Dioxygen and glucose force motion of the electron-transfer switch in the iron(III) flavohemoglobin-type nitric oxide dioxygenase

Kinetic and structural investigations of the flavohemoglobin-type NO dioxygenase have suggested critical roles for transient Fe(III)O2 complex formation and O2-forced movements affecting hydride transfer to the FAD cofactor and electron-transfer to the Fe(III)O2 complex. Stark-effect theory together with structural models and dipole and internal electrostatic field determinations provided a semi-quantitative spectroscopic method for investigating the proposed Fe(III)O2 complex and O2-forced movements. Deoxygenation of the enzyme causes Stark effects on the ferric heme Soret and charge-transfer bands revealing the Fe(III)O2 complex. Deoxygenation also elicits Stark effects on the FAD that expose forces and motions that create a more restricted NADH access to FAD for hydride transfer and switch electron-transfer off. Glucose also forces the enzyme toward an off state. Amino acid substitutions at the B10, E7, E11, G8, D5, and F7 positions influence the Stark effects of O2 on resting heme spin states and FAD consistent with the proposed roles of the side chains in the enzyme mechanism. Deoxygenation of ferric myoglobin and hemoglobin A also induces Stark effects on the hemes suggesting a common 'oxy-met' state. The ferric myoglobin and hemoglobin heme spectra are also glucose-responsive. A conserved glucose or glucose-6-phosphate binding site is found bridging the BC-corner and G-helix in flavohemoglobin and myoglobin suggesting novel allosteric effector roles for glucose or glucose-6-phosphate in the NO dioxygenase and O2 storage functions. The results support the proposed roles of a ferric O2 intermediate and protein motions in regulating electron-transfer during NO dioxygenase turnover.

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases

Nitric-oxide dioxygenases (NODs) activate and combine O₂ with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O₂ and NO, gates for NO and nitrate, transient O₂ association with ferric heme, and an O₂ and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b₅ oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b₅ oxidoreductase and cytochrome b₅. Cytoglobin interfaces weakly with cytochrome b₅ for O₂ and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O₂ binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

Allostery in the nitric oxide dioxygenase mechanism of flavohemoglobin

The substrates O₂ and NO cooperatively activate the NO dioxygenase function of Escherichia coli flavohemoglobin. Steady-state and transient kinetic measurements support a structure-based mechanistic model in which O₂ and NO movements and conserved amino acids at the E11, G8, E2, E7, B10, and F7 positions within the globin domain control activation. In the cooperative and allosteric mechanism, O₂ migrates to the catalytic heme site via a long hydrophobic tunnel and displaces LeuE11 away from the ferric iron, which forces open a short tunnel to the catalytic site gated by the ValG8/IleE15 pair and LeuE11. NO permeates this tunnel and leverages upon the gating side chains triggering the CD loop to furl, which moves the E and F-helices and switches an electron transfer gate formed by LysF7, GlnE7, and water. This allows FADH₂ to reduce the ferric iron, which forms the stable ferric–superoxide–TyrB10/GlnE7 complex. This complex reacts with internalized NO with a bimolecular rate constant of 10¹⁰ M⁻¹ s⁻¹ forming nitrate, which migrates to the CD loop and unfurls the spring-like structure. To restart the cycle, LeuE11 toggles back to the ferric iron. Actuating electron transfer with O₂ and NO movements averts irreversible NO poisoning and reductive inactivation of the enzyme. Together, structure snapshots and kinetic constants provide glimpses of intermediate conformational states, time scales for motion, and associated energies.

Globins Scavenge Sulfur Trioxide Anion Radical

Ferrous myoglobin was oxidized by sulfur trioxide anion radical (STAR) during the free radical chain oxidation of sulfite. Oxidation was inhibited by the STAR scavenger GSH and by the heme ligand CO. Bimolecular rate constants for the reaction of STAR with several ferrous globins and biomolecules were determined by kinetic competition. Reaction rate constants for myoglobin, hemoglobin, neuroglobin, and flavohemoglobin are large at 38, 120, 2,600, and ≥ 7,500 × 10(6) m(-1) s(-1), respectively, and correlate with redox potentials. Measured rate constants for O2, GSH, ascorbate, and NAD(P)H are also large at ∼100, 10, 130, and 30 × 10(6) m(-1) s(-1), respectively, but nevertheless allow for favorable competition by globins and a capacity for STAR scavenging in vivo. Saccharomyces cerevisiae lacking sulfite oxidase and deleted of flavohemoglobin showed an O2-dependent growth impairment with nonfermentable substrates that was exacerbated by sulfide, a precursor to mitochondrial sulfite formation. Higher O2 exposures inactivated the superoxide-sensitive mitochondrial aconitase in cells, and hypoxia elicited both aconitase and NADP(+)-isocitrate dehydrogenase activity losses. Roles for STAR-derived peroxysulfate radical, superoxide radical, and sulfo-NAD(P) in the mechanism of STAR toxicity and flavohemoglobin protection in yeast are suggested.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

Nitric-oxide dioxygenase function of human cytoglobin with cellular reductants and in rat hepatocytes

Cytoglobin (Cygb) was investigated for its capacity to function as a NO dioxygenase (NOD) in vitro and in hepatocytes. Ascorbate and cytochrome b(5) were found to support a high NOD activity. Cygb-NOD activity shows respective K(m) values for ascorbate, cytochrome b(5), NO, and O(2) of 0.25 mm, 0.3 microm, 40 nm, and approximately 20 microm and achieves a k(cat) of 0.5 s(-1). Ascorbate and cytochrome b(5) reduce the oxidized Cygb-NOD intermediate with apparent second order rate constants of 1000 m(-1) s(-1) and 3 x 10(6) m(-1) s(-1), respectively. In rat hepatocytes engineered to express human Cygb, Cygb-NOD activity shows a similar k(cat) of 1.2 s(-1), a K(m)(NO) of 40 nm, and a k(cat)/K(m)(NO) (k'(NOD)) value of 3 x 10(7) m(-1) s(-1), demonstrating the efficiency of catalysis. NO inhibits the activity at [NO]/[O(2)] ratios >1:500 and limits catalytic turnover. The activity is competitively inhibited by CO, is slowly inactivated by cyanide, and is distinct from the microsomal NOD activity. Cygb-NOD provides protection to the NO-sensitive aconitase. The results define the NOD function of Cygb and demonstrate roles for ascorbate and cytochrome b(5) as reductants.

Hemoglobins dioxygenate nitric oxide with high fidelity

Distantly related members of the hemoglobin (Hb) superfamily including red blood cell Hb, muscle myoglobin (Mb) and the microbial flavohemoglobin (flavoHb) dioxygenate nitric oxide (.NO). The reaction serves important roles in .NO metabolism and detoxification throughout the aerobic biosphere. Analysis of the stoichiometric product nitrate shows greater than 99% double O-atom incorporation from Hb(18)O(2), Mb(18)O(2) and flavoHb(18)O(2) demonstrating a conserved high fidelity .NO dioxygenation mechanism. Whereas, reactions of .NO with the structurally unrelated Turbo cornutus MbO(2) or free superoxide radical (-O.(2)) yield sub-stoichiometric nitrate showing low fidelity O-atom incorporation. These and other results support a .NO dioxygenation mechanism involving (1) rapid reaction of .NO with a Fe(III-)O.(2) intermediate to form Fe(III-)OONO and (2) rapid isomerization of the Fe(III-)OONO intermediate to form nitrate. A sub-microsecond isomerization event is hypothesized in which the O-O bond homolyzes to form a protein caged [Fe(IV)O .NO(2)] intermediate and ferryl oxygen attacks .NO(2) to form nitrate. Hb functions as a .NO dioxygenase by controlling O(2) binding and electrochemistry, guiding .NO diffusion and reaction, and shielding highly reactive intermediates from solvent water and biomolecules.

Inducible defense mechanism against nitric oxide in Candida albicans

The yeast Candida albicans is an opportunistic pathogen that threatens patients with compromised immune systems. Immune cell defenses against C. albicans are complex but typically involve the production of reactive oxygen species and nitrogen radicals such as nitric oxide (NO) that damage the yeast or inhibit its growth. Whether Candida defends itself against NO and the molecules responsible for this defense have yet to be determined. The defense against NO in various bacteria and the yeast Saccharomyces cerevisiae involves an NO-scavenging flavohemoglobin. The C. albicans genome contains three genes encoding flavohemoglobin-related proteins, CaYHB1, CaYHB4, and CaYHB5. To assess their roles in NO metabolism, we constructed strains lacking each of these genes and demonstrated that just one, CaYHB1, is responsible for NO consumption and detoxification. In C. albicans, NO metabolic activity and CaYHB1 mRNA levels are rapidly induced by NO and NO-generating agents. Loss of CaYHB1 increases the sensitivity of C. albicans to NO-mediated growth inhibition. In mice, infections with Candida strains lacking CaYHB1 still resulted in lethality, but virulence was decreased compared to that in wild-type strains. Thus, C. albicans possesses a rapid, specific, and highly inducible NO defense mechanism involving one of three putative flavohemoglobin genes.

Imidazole antibiotics inhibit the nitric oxide dioxygenase function of microbial flavohemoglobin

Flavohemoglobins metabolize nitric oxide (NO) to nitrate and protect bacteria and fungi from NO-mediated damage, growth inhibition, and killing by NO-releasing immune cells. Antimicrobial imidazoles were tested for their ability to coordinate flavohemoglobin and inhibit its NO dioxygenase (NOD) function. Miconazole, econazole, clotrimazole, and ketoconazole inhibited the NOD activity of Escherichia coli flavohemoglobin with apparent K(i) values of 80, 550, 1,300, and 5,000 nM, respectively. Saccharomyces cerevisiae, Candida albicans, and Alcaligenes eutrophus enzymes exhibited similar sensitivities to imidazoles. Imidazoles coordinated the heme iron atom, impaired ferric heme reduction, produced uncompetitive inhibition with respect to O(2) and NO, and inhibited NO metabolism by yeasts and bacteria. Nevertheless, these imidazoles were not sufficiently selective to fully mimic the NO-dependent growth stasis seen with NOD-deficient mutants. The results demonstrate a mechanism for NOD inhibition by imidazoles and suggest a target for imidazole engineering.

Nitric oxide dioxygenase function and mechanism of flavohemoglobin, hemoglobin, myoglobin and their associated reductases

Microbial flavohemoglobins (flavoHbs) and hemoglobins (Hbs) show large *NO dioxygenation rate constants ranging from 745 to 2900 microM(-1) s(-1) suggesting a primal *NO dioxygenase (NOD) (EC 1.14.12.17) function for the ancient Hb superfamily. Indeed, modern O2-transporting and storing mammalian red blood cell Hb and related muscle myoglobin (Mb) show vestigial *NO dioxygenation activity with rate constants of 34-89 microM(-1) s(-1). In support of a NOD function, microbial flavoHbs and Hbs catalyze O2-dependent cellular *NO metabolism, protect cells from *NO poisoning, and are induced by *NO exposures. Red blood cell Hb, myocyte Mb, and flavoHb-like activities metabolize *NO in the vascular lumen, muscle, and other mammalian cells, respectively, decreasing *NO signalling and toxicity. HbFe(III)-OO*, HbFe(III)-OONO and protein-caged [HbFe(III)-O**NO2] are proposed intermediates in a reaction mechanism that combines both O-atoms of O2 with *NO to form nitrate and HbFe(III). A conserved Hb heme pocket structure facilitates the dioxygenation reaction and efficient turnover is achieved through the univalent reduction of HbFe(III) by associated reductases. High affinity flavoHb and Hb heme ligands, and other inhibitors, may find application as antibiotics and antitumor agents that enhance the toxicity of immune cell-derived *NO or as vasorelaxants that increase *NO signalling.

Dioxygen-dependent metabolism of nitric oxide

Nitric oxide (NO) serves critical signaling, energetic, and toxic functions throughout the biosphere. NO steady-state levels and functions are controlled in part by NO metabolism or degradation. Dioxygen-dependent NO dioxygenases (EC 1.14.12.17) and dioxygen-independent NO reductases (EC 1.7.99.7) are being identified as major routes for NO metabolism in various life forms. Here we describe the use of the Clark-type NO electrode, mechanistic inhibitors, and nitrate/nitrite assays to measure, characterize, and identify major NO metabolic pathways and enzymes in bacteria, fungi, plants, mammalian cells, and organelles. The methods may prove to be particularly useful for mechanistic investigations and the development of inhibitors, inducers, and other novel NO-modulating therapeutics.

Nitric oxide metabolism in mammalian cells: substrate and inhibitor profiles of a NADPH-cytochrome P450 oxidoreductase-coupled microsomal nitric oxide dioxygenase

Human intestinal Caco-2 cells metabolize and detoxify NO via a dioxygen- and NADPH-dependent, cyanide- and CO-sensitive pathway that yields nitrate. Enzymes catalyzing NO dioxygenation fractionate with membranes and are enriched in microsomes. Microsomal NO metabolism shows apparent KM values for NO, O2, and NADPH of 0.3, 9, and 2 microM, respectively, values similar to those determined for intact or digitonin-permeabilized cells. Similar to cellular NO metabolism, microsomal NO metabolism is superoxide-independent and sensitive to heme-enzyme inhibitors including CO, cyanide, imidazoles, quercetin, and allicin-enriched garlic extract. Selective inhibitors of several cytochrome P450s and heme oxygenase fail to inhibit the activity, indicating limited roles for a subset of microsomal heme enzymes in NO metabolism. Diphenyleneiodonium and cytochrome c(III) inhibit NO metabolism, suggesting a role for the NADPH-cytochrome P450 oxidoreductase (CYPOR). Involvement of CYPOR is demonstrated by the specific inhibition of the NO metabolic activity by inhibitory anti-CYPOR IgG. In toto, the results suggest roles for a microsomal CYPOR-coupled and heme-dependent NO dioxygenase in NO metabolism, detoxification, and signal attenuation in mammalian cells.

Regulation of the nitric oxide reduction operon (norRVW) in Escherichia coli. Role of NorR and sigma54 in the nitric oxide stress response

Nitric oxide (NO) induces NO-detoxifying enzymes in Escherichia coli suggesting sensitive mechanisms for coordinate control of NO defense genes in response to NO stress. Exposure of E. coli to sub-micromolar NO levels under anaerobic conditions rapidly induced transcription of the NO reductase (NOR) structural genes, norV and norW, as monitored by lac gene fusions. Disruption of rpoN (sigma(54)) impaired the NO-mediated induction of norV and norW transcription and NOR expression, whereas disruption of the upstream regulatory gene, norR, completely ablated NOR induction. NOR inducibility was restored to NorR null mutants by expressing NorR in trans. Furthermore, an internal deletion of the N-terminal domain of NorR activated NOR expression independent of NO exposure. Neither NorR nor sigma(54) was essential for NO-mediated induction of the NO dioxygenase (flavohemoglobin) encoded by hmp. However, elevated NOR activity inhibited NO dioxygenase induction, and, in the presence of dioxygen, NO dioxygenase inhibited norV induction by NO. The results demonstrate the role of NorR as a sigma(54)-dependent regulator of norVW expression. A role for the NorR N-terminal domain as a transducer or sensor for NO is suggested.

Pseudomonas aeruginosa anaerobic respiration in biofilms: relationships to cystic fibrosis pathogenesis

Recent data indicate that cystic fibrosis (CF) airway mucus is anaerobic. This suggests that Pseudomonas aeruginosa infection in CF reflects biofilm formation and persistence in an anaerobic environment. P. aeruginosa formed robust anaerobic biofilms, the viability of which requires rhl quorum sensing and nitric oxide (NO) reductase to modulate or prevent accumulation of toxic NO, a byproduct of anaerobic respiration. Proteomic analyses identified an outer membrane protein, OprF, that was upregulated approximately 40-fold under anaerobic versus aerobic conditions. Further, OprF exists in CF mucus, and CF patients raise antisera to OprF. An oprF mutant formed poor anaerobic biofilms, due, in part, to defects in anaerobic respiration. Thus, future investigations of CF pathogenesis and therapy should include a better understanding of anaerobic metabolism and biofilm development by P. aeruginosa.

Nitric oxide scavenging and detoxification by the Mycobacterium tuberculosis haemoglobin, HbN in Escherichia coli

Nitric oxide (NO), generated in large amounts within the macrophages, controls and restricts the growth of internalized human pathogen, Mycobacterium tuberculosis H37Rv. The molecular mechanism by which tubercle bacilli survive within macrophages is currently of intense interest. In this work, we have demonstrated that dimeric haemoglobin, HbN, from M. tuberculosis exhibits distinct nitric oxide dioxygenase (NOD) activity and protects growth and cellular respiration of heterologous hosts, Escherichia coli and Mycobacterium smegmatis, from the toxic effect of exogenous NO and the NO-releasing compounds. A flavohaemoglobin (HMP)-deficient mutant of E. coli, unable to metabolize NO, acquired an oxygen-dependent NO consumption activity in the presence of HbN. On the basis of cellular haem content, the specific NOD activity of HbN was nearly 35-fold higher than the single-domain Vitreoscilla haemoglobin (VHb) but was sevenfold lower than the two-domain flavohaemoglobin. HbN-dependent NO consumption was sustained with repeated addition of NO, demonstrating that HbN is catalytically reduced within E. coli. Aerobic growth and respiration of a flavohaemoglobin (HMP) mutant of E. coli was inhibited in the presence of exogenous NO but remained insensitive to NO inhibition when these cells produced HbN, VHb or flavohaemoglobin. M. smegmatis, carrying a native HbN very similar to M. tuberculosis HbN, exhibited a 7.5-fold increase in NO uptake when exposed to gaseous NO, suggesting NO-induced NOD activity in these cells. In addition, expression of plasmid-encoded HbN of M. tuberculosis in M. smegmatis resulted in 100-fold higher NO consumption activity than the isogenic control cells. These results provide strong experimental evidence in support of NO scavenging and detoxification function for the M. tuberculosis HbN. The catalytic NO scavenging by HbN may be highly advantageous for the survival of tubercle bacilli during infection and pathogenesis.

Flavorubredoxin, an inducible catalyst for nitric oxide reduction and detoxification in Escherichia coli

Nitric oxide (NO) is a poison, and organisms employ diverse systems to protect against its harmful effects. In Escherichia coli, ygaA encodes a transcription regulator (b2709) controlling anaerobic NO reduction and detoxification. Adjacent to ygaA and oppositely transcribed are ygaK (encoding a flavorubredoxin (flavoRb) (b2710) with a NO-binding non-heme diiron center) and ygbD (encoding a NADH:(flavo)Rb oxidoreductase (b2711)), which function in NO reduction and detoxification. Mutation of either ygaA or ygaK eliminated inducible anaerobic NO metabolism, whereas ygbD disruption partly impaired the activity. NO-sensitive [4Fe-4S] (de)hydratases, including the Krebs cycle aconitase and the Entner-Doudoroff pathway 6-phosphogluconate dehydratase, were more susceptible to inactivation in ygaK or ygaA mutants than in the parental strain, and these metabolic poisonings were associated with conditional growth inhibitions. flavoRb (NO reductase) and flavohemoglobin (NO dioxygenase) maximally metabolized and detoxified NO in anaerobic and aerobic E. coli, respectively, whereas both enzymes scavenged NO under microaerobic conditions. We suggest designation of the ygaA-ygaK-ygbD gene cluster as the norRVW modulon for NO reduction and detoxification.

Flavohemoglobin detoxifies nitric oxide in aerobic, but not anaerobic, Escherichia coli. Evidence for a novel inducible anaerobic nitric oxide-scavenging activity

Nitric-oxide dioxygenase (NOD) and reductase (NOR) activities of flavohemoglobin (flavoHb) have been suggested as mechanisms for NO metabolism and detoxification in a variety of microbes. Mechanisms of NO detoxification were tested in Escherichia coli using flavoHb-deficient mutants and overexpressors. flavoHb showed negligible anaerobic NOR activity and afforded no protection to the NO-sensitive aconitase or the growth of anoxic E. coli, whereas the NOD activity and the protection afforded with O(2) were substantial. A NO-inducible, O(2)-sensitive, and cyanide-resistant NOR activity efficiently metabolized NO and protected anaerobic cells from NO toxicity independent of the NOR activity of flavoHb. flavoHb possesses nitrosoglutathione and nitrite reductase activities that may account for the protection it affords against these agents. NO detoxification by flavoHb occurs most effectively via O(2)-dependent NO dioxygenation.

Dioxygen-dependent metabolism of nitric oxide in mammalian cells

Steady-state gradients of NO within tissues and cells are controlled by rates of NO synthesis, diffusion, and decomposition. Mammalian cells and tissues actively decompose NO. Of several cell lines examined, the human colon CaCo-2 cell produces the most robust NO consumption activity. Cellular NO metabolism is mostly O2-dependent, produces near stoichiometric NO3-, and is inhibited by the heme poisons CN-, CO (K(I) approximately 3 microM), phenylhydrazine, and NO and the flavoenzyme inhibitor diphenylene iodonium. NO consumption is saturable by O2 and NO and shows apparent K(M) values for O2 and NO of 17 and 0.2 microM, respectively. Mitochondrial respiration, O2*-, and H2O2 are neither sufficient nor necessary for O2-dependent NO metabolism by cells. The existence of an efficient mammalian heme and flavin-dependent NO dioxygenase is suggested. NO dioxygenation protects the NO-sensitive aconitases, cytochrome c oxidase, and cellular respiration from inhibition, and may serve a dual function in cells by limiting NO toxicity and by spatially coupling NO and O2 gradients.

Nitric-oxide dioxygenase activity and function of flavohemoglobins. sensitivity to nitric oxide and carbon monoxide inhibition

Widely distributed flavohemoglobins (flavoHbs) function as NO dioxygenases and confer upon cells a resistance to NO toxicity. FlavoHbs from Saccharomyces cerevisiae, Alcaligenes eutrophus, and Escherichia coli share similar spectra, O(2), NO, and CO binding kinetics, and steady-state NO dioxygenation kinetics. Turnover numbers (V(max)) for S. cerevisiae, A. eutrophus, and E. coli flavoHbs are 112, 290, and 365 NO heme(-1) s(-1), respectively, at 37 degrees C with 200 microm O(2). The K(M) values for NO are low and range from 0.1 to 0.25 microm. V(max)/K(M)(NO) ratios of 900-2900 microm(-1) s(-1) indicate an extremely efficient dioxygenation mechanism. Approximate K(M) values for O(2) range from 60 to 90 microm. NO inhibits the dioxygenases at NO:O(2) ratios of > or =1:100 and makes true K(M)(O(2)) values difficult to determine. High and roughly equal second order rate constants for O(2) and NO association with the reduced flavoHbs (17-50 microm(-1) s(-1)) and small NO dissociation rate constants suggest that NO inhibits the dioxygenase reaction by forming inactive flavoHbNO complexes. Carbon monoxide also binds reduced flavoHbs with high affinity and competitively inhibits NO dioxygenases with respect to O(2) (K(I)(CO) = approximately 1 microm). These results suggest that flavoHbs and related hemoglobins evolved as NO detoxifying components of nitrogen metabolism capable of discriminating O(2) from inhibitory NO and CO.

Steady-state and transient kinetics of Escherichia coli nitric-oxide dioxygenase (flavohemoglobin). The B10 tyrosine hydroxyl is essential for dioxygen binding and catalysis

Escherichia coli expresses an inducible flavohemoglobin possessing robust NO dioxygenase activity. At 37 degrees C, the enzyme shows a maximal turnover number (V(max)) of 670 s(-1) and K(m) values for NADH, NO, and O(2) equal to 4.8, 0.28, and approximately 100 microM, respectively. Individual reduction, ligand binding, and NO dioxygenation reactions were examined at 20 degrees C, where V(max) is approximately 94 s(-1). Reduction by NADH occurs in two steps. NADH reduces bound FAD with a rate constant of approximately 15 microM(-1) s(-1), and heme iron is reduced by FADH(2) with a rate constant of 150 s(-1). Dioxygen binds tightly to reduced flavohemoglobin, with association and dissociation rate constants equal to 38 microM(-1) s(-1) and 0.44 s(-1), respectively, and the oxygenated flavohemoglobin dioxygenates NO to form nitrate. NO also binds reversibly to reduced flavohemoglobin in competition with O(2), dissociates slowly, and inhibits NO dioxygenase activity at [NO]/[O(2)] ratios of 1:100. Replacement of the heme pocket B10 tyrosine with phenylalanine increases the O(2) dissociation rate constant approximately 80-fold and reduces NO dioxygenase activity approximately 30-fold, demonstrating the importance of the tyrosine hydroxyl for O(2) affinity and NO scavenging activity. At 37 degrees C, V(max)/K(m)(NO) is 2,400 microM(-1) s(-1), demonstrating that the enzyme is extremely efficient at converting toxic NO into nitrate under physiological conditions.

Constitutive and adaptive detoxification of nitric oxide in Escherichia coli. Role of nitric-oxide dioxygenase in the protection of aconitase

Nitric oxide (NO.) is a naturally occurring toxin that some organisms adaptively resist. In aerobic or anaerobic Escherichia coli, low levels of NO. exposure inactivated the NO.-sensitive citric acid cycle enzyme aconitase, and inactivation was more effective when the adaptive synthesis of NO.-defensive proteins was blocked with chloramphenicol. Protection of aconitase in aerobically grown E. coli was dependent upon O2, was potently inhibited by cyanide, and was correlated with an induced rate of cellular NO. consumption. Constitutive and adaptive cellular NO. consumption in aerobic cells was also dependent upon O2 and inhibited by cyanide. Exposure of aerobic cells to NO. accordingly elevated the activity of the O2-dependent and cyanide-sensitive NO. dioxygenase (NOD). Anaerobic E. coli exposed to NO. or nitrate induced a modest O2-independent and cyanide-resistant NO.-metabolizing activity and a more robust O2-stimulated cyanide-sensitive activity. The latter activity was attributed to NOD. The results support a role for NOD in the aerobic detoxification of NO. and suggest functions for NOD and a cyanide-resistant NO. scavenging activity in anaerobic cells.

Nitric oxide dioxygenase: an enzymic function for flavohemoglobin

Nitric oxide (NO*) is a toxin, and various life forms appear to have evolved strategies for its detoxification. NO*-resistant mutants of Escherichia coli were isolated that rapidly consumed NO*. An NO*-converting activity was reconstituted in extracts that required NADPH, FAD, and O2, was cyanide-sensitive, and produced NO3-. This nitric oxide dioxygenase (NOD) contained 19 of 20 N-terminal amino acids identical to those of the E. coli flavohemoglobin. Furthermore, NOD activity was produced by the flavohemoglobin gene and was inducible by NO*. Flavohemoglobin/NOD-deficient mutants were also sensitive to growth inhibition by gaseous NO*. The results identify a function for the evolutionarily conserved flavohemoglobins and, moreover, suggest that NO* detoxification may be a more ancient function for the widely distributed hemoglobins, and associated methemoglobin reductases, than dioxygen transport and storage.

Nitric oxide sensitivity of the aconitases

Aconitases are important cellular targets of nitric oxide (NO.) toxicity, and NO.-derived species, rather than NO. per se, have been proposed to mediate their inactivation. NO.-mediated inactivation of the Escherichia coli aconitase and the porcine mitochondrial aconitase was investigated. In E. coli, aconitase activity decreased by approximately 70% during a 2-h exposure to an atmosphere containing 120 ppm NO. in N2. The NO.-inactivated aconitase reactivated poorly in E. coli under anaerobic or aerobic conditions. Elevated superoxide dismutase activity did not affect the aerobic inactivation of aconitase by NO., thus indicating a limited role of the NO.- and superoxide-derived species peroxynitrite. Glutathione-deficient and glutathione-containing E. coli were comparably sensitive to NO.-mediated aconitase inactivation, thus excluding the participation of S-nitrosoglutathione or more oxidizing NO.-derived species. NO. progressively decreased aconitase activity in extracts in the presence of substrates, and inactivation was greatest at an acidic pH with cis-aconitate. The porcine mitochondrial aconitase was sensitive to NO. when exposed at pH 6.5, but not at pH 7.5, and irreversible inactivation occurred during catalysis. The requirement of an acidic pH or substrates for sensitivity may explain the reported resistance of aconitases to NO. in vitro (Castro, L., Rodriguez, M., and Radi, R. (1994) J. Biol. Chem. 269, 29409-29415; Hausladen, A., and Fridovich, I. (1994) J. Biol. Chem. 269, 29405-29408). An S-nitrosation of the aconitase [4Fe-4S] center catalyzed by the solvent-exposed electron withdrawing iron atom (Fea) is proposed.

Superoxide-driven aconitase FE-S center cycling

O2- produced by the autoxidation of respiratory chain electron carriers, and other cellular reductants, inactivates bacterial and mammalian iron-sulfur-containing (de)hydratases including the citric acid cycle enzyme aconitase. Release of the solvent-exposed iron atom and oxidation of the [4Fe-4S]2+ cluster accompanies loss of catalytic activity. Rapid reactivation is achieved by iron-sulfur cluster reduction and Fe2+ insertion. Inactivation-reactivation is a dynamic and cyclical process which modulates aconitase and (de)hydratase activities in Escherichia coli and mammalian cells. The balance of inactive and active aconitase provides a sensitive measure of the changes in steady-state O2- levels occurring in living cells and mitochondria under stress conditions. Aconitases are also inactivated by other oxidants including O2, H2O2, NO, and ONOO- which are associated with inflammation, hyperoxia and other pathophysiological conditions. Loss of aconitase activity during oxidant stress may impair energy production, and the liberation of reactive iron may further enhance oxidative damage. Iron-sulfur center cycling may also serve adaptive functions by modulating gene expression or by signaling metabolic quiescence.

Failure of tumor necrosis factor and interleukin-1 to elicit superoxide production in the mitochondrial matrices of mammalian cells

Subversion of mitochondrial electron transport to the production of O2.- has been proposed as a mechanism of tumor necrosis factor (TNF)-mediated cell killing and to a lesser extent interleukin-1 (IL-1) and lipopolysaccharide (LPS) cytotoxicity. We utilized the O2.- -sensitive aconitases to measure changes in steady-state 02.- levels in the mitochondrial matrix and cytoplasm of cultured mammalian cells in response to these inflammatory mediators. TNF alpha did not measurably affect aconitase activity, and thus mitochondrial 02.- production, in either cultured human A549 cells or murine L929 cells while TNF alpha clearly caused cytotoxicity as revealed by impaired mitochondrial respiration. IL-1 alpha and Escherichia coli LPS also failed to affect the aconitase activity in A549 cells. Neither the O2.- scavenger Mn(III) TMPyP nor the H2O2 scavenger catalase protected L929 cells against the cytotoxicity of TNF alpha. In conclusion, TNF, IL-1, and LPS do not appear to exert cytotoxicity, or MnSOD gene induction effects, by eliciting mitochondrial O2.- production.

Superoxide production by the mycobacterial and pseudomonad quinoid pigments phthiocol and pyocyanine in human lung cells

The quinoid pigments pthiocol, produced by Mycobacterium tuberculosis, and pyocyanine, produced by Pseudomonas aeruginosa, were examined for their effects on O2.- production in cultured human lung epithelial-like A549 cells. Intracellular O2.- levels were measured using the O2.-sensitive aconitase(s), and rates of O2.- generation were assessed from rates of antimycin-resistant respiration. Elevated O2.- was detected in cells exposed to < 25 microM phthiocol and < 2 microM pyocyanine in neutral pH medium, and both agents impaired cell growth. The O2.- scavenging manganoporphyrin, Mn(III)TMPyP, partially protected cells against pyocyanine and phthiocol-mediated growth inhibition. O2.- production by phthiocol and pyocyanine was enhanced by acidification of the growth medium. Surprisingly, the dicumarol-inhibitable quinoid detoxification enzyme DT-diaphorase was a significant source of phthiocol and pyocyanine-mediated O2.- generation in cells. O2.- production in macrophages by the phthiocol analog, menadione, was shown to impair macrophage mitochondrial respiration and bactericidal activity toward Escherichia coli. Phthiocol and pyocyanine, by producing O2.-/H2O2, and inhibiting host cell aconitase activity, energetics, and other host cell functions, may contribute to the pathogenicity of M. tuberculosis and P. aeruginosa.

Superoxide scavenging by Mn(II/III) tetrakis (1-methyl-4-pyridyl) porphyrin in mammalian cells

The superoxide dismutase mimic Mn(II/III) tetrakis (1-methyl-4-pyridyl) porphyrin (Mn(II/III)TMPyP) was examined for its superoxide radical (O2.-)-scavenging ability in cultured mammalian cells. Mn(III)TMPyP (< 5 microM) added to culture media relieved growth inhibition and decreased the inactivation of the O2(.-)-sensitive enzyme aconitase in cells exposed to the O2(.-)-generating phenazine pyocyanine. Treatment of cells with Mn(III)TMPyP did not measurably affect cellular O2.- production as revealed by rates of cyanide-resistant respiration with or without added pyocyanine. In contrast, Mn(II/III)TMPyP enhanced O2.- production in cells when the redox-active naphthoquinone menadione was present as measured by both increased cyanide-resistant respiration rates and aconitase inactivation. In vitro, Mn(II/III)TMPyP catalyzed the oxidation of ascorbate, and menadione enhanced this effect. Mn(III)TMPyP did not protect aconitase when O2.- production was elicited in mitochondria by antimycin A and the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone. The results support a reductant-O2.-:oxidoreductase mechanism for O2.- scavenging by Mn(II/III)TMPyP in the mammalian cytosol as proposed for its action in Escherichia coli, but also indicate that Mn(II/III)TMPyP can either scavenge or produce O2.- in cells depending upon the prevailing pathways of Mn(II/III)TMPyP oxidation-reduction.

Superoxide radical and iron modulate aconitase activity in mammalian cells

Aconitase is a member of a family of iron-sulfur-containing (de)hydratases whose activities are modulated in bacteria by superoxide radical (O2-.)-mediated inactivation and iron-dependent reactivation. The inactivation-reactivation of aconitase(s) in cultured mammalian cells was explored since these reactions may impact important and diverse aconitase functions in the cytoplasm and mitochondria. Conditions which increase O2-. production including exposure to the redox-cycling agent phenazine methosulfate (PMS), inhibitors of mitochondrial ubiquinol-cytochrome c oxidoreductase, or hyperoxia inactivated aconitase in mammalian cells. Overproduction of mitochondrial Mn-superoxide dismutase protected aconitase from inactivation by PMS or inhibitors of ubiquinol-cytochrome c oxidoreductase, but not from normobaric hyperoxia. Aconitase activity was reactivated (t1/2 of 12 +/- 3 min) upon removal of PMS. The iron chelator deferoxamine impaired reactivation and increased net inactivation of aconitase by O2-.. The ability of ubiquinol-cytochrome c oxidoreductase-generated O2-. to inactivate aconitase in several cell types correlated with the fraction of the aconitase activity localized in mitochondria. Extracellular O2-. generated with xanthine oxidase did not affect aconitase activity nor did exogenous superoxide dismutase decrease aconitase inactivation by PMS. The results demonstrate a dynamic and cyclical O2-.-mediated inactivation and iron-dependent reactivation of the mammalian [4Fe-4S] aconitases under normal and stress conditions and provide further evidence for the membrane compartmentalization of O2-..

Aconitase is a sensitive and critical target of oxygen poisoning in cultured mammalian cells and in rat lungs

The effect of hyperoxia on activity of the superoxide-sensitive citric acid cycle enzyme aconitase was measured in cultured human epithelial-like A549 cells and in rat lungs. Rapid and progressive loss of > 80% of the aconitase activity in A549 cells was seen during a 24-hr exposure to a PO2 of 600 mmHg (1 mmHg = 133 Pa). Inhibition of mitochondrial respiratory capacity correlated with loss of aconitase activity in A549 cells exposed to hyperoxia, and this effect could be mimicked by fluoroacetate (or fluorocitrate), a metabolic poison of aconitase. Exposure of rats to an atmospheric PO2 of 760 mmHg or 635 mmHg for 24 hr caused respective 73% and 61% decreases in total lung aconitase activity. We propose that early inactivation of aconitase and inhibition of the energy-producing and biosynthetic reactions of the citric acid cycle contribute to the sequelae of lung damage and edema seen during exposure to hyperoxia.

NADPH inhibits transcription of the Escherichia coli manganese superoxide dismutase gene (sodA) in vitro

We have previously reported that the thiols glutathione, dithiothreitol, and beta-mercaptoethanol suppress transcription of the Escherichia coli manganese-containing superoxide dismutase gene (sodA) in an in vitro coupled transcription plus translation system (Gardner, P. R., and Fridovich, I. (1987) J. Biol. Chem. 262, 17591-17595). We now report that NADPH, but not NADH, selectively decreases transcription of sodA in vitro and that an NADPH generating system utilizing glucose 6-phosphate and the corresponding dehydrogenase markedly augments this suppressive effect. A redox buffer containing various ratios of oxidized and reduced glutathione also modulated transcription of sodA thus demonstrating the existence of a redox-sensitive mechanism controlling sodA transcription. Fusion of a 120-base pair fragment, containing 90 base pairs of DNA upstream of the sodA transcription initiation site, to a promoterless galactokinase gene (galK) conferred redox-sensitivity to GalK synthesis. We propose that these redox effects act through a redox-sensitive regulator of sodA and that the anabolic reduction charge, [NADPH]/([NADPH]+[NADP+]), is one cellular signal controlling sodA transcription.

Effect of glutathione on aconitase in Escherichia coli

The effect of glutathione (GSH) on the superoxide-sensitive [4Fe-4S]-containing aconitase of Escherichia coli was explored. A mutant deficient in GSH biosynthesis, designated gshA, grew slower in a defined medium than did the parental strain and this effect was more pronounced when succinate was supplied as the carbon source in place of glucose. This suggested that the citric acid cycle was compromised in the gshA strain. Aconitase activity was approximately 25% lower in GSH-deficient cells growing on either glucose or succinate, and was lower still in strains producing less superoxide dismutase. Addition of GSH to the medium stimulated growth of the gshA strain on succinate. It also elevated the aconitase activity in the presence of chloramphenicol, which was added to block protein synthesis. Dithiothreitol and 2-mercaptoethanol were much less effective in this regard than was GSH. Exposure of cultures to 4.2 atm O2 caused a rapid decline in aconitase activity and this was the case in both GSH-proficient and GSH-deficient E. coli; however, the reactivation which was seen when the hyperoxic exposure was terminated was significantly impaired in the gshA strain. There is a dynamic balance between inactivation of aconitase by superoxide and reactivation by Fe(II) and this balance is altered in GSH-deficient E. coli. GSH may facilitate reactivation of aconitase, and of other [4Fe-4S]-containing dehydratases, by increasing the rate of transfer of Fe(II) to the [3Fe-4S] site.

Inactivation-reactivation of aconitase in Escherichia coli. A sensitive measure of superoxide radical

The rapid inactivation of aconitase by O2-, previously seen to occur in vitro, was explored in vivo. A fraction of the aconitase in growing, aerobic, Escherichia coli is inactive at any instant but can be activated by imposition of anaerobic conditions. This reactivation occurred in the absence of protein synthesis and was inhibited by the ferrous chelator alpha,alpha'-dipyridyl. This fraction of inactive, but activatable, aconitase was increased by augmenting O2- production with paraquat, decreased by elevation of superoxide dismutase, and increased by inhibiting reactivation with alpha,alpha'-dipyridyl. The balance between inactive and active aconitase thus represented a pseudoequilibrium between inactivation by O2- and reactivation by restoration of Fe(II), and it provided, for the first time, a measure of the steady-state concentration of O2- within E. coli. On this basis, [O2-] was estimated to be approximately 20-40 pM in aerobic log phase E. coli containing wild type levels of superoxide dismutase and approximately 300 pM in a mutant strain lacking superoxide dismutase.

Superoxide sensitivity of the Escherichia coli aconitase

Mutants of Escherichia coli lacking superoxide dismutase (SOD) activity were used to explore the sensitivity of aconitase toward O2 and O2-. The aconitase activity in SOD-free extracts was rapidly lost under aerobic conditions and exogenous SOD afforded a concentration-dependent protection. The rate of the inactivating reaction between O2- and aconitase was estimated to be of the order of 10(9) M-1 s-1. The competitive inhibitors fluorocitrate and tricarballylate provided some protection, and at saturating concentrations, they decreased the rate of the inactivating reaction by 100- and 10-fold, respectively. Aconitase was markedly less sensitive to O2 than it was to O2-. Aerobic growth on succinate involves a greater dependence upon aconitase than does growth on glucose and, as expected, the deleterious consequences of SOD deficiency were more pronounced on succinate than on glucose. Moreover, aconitase activity was lower in extracts of aerobically grown SOD mutants, than it was in the parental strain. We suppose that inactivation of aconitase by O2- involves oxidative attack on the prosthetic iron-sulfur cluster. The extreme sensitivity of aconitase to inactivation by O2- suggests that its inactivation will be an early event in the oxidative stress imposed by hyperoxia, ultraviolet irradiation or redox-cycling agents, such as viologens or quinones.

Superoxide sensitivity of the Escherichia coli 6-phosphogluconate dehydratase

The activity of 6-phosphogluconate dehydratase was significantly lower in extracts of aerobically grown Escherichia coli deficient in superoxide dismutase (sodAsodB) and in mutants lacking the inducible manganese-containing superoxide dismutase (sodA), exposed to the redox-cycling agent paraquat, than in the parental strain. Growth of these strains on a gluconate minimal medium was also impaired under these conditions. The enzyme was most susceptible to dioxygen in superoxide dismutase (SOD)-free extracts, and exogenous SOD afforded a concentration-dependent protection against inactivation. The amount of SOD necessary for full protection was comparable to the amount normally present in extracts of aerobic E. coli (7-36 units/mg protein), and the rate of reaction of O2- with the dehydratase was estimated to be approximately 2.0 x 10(8) M-1 s-1. The dehydratase was much less sensitive to O2 or H2O2 than to O2-. The virtual substrate, alpha-glycerophosphate, provided partial protection. Iron chelators, thiol-reactive reagents, and oxidants, including nitrite and diamide, inactivated the enzyme. Fluoride ions stabilized the dehydratase and blocked the effect of oxidants. The O2(-)-sensitive target site is proposed to be an iron-sulfur cluster which is readily destroyed by oxidation.

Quinolinate synthetase: the oxygen-sensitive site of de novo NAD(P)+ biosynthesis

The ability of niacin to relieve the growth-inhibiting effect of hyperoxia on Escherichia coli can be attributed to the dioxygen sensitivity of quinolinate synthetase. The activity of this enzyme within E. coli was diminished by exposure of the cells to 4.2 atm O2, while the activity in extracts was rapidly decreased by 0.2 atm O2. Neither catalase nor superoxide dismutase afforded detectable protection against the inactivating effect of O2, indicating that H2O2 and O2- were not significant intermediates in this process. Nevertheless, H2O2 at 1.0 mM did inactivate quinolinate synthetase, even under anaerobic conditions and in the absence of catalatic activity which might have generated O2. Addition of paraquat to aerobic cultures of E. coli caused an inactivation of quinolinate synthetase, which may be explained in terms of an increase in the production of H2O2. The O2-dependent inactivation of quinolinate synthetase in extracts was gradually reversed during anaerobic incubation and this reactivation was blocked by alpha, alpha'-dipyridyl or by 1,10-phenanthroline. The sequence of the quinolinate synthetase "A" protein contains a--cys-w-x-cys-y-z-cys--sequence, which is characteristic of (Fe-S)4-containing proteins. This sequence, together with the effect of the Fe(II)-chelating agents, suggests that the O2-sensitive site of quinolinate synthetase is an iron-sulfur cluster which is essential for the dehydration reaction catalyzed by the A protein.

Quinolinate phosphoribosyl transferase is not the oxygen-sensitive site of nicotinamide adenine dinucleotide biosynthesis

Quinolinate phosphoribosyl transferase (QPT) activity was not affected when Escherichia coli were treated with hyperbaric oxygen. This result is not in accord with a previous report (Biochem. Biophys. Res. Comm. 91:982-990; 1979) in which the enzyme was shown to be rapidly inactivated in E. Coli exposed to 4.2 atmospheres of oxygen. Our data rule out QPT as a site of oxygen toxicity and suggest other mechanisms for the inhibitory effects of the hyperbaric oxygen on pyridine nucleotide biosynthesis.

Controls on the biosynthesis of the manganese-containing superoxide dismutase of Escherichia coli. Effects of thiols

In vitro synthesis of Escherichia coli manganese-containing superoxide dismutase, directed by the plasmid pDT1-5, has been achieved. The Mn superoxide dismutase polypeptide was identified by electrophoresis on polyacrylamide gels, immunoprecipitation, and the competitive immunoprecipitation effect of pure, active E. coli Mn superoxide dismutase. Dithiothreitol and glutathione, but not cysteine, suppressed in vitro synthesis of Mn superoxide dismutase. The parallel syntheses of beta-lactamase and of another unidentified polypeptide were not suppressed by thiols. In vitro transcription of the E. coli Mn superoxide dismutase gene was similarly suppressed by glutathione, dithiothreitol, and beta-mercaptoethanol; but not by L-cysteine or thioglycolate. Compounds, such as diamide, 1-chloro-2,4-dinitrobenzene, potassium ferricyanide, and methylene blue, which are expected to deplete intracellular glutathione, caused the induction of Mn superoxide dismutase in anaerobic E. coli.

Use of a synthetic peptide antigen to generate antisera reactive with a proteolytic processing site in native human proinsulin: demonstration of cleavage within clathrin-coated (pro)secretory vesicles

Polyclonal antibodies reactive with a cleavage site in human proinsulin (HPI) (C-peptide-A-chain junction) have been raised (rabbit, guinea pig) using a synthetic peptide antigen coupled with keyhole limpet hemocyanin. These antisera recognize native HPI and des-31,32-HPI equally well but react 20-50 times less well with des-64,65-HPI, the intermediate cleaved at the C-peptide-A-chain junction and lacking the Lys-Arg pair. The guinea pig antisera did not recognize insulin but reacted weakly with C peptide at high concentrations; the rabbit antisera reacted with neither insulin nor C peptide. Immunocytochemical studies with human islet tissue localized the immunoreactivity of these antisera to clathrin-coated (pro)secretory vesicles derived from the trans Golgi, indicating that cleavage of the C-peptide-A-chain junction of proinsulin occurs mainly, if not exclusively, in this compartment of the beta cell.

Amino acid sequence of the mRNA cap-binding protein from human tissues

The 25-kDa mRNA cap-binding protein (CBP) involved in translation was purified by affinity chromatography from human erythrocytes and rabbit reticulocytes. The sequences of eight human and seven rabbit tryptic and V8 proteolytic peptides were determined. Based on the peptide sequence data, oligodeoxynucleotide probes were synthesized and used to screen human fibroblast and lymphocyte lambda cDNA libraries. The DNA sequence obtained from recombinant lambda phage inserts was found to code for all but one peptide. A 23-base oligonucleotide was synthesized based on the DNA sequence and used to prime synthesis of cDNA from human placental mRNA to construct a third library in lambda gt10. Screening with a 22-base oligonucleotide, whose sequence was upstream from the 23-base primer, yielded numerous recombinant phages with approximately equal to 250-base inserts. The 1900-base-pair cDNA sequence compiled from all phage inserts appeared to represent the entire primary sequence of CBP (Mr 25,117). Blot analysis of human placental and HeLa mRNA revealed multiple CBP mRNA species ranging from 1925 to 2250 bases. The amino acid sequence of CBP showed homology to the cap-binding PB2 protein of influenza virus.

Biopsychosocial approach to premenstrual syndrome

Premenstrual syndrome is a constellation of behavioral, physical and psychologic symptoms that occur every 28 days in some women of childbearing age. A multidimensional, biopsychosocial approach, combining stress reduction, increased exercise and proper diet, has been found effective in treating this syndrome. Medications can be added to relieve symptoms. The cornerstone of this approach is empathic understanding.

Structural analysis of the messenger RNA cap-binding protein. Presence of phosphate, sulfhydryl, and disulfide groups

The messenger RNA cap-binding protein (CBP) was isolated from human erythrocyte, rabbit erythrocyte, and rabbit reticulocyte lysate by affinity chromatography on 7-methylguanosine 5'-triphosphate-Sepharose. The specific activity of binding to capped oligonucleotides was similar for the human erythrocyte and rabbit reticulocyte CBPs. Isoelectric focusing of human and rabbit preparations revealed that each was composed of up to five species. The pI values of human and rabbit CBPs ranged from 5.7 to 6.5. The predominant form in erythrocytes had a pI of 6.3 while in reticulocytes, two major species, having pI values of 5.9 and 6.3, were present. Labeling of rabbit reticulocytes with [32P]orthophosphate revealed that the pI 5.9 but not the pI 6.3 form contained phosphate. All of the phosphate was found in phosphoserine residues. The amino acid compositions of human erythrocyte and rabbit reticulocyte CBPs were quite similar. Both proteins had 7 tryptophanyl and 6 cysteinyl residues. Labeling with [1-14C]iodoacetic acid under native and denaturing conditions provided evidence that 2 of the cysteinyl residues are present in the reduced form and 4 in disulfide bridges. Species of CBP with faster or slower electrophoretic mobilities could be generated by treatment of the protein either with O2 in the presence of a catalyst or with dithiothreitol. The predominant form of the untreated protein migrated between these two forms.

Spin?Parity Combinations in 32S

Inelastically scattered IX-particles from the reaction 32S(IX, 1X')32S have been studied with solid state counters at extreme backward angles in order to determine spin-parity combinations for levels in 32S at excitation energies Ex up to 7 �15 MeV. The results confirm the well-established spin and parity values, show that the 5� 798 MeV spin 1 state has negative parity, and provide narrow limits for the possible spin and parity values of the 6'410,6' 666,6' 762, and 6� 854 MeV levels. A previously unreported natural parity level was found at Ex = 6�58 MeV. Magnetic analysis of the reaction 32S(p, p')32S confirmed the existence of this level and established its excitation energy as 6�581�0�003 MeV. Particle-y-ray coincidence studies showed that this level decays predominantly by y-ray transitions to the 2�23 MeV 2 + state.

Gamma Ray Decay Schemes of Levels at Intermediate Energies in 32S

The reaction 32S(p,p'y)32S has been studied with a 12'7 cm by 10�2 cm NaI(TI) y-ray detector in conjunction with a 61 cm double-focusing magnetic spectrometer to determine the y-ray decay schemes of all known levels in 32S between the excitation energies of 5�40 and 7� 15 MeV.

Polarization Studies of Ground?State Radiation from the 2·21 and 2·73 MeV Levels of 27Al

The E2/Ml mixing ratios of the ground-state )i-ray transitions from the 2�21 and 2� 73 MeV levels of 27 Al have been measured using a three-crystal Compton polarimeter in order to resolve discrepancies between previous experimental results for the 2� 73 MeV transition.