Expression of prostaglandin endoperoxide H synthase-2 induced by nitric oxide in conditionally immortalized murine colonic epithelial cells.

Increased expression of prostaglandin endoperoxide H synthase-2 (PGHS-2) has been implicated in pathological conditions such as inflammatory bowel diseases and colon cancer. Recently, it has been demonstrated that inducible nitric oxide synthase (NOS II) expression and nitric oxide (NO) production are up-regulated in these diseases as well. However, the apparent link between PGHS-2 and NOS II has not been thoroughly investigated in nontransformed and nontumorigenic colonic epithelial cells. In the present study, we examined the concomitant expression of PGHS-2 and NOS II as well as the production of prostaglandin E2 (PGE2) and NO in conditionally immortalized mouse colonic epithelial cells, namely YAMC (Apc(+/+)). We found that the induction of PGHS-2 and generation of PGE2 in these cells by IFN-gamma and lipopolysaccharide (LPS) were greatly reduced by two selective NOS II inhibitors, L-NIL and SMT. To ascertain the effect of NO on PGHS-2 overexpression, we tested NO-releasing compounds, NOR-1 and SNAP, and found that they caused PGHS-2 expression and PGE2 production. This effect was abolished by hemoglobin, a NO scavenger. Using electrophoretic mobility shift assays, we found that both NOR-1 and SNAP caused beta-catenin/LEF-1 DNA complex formation. Super-shift by anti-beta-catenin antibody confirmed the presence of beta-catenin in the complex. Cell fractionation studies indicated that NO donors caused an increase in free soluble cytoplasmic beta-catenin. This is further corroborated by the immunocytochemistry data showing the redistribution of beta-catenin from the predominantly membrane localization into the cytoplasm and nucleus after treatment with NO donors. To further explore the possible connection between PGHS-2 expression and beta-catenin/LEF-1 DNA complex formation, we studied IMCE (Apc(Min/+)) cells, a sister cell line of YAMC with similar genetic background but differing in Apc genotype and, consequently, their beta-catenin levels. We found that IMCE cells, in comparison with YAMC cells, had markedly higher beta-catenin/LEF-1 DNA complex formation under both resting conditions as well as after induction with NO. In parallel fashion, IMCE cells expressed significantly higher levels of PGHS-2 mRNA and protein, and generated more PGE2. Overall, this study suggests that NO may be involved in PGHS-2 overexpression in conditionally immortalized mouse colonic epithelial cells. Although the molecular mechanism of the link is still under investigation, this effect of NO appears directly or indirectly to be a result of the increase in free soluble beta-catenin and the formation of nuclear beta-catenin/LEF-1 DNA complex.     

Topography of the prostaglandin endoperoxide H2 synthase-2 in membranes

The topology of association of the monotopic protein cyclooxygenase-2 (COX-2) with membranes has been examined using EPR spectroscopy of spin-labeled recombinant human COX-2. Twenty-four mutants, each containing a single free cysteine substituted for an amino acid in the COX-2 membrane binding domain were expressed using the baculovirus system and purified, then conjugated with a nitroxide spin label and reconstituted into liposomes. Determining the relative accessibility of the nitroxide-tagged amino acid side chains for the solubilized COX-2 mutants, or COX-2 reconstituted into liposomes to nonpolar (oxygen) and polar (NiEDDA or CrOx) paramagnetic reagents allowed us to map the topology of COX-2 interaction with the lipid bilayer. When spin-labeled COX-2 was reconstituted into liposomes, EPR power saturation curves showed that side chains for all but two of the 24 mutants tested had limited accessibility to both polar and nonpolar paramagnetic relaxation agents, indicating that COX-2 associates primarily with the interfacial membrane region near the glycerol backbone and phospholipid head groups. Two amino acids, Phe(66) and Leu(67), were readily accessible to the non-polar relaxation agent oxygen, and thus likely inserted into the hydrophobic core of the lipid bilayer. However these residues are co-linear with amino acids in the interfacial region, so their extension into the hydrophobic core must be relatively shallow. EPR and structural data suggest that membrane interaction of COX-2 is also aided by partitioning of 4 aromatic amino acids, Phe(59), Phe(66), Tyr(76), and Phe(84) to the interfacial region, and by the electrostatic interactions of two basic amino acids, Arg(62) and Lys(64), with the phospholipid head groups.     

Interactions between nitric oxide and peroxynitrite during prostaglandin endoperoxide H synthase-1 catalysis: a free radical mechanism of inactivation

Peroxynitrite (ONOO(-)) can serve either as a peroxide substrate or as an inactivator of prostaglandin endoperoxide H synthase-1 (PGHS-1). Herein, the mechanism of PGHS-1 inactivation by ONOO(-) and the modulatory role that nitric oxide (*NO) plays in this process were studied. PGHS-1 reacted with ONOO(-) with a second-order rate constant of 1.7 x 10(7) M(-1) s(-1) at pH 7.0 and 8 degrees C. In the absence of substrates, the enzyme was dose-dependently inactivated by ONOO(-) in parallel with 3-nitrotyrosine formation. However, when PGHS-1 was incubated with ONOO(-) in the presence of substrates, the direct reaction with ONOO(-) was less relevant and ONOO(-)-derived radicals became involved in enzyme inactivation. Bicarbonate at physiologically relevant concentrations enhanced PGHS-1 inactivation and nitration by ONOO(-), further supporting a free radical mechanism. Importantly, *NO (0.4-1.5 microM min(-1)) was able to spare the peroxidase activity of PGHS-1 but it enhanced ONOO(-)-mediated inactivation of cyclooxygenase. The observed differential effects of *NO on ONOO(-)-mediated PGHS-1 inactivation emphasize a novel aspect of the complex modulatory role that *NO plays during inflammatory processes. We conclude that ONOO(-)-derived radicals inactivate both peroxidase and cyclooxygenase activities of PGHS-1 during enzyme turnover. Finally, our results reconcile the proposed alternative effects of ONOO(-) on PGHS-1 (activation versus inactivation).     

Different substrate utilization between prostaglandin endoperoxide H synthase-1 and -2 in NIH3T3 fibroblasts

Recent studies suggested that prostaglandin endoperoxide H synthase-1 and prostaglandin endoperoxide H synthase-2 (PGHS-1 and PGHS-2) utilize different pools of arachidonic acid for synthesizing prostanoids. Using cultured murine NIH3T3 fibroblasts, we investigated the mechanism for the different utilization of arachidonic acid between PGHS-1 and -2. Histofluorescence staining for PGHS activity in intact cells demonstrated that quiescent 3T3 cells expressed only PGHS-1 activity and serum-activated 3T3 cells pretreated with aspirin expressed only PGHS-2 activity. Endogenous arachidonic acid released by calcium ionophore A23187 was not converted by PGHS-1 but exclusively converted by PGHS-2. In the cell free system, the kinetics of PGHS-1 were not so much different from those of PGHS-2. However, in intact cells, arachidonic acid at concentrations lower than 2.5 μM was converted by PGHS-2 alone but not by PGHS-1. Our findings indicated that this small amount of arachidonic acid as released by some stimuli is converted exclusively by PGHS-2. Furthermore, treating the PGHS-2-expressing cells with sodium selenite or ebselen, reducing agents of intracellular peroxides, only decreased PGHS-2 activity. We speculate that only PGHS-2 has been activated by intracellular peroxides and subsequently, it can convert the arachidonic acid released endogenously.     

Unique peroxidase reaction mechanism in prostaglandin endoperoxide H synthase-2: compound I in prostaglandin endoperoxide H synthase-2 can be formed without assistance by distal glutamine residue

Prostaglandin-endoperoxide H synthase-2 (PGHS-2) shows peroxidase activity to promote the cyclooxygenase reaction for prostaglandin H2, but one of the highly conserved amino acid residues in peroxidases, distal Arg, stabilizing the developing negative charge on the peroxide through a hydrogen-bonding interaction, is replaced with a neutral amino acid residue, Gln. To characterize the peroxidase reaction in PGHS-2, we prepared three distal glutamine (Gln-189) mutants, Arg (Gln-->Arg), Asn (Gln-->Asn), and Val (Gln-->Val) mutants, and examined their peroxidase activity together with their structural characterization by absorption and resonance Raman spectra. Although a previous study (Landino, L. M., Crews, B. C., Gierse, J. K., Hauser, S. D., and Marnett, L. (1997) J. Biol. Chem. 272, 21565-21574) suggested that the Gln residue might serve as a functionally equivalent residue to Arg, our current results clearly showed that the peroxidase activity of the Val and Asn mutants was comparable with that of the wild-type enzyme. In addition, the Fe-C and C-O stretching modes in the CO adduct were almost unperturbed by the mutation, implying that Gln-189 might not directly interact with the heme-ligated peroxide. Rather, the peroxidase activity of the Arg mutant was depressed, concomitant with the heme environmental change from a six-coordinate to a five-coordinate structure. Introduction of the bulky amino acid residue, Arg, would interfere with the ligation of a water molecule to the heme iron, suggesting that the side chain volume, and not the amide group, at position 189 is essential for the peroxidase activity of PGHS-2. Thus, we can conclude that the O-O bond cleavage in PGHS-2 is promoted without interactions with charged side chains at the peroxide binding site, which is significantly different from that in typical plant peroxidases.     

Catalytic consumption of nitric oxide by prostaglandin H synthase-1 regulates platelet function

Nitric oxide (( small middle dot)NO) plays a central role in vascular homeostasis via regulation of smooth muscle relaxation and platelet aggregation. Although mechanisms for ( small middle dot)NO formation are well known, removal pathways are less well characterized, particularly in cells that respond to ( small middle dot)NO through activation of soluble guanylate cyclase. Herein, we report that ( small middle dot)NO is catalytically consumed by prostaglandin H synthase-1 (PGHS-1) through acting as a reducing peroxidase substrate. With purified ovine PGHS-1, ( small middle dot)NO consumption requires peroxide (LOOH or H(2)O(2)), with a K(m)( (app)) for 15(S)hydroperoxyeicosatetraenoic acid (HPETE) of 3. 27 +/- 0.35 microm. During this, 2 mol ( small middle dot)NO are consumed per mol HPETE, and loss of HPETE hydroperoxy group occurs with retention of the conjugated diene spectrum. Hydroperoxide-stimulated ( small middle dot)NO consumption requires heme incorporation, is not inhibited by indomethacin, and is further stimulated by the reducing peroxidase substrate, phenol. PGHS-1-dependent ( small middle dot)NO consumption also occurs during arachidonate, thrombin, or activation of platelets (1-2 microm.min(-1) for typical plasma platelet concentrations) and prevents ( small middle dot)NO stimulation of platelet soluble guanylate cyclase. Platelet sensitivity to ( small middle dot)NO as an inhibitor of aggregation is greater using a platelet-activating stimulus () that does not cause ( small middle dot)NO consumption, indicating that this mechanism overcomes the anti-aggregatory effects of ( small middle dot)NO. Catalytic consumption of ( small middle dot)NO during eicosanoid synthesis thus represents both a novel proaggregatory function for PGHS-1 and a regulated mechanism for vascular ( small middle dot)NO removal.     

Peroxidase activity in prostaglandin endoperoxide H synthase-1 occurs with a neutral histidine proximal heme ligand

Prostaglandin endoperoxide H synthases-1 and -2 (PGHS-1 and -2) convert arachidonic acid to prostaglandin H(2) (PGH(2)), the committed step in prostaglandin and thromboxane formation. Interaction of peroxides with the heme sites in PGHSs generates a tyrosyl radical that catalyzes subsequent cyclooxygenase chemistry. To study the peroxidase reaction of ovine oPGHS-1, we combined spectroscopic and directed mutagenesis data with X-ray crystallographic refinement of the heme site. Optical and Raman spectroscopy of oxidized oPGHS-1 indicate that its heme iron (Fe(3+)) exists exclusively as a high-spin, six-coordinate species in the holoenzyme and in heme-reconstituted apoenzyme. The sixth ligand is most likely water. The cyanide complex of oxidized oPGHS-1 has a six-coordinate, low-spin ferric iron with a v[Fe-CN] frequency at 445 cm(-)(1); a monotonic sensitivity to cyanide isotopomers that indicates the Fe-CN adduct has a linear geometry. The ferrous iron in reduced oPGHS-1 adopts a high-spin, five-coordinate state that is converted to a six-coordinate, low-spin geometry by CO. The low-frequency Raman spectrum of reduced oPGHS-1 reveals two v[Fe-His] frequencies at 206 and 222 cm(-)(1). These vibrations, which disappear upon addition of CO, are consistent with a neutral histidine (His388) as the proximal heme ligand. The refined crystal structure shows that there is a water molecule located between His388 and Tyr504 that can hydrogen bond to both residues. However, substitution of Tyr504 with alanine yields a mutant having 46% of the peroxidase activity of native oPGHS-1, establishing that bonding of Tyr504 to this water is not critical for catalysis. Collectively, our results show that the proximal histidine ligand in oPGHS-1 is electrostatically neutral. Thus, in contrast to most other peroxidases, a strongly basic proximal ligand is not necessary for peroxidase catalysis by oPGHS-1.     

Comparison of the properties of prostaglandin H synthase-1 and -2

Biosynthesis of prostanoid lipid signaling agents from arachidonic acid begins with prostaglandin H synthase (PGHS), a hemoprotein in the myeloperoxidase family. Vertebrates from humans to fish have two principal isoforms of PGHS, termed PGHS-1 and-2. These two isoforms are structurally quite similar, but they have very different pathophysiological roles and are regulated very differently at the level of catalysis. The focus of this review is on the structural and biochemical distinctions between PGHS-1 and-2, and how these differences relate to the functional divergence between the two isoforms.     

Structure of eicosapentaenoic and linoleic acids in the cyclooxygenase site of prostaglandin endoperoxide H synthase-1

Prostaglandin endoperoxide H synthases-1 and -2 (PGHSs) can oxygenate 18-22 carbon polyunsaturated fatty acids, albeit with varying efficiencies. Here we report the crystal structures of eicosapentaenoic acid (EPA, 20:5 n-3) and linoleic acid (LA, 18:2 n-6) bound in the cyclooxygenase active site of Co(3+) protoporphyrin IX-reconstituted ovine PGHS-1 (Co(3+)-oPGHS-1) and compare the effects of active site substitutions on the rates of oxygenation of EPA, LA, and arachidonic acid (AA). Both EPA and LA bind in the active site with orientations similar to those seen previously with AA and dihomo-gamma-linolenic acid (DHLA). For EPA, the presence of an additional double bond (C-17/C-18) causes this substrate to bind in a "strained" conformation in which C-13 is misaligned with respect to Tyr-385, the residue that abstracts hydrogen from substrate fatty acids. Presumably, this misalignment is responsible for the low rate of EPA oxygenation. For LA, the carboxyl half binds in a more extended configuration than AA, which results in positioning C-11 next to Tyr-385. Val-349 and Ser-530, recently identified as important determinants for efficient oxygenation of DHLA by PGHS-1, play similar roles in the oxygenation of EPA and LA. Approximately 750- and 175-fold reductions in the oxygenation efficiency of EPA and LA were observed with V349A oPGHS-1, compared with a 2-fold change for AA. Val-349 contacts C-2 and C-3 of EPA and C-4 of LA orienting the carboxyl halves of these substrates so that the omega-ends are aligned properly for hydrogen abstraction. An S530T substitution decreases the V(max)/K(m) of EPA and LA by 375- and 140-fold. Ser-530 makes six contacts with EPA and four with LA involving C-8 through C-16; these interactions influence the alignment of the substrate for hydrogen abstraction. Interestingly, replacement of Phe-205 increases the volume of the cyclooxygenase site allowing EPA to be oxygenated more efficiently than with native oPGHS-1.     

Differential effects of ibuprofen,indomethacin, and meclofenamate on prostaglandin endoperoxide H2 metabolism

Summary Previous studies have suggested the possibility that the non-steroidal antiflammatory drug (NSAID), ibuprofen, may inhibit thromboxane (TX) A₂ synthase activity in addition to inhibiting cyclooxygenae activity. Microsomal fractions isolated from the cat lung contain cyclooxygenase as well as prostacyclin (PGI₂) synthase, TX synthase, and a GSH-dependent prostaglandin (PG) E₂ isomerase activities. When [1-¹⁴C] PG endoperoxide H₂ (PGH₂) was used as substrate, ibuprofen, indomethacin, and meclofenamate exhibited differential effects on terminal enzyme activities. Ibuprofen, at concentrations up to 1 mM, had no effect on the activities of PGI₂ synthase, TXA₂ synthase of GSH-dependent PGE₂ isomerase, whereas indomethacin selectively inhibited PGI₂ synthase activity at 5 x 10^–4 M and 10^–3 M. Meclofenamate selectively inhibited TXA₂ synthase activity at 5 x 10^–4 M and 10^–3 M. At concentrations of 5 x 10^–3 M, this selectivity was not oberved, and indomethacin and meclofenamate decreased the formation of both 6-keto-PGF₁ and TXB₂. These data indicate that the choice of NSAID and the concentration employed may specifically alter PGH₂ metabolism. This action may affect the physiologic consequences of the exchange of PGH₂ between cells. The data further indicate that indomethacin has the potential for use as a tool to specifically attenuate PGI₂ synthase activity in vitro.     

Effects of nitric oxide and nitric oxide-derived species on prostaglandin endoperoxide synthase and prostaglandin biosynthesis.

Prostaglandins and NO. are important mediators of inflammation and other physiological and pathophysiological processes. Continuous production of these molecules in chronic inflammatory conditions has been linked to development of autoimmune disorders, coronary artery disease, and cancer. There is mounting evidence for a biological relationship between prostanoid biosynthesis and NO. biosynthesis. Upon stimulation, many cells express high levels of nitric oxide synthase (NOS) and prostaglandin endoperoxide synthase (PGHS). There are reports of stimulation of prostaglandin biosynthesis in these cells by direct interaction between NO. and PGHS, but this is not universally observed. Clarification of the role of NO. in PGHS catalysis has been attempted by examining NO. interactions with purified PGHS, including binding to its heme prosthetic group, cysteines, and tyrosyl radicals. However, a clear picture of the mechanism of PGHS stimulation by NO. has not yet emerged. Available studies suggest that NO. may only be a precursor to the molecule that interacts with PGHS. Peroxynitrite (from O2.-+NO.) reacts directly with PGHS to activate prostaglandin synthesis. Furthermore, removal of O2.- from RAW 267.4 cells that produce NO. and PGHS inhibits prostaglandin biosynthesis to the same extent as NOS inhibitors. This interaction between reactive nitrogen species and PGHS may provide new approaches to the control of inflammation in acute and chronic settings.     

Expression of prostaglandin endoperoxide synthase-1 in a baculovirus system.

A cDNA coding for ovine prostaglandin endoperoxide (PGH) synthase-1 was used to construct a recombinant baculovirus which was expressed in Spodoptera frugiperda (Sf9) insect cells. Two proteins reactive with anti-PGH synthase antibody were produced. A larger protein (Mr = 72,000) coelectrophoresed with native enzyme; a smaller, more abundant protein (Mr = 66,000) was unglycosylated enzyme. About 90% of both the immunoreactivity and the cyclooxygenase activity were present in a low speed (10(5) g x min) pellet; variable but low peroxidase activities were observed in this fraction. The specific cyclooxygenase activity of solubilized PGH synthase-1 from Sf9 cells was 56 units/mg versus 112 units/mg for the same cDNA expressed in cos-1 cells. The baculovirus-insect cell system is not ideal for generating large amounts of active PGH synthase-1 apparently because of inefficient N-glycosylation.     

A fluorescence microscopy method for quantifying the detection of prostaglandin endoperoxide H synthase-1 and CD-41 in MEG-01 cells

In platelets, PGHS-1-dependant formation of thromboxane A₂ is an important modulator of platelet function and a target for pharmacological inhibition of platelet function by aspirin. Since platelets are anucleated cells, we have used the immortalized human megakaryoblastic cell line MEG-01, which can be induced to differentiate into platelet-like structures upon addition of TPA as a model system to study PGHS-1 gene expression. Using a specific antibody to PGHS-1 we have developed a technique using immunofluorescence microscopy and analysis of multiple digital images to monitor PGHS-1 protein expression as MEG-01 cells were induced to differentiate by a single addition of TPA (1.6 × 10⁻⁸ M) over a period of 8 days. The method represents a rapid and economical alternative to flow cytometry. Using this technique we observed that TPA induced adherence of MEG-01 cells, and only the non-adherent TPA-stimulated cells demonstrated compromised viability. The differentiation of MEG-01 cells was evaluated by the expression of the platelet-specific cell surface antigen, CD-41. The latter was expressed in MEG-01 cells at the later stages of differentiation. We demonstrated a good correlation between PGHS-1 expression and the overall level of cellular differentiation of MEG-01 cells. Furthermore, PGHS-1 protein expression, which shows a consistent increase over the entire course of differentiation can be used as an additional and better index by which to monitor megakaryocyte differentiation. Published: December 12, 2001     

Different suicide inactivation processes for the peroxidase and cyclooxygenase activities of prostaglandin endoperoxide H synthase-1.

Prostaglandin endoperoxide H synthases (PGHSs)-1 and -2 have a cyclooxygenase (COX) activity involved in forming prostaglandin G2 (PGG2) from arachidonic acid and an associated peroxidase (POX) activity that reduces PGG2 to PGH2. Suicide inactivation processes are observed for both POX and COX reactions. Here we report COX reaction conditions for PGHS-1 under which complete COX inactivation occurs but with > or = 60% retention of POX activity. The rates of POX inactivation were compared for native oPGHS-1 versus Y385F oPGHS-1, a mutant that cannot form the Tyr385 radical of COX Intermediate II; the rates were the same for both native and Y385F oPGHS-1. Our data indicate that a COX Intermediate II/acyl or product complex is the precursor in COX inactivation. However, another species, probably an Intermediate II-like species but with a radical centered on a tyrosine other than Tyr385, is the immediate precursor for POX inactivation.     

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.     

Hydroperoxide dependence and cooperative cyclooxygenase kinetics in prostaglandin H synthase-1 and -2.

Prostaglandin H synthase isoform-1 (PGHS-1) cyclooxygenase activity has a cooperative response to arachidonate concentration, whereas the second isoform, PGHS-2, exhibits saturable kinetics. The basis for the cooperative PGHS-1 behavior and for the difference in cooperativity between the isoforms was unclear. The two cyclooxygenase activities have different efficiencies of feedback activation by hydroperoxide. To determine whether the cooperative kinetics were governed by the feedback activation characteristics, we examined the cyclooxygenase activities under conditions where feedback activation was either assisted (by exogenous peroxide) or impaired (by replacement of heme with mangano protoporphyrin IX to form MnPGHS-1 and -2). Heme replacement increased PGHS-1 cyclooxygenase cooperativity and changed PGHS-2 cyclooxygenase kinetics from saturable to cooperative. Peroxide addition decreased or abolished cyclooxygenase cooperativity in PGHS-1, MnPGHS-1, and MnPGHS-2. Kinetic simulations predicted that cyclooxygenase cooperativity depends on the hydroperoxide activator requirement and initial peroxide concentration, consistent with observed behavior. The results indicate that PGHS-1 cyclooxygenase cooperativity originates in the feedback activation kinetics and that the cooperativity difference between the isoforms can be explained by the difference in feedback activation loop efficiency. This linkage between activation efficiency and cyclooxygenase cooperativity indicates an interdependence between fatty acid and hydroperoxide levels in controlling the synthesis of potent prostanoid mediators.