Designing for selectivity: weak interactions and the competition for reactive sites on gold catalysts.

A major challenge in heterogeneous catalysis is controlling reaction selectivity, especially in complex environments. When more than one species is present in the gas mixture, the competition for binding sites on the surface of a catalyst is an important factor in determining reaction selectivity and activity. We establish an experimental hierarchy for the binding of a series of reaction intermediates on Au(111) and demonstrate that this hierarchy accounts for reaction selectivity on both the single crystal surface and under operating catalytic conditions at atmospheric pressure using a nanoporous Au catalyst. A partial set of measurements of relative binding has been measured by others on other catalyst materials, including Ag, Pd and metal oxide surfaces; a comparison demonstrates the generality of this concept and identifies differences in the trends. Theoretical calculations for a subset of reactants on Au(111) show that weak van der Waals interactions are key to predicting the hierarchy of binding strengths for alkoxides bound to Au(111). This hierarchy is key to the control of the selectivity for partial oxidation of alcohols to esters on both Au surfaces and under working catalytic conditions using nanoporous gold. The selectivity depends on the competition for active sites among key intermediates. New results probing the effect of fluorine substitution are also presented to extend the relation of reaction selectivity to the hierarchy of binding. Motivated by an interest in synthetic manipulation of fluorinated organics, we specifically investigated the influence of the -CF3 group on alcohol reactivity and selectivity. 2,2,2-Trifluoroethanol couples on O-covered Au(111) to yield CF3CH2O-C([double bond, length as m-dash]O)(CF3), but in the presence of methanol or ethanol it preferentially forms the respective 2,2,2-trifluoroethoxy-esters. The ester is not the dominant product in any of these cases, though, indicating that the rate of ß-H elimination from adsorbed trifluoroethoxy is slower than that for either adsorbed methoxy or ethoxy, consistent with their relative estimated ß-C-H bond strengths. The measured equilibrium constants for the competition for binding to the surface are 2.9 and 0.38 for ethanol and methanol, respectively, vs. 2,2,2-trifluoroethanol, indicating that the binding strength of 2,2,2-trifluoroethoxy is weaker than ethoxy, but stronger than methoxy. These results are consistent with weakening of the interactions between the surface and the alkyl group due to Pauli repulsion of the electron-rich CF3 group from the surface, which offsets the van der Waals attraction. These experiments provide guiding principles for understanding the effect of fluorination on heterogeneous synthesis and further demonstrate the key role of molecular structure in determining reaction selectivity.

Active site densities, oxygen activation and adsorbed reactive oxygen in alcohol activation on npAu catalysts.

The activation of molecular O2 as well as the reactivity of adsorbed oxygen species is of central importance in aerobic selective oxidation chemistry on Au-based catalysts. Herein, we address the issue of O2 activation on unsupported nanoporous gold (npAu) catalysts by applying a transient pressure technique, a temporal analysis of products (TAP) reactor, to measure the saturation coverage of atomic oxygen, its collisional dissociation probability, the activation barrier for O2 dissociation, and the facility with which adsorbed O species activate methanol, the initial step in the catalytic cycle of esterification. The results from these experiments indicate that molecular O2 dissociation is associated with surface silver, that the density of reactive sites is quite low, that adsorbed oxygen atoms do not spill over from the sites of activation onto the surrounding surface, and that methanol reacts quite facilely with the adsorbed oxygen atoms. In addition, the O species from O2 dissociation exhibits reactivity for the selective oxidation of methanol but not for CO. The TAP experiments also revealed that the surface of the npAu catalyst is saturated with adsorbed O under steady state reaction conditions, at least for the pulse reaction.

Evidence for transformation of spleen cells one day after infection of mice with friend leukemia virus.

Proliferation and erythroid differentiation of transplanted DBA/2 marrow cells and Friend virus-induced leukemic cells were assessed in syngeneic, allogeneic (H-2 compatible), and (BALB/c x DBA/2)F(1) hybrid mice (CDF(1)). Measurements were made 5 days after transplantation of donor cells into nonirradiated or X-irradiated mice by the spleen colony or the (125)IUdR-(59)Fe uptake methods. Growth of DBA/2J (Jackson subline) marrow grafts was poor in irradiated CDF(1)J hybrids as compared with growth in syngeneic and allogeneic hosts. The DBA/2J transplants proliferated, however, without impairment in irradiated CDF(1) hybrids which were the progeny of DBA/2 male parents of other sublines, e.g. DBA/2Ha, DBA/2Cr, and DBA/2Cum. In contrast, tissue-cultured Friend leukemic cells of DBA/2J origin grew deficiently in all CDF(1) hybrids tested, regardless of irradiation and of the DBA/2 parent's subline. The growth pattern of transplanted DBA/2J cells was a manifestation of hybrid resistance. The results with DBA/2J and other DBA/2 subline grafts suggested that hybrid histocompatibility alleles were expressed to a greater extent in leukemic than in normal marrow cells, for the former were consistently recognized as "nonself" by CDF(1) mice, but not the latter cells. The property of deficient growth in irradiated CDF(1)Ha hybrids was acquired by DBA/2J hemopoietic cells within 6 hr from infection in vivo with Friend leukemia virus, and persisted during the following 8 days. It was ascribed to enhanced expression of hybrid histocompatibility gene(s) (Hh) induced by the virus. Autonomous growth potential of hemopoietic cells, manifested by proliferation in nonirradiated recipients, was first detected 24 hr from infection, and likewise persisted at the later intervals. At the same time, the infected cells grew deficiently also in nonirradiated CDF(1)Ha mice. The two irreversible cellular changes were regarded as the earliest signals of virus-induced transformation.

Phosphoribosylamidotransferase: regulation of activity in virus-induced murine leukemia by purine nucleotides.

Infection with Friend leukemia virus causes a marked increase in the activity of splenic phosphoribosylamidotransferase in mice. Intraperitoneal injection of purine nucleotides and their free bases inhibits this enzyme. This is the first example of the control of phosphoribosylamidostransferase in vivo in the mammalian system as well as in virus-induced leukemia. Experiments in vitro support the findings in vivo.

Lymphomas in mice: failure of induction after a graft-versus-host reaction.

A mild graft-versus-host reaction induced in (BALB/cJ x DBA/2J) F(1) mice by the administration of parental spleen cells that differ at several weak histocompatibility loci did not influence the development of lymphomas in these animals. Rous sarcoma virus also failed to induce tumors in the runt and control animals. Breast carcinomas, presumably due to contamination of the inoculums with mammary tumor virus, occurred in those experimental groups given parental cells, whether or not they were viable or immunologically competent. We found no evidence that the immunologic process-as represented by the graft-versus-host reaction-is causally related to the induction of neoplasia.

Transient hydroxyl formation from water on oxygen-covered Au(111).

We present evidence for the formation of transient hydroxyls from the reaction of water with atomic oxygen on Au(111) and investigate the effect of adsorbed oxygen on the hydrogen bonding of water. Water is evolved in peaks at 175 and 195 K in temperature programed reaction experiments following adsorption of water on oxygen-covered Au(111). The peak at 175 K is ascribed to sublimation of multilayers of water, whereas the peak at 195 K is associated with oxygen-stabilized water or a water-hydroxyl surface complex. Infrared reflection absorption spectra are consistent with the presence of molecular water over the entire range of coverages studied, indicating that isolated stable hydroxyls are not formed. Isotopic exchange of adsorbed (16)O with H(2)(18)O following adsorption and subsequent temperature programed reaction, however, indicates that transient OH species are formed. The extent of oxygen exchange was considerable--up to 70%. The degree of oxygen exchange depends on the initial coverage of oxygen, the surface temperature when preparing oxygen adatoms, and the H(2)(18)O coverage. The hydroxyls are short-lived, forming and disproportionating multiple times before water desorption during temperature programed reaction. It was also found that chemisorbed oxygen is critical in the formation of hydroxyls and stabilizing water, whereas gold oxide does not contribute to these effects. These results identify transient hydroxyls as species that could play a critical role in oxidative chemical reactions on gold, especially in ambient water vapor. The crystallinity of adsorbed water also depended on the degree of surface ordering and chemical modification based on scanning tunneling microscopy and infrared spectra. These results demonstrate that oxidation of interfaces has a major impact on their interaction with water.

Efficient CO oxidation at low temperature on Au(111).

The rate of CO oxidation to CO2 depends strongly on the reaction temperature and characteristics of the oxygen overlayer on Au(111). The factors that contribute to the temperature dependence in the oxidation rate are (1) the residence time of CO on the surface, (2) the island size containing Au-O complexes, and (3) the local properties, including the degree of order of the oxygen layer. Three different types of oxygen--defined as chemisorbed oxygen, a surface oxide, and a bulk oxide--are identified and shown to have different reactivity. The relative populations of the various oxygen species depend on the preparation temperature and the oxygen coverage. The highest rate of CO oxidation was observed for an initial oxygen coverage of 0.5 monolayers that was deposited at 200 K where the density of chemisorbed oxygen is maximized. The rate decreases when two-dimensional islands of the surface oxide are populated and further decreases when three-dimensional bulk gold oxide forms. Our results are significant for designing catalytic processes that use Au for CO oxidation, because they suggest that the most efficient oxidation of CO occurs at low temperature--even below room temperature--as long as oxygen could be adsorbed on the surface.

The spleen in Friend leukemia. I. Prolonged survival of leukemic mice after autoimplantation of spleen tissue.

Friend virus infection of susceptible mice led rapidly to fulminant erythroleukemia and death. Subcutaneous implantation of leukemia spleen bits into splenectomized normal animals led to their early death from Friend leukemia. In contrast, bits of leukemic spleen implanted sc into splenectomized leukemic mice prolonged the survival of these animals. Concomitant with this survival was a reversal of the virus-induced immunosuppression and an increase in the levels of circulating, neutralizing, antivirus activity. This marked difference in response to leukemic spleen implants by leukemic as compared to normal mice reflected previous contact of the former with Friend Virus. Our studies indicated that the Friend virus-infected mouse mounted a resistance to the virus infection, which under certain conditions is capable of reversing the disease process.

Early and late changes in the glycoproteins of Friend erythroleukemia cells induced to differentiate.

Incorporation of labeled fucose and galactose into Friend virus-transformed murine erythroleukemia cells, uninduced and induced to differentiate by hexamethylene bisacetamide, was followed during the first 4 days of treatment. The plasma membrane and microsomal-mitochondrial fractions of the cells were isolated and purified. The rate of fucose and galactose incorporation into membranal proteins was markedly stimulated in the induced cells as compared to the uninduced cells. However, the total amount of the label incorporated was decreased. Binding of wheat germ agglutinin to M.W. 140,000 and 90,000 bands of the plasma membrane was increased within 24 hr of induction and occurred at the same time as increased fucose incorporation into protein bands of similar size. Stimulation of concanavalin A binding to the plasma membrane proteins within 74 hr of induction was correlated with increased galactose incorporation into M.W. 32,000 and 64,000 bands which were tentatively identified as the glycophorin bands.

Polyamine biosynthesis enzymes in the induction and inhibition of differentiation in Friend erythroleukemia cells.

O-Ornithine decarboxylase (ODC) activity in cultures of Friend erythroleukemia cells induced to differentiate with various compounds was examined. Based on ability to stimulate ODC activity, the inducers tested could be divided into two categories. Inducers of the first class, among which were dimethyl sulfoxide and hexamethylene bisacetamide, stimulated ODC activity and were inhibited by dexamethasone and the phorbol diester, 12-O-tetradecanoylphorbol-13-acetate. Specific inhibitors of polyamine biosynthesis, such as methylglyoxal bis(guanylhydrazone), an inhibitor of S-adenosylmethionine decarboxylase, and the ornithine analogs, alpha-methylornithine and alpha-hydrazinoornithine, also inhibited the induction of Friend erythroleukemia cell differentiation. The inhibition of induced differentiation by this class of compounds could be abrogated by spermine, spermidine, or putrescine. Inducers of the second class, among which were sodium butyrate, actinomycin D, and aminonucleoside of puromycin, had little or no stimulatory effect on ODC and were inhibited only by bromodeoxyuridine. The effect of bromodeoxyuridine, which inhibits inducers of both classes, was not abrogated by polyamines.

Granulocytic stem cells in Friend leukemia.

Friend virus induces a leukemia characterized by the proliferation of neoplastic hematopoietic cells believed to be erythroid precursors. In vitro studies were conducted with spleen cells from mice with terminal Firend leukemia in order to determine their capacity for leukocytic differentiation. Spleen cells were obtained from leukemic DBA/2 mice 1 to 2 days before anticipated death and cultured in the presence or absence of colony-stimulating activity (CSA). Growth in liquid culture in dissusion chambers was dependent on CSA and resulted in the generation of normally differentiated granulocytes and macrophages. Colony formation in agar was also dependent on CSA, and the cloning efficiency of leukemic spleen cells was found to be approximately 10 times normal. The colonies formed were composed of leukocytes, which appeared morphologically normal. Total in vitro colony-forming units per leukemic spleen exceeded normal by more than 300-fold, but cells elaborating CSA were decreased. Although it is uncertain whether the stem cells stimulated by CSA are "normal" or leukemic," it is clear that Friend leukemia has profound effects on the proliferation and differentiation of nonerythroid stem cells.

Depletion of sodium butyrate from the culture medium of Friend erythroleukemia cells undergoing differentiation.

Friend erythroleukemia cells can be induced to undergo erythroid differentiation by a variety of unrelated compounds. The fact that sodium butyrate causes reversible alterations in growth, morphology, and biochemistry in many cell systems prompted us to reexamine its pattern of induction of differentiation and to compare it to that of dimethyl sulfoxide (DMSO) and hexamethylbisacetamide (HMBA). By the fourth day of induction, a peak in hemoglobin accumulation was reached in the cultures treated with each of these potent inducers. Differences, however, were noted in cultures in which there had been no change of medium for 7 days. Whereas DMSO or HMBA induced cultures reached a stationary stage of growth and maintained a high percentage of benzidine positive cells, butyrate treated cultures resumed active growth and showed a marked decrease in the percentage of benzidine positive cells. However, the actual number of terminally differentiated cells remained relatively constant. The addition of fresh butyrate to 4-day treated cultures prevented the decrease in the percentage of benzidine positive cells. Measurement of [14C]butyrate uptake into the cells showed a decrease in the incorporation of the inducer with time coincident with the decrease in the percentage of benzidine positive cells and of the butyrate in the medium. Incorporation of [3H]thymidine into cells undergoing differentiation for 4 days indicated that butyrate treated cells, but not cells treated with DMSO or HMBA were capable of active DNA synthesis and growth after removal of the inducers. These data suggest that butyrate, a natural fatty acid, is metabolized by the cells and with time its concentration is reduced to a level below that required to stimulate differentiation. Additional evidence to support this notion are the results obtained with conditioned medium (CM) from induced cultures. CM-DMSO and CM-HMBA retained the capacity to induce differentiation whereas CM-butyrate lost its potency with time.

Observations on cell lines derived from a patient with Hodgkin's disease.

Permanent cell lines have been established from a spleen nodule and lymph node of a male Hodgkin's disease (HD) patient whose father has the same disease. Th in vitro growth pattern morphological and cytogenetic characteristics of these lines maintained continuously for over 2 years are described. The cultures contain a population of mixed cell types that grow in suspension. Between 5 and 10% of the cells have surface immunoglobulins M and D. B-cell alloantigens are also detectable. While the cultures are predominantly lymphoid, some of the large cells, by light and electron microscopy, resemble the Reed-Sternberg and Hodgkin's cells of the original biopsies. Although the cells maintain the human diploid karyotype, they are heterotransplantable in nude mice. After 14 months of culture, chromosome rearrangement and losses, commonly seen in leukemic bone marrow, occurred. Close to 100% of the cells are Epstein-Barr nuclear antigen positive, but they lack Epstein-Barr viral (EBV) capsid antigen and EBV-induced early antigen. Nucleic acid hybridization tests indicated that there were no more than two EBV genome equivalents per cell. Tests with HD sera free of anti-EBV were negative. Electron microscope examination of the cells revealed the presence of intracellular as well as extracellular rare pleomorphic particles ranging from 400 to 1200 A. The nature of these particles, which increased in number after the cultures were treated with halogenated pyrimidines but not with dimethyl sulfoxide, remains questionable. The cultures derived from the mouse-passaged HD cells, however, had reverse transcriptase activity and readily identifiable type C particles which were probably of murine origin. These cultures have some unique features that make them useful in studying the perplexing pathological entity of HD.

Promotion of formaldehyde production from adsorbed methoxy by oxidation of Mo(110) with NO2.

The reaction of methoxy (OCH3) in the presence of NO2 is studied on a thin-film oxide of Mo(110) for its relevance to the alkane-assisted reduction of NO(x). Temperature-programmed reaction indicates that oxygen deposited by NO2 dissociation promotes formaldehyde evolution. This pathway is not observed in any appreciable amount for methoxy on the thin-film oxide of Mo(110), nor for the reaction of methoxy in the presence of NO or O2. No new intermediates, in particular those containing C-N bonds, are detected after NO2 is exposed to the surface containing methoxy. Furthermore, infrared spectra provide evidence that the presence of NO2 does not significantly perturb the methoxy intermediate. These results indicate that surface oxidation rather than intermolecular complexation is the most likely mechanism by which NO2 promotes the evolution of oxygenates. In addition, the presence of methoxy decreases the capacity of the Mo surface to reduce NO2. No N2 is produced, and molecular desorption predominates. There are also no NO(x) species present after heating to 500 K when NO2 reacts in the presence of methoxy, whereas monomeric NO and dinitrosyl are present when NO2 reacts alone. These results are discussed in the context of CH4-assisted NO(x) reduction.

Low-temperature reduction of NO(2) on oxidized Mo(110).

The reactions of nitrogen dioxide (NO(2)) were investigated on oxidized Mo(110) containing both chemisorbed oxygen and a thin film oxide. NO(2) reacts on both oxidized Mo(110) surfaces via a combination of reversible adsorption and reduction to NO, N(2), and trace amounts of N(2)O below 200 K. On the surface containing chemisorbed O, there is some complete dissociation of NO(2) to yield N(a) and O(a). N(2) forms at high temperatures through atom combination. On both surfaces, NO is the predominant product of NO(2) reduction. However, the chemisorbed layer which has a low oxidation state, and hence a greater capacity to accept oxygen, more effectively reduces NO(2). The selectivity for N(2) formation over N(2)O is greater for NO(2) as compared with NO on both surfaces studied. The selectivity changes are largely attributed to an increase in the concentration of Mo=O species and a change in the distribution of oxygen on the surface. Notably, more oxygen, in particular Mo=O moieties, is deposited by NO(2) reaction than by O(2) reaction, indicating that NO(2) is a stronger oxidant. The fact that there are several N-containing species on the surface at low temperatures may also affect the product distribution. On both surfaces, N(2)O(4), NO(2), and NO are identified by infrared spectroscopy upon adsorption at 100 K. All N(2)O(4) desorbs by 200 K, leaving only NO(2) and NO on the surface. Infrared spectroscopy of NO(2) on (18)O-labeled surfaces provides evidence for oxygen transfer or exchange between different types of sites even at low temperatures.

Water dissociation associated with NO2 coadsorption on Mo(110)-(1 x 6)-O: effect of coverage and electronic properties of oxygen.

Water dissociation on an oxygen-covered Mo(110) surface was investigated using temperature-programmed reaction spectroscopy (TPRS) and infrared reflectance absorbance spectroscopy (IRAS). Adsorbed hydroxyl formation is enhanced by increasing the coverage of chemisorbed oxygen prior to exposure to water up to saturation (0.66 ML). Additional oxidation of the surface using NO(2) suppresses the formation of hydroxyl species (OH). There is no detectable change in the reaction of NO(2) on Mo(110)-(1 x 6)-O when either the water or hydroxyl is adsorbed on the Mo(110)-(1 x 6)-O surface prior to NO(2) adsorption. In contrast, NO(2) induces the displacement of water into the gas phase and the conversion of hydroxyl species to molecular water. Infrared spectra show that the dissociation of NO(2) populates three types of terminal oxygen sites on Mo(110)-(1 x 6)-O, and the population of the terminal oxygen at step sites increases with respect to the amount of NO(2) deposited. Overall, these results suggest that the oxidic property of oxygen results in a lack of activity for the water dissociation.

Increased mouse minor hemoglobin during erythroid stress: a model for hemoglobin regulation.

In mice with "diffuse" hemoglobin (Hb), the decrease in the proportion of minor Hb during ontogeny qualitatively resembles the decline observed in human Hb F. Since Hb F reappears during some forms of erythroid stress, we investigated the effect of hematopoietic stress on minor Hb in DBA/2 mice. The stresses were acetlyphenylhydrazine-induced hemolysis, phlebotomy, or infection with Friend erythroleukemia virus. Recovery from anemia was associated with a transient increase in the synthesis of minor Hb similar to the reappearance of Hb F in man. Minor Hb synthesis also increased during the evolution of erythroleukemia induced by both the anemic and the polycythemic strains of virus. Thus, the mouse model can be used to study Hb regulation, since changes in the modulation of minor Hb synthesis occur under conditions which are associated with alterations in Hb F synthesis in humans.

Induction of differentiation in Friend erythroleukemia cells with dimethyl sulfoxide, hexamethylene bisacetamide and sodium butyrate is not accompanied by changes in proviral DNA or its expression.

The integration pattern, copy number and DNA expression of the Friend virus genome was examined in Friend erythroleukemia cells induced to differentiate with dimethyl sulfoxide, hexamethylene bisacetamide and sodium butyrate. The integrated proviral DNA in Friend erythroleukemia cells was examined by Southern hybridization with a cloned Friend virus (F-MuLV) probe at days 1 and 4 following inducer treatment, as well as on day 6 at which time the cells had remained for 48 h in inducer free medium. KpnI fragments 9 and 5.7 kb long were observed. The copy number of each fragment remained constant throughout the erythroid differentiation process. EcoRI digestion of DNA isolated from cells at different times following the inducer treatment demonstrated multiple integration sites of the proviral genome, which also remained constant during the differentiation process. Proviral DNA expression was examined at 4 h and 4 days following inducer treatment as well as on day 6 by which time cells remained for 48 h in inducer free medium. Northern blot hybridization to the F-MuLV probe indicated no change in the provirus gene expression independent of the class of inducers. These observations reinforce our conclusions that the viral genome and its transcription product do not play a major role in the differentiation process of Friend erythroleukemic cells.

Vaccinia virus DNA sequences in the nucleus of persistently infected Friend erythroleukemia cells.

FL vac cell lines are Friend erythroleukemia cells persistently infected with vaccinia virus. These cells produce attenuated leukemia virus, virulent poxvirus, resist superinfection with vaccinia, and show high levels of spontaneous erythrodifferentiation and decreased tumorigenicity in syngeneic hosts (Pogo, G.T. and Friend, C. (1982) Proc. Natl. Acad. Sci. USA 79, 4805-4809). To determine whether resistance to superinfection was associated with the presence of vaccinia DNA in the nucleus, DNA from cells at different passage levels was hybridized to a vaccinia DNA probe. Vaccinia DNA sequences were detected in the nucleus of cells of lines that were productively infected with vaccinia. No such sequences were detected in productively infected with vaccinia. No such sequences were detected in productively infected L cells nor in persistently infected cell lines that no longer produced infectious particles but were resistant to superinfection. Although no evidence of integration of vaccinia DNA was observed, differences in the restriction patterns were detected at some passage levels. The presence of vaccinia virus DNA sequences in the nucleus apparently did not affect the size of the provirus, the integration pattern or the expression of the leukemia virus.

Inhibition of nitric oxide with aminoguanidine reduces bacterial translocation after endotoxin challenge in vivo.

BACKGROUND: Administration of lipopolysaccharide (LPS) has been shown to increase bacterial translocation (BT) in vivo and in vitro. In addition, LPS upregulates inducible nitric oxide synthase expression in the intestinal epithelium-a phenomenon that can either enhance microbial killing, or alternatively, promote BT by impairing the gut barrier. OBJECTIVE: To determine the effect, if any, of an inhibitor of nitric oxide synthase, namely, aminoguanidine (AG), on BT after LPS challenge. DESIGN: Sprague-Dawley rats were randomized to receive either AG or normal saline solution via subcutaneously placed osmotic pumps (Alzet), followed 18 hours later by LPS injection (5 mg/kg or 20 mg/kg intraperitoneally). Quantitative cultures of the cecum, mesenteric lymph nodes, liver, and spleen were obtained, and plasma nitrite and nitrate levels were measured at 24 hours. Transmembrane potential difference and mucosal permeability to fluorescein isothiocyanate-labeled dextran and fluorescein isothiocyanate-labeled Escherichia coli C25 were measured in the Using chamber. The intestinal membrane was examined by light, transmission electron, and confocal laser microscopy. RESULTS: Rats that were given high-dose LPS had elevated levels of nitrite and nitrate and a 100% incidence of BT. In contrast, AG infusion significantly reduced both BT (22%) and nitrite and nitrate levels. Animals that received LPS and normal saline solution had a significantly lower transmembrane potential difference than those that received LPS and AG. High-dose LPS resulted in sloughing of the apical enterocytes at the villus tips where bacterial entry seemed to occur, as seen with confocal laser microscopy. CONCLUSIONS: Inhibition of nitric oxide production with AG decreases BT after high-dose LPS challenge. The mechanism may involve increased cellular viability and decreased damage to the gut mucosal barrier in rats that receive AG.