A Randomized, Double-Blind, Parallel Design Thorough QT Study With a Nested Crossover to Compare the Cardiac Safety of Amiselimod With Placebo and Positive Control in Healthy Volunteers.

This double-blind study evaluated the cardiac safety of amiselimod. Healthy adults (n = 190) were randomized (2:1:1) to receive (1) oral placebo (day -1), followed by oral amiselimod (days 1-26), which was upwardly titrated from 0.4 to 1.6 mg once daily to achieve steady-state concentrations comparable with 0.4 (therapeutic) and 0.8 mg (supratherapeutic) once daily, and placebo (day 27); (2) placebo (day -1), oral moxifloxacin 400 mg (day 1; positive control), followed by placebo (days 1-27); or (3) placebo (days -1 to 26), followed by moxifloxacin 400 mg (day 27). No participant had a corrected QT interval by Fredericia (QTcF) >500 milliseconds or a change from baseline (dQTcF) >60 milliseconds. The upper limits of the 90%CIs for the differences in least-squares mean difference in dQTcF between amiselimod and placebo on days 13 and 26 were <10 milliseconds. Area under the concentration-time curve from 0 to 23.5 hours after dosing and maximum plasma concentration of amiselimod and amiselimod-P (active metabolite) at steady-state concentrations for the 0.8-mg dose on day 26 were approximately double that observed with the 0.4-mg dose on day 13. All adverse events were mild to moderate in severity, and no deaths occurred. Amiselimod did not have any clinically relevant effect on the QTcF interval.

Rheological Properties, Dissolution Kinetics, and Ocular Pharmacokinetics of Loteprednol Etabonate (Submicron) Ophthalmic Gel 0.38.

Purpose: To evaluate rheological properties, in vitro dissolution, and in vivo ocular pharmacokinetics of loteprednol etabonate (LE) (submicron) ophthalmic gel 0.38%. Methods: The viscosity of the LE gel 0.38% formulation was measured with a controlled stress rheometer. Dissolution kinetics were evaluated in a fixed-volume and flow-through assay. Rabbits received a single instillation of LE (submicron) gel 0.38% (both eyes), and concentrations of LE in ocular tissues were determined through 24 h by liquid chromatography with tandem mass spectrometry. Where indicated, comparators included micronized LE gel 0.38%, 0.5% (Lotemax® gel), and 0.75%. Results: LE (submicron) gel 0.38% exhibited shear-thinning characteristics similar to LE gel 0.5% with nearly identical yield stress. LE (submicron) gel 0.38% released 2.6-fold more LE into the dissolution medium than micronized LE gel 0.5% over 30 s in the fixed-volume dissolution assay, and submicron LE attained higher concentrations of dissolved LE than micronized LE gel 0.38% in the flow-through dissolution assay. In rabbits, the maximal concentration and area-under-the-curve over 24 h for LE in aqueous humor were 2.5- and 1.8-fold higher, respectively, for LE (submicron) gel 0.38% versus micronized LE gel 0.5% (both P < 0.001). Pharmacokinetic parameters were similar for most other tissues. Conclusions: LE (submicron) gel 0.38% demonstrated similar rheological properties to micronized LE gel 0.5% but faster dissolution, thus providing similar or higher LE concentrations in the aqueous humor, cornea, and iris-ciliary body after ocular dosing in rabbits despite a lowered concentration of drug in the formulation.

Ocular pharmacokinetics of mapracorat, a novel, selective glucocorticoid receptor agonist, in rabbits and monkeys.

Mapracorat is a selective glucocorticoid receptor agonist in development for the treatment of a variety of ocular diseases. The purpose of this investigation was to evaluate the ocular pharmacokinetics of mapracorat after topical dosing over a range of dose levels in rabbits and monkeys. Mapracorat was administered over a range of doses from 0.01 to 3000 µg/eye (rabbit) or 50 to 3000 µg/eye (monkey). All animals received a single instillation, and monkeys also received repeated (three times per day for 4 days) instillations. At predetermined intervals through at least 24 h after dosing, ocular tissues and plasma were collected and analyzed for mapracorat by liquid chromatography-tandem mass spectrometry. Mapracorat was rapidly absorbed and widely distributed into ocular tissues after topical ocular administration, with measurable levels sustained through ≥24 h. In both species, mapracorat concentrations were highest in tears followed by conjunctiva and cornea, with lower levels observed in iris/ciliary body and aqueous humor. Mapracorat concentrations in conjunctiva, cornea, and iris/ciliary body increased linearly with increasing dose levels. Ocular exposure was higher after repeated dosing to monkeys than after a single dose. Systemic exposure to mapracorat was low after a single administration, with an average maximal concentration of ≤2.0 ng/ml at the highest dose tested (3000 µg/eye). In comparison with the traditional glucocorticoids, dexamethasone (0.1%) and prednisolone acetate (1%), mapracorat (3%) demonstrated similar or higher levels in ocular tissues with lower systemic exposure. The favorable pharmacokinetic profile of mapracorat supports further clinical investigation and suggests that a convenient daily dosing regimen may be efficacious for this novel ophthalmic anti-inflammatory therapy.

In vitro metabolism, glutathione conjugation, and CYP isoform specificity of epoxidation of 4-vinylphenol.

4-Vinylphenol (4VP) has been identified as a minor urinary metabolite of styrene in rat and human volunteers. This compound has been shown to be more hepatotoxic and pneumotoxic than both styrene and styrene oxide at lower doses in rats and mice. To explore the possible toxicity mechanism of 4VP, the current study was conducted to investigate the metabolism of 4VP, the glutathione (GSH) conjugation of the metabolites of 4VP and its cytochrome P(450) (CYP) specificity in epoxidation in different microsomes in vitro. Incubations of 4VP with mouse lung microsomes afforded two major metabolites which were identified as 4-(2-oxiranyl)-phenol of 4VP (4VPO) and 4VP catechol. 4VPO was found to react with GSH to form GSH conjugate and 4VP catechol was found to further be metabolized to electrophilic species which react with GSH to form the corresponding 4VP catechol GSH conjugates. Relative formation rates for those GSH conjugates and the regioisomer formation of 4VPO-GSH conjugates with both inhibitors of CYP 2F2 and CYP 2E1 in microsomal incubation condition were also investigated. This present study provides better insight on the lung toxicity seen with 4VP, the toxic metabolite of commercial styrene.

The effect of plasma lipids on the pharmacokinetics of chlorpyrifos and the impact on interpretation of blood biomonitoring data.

Lipophilic molecules, like chlorpyrifos (CPF), present a special problem for interpretation of biomonitoring data because both the environmental dose of CPF and the physiological (pregnancy, diet, etc.) or pathological levels of blood lipids will affect the concentrations of CPF measured in blood. The objective of this study was to investigate the distribution of CPF between plasma and tissues when lipid levels are altered in late pregnancy. CPF was sequestered more in the low-density lipid fraction of the blood during the late stages of gestation in the rat and returned to nonpregnant patterns in the dam after birth. Plasma partitioning of CPF increased with increases in plasma lipid levels and the increased partitioning of CPF into plasma lipids resulted in less CPF in other tissue compartments. Gavage dosing with corn oil also increased plasma lipids that led to a moderate increase of CPF partitioning into the plasma. To mechanistically investigate the potential pharmacokinetic effects of blood lipid changes, an existing CPF physiologically based pharmacokinetic/pharmacodynamic model for rats and humans was modified to account for altered lipid-tissue partition coefficients and for major physiological and biochemical changes of pregnancy. The model indicated that plasma CPF levels are expected to be proportional to the well-known changes in plasma lipids during gestation. There is a rapidly growing literature on the relationship of lipid profiles with different disease conditions and on birth outcomes. Increased blood concentrations of lipophilic chemicals like CPF may point to altered lipid status, as well as possibly higher levels of exposure. Thus, proper interpretation of blood biomonitoring data of lipophilic chemicals requires a careful consideration of blood lipids.

Dynamic changes in lipids and proteins of maternal, fetal, and pup blood and milk during perinatal development in CD and Wistar rats.

An understanding of the physiological factors that regulate perinatal dosimetry is essential to improve the ability of physiologically based (PB) pharmacokinetic (PK) models to predict chemical risks to children. However, the impact of changing maternal/offspring physiology on PK during gestation and lactation remains poorly understood. This research determined lipid and protein changes in blood, milk and amniotic fluid of CD and Wistar dams, fetuses and neonates to improve the precision of perinatal PBPK modeling. Samples were collected from time-mated CD dams, fetuses, and pups on gestation day (GD) 18 and 20 (sperm positive = GD 0) or lactation day 0 (day of birth), 1, 3, 5, 10, 15, and 20 (n > or = 5 per time point). Fewer time points were sampled in Wistar rats, which showed similar patterns to CDs. Relative to nonpregnant dams, maternal serum protein levels (albumin, total protein and globulin) each decreased by approximately 20% during late gestation, whereas maternal serum lipids (triglycerides, low density lipoproteins, and phospholipids) increased up to fourfold. These physiological changes can impact maternal PK of both protein-bound and lipophilic chemicals. During lactation, triglycerides in milk were greater than 100-fold higher than maternal serum, favoring the disposition of lipophilic chemicals into milk and potentially increasing neonatal rodent exposure during critical stages of postnatal development. Serum protein levels in pups were two- to threefold lower than adults at birth, which may increase the bioavailability of protein-bound compounds. These data will aid in the interpretation of perinatal toxicity studies and improve the accuracy of predictive perinatal PBPK models.

Investigations of oxidative stress, antioxidant response, and protein binding in chlorpyrifos exposed rat neuronal PC12 cells.

ABSTRACT Chlorpyrifos (CPF) is a widely used organophosphate insecticide. In addition to its known properties of cholinesterase inhibition, the production of reactive oxygen species (ROS) has been suggested as a possible toxic mechanism. To investigate CPF-generated ROS, rat neuronal PC12 cells were exposed to CPF concentrations of 0 to 5000 mug/mL in Krebs buffered media (KRH), KRH + 4% bovine serum albumin (BSA), and KRH + 25 muM of the antioxidant Trolox for 0 to 5 h. Paraquat served as a positive control for ROS. The fluorescent probe 2,7-dichlorodihydro-fluorescein and the MTS assay were used to measure ROS and cytotoxicity, respectively. Examinations into CPF-albumin binding were also conducted. CPF was not strongly cytotoxic to PC12 cells, causing only mild cytotoxicity at 5000 mug/ml. In KRH media, CPF-generated ROS was observed at 4 and 5 h at 500 and 1000 mug/mL, and at 1 to 5 h at 5000 mug/mL CPF. In KRH + 4% BSA, ROS was seen only at 5 h in 5000 mug/mL CPF. Trolox significantly reduced CPF- and paraquat-induced ROS. Calculated CPF-albumin binding at 1, 10, and 100 mug/mL CPF in 4% BSA was 96%, 75%, and 15%. These data show CPF at >/=500 mug/mL induced ROS in PC12 cells, but the addition of the antioxidant Trolox and 4% BSA dramatically reduced ROS levels.

Plant-Derived Small Molecule Inhibitors of Neuronal NO-Synthase: Potential Effects on Protein Degradation.

Cigarette smoking is known to cause a decrease in NO production in man resulting in a variety of pathological effects, including vascular dysfunction. Aqueous extracts of cigarette and cigarette smoke contain chemical inhibitors to NO-synthases, a heme-containing cytochrome P450 enzymes. More recently, it was shown that freshly harvested leaves from the tobacco plant (Nicotiana tabacum, Solanaceae) also contain chemical inhibitors to neuronal NO-synthase (nNOS). Examination of leaves from 32 other plants representing diverse members of the plant kingdom showed that 17 other plants, besides tobacco, contain these chemical inhibitors. Of all these plants, 16 are members of the core asterids flowering plant group and 6 are members of the Solanaceae family. Although the identity of the chemicals is not known, perhaps the closely related plants contain the same or similar compounds that inhibit nNOS. The inhibitory effects are not attributable to nicotine. The discovery of these chemicals and their further characterization may help to explain the loss of nNOS in smokers. In this addendum, we discuss these results in light of the effect of tobacco-derived chemicals in inhibiting P450 cytochromes, as well as our thoughts on how the inactivation of nNOS leads to its selective downregulation through proteolytic degradation.

Naturally occurring neuronal NO-synthase inactivators found in Nicotiana tabacum (Solanaceae) and other plants.

NO-synthase (NOS) is a heme-containing enzyme that catalyzes the oxidation of L-arginine to nitric oxide, an important cellular signaling molecule. Recently, it was found that aqueous extracts of tobacco cigarettes cause the inactivation of the neuronal isoform of NOS (nNOS) and that this may explain some of the toxicological effects of smoking. Although the exact identity of the chemical inactivator(s) is not known, we wondered if extracts prepared from other plants, including those closely related to tobacco, Nicotiana tabacum (Solanaceae), would similarly inactivate nNOS. We examined 33 plants, representing diverse members of the plant kingdom ranging from whisk fern, Psilotum nudum (Psilotaceae) to tobacco and discovered 18 plants that contain a chemical inactivator(s) of nNOS. Of these plants, 16 are members of the core asterids flowering plant group. Of these asterids, 6 are members of the Solanaceae family, of which tobacco is a member. Based on the phylogenetic relationship of the plants, it is possible that the same chemical or related chemical inactivator(s) exist. This, in turn, may help elucidate the structure of the chemical(s), as well as provide a source of a potentially novel inactivator of nNOS. The alkaloid nicotine can be excluded as putative nNOS inhibitor.

Tetrahydrobiopterin protects against guanabenz-mediated inhibition of neuronal nitric-oxide synthase in vitro and in vivo.

It is established that guanabenz inhibits neuronal nitric-oxide (NO) synthase (nNOS) and causes the enhanced proteasomal degradation of nNOS in vivo. Although the time- and NADPH-dependent inhibition of nNOS has been reported in studies where guanabenz was incubated with crude cytosolic preparations of nNOS, the exact mechanism for inhibition is not known. Moreover, even less is known about how the inhibition of nNOS triggers its proteasomal degradation. In the current study, we show, with the use of purified nNOS, that guanabenz treatment leads to the oxidation of tetrahydrobiopterin and formation of a pterin-depleted nNOS, which is not able to form NO. With the use of 14C-labeled guanabenz, we were unable to detect any guanabenz metabolites or guanabenz-nNOS adducts, indicating that reactive intermediates of guanabenz probably do not play a role in the inhibition. Superoxide dismutase, however, prevents the guanabenz-mediated oxidation of tetrahydrobiopterin and inhibition of nNOS, suggesting the role of superoxide as an intermediate. Studies in rats show that administration of tetrahydrobiopterin prevents the inhibition and loss of penile nNOS due to guanabenz, indicating that the loss of tetrahydrobiopterin plays a major role in the effects of guanabenz in vivo. Our findings are consistent with the destabilization and enhanced degradation of nNOS found after tetrahydrobiopterin depletion. These studies suggest that drug-mediated destabilization and subsequent enhanced degradation of protein targets will likely be an important toxicological consideration.

Tetrahydrobiopterin depletion and ubiquitylation of neuronal nitric oxide synthase.

Tetrahydrobiopterin is a necessary cofactor for the synthesis of nitric oxide by the hemeprotein enzyme, NO-synthase (NOS). It is widely thought that inadequate levels of tetrahydrobiopterin lead to tissue injury and organ dysfunction due, in part, to formation of superoxide from pterin-deficient NOS. In the course of studies on the ubiquitylation of neuronal NOS (nNOS), we have found that certain substrate analogs, such as N(G)-nitro-L-arginine, stabilize the dimeric form of nNOS and protect the enzyme from ubiquitylation. Since tetrahydrobiopterin is known to bind near heme and confers stability to the active dimeric structure of nNOS, we wondered if the loss of tetrahydrobiopterin could be an endogenous signal for nNOS ubiquitylation and degradation. We show here in HEK293 cells stably transfected with nNOS that depletion of tetrahydrobiopterin by treatment with 2,4-diamino-6-hydroxypyrimidine leads to destabilization of the dimeric form and enhances ubiquitylation of nNOS. Sepiapterin, a precursor to tetrahydrobiopterin in the salvage pathway, completely reverses the effect of 2,4-diamino-6-hydroxypyrimidine on nNOS ubiquitylation. Consistent with that found in cells, the in vitro ubiquitylation of nNOS by reticulocyte proteins decreases when tetrahydrobiopterin is present. Thus, inadequate amounts of tetrahydrobiopterin may lead to a sustained decrease in the steady state level of nNOS that is not readily reversed.

Time-dependent inhibition and tetrahydrobiopterin depletion of endothelial nitric-oxide synthase caused by cigarettes.

Smoking causes a dysfunction in endothelial nitric-oxide synthase (eNOS), which is ameliorated, in part, by administration of tetrahydrobiopterin (BH(4)). The exact mechanism by which the nitric oxide deficit occurs is unknown. We have previously shown that aqueous extracts of chemicals in cigarettes (CE) cause the suicide inactivation of neuronal NO synthase (nNOS) by interacting at the substrate-binding site. In the current study, we have found that CE directly inactivates eNOS by a process that is not affected by the natural substrate l-arginine and is distinct from the mechanism of inactivation of nNOS. We discovered that CE causes a time-, concentration-, and NADPH-dependent inactivation of eNOS in an in vitro system containing the purified enzyme, indicating a metabolic component to the inactivation. The CE-treated eNOS but not nNOS was nearly fully reactivated upon incubation with excess BH(4), suggesting that BH(4) depletion is a potential mechanism of inactivation. Moreover, in the presence of CE, eNOS catalyzed the oxidation of BH(4) to dihydrobiopterin and biopterin by a process attenuated by high concentrations of superoxide dismutase but not catalase. We speculate that a redox active component in CE, perhaps a quinone compound, causes oxidative uncoupling of eNOS to form superoxide, which in turn oxidizes BH(4). The discovery of a direct inactivation of eNOS by a compound(s) present in tobacco provides a basis not only for further study of the mechanisms responsible for the biological effects of tobacco but also a search for a potentially novel inactivator of eNOS.

Ubiquitination and degradation of neuronal nitric-oxide synthase in vitro: dimer stabilization protects the enzyme from proteolysis.

It is established that neuronal NO synthase (nNOS) is ubiquitinated and proteasomally degraded. The metabolism-based inactivation of nNOS and the inhibition of heat shock protein 90 (hsp90)-based chaperones, which are known to regulate nNOS, both lead to enhanced proteasomal degradation of nNOS. The mechanism of this selective proteolytic degradation, or in essence how the nNOS becomes labilized and recognized for ubiquitination and subsequent degradation, has not been determined. In the current study, we used a crude preparation of reticulocyte proteins, which contains ubiquitin-conjugating enzymes and the proteasome, to determine how nNOS is labilized. We found that the inactive monomeric heme-deficient nNOS (apo-nNOS) is rapidly degraded in vitro, consistent with the finding that both metabolism-based inactivation and inhibition of hsp90-based chaperones cause the formation of apo-nNOS and enhance its degradation in vivo. In the current study, we discovered that destabilization of the dimeric nNOS, as determined by measuring the SDS-resistant dimer, is sufficient to trigger ubiquitin-proteasomal degradation. Treatment of nNOS with NG-nitro-L-arginine or 7-nitroindazole led to stabilization of the dimeric nNOS and decreased proteasomal degradation of the enzyme, consistent with that observed in cells. Thus, it seems that the dimeric structure is a major determinant of nNOS stability and proteolysis.

The hsp90 cochaperone p23 is the limiting component of the multiprotein hsp90/hsp70-based chaperone system in vivo where it acts to stabilize the client protein: hsp90 complex.

A variety of signaling proteins form heterocomplexes with and are regulated by the heat shock protein chaperone hsp90. These complexes are formed by a multiprotein machinery, including hsp90 and hsp70 as essential and abundant components and Hop, hsp40, and p23 as non-essential cochaperones that are present in much lower abundance in cells. Overexpression of signaling proteins can overwhelm the capacity of this machinery to properly assemble heterocomplexes with hsp90. Here, we show that the limiting component of this assembly machinery in vitro in reticulocyte lysate and in vivo in Sf9 cells is p23. Only a fraction of glucocorticoid receptors (GR) overexpressed in Sf9 cells are in heterocomplex with hsp90 and have steroid binding activity, with the majority of the receptors present as both insoluble and cytosolic GR aggregates. Coexpression of p23 with the GR increases the proportion of cytosolic receptors that are in stable GR.hsp90 heterocomplexes with steroid binding activity, a strictly hsp90-dependent activity for the GR. Coexpression of p23 eliminates the insoluble GR aggregates and shifts the cytosolic receptor from very large aggregates without steroid binding activity to approximately 600-kDa heterocomplexes with steroid binding activity. These data lead us to conclude that p23 acts in vivo to stabilize hsp90 binding to client protein.

Metabolism-based inactivation of neuronal nitric-oxide synthase by components of cigarette and cigarette smoke.

It has been shown that administration of cigarette smoke to rats leads to loss of neuronal nitric-oxide synthase (nNOS) activity and nNOS protein in penile tissue. The exact mechanism for this loss of activity and protein is not known. In the current study, we investigated whether extracts prepared from cigarette smoke or from the cigarette itself could directly inhibit nNOS activity. We discovered that the cigarette smoke extract and the cigarette extract cause a time-, concentration-, and calmodulin-dependent inactivation of nNOS in an in vitro system containing the purified enzyme. L-Arginine, but not D-arginine, protects nNOS from this time-dependent inactivation, suggesting an active site directed event. The kinetics of inactivation are consistent with the metabolism-based or suicide inactivation of nNOS. Based on studies with other metabolism-based inactivators, this cigarette-mediated inactivation may render nNOS more susceptible to proteasomal degradation and thereby may explain the loss of nNOS protein in vivo. The component(s) responsible for nNOS inactivation is not volatile, is not retained by a 3,000 molecular weight cut-off membrane, binds to activated charcoal, and is highly water-soluble under both acidic and basic conditions. The discovery of a direct inactivation of nNOS by an organic, cationic compound(s) present in tobacco and tobacco smoke provides a basis for further study of not only the mechanisms responsible for the biological effects of tobacco but also a search for a potentially novel inactivator of nNOS.

Proteolytic degradation of nitric oxide synthase: effect of inhibitors and role of hsp90-based chaperones.

Nitric oxide synthase (NOS) is a highly regulated enzyme that produces nitric oxide, a critical messenger in many physiological processes. In this perspective, we explore the role of proteolytic degradation of NOS, in particular the inducible and neuronal isoforms of NOS, as a mechanism of regulation of the enzyme. The ubiquitin-proteasome and calpain pathways are the major proteolytic systems identified to date that are responsible for this regulated degradation. The degradation of NOS is affected by diverse agents, including glucocorticoids, caveolin, neurotoxic compounds, and certain NOS inhibitors. Some irreversible inactivators of NOS enhance the proteolytic degradation of the enzyme, and this property may be of great importance in understanding the biological effects of these inhibitors, some of which are being developed for clinical use. Analogies with the regulated degradation of liver microsomal cytochromes P450, which are related to NOS, provide a framework for understanding these processes. Finally, a new perspective on the regulation of NOS by hsp90-based chaperones is presented that involves facilitated heme insertion into the enzyme.

Alteration of the heme prosthetic group of neuronal nitric-oxide synthase during inactivation by N(G)-amino-L-arginine in vitro and in vivo.

It is established that N(G)-amino-L-arginine (NAA) is a metabolism-based inactivator of all three major nitric-oxide synthase (NOS) isoforms. The mechanism by which this inactivation occurs, however, is not well understood. In the current study, we discovered that inactivation of the neuronal isoform of NOS (nNOS) by NAA in vitro results in covalent alteration of the heme prosthetic group, in part, to products that contain an intact porphyrin ring and are either dissociable from or irreversibly bound to the protein. The alteration of the heme is concomitant with the loss of nNOS activity. Studies with nNOS containing a 14C-labeled prosthetic heme moiety indicate that the major dissociable product and the irreversibly bound heme adduct account for 21 and 28%, respectively, of the heme that is altered. Mass spectral analysis of the major dissociable product gave a molecular ion of m/z 775.3 that is consistent with the mass of an adduct of heme and NAA minus a hydrazine group. Peptide mapping of the irreversibly bound heme adduct indicates that the heme is bound to a residue in the oxygenase domain of nNOS. We show for the first time that metabolism-based inactivation of nNOS occurs in vivo as highly similar heme products are formed. Because inactivation and alteration may trigger ubiquitination and proteasomal degradation of nNOS, NAA may be a useful biochemical tool for the study of these basic regulatory processes.

hsp90 is required for heme binding and activation of apo-neuronal nitric-oxide synthase: geldanamycin-mediated oxidant generation is unrelated to any action of hsp90.

It is established that neuronal NO synthase (nNOS) is associated with the chaperone hsp90, although the functional role for this interaction has not been defined. We have discovered that inhibition of hsp90 by radicicol or geldanamycin nearly prevents the heme-mediated activation and assembly of heme-deficient apo-nNOS in insect cells. This effect is concentration-dependent with over 75% inhibition achieved at 20 microm radicicol. The ferrous carbonyl complex of nNOS is not formed when hsp90 is inhibited, indicating that functional heme insertion is prevented. We propose that the hsp90-based chaperone machinery facilitates functional heme entry into apo-nNOS by the opening of the hydrophobic heme-binding cleft in the protein. Previously, it has been reported that the hsp90 inhibitor geldanamycin uncouples endothelial NOS activity and increases endothelial NOS-dependent O(2)() production. Geldanamycin is an ansamycin benzoquinone, and we show here that it causes oxidant production from nNOS in insect cells as well as with the purified protein. At a concentration of 20 microm, geldanamycin causes a 3-fold increase in NADPH oxidation and hydrogen peroxide formation from purified nNOS, whereas the non-quinone hsp90 inhibitor radicicol had no effect. Thus, consistent with the known propensity of other quinones, geldanamycin directly redox cycles with nNOS by a process independent of any action on hsp90, cautioning against the use of geldanamycin as a specific inhibitor of hsp90 in redox-active systems.