intermediate, a protein-linked free radical, and prostaglandins. The radical signal seen with Mn-PGHS (singlet, 36 G peak-to-trough) was distinct from those observed with Fe-PGHS, but the kinetics of the Mn-PGHS radical were consistent with participation in cyclooxygenase catalysis.

Prostaglandin H synthase. Kinetics of tyrosyl radical formation and of cyclooxygenase catalysis

Hydroperoxides are known to induce the formation of tyrosyl free radicals in prostaglandin (PG) H synthase. To evaluate the role of these radicals in cyclooxygenase catalysis we have analyzed the temporal correlation between radical formation and substrate conversion during reaction of the synthase with arachidonic acid. PGH synthase reacted with equimolar levels of arachidonic acid generated sequentially the wide doublet (34 G peak-to-trough) and wide singlet (32 G peak-to-trough) tyrosyl radical signals previously reported for reaction with hydroperoxide. The kinetics of formation and decay of the doublet signal corresponded reasonably well with those of cyclooxygenase activity. However, the wide singlet free radical signal accumulated only after prostaglandin formation had ceased, indicating that the wide singlet is not likely to be an intermediate in cyclooxygenase catalysis. When PGH synthase was reacted with 25 equivalents of arachidonic acid, the wide doublet and wide singlet radical signals were not observed. Instead, a narrower singlet (24 G peak-to-trough) tyrosyl radical was generated, similar to that found upon reaction of indomethacin-treated synthase with hydroperoxide. Only about 11 mol of prostaglandin were formed per mol of synthase before complete self-inactivation of the cyclooxygenase, far less than the 170 mol/mol synthase produced under standard assay conditions. Phenol (0.5 mM) increased the extent of cyclooxygenase reaction by only about 50%, in contrast to the 460% stimulation seen under standard assay conditions. These results indicate that the narrow singlet tyrosyl radical observed in the reaction with high levels of arachidonate in this study and by Lassmann et al. (Lassmann, G., Odenwaller, R., Curtis, J.F., DeGray, J.A., Mason, R.P., Marnett, L.J., and Eling, T.E. (1991) J. Biol. Chem. 266, 20045-20055) is associated with abnormal cyclooxygenase activity and is probably nonphysiological. In titrations of the synthase with arachidonate or with hydroperoxide, the loss of enzyme activity and destruction of heme were linear functions of the amount of titrant added. Complete inactivation of cyclooxygenase activity was found at about 10 mol of arachidonate, ethyl hydrogen peroxide, or hydrogen peroxide per mol of synthase heme; maximal bleaching of the heme Soret absorbance peak was found with 10 mol of ethyl hydroperoxide or 20 mol of either arachidonate or hydrogen peroxide per mol of synthase heme. The peak concentration of the wide doublet tyrosyl radical did not change appreciably with increased levels of ethyl hydroperoxide. In contrast, higher levels of hydroperoxide gave higher levels of the wide singlet radical species, in parallel with enzyme inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

Temperature- and pH-dependent changes in the coordination sphere of the heme c group in the model peroxidase N alpha-acetyl microperoxidase-8

The pH- and temperature-dependent changes in the coordination sphere of the heme c group of N alpha-acetyl microperoxidase-8 (Ac-MP-8) have been studied by examining its optical, resonance Raman, electron paramagnetic resonance, and magnetic circular dichroism spectra. An optical titration indicates that Ac-MP-8 exists in three major ionization forms over the pH 1-12 range that are linked by pK alpha values of approximately 3 and 9. The acid form that is present at pH 1.5 exists as a mixture of five- and six-coordinate high-spin species and most likely has water or buffer ions as axial ligand(s). On titration to pH 7, the His18 residue is deprotonated and becomes the proximal ligand to the iron to give a six-coordinate neutral form that has water as the sixth ligand. This form exists in a thermal high-spin intermediate-spin state equilibrium. On raising the pH to 10, an alkaline form is generated which is predominantly a five-coordinate high-spin species. It is formed by ionization of the proximal His18 residue to its imidazolate form with concomitant dissociation of the water ligand at the sixth site. At concentrations of Ac-MP-8 greater than 10 microM, some six-coordinate low-spin species are formed that are attributed to a dimer in which a His18 residue from a second molecule of Ac-MP-8 coordinates to the sixth site of another to give a bis-His complex. Raising the pH to 11.5 does not produce an appreciable amount of the six-coordinate complex with hydroxide as the sixth ligand. These studies show that Ac-MP-8 is a good water-soluble model for the peroxidases that exhibits minimal aggregation at concentrations below 10 microM in the neutral and alkaline pH regions.

Chemical modification of prostaglandin H synthase with diethyl pyrocarbonate

The role of histidine in catalysis by prostaglandin H synthase has been investigated using chemical modification with diethyl pyrocarbonate (DEPC), an agent that has been found to rather selectively derivatize histidine residues in proteins under mild conditions. Incubation of the synthase apoprotein with DEPC at pH 7.2 resulted in a progressive loss of the capacity for both cyclooxygenase and peroxidase catalytic activities. The kinetics of inactivation of the cyclooxygenase activity were dependent on the concentration of DEPC; a second-order rate constant of 680 M-1 min-1 was estimated for reaction of the apoenzyme at pH 7.2 and 0 degrees C. The kinetics of inactivation of the cyclooxygenase by DEPC exhibited a sigmoidal dependence on the pH, indicating that deprotonation of a group with a pKa of 6.3 was required for inactivation. The presence of the heme prosthetic group slowed, but did not prevent, inactivation by DEPC. The stoichiometry of histidine modification of apoenzyme during inactivation determined from absorbance increases at 242 nm agreed well with the overall stoichiometry of derivatized residues determined with [14C]DEPC, indicating that modification by DEPC was quite selective for histidine residues on the synthase. Although modification of several histidine residues by DEPC was observed, only one of the histidine residues was essential for cyclooxygenase activity. Modification of the holoenzyme with DEPC altered the EPR signal of the hydroperoxide-induced tyrosyl free radical from the wide doublet (35 G, peak-to-trough) found with the native synthase to a narrower singlet (28 G, peak-to-trough) quite like that found in the indomethacin-synthase complex. Reaction of the indomethacin-synthase complex with DEPC was found to increase the cyclooxygenase velocity by 9 times its initial value, to about one-third of the uninhibited value, without displacement of the indomethacin; the peroxidase was significantly inactivated under the same conditions. Histidyl residues in the synthase are thus likely to have important roles not only in cyclooxygenase and peroxidase catalysis but also in the interaction of the synthase with indomethacin.

Regulation of PGI2 activity by serum proteins: serum albumin but not high density lipoprotein is the PGI2 binding and stabilizing protein in human blood

Although previous studies have shown that serum albumin binds PGI2 and protects it from rapid degradation, it remains debatable whether it is physiologically important due to its low binding affinity for PGI2. We were intrigued by the observations of Yui et al. (J. Clin. Invest. 82 (1988) 803-807) which suggested that apo A-I of the high density lipoprotein (HDL) is the "serum PGI2 stabilizing factor". To clarify this, we carried out experiments to determine the binding kinetics and parameters of HDL and albumin purified from normal pooled human serum. Despite the use of multiple binding assays, we could not detect any binding activity in HDL2, HDL3 or nascent HDL preparations, nor could we demonstrate any PGI2 protecting activity by these molecules. By contrast, purified albumin exhibited essentially identical binding parameters as the native serum from which the albumin was purified. The binding activity of various albumin preparations was not due to the contamination of apo A-I. Computer simulation analysis also failed to provide evidence to support the notion that HDL bound and prolonged PGI2 activity. To determine whether physiological concentrations of albumin influence PGI2 binding to platelet receptors, we measured PGI2 binding to platelet membrane in the absence and presence of albumin. Albumin at 40 mg/ml increased the KD of PGI2 binding to the receptors by 2-3 fold. These findings indicate that albumin plays a major role in protecting PGI2 activity and regulating its availability for platelet PGI2 receptors.

Prostaglandin H synthase: perturbation of the tyrosyl radical as a probe of anticyclooxygenase agents

EPR spectroscopy was used to study the effects of various nonsteroidal anti-inflammatory agents on the peroxidase-related tyrosyl radical present in prostaglandin H synthase (prostaglandin endoperoxide synthase; EC 1.14.99.1). Two types of perturbation of the tyrosyl radical by these anticyclooxygenase agents were observed. In the first case, aspirin, indomethacin, ibuprofen, (S)-flurbiprofen, and (S)-naproxen converted the doublet tyrosyl EPR signal seen on reaction of the uninhibited enzyme with ethyl hydroperoxide to a singlet bearing additional partially resolved hyperfine splittings. These compounds also decreased the maximum amount of radical generated, but they did not change the kinetics of formation and decay of the tyrosyl radical. In the second case, acetaminophen and three fenamate analogs (meclofenamate, flufenamate, and mefenamate) did not perturb the EPR line shape observed after reaction with hydroperoxide but did cause a more rapid decay of the tyrosine radical species. It would appear that, despite considerable variation in structure, the nonsteroidal anti-inflammatory agents may inhibit the cyclooxygenase activity of the synthase by two basic mechanisms.

Conformation of receptor-associated PGI2: an investigation by molecular modeling

To elucidate the conformation of receptor-associated prostacyclin (PGI2), we first performed structure-activity correlation analysis of over 200 PGI2 analogues and derived from this analysis several crucial features pertaining to structural requirements for PGI2 activity [Ah-lim Tsai and Kenneth K. Wu, Eicosanoids, 2 (1989) 131-143]. These structural features proved to be useful guidelines for selecting 'model molecules' for further investigations by molecular mechanics. By properly selecting four analogues with either rigid or uniquely oriented alpha-side chain structure for geometric fitting, we succeeded in maximally minimizing the degree of freedom of the carboxylate terminus of PGI2. We were able to define the spatial relationship among the four critical functional groups, i.e., C1-COOH, C6a-O, C11-OH and C15-OH. More information is needed, however, to define the geometry of the omega-side chain, particularly for the moiety beyond C15. Nevertheless, results from structure-activity correlation analysis and molecular modeling provide useful information regarding the conformation of receptor-associated PGI2, which assumes an 'elongated' conformation instead of the traditional 'hairpin' structure.

Aspirin inhibits interleukin 1-induced prostaglandin H synthase expression in cultured endothelial cells

Prostaglandin H (PGH) synthase (EC 1.14.99.1) is a key enzyme in the biosynthesis of prostaglandins, thromboxane, and prostacyclin. In cultured human umbilical vein endothelial cells, interleukin 1 (IL-1) is known to induce the synthesis of this enzyme, thereby raising the level of PGH synthase protein severalfold over the basal level. Pretreatment with aspirin at low concentrations (0.1-1 micrograms/ml) inhibited more than 60% of the enzyme mass and also the cyclooxygenase activity in IL-1-induced cells with only minimal effects on the basal level of the synthase enzyme in cells without IL-1. Sodium salicylate exhibited a similar inhibitory action whereas indomethacin had no apparent effect. Similarly low levels of aspirin inhibited the increased L-[35S]methionine incorporation into PGH synthase that was induced by IL-1 and also suppressed expression of the 2.7-kilobase PGH synthase mRNA. These results suggest that in cultured endothelial cells a potent inhibition of eicosanoid biosynthetic capacity can be effected by aspirin or salicylate at the level of PGH synthase gene expression. The aspirin effect may well be due to degradation of salicylate.

Prostaglandin H synthase: spectroscopic studies of the interaction with hydroperoxides and with indomethacin

Prostaglandin H synthase has both a heme-dependent peroxidase activity and a cyclooxygenase activity. A current hypothesis considers the cyclooxygenase reaction to be a free radical chain reaction, initiated by an interaction of the synthase peroxidase with hydroperoxides leading to the production of a tyrosyl free radical [Stubbe, J. A. (1989) Annu. Rev. Biochem. 58, 257-285]. We have examined the kinetics of radical formation with both ethyl hydroperoxide (EtOOH) and 15-hydroperoxyeicosatetraenoic acid (15-HPETE) and have analyzed the effects of indomethacin (a selective cyclooxygenase inhibitor) and tetranitromethane (TNM; a selective agent for nitration of tyrosyl residues) on the synthase. At -14 degrees C both EtOOH and 15-HPETE generated within 5 s a free radical species whose electron paramagnetic resonance spectrum was dominated by a doublet centered at g = 2.005 (splitting of approximately 16 G; overall peak-to-trough width of 35 G) that has been attributed to tyrosyl radical. The doublet subsequently gave way to a singlet with a similar peak-to-trough width; the doublet-to-singlet transition was complete in 20-60 s. The intensity of the doublet/singlet combination peaked at 0.6 spins/heme after 120 s with EtOOH and at about 0.3 spins/heme after 20 s with 15-HPETE; the radical intensity declined slowly with EtOOH but more rapidly with 15-HPETE. Reaction of the indomethacin-synthase complex with EtOOH resulted in a narrower (peak-to-trough width of 24 G) singlet free radical signal, with no evidence of an earlier doublet; the intensity of the singlet peaked at 0.45 spins/heme after about 300 s. Reaction of TNM-treated synthase with EtOOH resulted in a singlet almost identical with that seen for the indomethacin-synthase complex. Reaction of the synthase holoenzyme with TNM at pH 8.0 led to inactivation of both cyclooxygenase and peroxidase activity, with the former being lost rapidly and completely while the latter was lost slowly and to about 50%. Ibuprofen, a competitive cyclooxygenase inhibitor, slowed the rate of inactivation of the cyclooxygenase by about 20-fold. The rate of inactivation of the cyclooxygenase activity in synthase apoenzyme by TNM was also about 20-fold less than that observed with the holoenzyme. Amino acid analyses revealed that TNM-reacted holoenzyme with less than 10% residual activity contained 1.8 nitrotyrosines/subunit; apoenzyme reacted under the same conditions had greater than 80% of the original activity and contained 0.7 nitrotyrosine/subunit.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the interaction between prostacyclin and human serum albumin using a fluorescent analogue, 2,6-dichloro-4-aminophenol iloprost

We synthesized a fluorescent probe, 2,6-dichloro-4-aminophenol iloprost or dichlorohydroxyphenylamide of iloprost (DCHPA-iloprost) by reacting the stable prostacyclin analog, iloprost (ZK 35 374), with 2,6-dichloro-4-aminophenol with a yield of 60%. This probe exhibited an optical spectrum which overlapped with the emission spectrum of the sole tryptophan of human serum albumin (HSA). Energy transfer from the tryptophan residue to the phenol moiety of DCHPA-iloprost was observed. We utilized this donor-quenching phenomenon to quantitate the binding stoichiometry and affinity as well as the association rate of DCHPA-iloprost binding to HSA. As DCHPA-iloprost showed similar binding characteristics similar to those of iloprost and prostacyclin and competed with iloprost for HSA binding sites, we used DCHPA-iloprost as a probe to locate the binding domain of prostacyclin (PGI2) in HSA. The distance between the tryptophan indole and the phenol group of DCHPA-iloprost was estimated to be 15-18 A. Because iloprost binding to HSA was competitive with warfarin and not with free fatty acid, we propose that PGI2 binds to the 'domain 2' of HSA was competitive with warfarin and not with free fatty acid, we propose that PGI2 binds to the 'domain 2' of HSA molecules. A possible molecular mechanism by which HSA reduces the chemical degradation of PGI2 and stabilizes its activity could be derived from this model.

Solubilization of prostacyclin membrane receptors from human platelets

Prostacyclin (PGI2) receptors have been identified on platelets and other tissues but their physicochemical properties remain unknown due to difficulties in obtaining active solubilized receptors. We evaluated the ability of several detergents to release the receptors from platelet membrane preparations. In contrast to the results of Dutta-Roy and Sinha (Dutta-Roy, A. K., and Sinha, A. K. (1987) J. Biol. Chem. 262, 12685-12691) which revealed selective solubilization of PGE1/PGI2 receptors by 0.05% Triton X-100, we found that CHAPS (3-[(3-chlamidopropyl)dimethylammonio]-1-propanesulfonic acid) (10 mM) was far superior in releasing the PGI2 receptors. In fact, Triton X-100 failed to release detectable PGI2 binding activity into the supernatant. The CHAPS-solubilized receptor degraded rapidly unless 30% glycerol was added which greatly enhanced its stability. By employing an improved binding assay using [3H]iloprost as the ligand and selective membrane filters (AP-15 or GF/B) pretreated with polyethyleneimine for achieving a higher trapping efficiency, we showed by equilibrium binding measurements that the solubilized receptors exhibited a single class of binding sites with a KD of 18.5 nM and Bmax 0.5 pmol/mg. These values were similar to those of the membrane receptors, i.e. KD of 16.6 nM and Bmax 1.0 pmol/mg. Kinetic binding measurements of the solubilized receptors revealed an association rate constant of 0.51 x 10(6) M-1 s-1 and dissociation rate constant of 0.0041 s-1 yielding a calculated KD of 8.0 nM. Displacement of [3H]iloprost (Ki values) from the solubilized and the membrane receptors by diversified eicosanoids was parallel. Our data demonstrate for the first time a successful solubilization of platelet PGI2 receptors. The solubilized receptors retained almost identical binding characteristics as the native membrane receptors.

Quantitation of serum prostacyclin-binding in thrombotic thrombocytopenic purpura

We quantitated serum PGI2 binding of 8 normal subjects and two TTP (thrombotic thrombocytopenic purpura) patients by gel filtration and gel partition methods using a stable PGI2 analogue, iloprost. The dissociation constant (KD) and the binding capacity (or binding stoichiometry) determined for the normals were 94 +/- S.D. 19 microM and 1.8 +/- S.D. 0.5 mM (or 2.0 +/- .6, iloprost:HSA). Corresponding values for serum samples obtained from TTP patient I were KD 200 microM, and Bmax 2.3 mM in the acute phase, and 75 microM and 1.8 mM respectively in the remission phase. The serum samples from TTP patient II exhibited a higher KD. Values of 299 microM (acute phase) and 147 microM (remission phase) were obtained. The corresponding binding capacities were 2.1 mM and 1.5 mM. Binding affinity change appears to be the main factor which resulted in the PGI2 binding defect in TTP.

Interaction between platelet receptor and iloprost isomers

Iloprost, a stable analog of prostacyclin, has been used for studying the interaction between prostacyclin and its effector cells such as platelets and vascular cells. The compound is usually prepared as a mixture of 16(S) and 16(R) stereoisomers. In this work, we compared the biological activity and platelet receptor binding characteristics between the two isomers. The 16(S) isomer was 20-times more potent than the 16(R) in inhibiting collagen-induced platelet aggregation. Equilibrium binding of iloprost isomers to platelet membrane receptors measured by rapid filtration method revealed that the specific binding data of 16(S) isomer was fit for a single binding species with Kd of 13.4 nM and Bmax 665 fmol/mg protein. By contrast, the Kd and Bmax of 16(R) isomer were 288 nM and 425 fmol/mg, respectively. To further assess different binding behavior of these two isomers, association rate was measured. The observed association rate of the S isomer was 0.036 s-1 and 0.001 s-1 for the R isomer at 15 nM iloprost and 2 mg/ml platelet membrane proteins. We postulate that the striking difference in the association rate with resultant difference in binding affinity and biologic activity between the two isomers was due to fitting of the molecule to the receptor channel. The 16(S) form has a more favorable orientation for fitting into the receptor. We conclude that the two iloprost isomers must be considered as two entirely different compounds when iloprost is used as the ligand for quantifying prostacyclin receptor binding.

Heme spin states and peroxide-induced radical species in prostaglandin H synthase

We have examined the optical, magnetic circular dichroism, and electron paramagnetic resonance (EPR) spectra of pure ovine prostaglandin H synthase in its resting (ferric) and ferrous states and after addition of hydrogen peroxide or 15-hydroperoxyeicosatetraenoic acid. In resting synthase, the distribution of heme between high- and low-spin forms was temperature-dependent: 20% of the heme was low-spin at room temperature whereas 50% was low-spin at 12 K. Two histidine residues were coordinated to the heme iron in the low-spin species. Anaerobic reduction of the synthase with dithionite produced a high-spin ferrous species that had no EPR signals. Upon reaction with the resting synthase, both hydroperoxides quickly generated intense (20-40% of the synthase heme) and complex EPR signals around g = 2 that were accompanied by corresponding decreases in the intensity of the signals from ferric heme at g = 3 and g = 6. The signal generated by HOOH had a doublet at g = 2.003, split by 22 G, superimposed on a broad component with a peak at g = 2.085 and a trough at g = 1.95. The lipid hydroperoxide generated a singlet at g = 2.003, with a linewidth of 25 G, superimposed on a broad background with a peak at g = 2.095 and a trough around g = 1.9. These EPR signals induced by hydroperoxide may reflect synthase heme in the ferryl state complexed with a free radical derived from hydroperoxide or fragments of hydroperoxide.

The kinetics of reoxidation of yeast complex III. An evaluation of the Q-cycle

The reoxidation of reduced yeast Complex III by oxidants believed to react with cytochrome c1 exhibited multiple phases for both cytochrome c1 and the cytochromes b; the reoxidation of cytochrome b, but not cytochrome c1, was markedly slowed by the presence of antimycin. The data are consistent with the Q-cycle or any other scheme which proposes a branched path for electron transport between the cytochrome b centers and the endogenous Q6, provided certain constraints are relaxed. The reoxidation of the endogenous quinone proceeded at a rate comparable to that of the rapidly reacting cytochrome b and appeared to be complete within 100 ms. Removal of the endogenous quinone did not change the rate or extent of reoxidation of any of the heme centers, demonstrating that quinone is not required for electron transport between cytochromes b and the iron-sulfur cluster. This result is inconsistent with the requirements of the Q-cycle. Funiculosin completely inhibited the reoxidation of cytochrome b whereas the reoxidation of cytochrome c1 exhibited simple first-order kinetics in the presence of this inhibitor, implying that the iron-sulfur cluster is on the direct path of electron transfer from cytochrome b to cytochrome c1. Potent inhibition of cytochrome b oxidation was also observed with myxothiazol and mucidin. The reaction of reduced Complex III with Q1 also exhibited multiple phases in the oxidation of the cytochrome b centers; these phases were unaffected by the presence of myxothiazol. Addition of antimycin, or removal of the endogenous quinone, eliminated the rapid phases; only one of the cytochrome b centers was oxidized under these conditions. Epr showed that it is the low-potential cytochrome b that is the species rapidly oxidized.