DNAJB6 chaperones PP2A mediated dephosphorylation of GSK3β to downregulate β-catenin transcription target, osteopontin

Elevated levels of the oncoprotein, osteopontin (OPN), are associated with poor outcome of several types of cancers including melanoma. We have previously reported an important involvement of DNAJB6, a member of heat-shock protein 40 (HSP40) family, in negatively impacting tumor growth. The current study was prompted by our observations reported here which revealed a reciprocal relationship between DNAJB6 and OPN in melanoma specimens. The 'J domain' is the most conserved domain of HSP40 family of proteins. Hence, we assessed the functional role of the J domain in activities of DNAJB6. We report that the J domain of DNAJB6 is involved in mediating OPN suppression. Deletion of the J domain renders DNAJB6 incapable of impeding malignancy and suppressing OPN. Our mechanistic investigations reveal that DNAJB6 binds HSPA8 (heat-shock cognate protein, HSC70) and causes dephosphorylation of glycogen synthase kinase 3β (GSK3β) at Ser 9 by recruiting protein phosphatase, PP2A. This dephosphorylation activates GSK3β, leading to degradation of β-catenin and subsequent loss of TCF/LEF (T cell factor1/lymphoid enhancer factor1) activity. Deletion of the J domain abrogates assembly of this multiprotein complex and renders GSK3β inactive, thus, stabilizing β-catenin, a transcription co-activator for OPN expression. Our in-vitro and in-vivo functional analyses show that silencing OPN expression in the background of deletion of the J domain renders the resultant tumor cells less malignant despite the presence of stabilized β-catenin. Thus, we have uncovered a new mechanism for regulation of GSK3β activity leading to inhibition of Wnt/β-catenin signaling.

Increased vascularity and spontaneous metastasis of breast cancer by hedgehog signaling mediated upregulation of cyr61

The Hedgehog (Hh) pathway is well known for its involvement in angiogenesis and vasculogenesis during ontogeny. The ligand, Sonic hedgehog (SHH), plays an important role in vascular formation during development. However, SHH expression is upregulated on tumor cells and can impact the tumor microenvironment. We have investigated the effects of autocrine as well as paracrine Hh signaling on tumor cells as well as on endothelial cells, respectively.

Upon constitutive expression of SHH, breast cancer cells showed aggressive behavior and rapid xenograft growth characterized by highly angiogenic tumors that were spontaneously metastatic. In these cells, SHH caused activation of the Hh transcription factor, GLI1, leading to upregulated expression of the potent pro-angiogenic secreted molecule, CYR61 (cysteine rich angiogenic inducer 61). Silencing of CYR61 from these SHH-expressing Hh activated cells blunted the malignant behavior of the tumor cells and resulted in reduced tumor vasculature and limited hematogenous metastases. Thus, CYR61 is a critical downstream contributor to the Hh influenced pro-angiogenic tumor microenvironment. We also observed concomitant upregulation of SHH and CYR61 transcripts in tumors from patients with advanced breast cancer, further ratifying the clinical relevance of our findings. In summary, we have defined a novel, VEGF-independent, clinically relevant, pro-angiogenic factor, CYR61, that is a transcriptional target of Hh-GLI signaling.

4-hydroxy-2-nonylquinoline: a novel iron chelator isolated from a bacterial cell membrane

The membrane associated iron chelator of Pseudomonas aeruginosa has been extracted from membranes of iron-rich cells with ethanol and purified by reverse phase HPLC. Using 13C NMR and FAB mass spectroscopy, the structure of the chelator has been determined to be 4-hydroxy-2-nonylquinoline. This compound has been previously isolated and named pseudan IX, a name which we use here. We synthesized pseudan IX and show that the spectral properties of the synthesized compound and the purified compound are nearly identical. Also purified from the ethanol extract of membranes is 4-hydroxy-2-heptylquinoline, i.e., pseudan VII. Bacterially purified pseudan IX binds iron as indicated by the incorporation of radiolabeled iron into the chelator and by the formation of pink micelles in a concentrated ethanol extract. The formation of pink micelles upon addition of iron to the synthesized compound indicates that it binds iron.

Solid-state NMR data support a helix-loop-helix structural model for the N-terminal half of HIV-1 Rev in fibrillar form

Rev is a 116 residue basic protein encoded by the genome of human immunodeficiency virus type 1 (HIV-1) that binds to multiple sites in the Rev response element (RRE) of viral mRNA transcripts in nuclei of host cells, leading to transport of incompletely spliced and unspliced viral mRNA to the cytoplasm of host cells in the latter phases of the HIV-1 life cycle. Rev is absolutely required for viral replication. Because Rev aggregates and fibrillizes in solution at concentrations required for crystal growth or liquid state NMR measurements, high-resolution structural characterization of full-length Rev has not been possible. Previously, circular dichroism studies have shown that approximately 50 % of the Rev sequence adopts helical secondary structure, predicted to correspond to a helix-loop-helix structural motif in the N-terminal half of the protein. We describe the application of solid-state NMR techniques to Rev fibrils as a means of obtaining site-specific, atomic-level structural constraints without requiring a high degree of solubility or crystallinity. Solid-state NMR measurements, using the double-quantum chemical shift anisotropy and constant-time double-quantum-filtered dipolar recoupling techniques, provide constraints on the phi and psi backbone dihedral angles at sites in which consecutive backbone carbonyl groups are labeled with (13)C. Quantitative analysis of the solid-state NMR data, by comparison with numerical simulations, indicates helical phi and psi angles at residues Leu13 and Val16 in the predicted helix 1 segment, and at residues Arg39, Arg 42, Arg43, and Arg44 in the predicted helix 2 segment. These data represent the first site-specific structural constraints from NMR spectroscopy on full-length Rev, and support the helix-loop-helix structural model for its N-terminal half.

In vivo mechanism-based inactivation of S-adenosylmethionine decarboxylases from Escherichia coli, Salmonella typhimurium, and Saccharomyces cerevisiae

S-adenosylmethionine decarboxylase (AdoMetDC), a key enzyme in the biosynthesis of spermidine and spermine, is first synthesized as a proenzyme, which is cleaved posttranslationally to form alpha and beta subunits. The alpha subunit contains a covalently bound pyruvoyl group derived from serine that is essential for activity. With the use of an Escherichia coli overexpression system, we have purified AdoMetDCs encoded by the E. coli, Saccharomyces cerevisiae, and Salmonella typhimurium genes. Unexpectedly we found by mass spectrometry that these enzymes had been modified posttranslationally in vivo by a mechanism-based "suicide" inactivation. A large percentage of the alpha subunit of each enzyme had been modified in vivo to give peaks with masses m/z = 57 +/- 1 and m/z = 75 +/- 1 daltons higher than the parent peak. AdoMetDC activity decreased markedly during overexpression concurrently with the increase of the additional peaks for the alpha subunit. Sequencing of a tryptic fragment by tandem mass spectrometry showed that Cys-140 was modified with a +75 +/- 1 adduct, which is probably derived from the reaction product. Comparable modification of the alpha subunit was also observed in in vitro experiments after incubation with the substrate or with the reaction product, which is consistent with the in vitro alkylation of E. coli AdoMetDC reported by Diaz and Anton [Diaz, E. & Anton, D. L. (1991) Biochemistry 30, 4078-4081].

Isolation of an anti-HIV diprenylated bibenzyl from Glycyrrhiza lepidota

The organic soluble extract from the leaves of Glycyrrhiza lepidota showed moderate activity in the US National Cancer Institute in vitro anti-HIV-1 bioassay. Chromatographic separation of this extract resulted in the identification of a new diprenylated bibenzyl as the compound responsible for the observed anti-viral activity. Extensive spectroscopic experiments provide the complete 1H NMR and 13C NMR spectral assignments to support the proposed structure. Known compounds glepidotin B and glepidotin A were also isolated from the extract and shown to be inactive in the anti-viral assay.

Patterns of resistance to exonuclease digestion of oligonucleotides containing polycyclic aromatic hydrocarbon diol epoxide adducts at N6 of deoxyadenosine

The effect of adduct stereochemistry on the susceptibility to hydrolysis by snake venom (VPD) and bovine spleen (SPD) phosphodiesterases was investigated with short deoxyoligonucleotides containing defined adducts derived from alkylation of the exocyclic 6-amino group of dA by polycyclic aromatic hydrocarbon diol epoxides (DEs). In accordance with several earlier reports, we have found that adducts with R configuration at the site of attachment of dA to the DE moiety derived from either benzo[a]pyrene (BaP) or benzo[c]phenanthrene (BcPh) are generally more resistant to hydrolysis by VPD than are their (S)-diastereomers. The reaction with VPD initially yields a fragment containing the adducted dA residue at its 3'-end, which slowly hydrolyzes to a dimer (pXpA*) with an intact 5'-phosphodiester bond to the adducted dA. With several of the adducts studied, this dimer underwent cleavage to release eventually the monomeric adduct p(dA*). Adducts derived from cis opening of the epoxide ring of both BaP and BcPh DEs were considerably more resistant to VPD than the corresponding trans-opened adducts. Although several previous investigations had suggested that oligonucleotides containing adducts which have S configuration at the site of attachment of the hydrocarbon to adenine are more resistant to cleavage by SPD than are their (R)-diastereomers, the present results with a more extensive set of oligonucleotides indicate that SPD, in contrast to VPD, exhibits little discrimination between adducts with R and S configuration at the site of attachment to the base. Notably, for both enzymes, the most resistant internucleotide linkage (the bond 3'-sugar to phosphate for VPD and 5'-sugar to phosphate for SPD) is between the modified base and the base immediately 5' to it, regardless of the configuration of the adduct.

Polychlorinated acetamides from the cyanobacterium Microcoleus lyngbyaceus

Several new compounds were isolated from the organic extract of the cyanobacterium Microcoleus lyngbyaceus, and their structures were determined by spectroscopic means. Polychlorinated acetamidoalkynes and alkanes were the major metabolites. 6-Acetamido-1,1,1-trichloroundecane, a positional isomer of the naturally occurring 5-acetamido-1,1,1-trichloroundecane, was synthesized in six steps from delta-decanolactone.

A novel anti-HIV macrocyclic peptide from Palicourea condensata

A 37 amino acid cyclic polypeptide has been isolated from the organic extract of the tropical tree Palicourea condensata. Palicourein (1) is the largest of a growing family of plant peptides that contain a cyclized amino acid backbone cross-linked via three internal disulfide bridges. Palicourein inhibits the in vitro cytopathic effects of HIV-1RF infection of CEM-SS cells with an EC50 value of 0.1 microM and an IC50 value of 1.5 microM.

Microspinosamide, a new HIV-inhibitory cyclic depsipeptide from the marine sponge Sidonops microspinosa

Microspinosamide (1), a new cyclic depsipeptide incorporating 13 amino acid residues, was isolated from extracts of an Indonesian collection of the marine sponge Sidonops microspinosa. Its structure was elucidated by extensive NMR and mass spectral analyses, and by chemical degradation and derivatization studies. The tridecapeptide 1 incorporates numerous uncommon amino acids, and it is the first naturally occurring peptide to contain a beta-hydroxy-p-bromophenylalanine residue. Microspinosamide (1) inhibited the cytopathic effect of HIV-1 infection in an XTT-based in vitro assay with an EC(50) value of approximately 0.2 microg/mL.

Expression, purification, refolding, and characterization of recombinant human interleukin-13: utilization of intracellular processing

Interleukin-13 (IL-13) is a pleiotropic cytokine that elicits both proinflammatory and anti-inflammatory immune responses. Recent studies underscore its role in several diseases, including asthma and cancer. Solution studies of IL-13 and its soluble receptors may facilitate the design of antagonists/agonists which would require milligram quantities of specifically labeled protein. A synthetic gene encoding human IL-13 (hIL-13) was inserted into the pMAL-c2 vector with a cleavage site for the tobacco etch virus (TEV) protease. Coexpression of the fusion protein and TEV protease led to in vivo cleavage, resulting in high levels of hIL-13 production. hIL-13, localized to inclusion bodies, was purified and refolded to yield approximately 2 mg per liter of bacteria grown in minimal media. Subsequent biochemical and biophysical analysis of both the unlabeled and (15)N-labeled protein revealed a bioactive helical monomer. In addition, the two disulfide bonds were unambiguously demonstrated to be Cys29-Cys57 and Cys45-Cys71 by a combined proteolytic digestion and mass spectrometric analysis.

Stolonic acids A and B, new cytotoxic cyclic peroxides from an Indian Ocean ascidian Stolonica species

Two new 3,6-epidioxy-7,10-tetrahydrofurano C(26) unsaturated fatty acids, stolonic acids A (1) and B (2), were isolated from a previously undescribed ascidian species, Stolonica sp. collected off the Maldive Islands in the Indian Ocean. The structures and relative stereochemistry of 1 and 2 were determined using conventional spectroscopic methods. Both compounds exhibited antiproliferative activity against selected human melanoma and ovarian tumor cell lines, with IC(50) values of approximately 0.05-0.1 microg/mL.

Nitric oxide-mediated heme oxidation and selective beta-globin nitrosation of hemoglobin from normal and sickle erythrocytes

Nitric oxide (NO) has been reported to modulate the oxygen affinity of blood from sickle cell patients (SS), but not that of normal adult blood (AA), with little or no heme oxidation. However, we had found that the NO donor compounds 2-(N, N-diethylamino)-diazenolate-2-oxide (DEANO) and S-nitrosocysteine (CysNO) caused increased oxygen affinity of red cells from both AA and SS individuals and also caused significant methemoglobin (metHb) formation. Rapid kinetic experiments in which HbA(0), AA, or SS erythrocytes were mixed with CysNO or DEANO showed biphasic time courses indicative of initial heme oxidation followed by reductive heme nitrosylation, respectively. Hemolysates treated with CysNO showed by electrospray mass spectrometry a peak corresponding to a 29 mass unit increase (consistent with NO binding) of both the beta(A) and beta(S) chains but not of the alpha chains. Therapeutic use of NO in sickle cell disease may ultimately require further optimization of these competing reactions, i.e., heme reactivity (nitrosylation or oxidation) versus direct S-nitrosation of hemoglobin on the beta-globin.

Relative role of heme nitrosylation and beta-cysteine 93 nitrosation in the transport and metabolism of nitric oxide by hemoglobin in the human circulation

To quantify the reactions of nitric oxide (NO) with hemoglobin under physiological conditions and to test models of NO transport on hemoglobin, we have developed an assay to measure NO-hemoglobin reaction products in normal volunteers, under basal conditions and during NO inhalation. NO inhalation markedly raised total nitrosylated hemoglobin levels, with a significant arterial-venous gradient, supporting a role for hemoglobin in the transport and delivery of NO. The predominant species accounting for this arterial-venous gradient is nitrosyl(heme)hemoglobin. NO breathing increases S-nitrosation of hemoglobin beta-chain cysteine 93, however only to a fraction of the level of nitrosyl(heme)hemoglobin and without a detectable arterial-venous gradient. A strong correlation between methemoglobin and plasma nitrate formation was observed, suggesting that NO metabolism is a primary physiological cause of hemoglobin oxidation. Our results demonstrate that NO-heme reaction pathways predominate in vivo, NO binding to heme groups is a rapidly reversible process, and S-nitrosohemoglobin formation is probably not a primary transport mechanism for NO but may facilitate NO release from heme.

Isolation and characterization of Myrianthus holstii lectin, a potent HIV-1 inhibitory protein from the plant Myrianthus holstii(1)

Aqueous extracts from the African plant Myrianthus holstii potently inhibited the infection of the T-lymphoblastoid cell line, CEM-SS, by human immunodeficiency virus-1(RF) (HIV-1(RF)). The active constituent, M. holstii lectin (MHL), was purified by LH-20 column chromatography and reversed phase HPLC. MHL, a 9284-Da cysteine-rich protein, was characterized by amino acid analysis, N-terminal sequencing, ESIMS, and matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry. Pure MHL had anti-HIV activity, with an EC(50) value of 150 nM. Delaying the addition of MHL for up to 8 h after initial exposure of CEM-SS cells to virus did not result in loss of the antiviral activity; however, if addition of the compound was delayed for 16 h or more, there was a marked decrease in the antiviral activity. MHL bound to a virus-free, soluble form of the viral envelope protein gp120 but did not inhibit the subsequent binding to a cell-free, soluble form of the cellular receptor CD4.

Peptidylglycine-alpha-hydroxylating monooxygenase generates two hydroxylated products from its mechanism-based suicide substrate, 4-phenyl-3-butenoic acid

The bifunctional enzyme peptidylglycine-alpha-amidating monooxygenase mediates the conversion of C-terminal glycine-extended peptides to their active alpha-amidated products. Peptidylglycine-alpha-hydroxylating monooxygenase (PHM, EC 1.14.17. 3) catalyzes the first reaction in this two-step process. The olefinic compound 4-phenyl-3-butenoic acid (PBA) is the most potent irreversible, mechanism-based PHM inactivator known. While the details of the inhibitory action of PBA on PHM remain undefined, covalent modification of the protein has been proposed as the underlying mechanism. We report here that, in the process of inactivating PHM, PBA itself serves as a substrate without covalently labeling the enzyme. Approximately 100 molecules of PBA are metabolized per molecule of PHM inactivated, under saturating conditions. The metabolism of PBA by PHM generates two hydroxylated products, 2-hydroxy-4-phenyl-3-butenoic acid and its allylic isomer, 4-hydroxy-4-phenyl-2-butenoic acid. While one enantiomer for each product is significantly favored in the reaction, both are produced. From these observations, we conclude that hydroxylated PBA products are formed by a delocalized free radical mechanism and that the lack of absolute stereospecificity indicates significant freedom of movement within the catalytic site. The ability of PHM to metabolize PBA suggests that the physiological functions of PHM may include the hydroxylation of substrates other than those containing terminal glycines.

Biophysical characterization of gp41 aggregates suggests a model for the molecular mechanism of HIV-associated neurological damage and dementia

In human immunodeficiency virus (HIV)-infected individuals, the level of the HIV envelope protein gp41 in brain tissue is correlated with neurological damage and dementia. In this paper we show by biochemical methods and electron microscopy that the extracellular ectodomain of purified HIV and simian immunodeficiency virus gp41 (e-gp41) forms a mixture of soluble high molecular weight aggregate and native trimer at physiological pH. The e-gp41 aggregate is shown to be largely alpha-helical and relatively stable to denaturants. The high molecular weight form of e-gp41 is variable in size ranging from 7 to 70 trimers, which associate by interactions at the interior of the aggregate involving the loop that connects the N- and C-terminal helices of the e-gp41 core. The trimers are predominantly arranged with their long axes oriented radially, and the width of the high molecular weight aggregate corresponds to the length of two e-gp41 trimers (approximately 200 A). Using both light and electron microscopy combined with immunohistochemistry we show that HIV gp41 accumulates as an extracellular aggregate in the brains of HIV-infected patients diagnosed with dementia. We postulate that the high molecular weight aggregates of e-gp41 are responsible for HIV-associated neurological damage and dementia, consistent with known mechanisms of encephalopathy.

Structures of five components of the actinomycin Z complex from Streptomyces fradiae, two of which contain 4-chlorothreonine

Structure elucidation of five components of the actinomycin Z complex (Z(1)-Z(5)) isolated from Streptomyces fradiae is described. The components were separated by Si gel column chromatography and TLC/PLC and analyzed by ESIMS, FABMS, LC-MS of derivatized hydrolysates, and 2D NMR techniques. This permitted determination of the complete structures of actinomycins Z(1)-Z(5). In Z(3) and Z(5,) site 1 of the beta-depsipeptide is occupied by the rare 4-chloro-L-threonine, an amino acid not previously found in an actinomycin. The structural variants of the actinomycin Z complex have the molecular architecture typical of other actinomycins but possess greater structural diversity resulting from the presence of several highly unusual amino acids. Actinomycins Z(3) and Z(5,) but not Z(1), were more potent than actinomycin D in cytotoxicity assays against three tumor cell lines.

New circulin macrocyclic polypeptides from Chassalia parvifolia

Four new macrocyclic polypeptides were isolated and identified from an extract of the tropical tree Chassalia parvifolia. Circulins C-F are 29-30 amino acid cyclic peptides in which the entire primary amino acid chain is covalently cyclized via peptide bonds. Their structures were deduced from a combination of FABMS analyses, N-terminal Edman degradation, endoproteinase digestion, and amino acid analyses. All the peptides share a high degree of sequence homology and contain six cysteine residues forming three intramolecular disulfide bridges. Circulins C-F inhibited the cytopathic effects of in vitro HIV-1 infection with EC(50) values of 50-275 nM.

Cycloviolins A-D, anti-HIV macrocyclic peptides from Leonia cymosa

Four novel anti-HIV macrocyclic peptides containing 28-31 amino acid residues, named cycloviolins A-D, have been isolated from the hitherto unstudied tropical plant Leonia cymosa. Their primary structure, including amino acid composition and sequence, was determined by a combination of MALDI-TOF and FAB MS and by enzymatic digestion of reduced derivatives, followed by Edman degradation and mass analyses. All of the cycloviolins contain six cysteines, which are present as three intramolecular disulfide bridges. Intriguingly, cycloviolins A-D showed high degrees of sequence homology to the known cyclopsychotride A and circulins A and B from the Rubiaceae family but much less homology to the varv peptides from Viola, a member of the same family (Violaceae).

Intermolecular aggregations are responsible for the slow kinetics observed in the folding of cytochrome c at neutral pH

Folding of equine cytochrome c at a low protein concentration (26 microM) eliminated a slow kinetic phase (time constant three seconds) that was observed in the previous hydrogen exchange pulse-labeling experiments at pH 6.2 and 10 degrees C. It was demonstrated that this slow folding phase was caused by intermolecular aggregations. Because heterogeneous kinetics is a very general feature in the folding of proteins characterized by pulsed hydrogen exchange coupled with two-dimensional NMR, our experimental results suggest aggregations might also be responsible for the complex folding kinetics of other proteins. This is possible since these experiments were performed at relatively high protein concentrations.

Inhaled nitric oxide augments nitric oxide transport on sickle cell hemoglobin without affecting oxygen affinity

Nitric oxide (NO) inhalation has been reported to increase the oxygen affinity of sickle cell erythrocytes. Also, proposed allosteric mechanisms for hemoglobin, based on S-nitrosation of beta-chain cysteine 93, raise the possibility of altering the pathophysiology of sickle cell disease by inhibiting polymerization or by increasing NO delivery to the tissue. We studied the effects of a 2-hour treatment, using varying concentrations of inhaled NO. Oxygen affinity, as measured by P(50), did not respond to inhaled NO, either in controls or in individuals with sickle cell disease. At baseline, the arterial and venous levels of nitrosylated hemoglobin were not significantly different, but NO inhalation led to a dose-dependent increase in mean nitrosylated hemoglobin, and at the highest dosage, a significant arterial-venous difference emerged. The levels of nitrosylated hemoglobin are too low to affect overall hemoglobin oxygen affinity, but augmented NO transport to the microvasculature seems a promising strategy for improving microvascular perfusion.

Characterization of an enterotoxigenic Escherichia coli strain from Africa expressing a putative colonization factor

An enterotoxigenic Escherichia coli (ETEC) strain of serotype O114:H- that expressed both heat-labile and heat-stable enterotoxins and tested negative for colonization factors (CF) was isolated from a child with diarrhea in Egypt. This strain, WS0115A, induced hemagglutination of bovine erythrocytes and adhered to the enterocyte-like cell line Caco-2, suggesting that it may elaborate novel fimbriae. Surface-expressed antigen purified by differential ammonium sulfate precipitation and column chromatography yielded a single protein band with M(r) 14,800 when resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (16% polyacrylamide). A monoclonal antibody against this putative fimbrial antigen was generated and reacted with strain WS0115A and also with CS1-, CS17-, and CS19-positive strains in a dot blot assay. Reactivity was temperature dependent, with cells displaying reactivity when grown at 37 degrees C but not when grown at 22 degrees C. Immunoblot analysis of a fimbrial preparation from strain WS0115A showed that the monoclonal antibody reacted with a single protein band. Electron microscopy and immunoelectron microscopy revealed fimbria-like structures on the surface of strain WS0115A. These structures were rigid and measured 6.8 to 7.4 nm in diameter. Electrospray mass-spectrometric analysis showed that the mass of the purified fimbria was 14,965 Da. The N-terminal sequence of the fimbria established that it was a member of the CFA/I family, with sequence identity to the amino terminus of CS19, a new CF recently identified in India. Cumulatively, our results suggest that this fimbria is CS19. Screening of a collection of ETEC strains isolated from children with diarrhea in Egypt found that 4.2% of strains originally reported as CF negative were positive for this CF, suggesting that it is biologically relevant in the pathogenesis of ETEC.

Salivary glands of the sand fly Phlebotomus papatasi contain pharmacologically active amounts of adenosine and 5′-AMP

Salivary gland homogenates of the sand fly Phlebotomus papatasi contain large amounts of adenosine and 5′-AMP, of the order of 1 nmol per pair of glands, as demonstrated by liquid chromatography, ultraviolet spectrometry, mass spectrometry and bioassays. These purines, 75–80 % of which are secreted from the glands following a blood meal, have vasodilatory and anti-platelet activities and probably help the fly to obtain a blood meal. Salivary 5′-AMP is also responsible for the previously reported protein phosphatase inhibitor in the salivary glands of P. papatasi, which is shown to be artifactual in nature as a result of allosteric modification by AMP of the phosphatase substrate used (phosphorylase a).

Occurrence and significance of decahydroquinolines from dendrobatid poison frogs and a myrmicine ant: use of 1H and 13C NMR in their conformational analysis

Structures for 2,5-disubstituted decahydroquinolines (DHQs) are reported for the two diastereomeric pairs cis-275B (14) and cis-275B' (15) and 5-epi-trans-269AB (18) and trans-269AB (19), all isolated from skin extracts of dendrobatid frogs, and for 5-epi-cis-275B' (16) and 5-epi-trans-275B (17) found in the extracts of virgin queens of a myrmicine ant [Solenopsis (Diplorhoptrum) azteca]. Detection of such DHQs in an ant, their first reported occurrence, strengthens a dietary hypothesis for the origin of the approximately 30 DHQs that have been detected in extracts of frog skin. NMR data on the two conformers of cis-decahydroquinoline permit assignment of ring conformations and stereochemistry to cis-DHQs of the "N-endo" type or the "N-exo" type. These conformations are also assigned on whether H-8a is equatorial or axial as determined with E-COSY or 1D-HOHAHA spectra.

Lack of anticonvulsant tolerance to the neuroactive steroid pregnanolone in mice

GABA-potentiating neuroactive steroids such as pregnanolone have potent protective effects in the pentylenetetrazol seizure test. We sought to determine if tolerance develops to the anticonvulsant activity of pregnanolone with chronic administration. Mice were treated with two daily injections of a 2 x ED50 dose of pregnanolone (25 mg/kg, i.p.) for 7 days. On the day after the chronic treatment protocol, the dose-response relationship for protection in the pentylenetetrazol seizure test was obtained. The ED50 value after the chronic treatment protocol was not significantly different from that in naive mice (12 mg/kg), indicating that tolerance does not develop to the anticonvulsant activity of pregnanolone. In subsequent experiments, we extended the chronic treatment protocol to 14 days with three daily injections of pregnanolone (25 mg/kg, i.p.). Again, no tolerance was observed (ED50, 13 mg/kg). The anticonvulsant activity of pregnanolone was well correlated with plasma levels in both the naive and chronically (14 day) treated mice. The estimated plasma concentrations of pregnanolone representing threshold (10%) protection (125-150 ng/ml) and 50% protection (575-700 ng/ml) were similar in naive and chronically treated animals. In both chronically treated and naive animals, plasma levels of pregnanolone declined rapidly (t1/2, 16-19 min) and there was a corresponding reduction in the anticonvulsant activity. Our results with pregnanolone suggest that tolerance does not develop to the anticonvulsant activity of neuroactive steroids as it does with other GABA potentiating drugs such as benzodiazepines, supporting the potential clinical utility of neuroactive steroids in chronic seizure therapy.

Theonellapeptolide IIIe, a new cyclic peptolide from the New Zealand deep water sponge, Lamellomorpha strongylata

The structure, stereochemistry, and conformation of theonellapeptolide IIIe (1), a new 36-membered ring cyclic peptolide from the New Zealand deep-water sponge Lamellomorpha strongylata, is described. The sequence of the cytotoxic peptolide was determined through a combination of NMR and MS-MS techniques and confirmed by X-ray crystal structure analysis, which, with chiral HPLC, established the absolute stereochemistry.

Role of the omega-loop in the activity, substrate specificity, and structure of class A beta-lactamase

The structure of class A beta-lactamases contains an omega-loop associated with the active site, which carries a key catalytic residue, Glu166. A 16-residue omega-loop deletion mutant of beta-lactamase from Staphylococcus aureus PC1, encompassing residues 163-178, was produced in order to examine the functional and structural role of the loop. The crystal structure was determined and refined at 2.3 A, and the kinetics of the mutant enzyme was characterized with a variety of beta-lactam antibiotics. In general, the wild-type beta-lactamase hydrolyzes penicillin compounds better than cephalosporins. In contrast, the deletion of the omega-loop led to a variant enzyme that acts only on cephalosporins, including third generation compounds. Kinetic measurements and electrospray mass spectrometry revealed that the first and third generation cephalosporins form stable acyl-enzyme complexes, except for the chromogenic cephalosporin, nitrocefin, which after acylating the enzyme undergoes hydrolysis at a 1000-fold slower rate than that with wild-type beta-lactamase. Hydrolysis of the acyl-enzyme adducts is prevented because the deletion of the omega-loop eliminates the deacylation apparatus comprising Glu166 and its associated nucleophilic water site. The crystal structure reveals that while the overall fold of the mutant enzyme is similar to that of the native beta-lactamase, local adjustments in the vicinity of the missing loop occurred. The altered beta-lactam specificity is attributed to these structural changes. In the native structure, the omega-loop restricts the conformation of a beta-strand at the edge of the active site depression. Removal of the loop provides the beta-strand with a new degree of conformational flexibility, such that it is displaced inward toward the active site space. Modeled Michaelis complexes with benzylpenicillin and cephaloridine show that the perturbed conformation of the beta-strand is inconsistent with penicillin binding because of steric clashes between the beta-lactam side chain substituent and the beta-strand. In contrast, no clashes occur upon cephalosporin binding. Recognition of third generation cephalosporins is possible because the bulky side chain substituents of the beta-lactam ring typical of these compounds can be accommodated in the space freed by the deletion of the omega-loop.

Recombinant production of cyanovirin-N, a potent human immunodeficiency virus-inactivating protein derived from a cultured cyanobacterium

Here we describe the recombinant production and purification of a novel anti-human immunodeficiency virus (HIV) protein, cyanovirin-N (CV-N), in Escherichia coli. Initial attempts to express CV-N using a vector containing an ompA signal peptide sequence resulted in production of an intractable mixture of the full-length (101 amino acid residue) protein and a truncated form lacking the first two N-terminal amino acids. The truncated protein was observed regardless of the host cell line, culture conditions, or induction time. These observations suggested that an as yet unidentified protease or peptidase was responsible for proteolytic cleavage between the second and third N-terminal amino acids of CV-N when presented as an ompA-CV-N fusion protein. When the ompA signal peptide sequence was replaced by a pelB signal peptide sequence, CV-N was produced in high yield as a single, homogeneous protein. This was confirmed by electrospray ionization mass spectrometry and N-terminal sequencing. This expression system provides a basis for large-scale production of clinical grade CV-N for further research and development as an anti-HIV microbicide.

Isolation of a novel Kunitz family protease inhibitor in association with Tethya hemolysin from the sponge Tethya ingalli

Aqueous extracts from the New Zealand sponge Tethya ingalli (Hadromerida) displayed potent cytotoxicity in the NCI's 60-cell-line human tumor panel. Fractionation of the extract by ammonium sulfate precipitation, gel filtration, ultrafiltration, and both hydrophobic interaction and reversed-phase chromatography resulted in the isolation of two biologically active proteins. The first protein, Tethya protease inhibitor (TPI), which was purified to homogeneity, inhibited trypsin with an EC50 of 65 nM. TPI had a molecular mass of 11,431 Da, and an isoelectric point of 8.2. A partial N-terminal amino acid sequence determined for TPI showed significant homology with protease inhibitors of the Kunitz family. The second isolated protein displayed potent cytotoxicity, with pronounced selectivity for certain tumor cell lines (e.g., ovarian, renal, CNS, and breast). The latter protein, which had an apparent molecular weight of 21 kDa (SDS-PAGE), also lysed human red blood cells (EC50 of 39 nM) and was similar to a hemolysin previously isolated from the sponge Tethya lycinurium.

Functional and biophysical characterization of recombinant human hepatocyte growth factor isoforms produced in Escherichia coli

Hepatocyte growth factor (HGF) is a pluripotent secreted protein that stimulates a wide array of cellular targets, including hepatocytes and other epithelial cells, melanocytes, endothelial and haematopoietic cells. Multiple mRNA species transcribed from a single HGF gene encode at least three distinct proteins: the full-length HGF protein and two truncated HGF isoforms that encompass the N-terminal (N) domain through kringle 1 (NK1) or through kringle 2 (NK2). We report the high-level expression in Escherichia coli of NK1 and NK2, as well as the individual kringle 1 (K1) and N domains of HGF. All proteins accumulated as insoluble aggregates that were solubilized, folded and purified in high yield using a simple procedure that included two gel-filtration steps. Characterization of the purified proteins indicated chemical and physical homogeneity, and analysis by CD suggested native conformations. Although the K1 and N-terminal domains of HGF have limited biological activity, spectroscopic evidence indicated that the conformation of each matched that observed when the domains were components of biologically active NK1. Both NK1 and NK2 produced in bacteria were functionally equivalent to proteins generated by eukaryotic systems, as indicated by mitogenicity, cell scatter, and receptor binding and activation assays. These data indicate that all four bacterially produced HGF derivatives are well suited for detailed structural analysis.

Probing the non-proline cis peptide bond in beta-lactamase from Staphylococcus aureus PC1 by the replacement Asn136 --> Ala

A non-proline cis peptide is present between Glu166 and Ile167 in the active site of beta-lactamase from Staphylococcus aureus PC1. To examine the role of the interaction between the side chain of Asn136 and the main chain of Glu166, the site-directed mutant N136A was produced. The enzyme shows no measurable hydrolytic activity toward a variety of penicillins or cephalosporins except for the chromogenic cephalosporin, nitrocefin. For nitrocefin, the progress curve exhibits a fast burst with a stoichiometry of 1 mol of degraded substrate per mole of enzyme followed by a slow phase with a hydrolysis rate that is reduced by approximately 700-fold compared with that of the wild-type enzyme. Thus, the mutant enzyme is deacylation defective. Monitoring the hydrolysis of nitrocefin after preincubation with a number of beta-lactam compounds shows that cephalosporins form stable acyl complexes with the enzyme, whereas penicillins do not. The molecular weight of the mutant was determined by electrospray mass spectrometry, and the presence of the stable acyl enzyme adducts with cephaloridine and cefotaxime was confirmed by both electrospray and MALDI mass spectrometry. Therefore, in addition to impairing deacylation, the acylation machinery has been altered compared with the wild-type enzyme to act on cephalosporins and not on penicillins. Urea denaturation and thermal unfolding studies show that the N136A mutant enzyme is less stable than the wild-type enzyme. However, stability against chemical denaturation of the mutant enzyme is enhanced in the presence of cephaloridine beyond the stability of the wild-type protein. This is attributed to accumulation of favorable interactions between the cephaloridine and the protein, which play a role in the folded state and not in the unfolded state.

Analysis of sequence requirements for biological activity of cyanovirin-N, a potent HIV (human immunodeficiency virus)-inactivating protein

Site-directed mutagenesis of DNA constructs coding for the novel, HIV-inactivating proteins cyanovirin-N (CV-N) and FLAG-cyanovirin-N (F-CV-N) was performed using mutagenic oligonucleotide primers in the polymerase chain reaction or by a restriction site elimination maneuver. The mutant constructs were expressed in Escherichia coli and the recombinant protein products were tested for binding to the HIV surface envelope glycoprotein gp 120 and for antiviral activity against infectious HIV. Results showed an overall very high correlation (r2 > 0.9) between the relative gp120 binding affinities and the anti-HIV activities of CV-N, F-CV-N, and the various mutants. An outlier, however, was a mutant which lacked one of the internal disulfide linkages normally present in CV-N and which showed modest gp120 binding but no antiviral activity against HIV. These findings are consistent with the view that gp120 binding is a necessary but not sufficient requirement for the HIV-inactivating activity of CV-N and related proteins; the sequence specificities for gp120 binding and anti-HIV activity are not identical.

Isolation, primary sequence determination, and disulfide bond structure of cyanovirin-N, an anti-HIV (human immunodeficiency virus) protein from the cyanobacterium Nostoc ellipsosporum

A novel anti-HIV protein, cyanovirin-N (CV-N), was isolated from an aqueous cellular extract of the cultured cyanobacterium (blue-green alga) Nostoc ellipsosporum, purified by reverse-phase HPLC, and sequenced by N-terminal Edman degradation of the intact protein and peptide fragments produced by endoproteinase digestions. CV-N consists of a single 101 amino acid chain which exhibits significant internal sequence duplication, but no significant homology to previously described proteins or to the transcription products of known nucleotide sequences. Alignment of residues 1-50 with residues 51-101 reveals 13 conservative amino acid changes as well as direct homology between 16 amino acid residues. CV-N contains four cysteines which form two intrachain disulfide bonds. The positions of the disulfide linkages were established by fast atom bombardment mass spectral studies of peptide fragments generated by a tryptic digestion of the native protein. Reductive cleavage of these crosslinks resulted in loss of anti-HIV activity.

Isolation and characterization of novel cytotoxic saponins from Archidendron ellipticum

A series of new ester saponins, elliptosides A-J, has been isolated from the tropical plant Archidendron ellipticum (Leguminosae). These saponins were particularly cytotoxic to certain renal and melanoma cancer cell lines in the NCI's 60-cell line human tumor screen. The structures of elliptosides A, E, and F were elucidated by spectroscopic and chemical means. Elliptoside A showed in vivo antitumor activity against the LOX melanoma cell line.

Crystalluria and urinary tract abnormalities associated with indinavir

BACKGROUND

Indinavir, a protease inhibitor widely used to treat patients with HIV infection, has been associated with nephrolithiasis. Distinctive urinary crystals and a spectrum of urologic disorders were noted in patients receiving indinavir.

OBJECTIVE

To determine the composition of urinary crystals and the frequency of asymptomatic crystalluria and urinary tract symptoms in patients receiving indinavir.

PATIENTS

Patients with HIV infection who were enrolled in studies conducted at the National Institutes of Health.

MEASUREMENTS

Microscopic urinalysis, high-performance liquid chromatography (HPLC) and mass spectrometry of urinary crystals and stones, and clinical evaluation of patients with urologic symptoms.

RESULTS

Of 240 patients receiving indinavir, 142 provided urine specimens for analysis. Twenty-nine (20%) had crystals consisting of plate-like rectangles and fan-shaped or starburst forms. Mass spectrometry and HPLC confirmed that these crystals were composed of indinavir. Of 40 patients who were not receiving indinavir, none had similar crystals (P < 0.001). Nineteen of the 240 patients receiving indinavir (8%) developed urologic symptoms. Of these, 7 (3%) had nephrolithiasis and the other 12 (5%) had previously undescribed syndromes: crystalluria associated with dysuria and crystalluria associated with back or flank pain. Four of the patients with the latter syndrome had radiographic evidence of intrarenal sludging.

CONCLUSIONS

Indinavir forms characteristic crystals in the urine. This crystalluria may be associated with dysuria and urinary frequency, with flank or back pain associated with intrarenal sludging, and with the classic syndrome of renal colic.

Discovery of cyanovirin-N, a novel human immunodeficiency virus-inactivating protein that binds viral surface envelope glycoprotein gp120: potential applications to microbicide development

We have isolated and sequenced a novel 11-kDa virucidal protein, named cyanovirin-N (CV-N), from cultures of the cyanobacterium (blue-green alga) Nostoc ellipsosporum. We also have produced CV-N recombinantly by expression of a corresponding DNA sequence in Escherichia coli. Low nanomolar concentrations of either natural or recombinant CV-N irreversibly inactivate diverse laboratory strains and primary isolates of human immunodeficiency virus (HIV) type 1 as well as strains of HIV type 2 and simian immunodeficiency virus. In addition, CV-N aborts cell-to-cell fusion and transmission of HIV-1 infection. Continuous, 2-day exposures of uninfected CEM-SS cells or peripheral blood lymphocytes to high concentrations (e.g., 9,000 nM) of CV-N were not lethal to these representative host cell types. The antiviral activity of CV-N is due, at least in part, to unique, high-affinity interactions of CV-N with the viral surface envelope glycoprotein gp120. The biological activity of CV-N is highly resistant to physicochemical denaturation, further enhancing its potential as an anti-HIV microbicide.

Isolation and characterization of niphatevirin, a human-immunodeficiency-virus-inhibitory glycoprotein from the marine sponge Niphates erecta

Anti-human immunodeficiency virus (HIV)-bioassay-guided fractionation of aqueous extracts of the Caribbean sponge Niphates erecta led to isolation of a novel anti-HIV protein, named niphatevirin. The protein was purified to homogeneity by ethanol precipitation, ammonium sulfate precipitation, gel-permeation chromatography and concanavalin-A-Sepharose affinity chromatography. Niphatevirin potently inhibited the cytopathic effects of HIV-1 infection in cultured human lymphoblastoid (CEM-SS) cells; the effective concentration of drug that results in 50% protection of the cells through inhibition of cell lethality, cell-cell fusion and syncytium formation was approximately 10 nM. Delay of addition of niphatevirin to infected cultures by two hours markedly decreased (approximately 50%) cytoprotection; delay of addition by eight hours resulted in no antiviral activity. Niphatevirin bound to CD4 in a manner that prevented the binding of gp120, but did not directly bind gp120. Niphatevirin (6.5 microM) was inactive in both hemagglutination and hemolysis assays. Niphatevirin had a molecular mass of about 19 kDa by matrix-assisted laser-desorption ionization-time of flight (MALDI-TOF) mass spectrometry, and a native molecular mass of approximately 18 kDa by gel-filtration chromatography. The protein had an acidic isoelectric point of 4.2-4.6, and was shown by periodate acid Schiff's staining to be glycosylated.

The structure of synenkephalin (pro-enkephalin 1-73) is dictated by three disulfide bridges

Mass spectrometry of fragments produced by limited proteolytic digestion of pro-enkephalin was used to locate the disulfide bridges in synenkephalin (pro-enkephalin 1-73), a domain which contains sorting information for targeting the pro-neuropeptide to the granules of the regulated secretory pathway in neuroendocrine cells. Mass spectrometric analysis was optimized by using chemicals that gave low interference with the ionization and desorption processes, and computer software which simplified the identification of all possible disulfide-linked peptide fragments. Three disulfide bridges between Cys2-Cys24, Cys6-Cys28, and Cys9-Cys41 were identified. Protein conformational prediction of synenkephalin1-42 shows beta-turns which facilitate the formation of these disulfide bonds.

Novel cytotoxic, alkylated hydroquinones from Lannea welwitschii

Two novel natural products, lanneaquinol (1) and 2'(R)-hydroxylanneaquinol (2), were isolated from the organic extract of the plant Lannea welwitschii (Hiern) Engl. Their structures were solved by spectroanalytical methods and confirmed by comparison to synthetic models. The absolute configuration of 2 was determined by the modified Mosher method. Both compounds exhibited modest cytotoxicity against the NCI panel of 60 human tumor cell lines. The structures of two isomeric 4,5-dihydroxy-5-alkyl-2-cyclohexenones (7 and 8), which appear to be biogenetic precursors of 1 and 2, were also elucidated.

Analysis of the disulfide linkage pattern in circulin A and B, HIV-inhibitory macrocyclic peptides

Circulin A and B are members of a family of macrocyclic peptides, originally isolated from the tropical tree Chassalia parvifolia, that have been shown to display anti-HIV activity. Complete structural elucidation of these highly constrained peptides was difficult due to their cyclic amide backbone and the presence of six disulfide-linked cysteines. In the present study, the disulfide pairing motif of circulin A and circulin B was determined. Since the circulins were resistant to enzymatic proteolysis, cysteine residue pairings were identified by analysis of the complex mixture of cleavage products that resulted from partial acid hydrolysis of the native peptides. Combined utilization of HPLC, fast atom bombardment mass spectrometry and peptide recognition software ("F-MASS" and "F-LINK" programs) were employed to identify the cleavage products. Thus, we were able to unambiguously identify the disulfide linkage pattern in circulin A and circulin B as Cys1-Cys4, Cys2-Cys5 and Cys3-Cys6, where the numbers on the cystine residues refer to their respective order in the peptides.

Three new naphthyldihydroisoquinoline alkaloids from Ancistrocladus tectorius

Three new 5--1'-linked naphthyldihydroisoquinoline alkaloids (1-3) have been isolated from the organic extract of Ancistrocladus tectorius. The gross structures of the compounds have been established using 1D and 2D NMR spectroscopy and difference NOE experiments. The absolute stereochemistry of 1, 2, and 3 was determined from CD spectral comparison and chemical degradation. Evidence is presented to show that two of the compounds exist exclusively in the keto form at C-8 of the isoquinoline system (2b, 3b).

Benzoylecgonine hydrazides: synthesis, coupling to horseradish peroxidase, and characterization of the conjugates

Benzoylecgonine-horseradish peroxidase conjugate (BE-HRP) can be used as a diagnostic reagent for the detection of cocaine in illicit drug samples and in biological fluids. This paper describes the preparation and characterization of BE-HRP. Two hydrazide derivatives of benzoylecgonine, N-2-(tert-butyloxycarbonyl)benzoylecgonine hydrazide and mono(N-2'-benzoylecgoninoyl)adipic dihydrazide, were synthesized by carbodiimide-activated coupling of benzoylecgonine to N-2-(tert-butyloxycarbonyl) hydrazide and adipic dihydrazide, respectively. Removal of the tert-butyloxycarbonyl protecting group in N-2-(tert-butyloxycarbonyl)benzoylecgonine hydrazide with anhydrous HCl yielded benzoylecgonine hydrazide hydrochloride. NMR and high-resolution mass spectral analyses demonstrated that the benzoyl group of benzoylecgonine remained intact under the conditions of both carbodiimide coupling and anhydrous HCl treatment. By aldehyde-hydrazide condensation, the hydrazides were covalently conjugated to the carbohydrate residues of horseradish peroxidase (HRP). Dot blot analysis of the conjugates employing antibodies specific to benzoylecgonine demonstrated the presence of bound benzoylecgonine in HRP. The stoichiometry of benzoylecgonine residues to HRP was determined by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Mono(N-2'-benzoylecgoninoyl)adipic dihydrazide gave a 2.5-3-fold higher coupling compared with benzoylecgonine hydrazide. Conjugates were also prepared by the coupling of the carbodiimide-activated benzoylecgonine to HRP that was derivatized with adipic dihydrazide.

The detection of intact double-stranded DNA by MALDI

DNA fragments have been analyzed by matrix-assisted laser desorption ionization (MALDI) and electrospray mass spectrometry. In many cases, only the single-stranded oligonucleotides have been detected. Recently, spectra of intact double-stranded DNA have been obtained in both electrospray and massive cluster impact ionization. We show here the first MALDI spectra of intact double-stranded DNA (EcoR1 adaptor 12/16) that is clearly not due to nonspecific dimer formation. 6-Aza-2-thiothymine was used as the matrix in the presence of ammonium citrate. Via the same procedure but with other matrices commonly employed for oligonucleotide analysis, the intact DNA duplex was not detected. No sign of the homodimer of either of the single strands is observed. Although the spectrum also shows peaks attributable to each of the single strands, these are demonstrated to arise from the DNA solution and not the sample preparation or desorption process.

Anticonvulsant efficacy of ADCI (5-aminocarbonyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine) after acute and chronic dosing in mice

ADCI (5-aminocarbonyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine), a low-affinity uncompetitive N-methyl-D-aspartate (NMDA) antagonist, is a broad-spectrum anticonvulsant with a favorable side-effect profile. In the present study, we sought to determine if tolerance develops to the anticonvulsant activity of ADCI, using the maximal electroshock (MES) test to assess seizure protection. Mice were treated with three daily injections of a 2 x ED50 dose for MES protection (18 mg/kg, intraperitoneally, i.p.) or vehicle for 7 or 14 days. On the day after the chronic treatment protocol, all animals received a challenge dose of ADCI (18 mg/kg) and 15 min later were evaluated in the MES test. In control animals, 83-94% of animals were protected and the ADCI plasma levels immediately after the MES test were 5.5-9.7 micrograms/ml. In treated animals, 29 and 0% of animals were protected at 7 and 14 days, respectively, and the ADCI plasma levels were 77 and 52% of the control values. [3H]Dizocilpine binding to brain NMDA receptors was unaltered by the chronic drug treatment. In subsequent experiments, we determined that 14-day chronically treated animals could be completely protected by increased doses of ADCI (ED50 28.9 mg/kg). In both naive and chronically treated animals receiving a challenge dose of ADCI, plasma drug levels decreased in two phases, the first with a time constant of approximately 55 min and the second with a much slower rate. The estimated plasma concentrations of ADCI reflecting threshold (3-5 micrograms/ml) and 50% protection (5-7.5 micrograms/mg) were similar in naive and chronic animals.(ABSTRACT TRUNCATED AT 250 WORDS)