N5-methylasparagine and energy-transfer efficiency in C-phycocyanin

A posttranslationally methylated asparagine residue, N5-methylasparagine, is found at the beta-72 site in many phycobiliproteins. Two mutations (Asp and Gln) in the beta-72 position of Agmenellum quadruplicatum C-phycocyanin were investigated to clarify the role of the wild-type N5-methylasparagine near the beta-84 "fluorescing" bilin tetrapyrrole chromophore. Chemical analysis for amide modification revealed that the beta-72Q protein was partially methylated with a stoichiometry of 0.27, suggesting that either the asparagine methyltransferase is nonspecific or a glutamine methyltransferase exists. Urea denaturation studies could detect no difference in protein stability for any of the C-phycocyanin species. Steady-state spectroscopic measurements demonstrate that Asp and Gln substitution for the C-phycocyanin beta-72 NMA affects both the ground to excited state transition and the excited-state characteristics of the beta-84 chromophore, while the rate of radiative energy transfer is unaffected. Energy-transfer efficiency within phycobilisomes (represented by steady-state fluorescence quantum yields) was also negatively impacted by the beta-72 substitutions. Time-resolved fluorescence emission spectroscopic studies with C-phycocyanin reveal three distinguishable fluorescence lifetimes. The longest fluorescence lifetime is diminished 7-10% by the Asp and Gln mutations in comparison to a control sample where beta-72 is NMA. Molecular dynamics calculations implicate a change in the bilin tetrapyrrole chromophore ring geometry as a likely source of the altered photophysics induced by the mutations. We conclude that N5-methylasparagine plays a special role in establishing the environment surrounding the beta-84 chromophore which minimizes the rates of nonradiative energy losses that would otherwise defeat the high quantum yield for energy transfer within the phycobilisomes.

A N-methyl-D-aspartate receptor-mediated neurotoxic effect of aspartate-based hydroxamate compounds in rat primary neuronal cultures

In a previous study (Lockhart et al., Brain Res., 630 (1993) 32-40) we demonstrated a NMDA receptor-mediated neurotoxic effect for the anti-HIV drugs D-aspartate beta-hydroxamate and structurally related analogues in rat primary cortical neurons. Herein, we have examined the neurotoxic action and pharmacology of a novel series of hydroxamate compounds, with potential anti-HIV activity, in a similar paradigm. In this aim, the aspartate-based hydroxamates L-VHS.126 and its stereoisomer D-VHS.126 selectively destroyed (EC50 = 300 microM) rat primary neurons. The D-VHS.126 analogue, VHS.134 was also neurotoxic (EC50 = 450 microM) whereas VHS.129 and VHS.137, or the D-aspartate beta-hydroxamate analogues VHS.128, VHS.135, VHS.132 and VHS.127 or hydroxyurea demonstrated no significant neurotoxicity. The neurotoxic action of L- and D-VHS.126, and VHS.134 was attenuated with MK-801, CGS-19755 but not with CNQX. These observations demonstrate a potent NMDA receptor-mediated excitotoxic action for novel aspartate-based hydroxamate compounds, and further extends this series of potential hydroxamate-based anti-HIV drugs lacking neurotoxicity in vitro.

L-glutamate uptake inhibitors may stimulate phosphoinositide hydrolysis in baby hamster kidney cells expressing mGluR1a via heteroexchange with L-glutamate without direct activation of mGluR1a

The functional efficacies of inhibitors of L-glutamate uptake for altering second messenger formation in baby hamster kidney cells expressing subtypes mGluR1a, mGluR2, and mGluR4 of the metabotropic glutamate receptor family were examined. L-Serine-O-sulfate was an agonist at mGluR1a (EC50 = 70 microM), mGluR2 (EC50 = 25 microM), and mGluR4 (EC50 = 324 microM). L-Cysteine sulfinate, 1-aminocyclobutane-trans-1,3-dicarboxylate, L-cysteine, and DL-threo-3-methylaspartate stimulated phosphoinositide hydrolysis in mGluR1a cells with EC50 values of 43, 64, 463, and 488 microM, respectively, and displaced L-[3H]glutamate binding from membranes prepared from these cells with respective IC50 values of 48, 44, 79, and 139 microM. However, D-aspartate, L-trans-pyrrolidine-2,4-dicarboxylate, L-threo-3-hydroxyaspartate, and L-aspartate-beta-hydroxamate stimulated phosphoinositide hydrolysis in mGluR1a cells (respective EC50 values of 73, 54, 57, and 430 microM) but did not displace L-[3H]glutamate binding. These compounds inhibited Na(+)-dependent L-glutamate uptake into baby hamster kidney cells with IC50 values similar to those for stimulation of phosphoinositide hydrolysis in mGluR1a cells. Phosphoinositide hydrolysis in mGluR1a cells, as stimulated by inhibitors of (or substrates for) this L-glutamate transporter, was significantly attenuated in the presence of L-glutamate decarboxylase (EC 4.1.1.15) or L-alanine aminotransferase (EC 2.6.1.2). Furthermore, incubation with 1 mM L-trans-pyrrolidine-2,4-dicarboxylate for 30 min increased the basal levels of free glutamate (1.5 +/- 0.2 microM) in the assay buffer four- to fivefold as measured by HPLC analysis. Thus, heteroexchange with endogenous L-glutamate may lead to erroneous estimations of the functional efficacies at mGluR1a.

Synergistic anti-human immunodeficiency virus type 1 effect of hydroxamate compounds with 2',3'-dideoxyinosine in infected resting human lymphocytes

The cellular models generally used in the in vitro evaluation of anti-human immunodeficiency virus compounds are dividing cells. A model constituted by resting lymphocytes may more accurately reflect a drug's future efficacy in humans, since viral DNA synthesis is known to take place in quiescent cells, creating a reservoir of infected cells awaiting activation to complete their viral replication cycle and to produce infectious virions. We report here the activity of 3'-azido-3'-deoxythymidine, 2',3'-dideoxyinosine, 2',3'-dideoxycytidine, and two hydroxamates, D-aspartic acid beta-hydroxamate and hydroxycarbamate (hydroxyurea), alone and in various combinations, in an in vitro model based on resting lymphocytes. In our model, resting peripheral blood lymphocytes were infected with human immunodeficiency virus type 1 and treated with drugs for 7 days, at which time drugs were removed and the cells were activated by phytohemagglutinin. We show that under these conditions 3'-azido-3'-deoxythymidine, 2',3'-dideoxyinosine, and 2',3'-dideoxycytidine, alone or in combination, neither fully inhibit viral production nor protect lymphocytes from the cytopathic effect of viral replication, at concentrations corresponding to the peak plasma levels observed in a typical treatment schedule in humans. In contrast, we report the synergistic effect of treatment by each hydroxamate with 2',3'-dideoxyinosine of infected resting lymphocytes, resulting in the total suppression of viral production, total protection against the cytopathic effect induced by viral replication, and no effect on the ability of the cells to replicate in this cell culture system.

Antiproliferative effect of D-aspartic acid beta-hydroxamate (DAH) on Friend virus-infected erythropoietic progenitor cells

D-aspartic beta-hydroxamate (DAH), an aspartic acid analog, exerts antitumoral activity on murine leukemia L5178Y, both in vitro and in vivo. In this study, we show that DAH is also active in vivo against Friend virus (FV-P)-induced erythroleukemia, and we report the effects of DAH in vivo an in vitro on FV-P target cells, i.e. the mature erythroid colony-forming cells (CFU-E). DAH treatment (2 g/kg/day) given for 95 days as a single daily i.p. injection to DBA/2 mice either 3 or 12 days following inoculation with a high dose (10(3) plaque-forming units) of FV-P resulted in a marked increase in the mean survival time of treated animals (212 and 191%, respectively). Since FV-P elicits spleen enlargement and polycythemia, we examined the effects of DAH on spleen size, spleen-nucleated cell number, and hematocrit, in normal and FV-P infected mice, at different times in the course of continuous DAH treatments. DAH treatment initiated 3 days after viral infection inhibits the virus-induced splenomegaly, with at day 26 p.i. 1.15 x 10(8) and 12.6 x 10(8) nucleated cells per spleen observed in DAH-treated mice and untreated mice respectively, whereas only 1.03 x 10(8) nucleated cells were observed in uninfected mice. Furthermore, DAH prevents virus-induced polycythemia: on day 26, an hematocrit of 39% was measured in DAH-treated mice as compared to 60% in untreated mice. DAH treatment initiated 12 days after viral infection reduces splenomegaly, the number of nucleated spleen cells and the hematocrit of infected mice. DAH treatment initiated 3 days after viral infection prevents the tremendous increase of CFU-E in the spleen of infected mice: on day 11, the spleen of infected mice contained 4.6 x 10(6) CFU-E, while the spleen of treated mice only contained 26 x 10(3) CFU-E, and on day 26 the spleen CFU-E numbers were 45.4 x 10(6) and 1.5 x 10(6) in untreated and treated infected mice, respectively. In control uninfected mice, DAH treatment induced a transient decrease in spleen CFU-E followed by a rebound phenomenon. In vitro, preincubation with DAH inhibits colony formation by FV-P infected CFU-E, at doses starting at 3 mM, as compared to uninfected CFU-E. These data show that DAH inhibits the expression of the retroviral infection, and appears to preferentially inhibit the proliferation of infected target cells (CFU-E) in vivo.

Purification and characterization of an endo-N-acetyl-beta-D-glucosaminidase from the culture medium of Stigmatella aurantiaca DW4

A novel endo-N-acetyl-beta-D-glucosaminidase (ENGase), acting on the di-N-acetylchitobiosyl part of N-linked glycans, was characterized in the culture medium of Stigmatella aurantiaca DW4. Purified to homogeneity by ammonium sulfate precipitation, gel filtration, and chromatofocusing, this ENGase presents, upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a molecular mass near 27 kDa. Optimal pH and pI were 4.0 and 6.8, respectively. The enzyme, named ENGase St, exhibits high activity on oligomannoside-type glycoasparagines and glycoproteins and could also hydrolyze hybrid- and complex-type glycoasparagines but does not acts as a murein hydrolase.

Therapeutic effects of D-aspartic acid beta-hydroxamate (DAH) on Friend erythroleukemia

D-aspartic acid beta-hydroxamate (DAH), an aspartic acid analogue, exerts anti-tumoral activity against murine leukemia L5178Y both in vitro and in vivo. We show here that DAH displays activity against Friend leukemia cells (FLC) in vitro: a concentration of 2 mM results in a total inhibition of cell growth. DAH is also active in vivo against Friend virus (FV-P)-induced erythroleukemia. Treatment with DAH, given for 95 days as a single daily i.p. injection to DBA/2 mice 3 days following FV-P inoculation, induced a marked increase of 212% in the mean survival time (MST) of treated animals. Since FV-P-induced erythroleukemia is characterized by the proliferation of mature erythroid precursors, we examined the effect of DAH treatment on erythroid colony-forming cells (CFU-E) and observed that the number of CFU-E per spleen was 30 times lower in DAH-treated mice than in the controls. To gain further insight into the early effects of DAH treatment on the early phase of Friend disease, we examined the effects of short DAH treatment on spleen size, hematocrit and viremia in FV-P-infected mice. DAH treatment initiated 3 days post infection (p.i.) inhibited splenomegaly, prevented virus-induced polycythemia, and reduced serum viremia. Late DAH treatment (18 days p.i.) induced regression of FVP-induced disease as evidenced by reduction of spleen weight.

Liquid chromatographic-mass spectrometric analysis of N-acetylamino acids in human urine

Liquid chromatography-atmospheric pressure chemical-ionization mass spectrometry (LC-APCI-MS) was used for the analysis of N-acetylamino acids that could not be determined with an amino acid analyzer. LC-APCI-MS could directly detect the protonated molecular ions of various synthetic N-acetylamino acids, distinguishing N-acetylserine from O-acetylserine and N(alpha)-acetyllysine from N(epsilon)- acetyllysine. Furthermore, N-acetylasparagine, N-acetylaspartic acid, N-acetylglutamine and N-acetylglutamic acid were identified in normal human urine. The assay data for N-acetylaspartic acid agree with a previous report using gas chromatography-mass spectrometry. These results demonstrate the usefulness of the apparatus described above for the analysis of N-acetylamino acids in biological samples.

Novel N-glycosylation in eukaryotes: laminin contains the linkage unit beta-glucosylasparagine

The linkage unit to protein of N-linked carbohydrate in eukaryotic glycoproteins consists of N-acetylglucosamine, coupled to the amido nitrogen of asparagine. Additional N-glycosyl linkage units have been unequivocally proven to exist only in the cell surface glycoproteins of various bacteria. Based on immunological analyses, isolation and chemical characterization, we report that one of these units, namely glucose linked to asparagine, exists in the mammalian protein laminin, an extracellular basement membrane component. This finding and the occurrence of identical disaccharide structures in archaebacterial cell surface glycoproteins and mammalian basement membrane protein complexes points towards a conserved and distinct function of these extracellular structural elements. In addition, a method is described to uncover a masked epitope in fixed tissues by chemical O-deglycosylation. This has allowed to morphologically localize the antigen beta-Glc-Asn by immunofluorescence to the basement membranes of kidney glomeruli.

Neurotoxic effect of the anti-HIV drug D-aspartate beta-hydroxamate for rat primary neuronal cultures: attenuation by N-methyl-D-aspartate (NMDA) antagonists

The anti-tumor drug D-aspartate beta-hydroxamate (D-A beta H), selectively destroys HIV-1 infected peripheral blood mononuclear cells, but produces anorexia and nausea during prolonged treatment to AIDS patients. Consequently, based on the structural similarity between D-A beta H and the excitotoxins L-aspartate and NMDA, we have investigated the potential neurotoxic action and pharmacology of D-A beta H and of a series of chemically related anti-tumor drugs on rat primary neuronal/glial cultures. In this aim, after a 30 min exposure to D-A beta H (1-2 mM), cortical neurons were selectively destroyed within 24 h. The stereoisomer L-A beta H (0.5-2 mM) was highly neurotoxic for both glial and neuronal cells in mixed cultures but demonstrated no toxicity in glial cell cultures alone. Furthermore, for a series of D-A beta H analogues, VHS.121 and VHS.122 demonstrated a reduced but significant neurotoxicity, whereas VHS.124 and VHS.125 showed no significant neurotoxic effect, and in the case of VHS.125 also prevented D-A beta H and glutamate-mediated neurotoxicity. The related anti-tumor drugs L- or D-glutamate gamma-monohydroxamate or keto-glutamate gamma-monohydroxamate (< or = 2 mM) were not neurotoxic for cortical neurons. The neurotoxic effect of D-A beta H and L-A beta H was attenuated by the NMDA antagonists MK-801, TCP, memantine, ifenprodil, pentamidine and CGS-19755. alpha-Difluoromethylornithine, an inhibitor of polyamine biosynthesis, also protected cultures against the neurotoxicity of L-A beta H and D-A beta H.(ABSTRACT TRUNCATED AT 250 WORDS)

(2S,3S,4R)-2-(carboxycyclopropyl)glycine, a potent and competitive inhibitor of both glial and neuronal uptake of glutamate

The effects of several diastereoisomers of L-2-(carboxycyclopropyl)glycine (CCG) on L-glutamate uptake were compared among three different preparations, glial plasmalemmal vesicles (GPV), synaptosomes and cultured astrocytes from rat hippocampus. The (2S,3S,4R)-isomer (L-CCG-III) inhibited a Na(+)-dependent high-affinity L-glutamate uptake in GPV and synaptosomes in a dose dependent manner at a micromolar range. The potency was quite similar to that of L-threo-beta-hydroxyaspartate in both subcellular fractions and much higher than L-aspartate-beta-hydroxamate, which were known as potent inhibitors of glutamate uptake. The (2S,3R,4S)-isomer (L-CCG-IV) also inhibited the glutamate uptake in GPV and synaptosomes, but it was about 100 times less active than L-CCG-III. The (2S,3S,4S)- and (2S,3R,4R)-isomers (L-CCG-I and L-CCG-II, respectively) hardly showed any inhibitory action on the glutamate uptake. Dixon plot analysis of the initial uptake rate revealed that the inhibition was in a competitive manner and the value of the inhibition constant (Ki) was about 1 microM in both GPV and synaptosomes. L-CCG-III effectively inhibited the glutamate uptake by cultured hippocampal astrocytes as well. These results suggested that L-CCG-III inhibited the glutamate uptake in both neurones and glial cells of the mammalian central nervous system in a similar manner.

Pharmacokinetic study of a tripeptide HIV-1 protease inhibitor, KNI-174, in rats after intravenous and intraduodenal administrations

Recently, as a new type of anti-AIDS drug, an HIV-1 protease inhibitor, KNI-174, has been synthesized; it shows a potent and selective HIV-1 protease inhibitory activity in vitro. In this study, we developed an HPLC assay system for KNI-174 in rat plasma and examined the pharmacokinetics of KNI-174 in rats using this assay method after both intravenous (i.v.) and intraduodenal (i.d.) administrations to obtain the disposition characteristics and bioavailability of this new anti-AIDS drug. This HPLC assay method is specific to KNI-174 and the standard curve was linear from 0.02 to 30 micrograms ml-1 plasma. After i.v. administration, 10.0 mg kg-1, KNI-174 disappeared from the rats' plasma in a three-exponential decay. The mean terminal elimination half-life, t1/2 lambda z, was 3.97 +/- 0.19 (S.E.) h, the total body clearance, CLtot, was 9.53 +/- 1.08 ml min-1 and the distribution volume at steady state, Vd,ss, was 7070 +/- 960 ml kg-1. In the case of the i.d. administration, 10.0 mg kg-1, the mean peak plasma concentration, Cmax, and the peak time, tmax, were 0.196 +/- 0.076 micrograms ml-1 and 0.444 +/- 0.193 h, respectively. The bioavailability of KNI-174 till infinity, BA(0-infinity), was 5.37 per cent. Because the IC50 of KNI-174 against HIV-1 in PHA-PBM was 138 ng ml-1, the time needed for maintaining the concentrations above IC50 after a single i.d. administration of KNI-174 is estimated to be 0.350 +/- 0.184 h.

A patch clamp study of excitatory amino acid effects on cortical astrocyte subtypes in culture

Electrophysiological effects of the excitatory amino acids (EAAs) glutamate (Glu) and kainate (KA) on membrane properties of confluent (> 7-9 day) astrocyte cultures were examined. The whole-cell patch clamp technique was employed to measure membrane currents. Cells were subdivided antigenically, morphologically and electrophysiologically into type-1-like and type-2 astrocytes. Lucifer yellow injection showed that type-1-like, but not type-2, astrocytes were electrically coupled in type-2 astrocytes, EAAs induced a cationic current by activating an ionotropic Glu receptor. The underlying receptor mechanism was KA-preferring and was blocked by the broad-spectrum EAA receptor channel antagonist kynurenate (Kyn). The current was dose-dependent and gave a Hill coefficient close to 2 for KA. In type-1-like astrocytes, EAA effects were agonist-dependent. Glu action involved an inward current mainly carried by an electrogenic Glu uptake system. This current was suppressed by the Glu uptake blocker DL-aspartate beta hydroxamate (ABH), but was not sensitive to Kyn. On the other hand, KA activated Kyn-sensitive receptors and was still able to induce this current in the presence of ABH. In type-1-like astrocytes, application of KA on average produced no conductance change. However, application of Ringer containing 5 mM Ba2+ caused a significant increase in input resistance and KA applied in the presence of Ba2+ consistently increased input conductance. In both subtypes of astrocyte, the KA-induced current was predominantly Na(+)-dependent, although in type-2 cells a small, Na(+)-independent current was also seen. These results support recent findings that type-1-like and type-2 astrocytes possess KA-preferring ionotropic receptors and type-1-like astrocytes also possess an electrogenic Glu uptake system.

Structure of a C-type mannose-binding protein complexed with an oligosaccharide

C-type (Ca(2+)-dependent) animal lectins such as mannose-binding proteins mediate many cell-surface carbohydrate-recognition events. The crystal structure at 1.7 A resolution of the carbohydrate-recognition domain of rat mannose-binding protein complexed with an oligomannose asparaginyl-oligosaccharide reveals that Ca2+ forms coordination bonds with the carbohydrate ligand. Carbohydrate specificity is determined by a network of coordination and hydrogen bonds that stabilizes the ternary complex of protein, Ca2+ and sugar. Two branches of the oligosaccharide crosslink neighbouring carbohydrate-recognition domains in the crystal, enabling multivalent binding to a single oligosaccharide chain to be visualized directly.

Fmoc-protected, glycosylated asparagines potentially useful as reagents in the solid-phase synthesis of N-glycopeptides

1-Amino 1-deoxy derivatives of unprotected O-beta-D-galactopyranosyl-(1-->3)-2-acetamido-2-deoxy-beta-D-glucopyrano se, 2-acetamido-2-deoxy-D-galactose, D-galactose, lactose, D-fucose, D-mannose, and 2-deoxy-D-arabino-hexose were prepared and acylated with N-fluorenylmethoxycarbonylaspartic acid alpha-tert-butyl ester. The anomeric configuration of the N-glycosyl bond (including that of the mannose derivative) in each of the purified compounds was shown to be beta. The probable stability of the N-glycosyl and glycosidic bonds during the conditions of solid-phase peptide synthesis was investigated by treatment of the glycosylated asparagine derivatives with different concentrations of trifluoroacetic acid. Based on their stability, we found that Fmoc-Asn(sugar)-OH derivatives are excellent candidates for automated synthesis of biologically active glycopeptides.

Multiple mechanisms for inhibition of excitatory amino acid receptors coupled to phosphoinositide hydrolysis

Excitatory amino acid (EAA) analogues activate receptors that are coupled to the increased hydrolysis of phosphoinositides (PIs). In these studies, hippocampal slices were prepared from neonatal rats (6-11 days old) to characterize the effects of EAA analogues on these receptors. The concentrations of ibotenate and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) required to evoke half-maximal stimulation (EC50 values) were 28 and 51 microM, respectively. Although the data for stimulation of PI hydrolysis by ibotenate and trans-ACPD were best fit to theoretical curves that had Hill slopes of 1, data for stimulation of PI hydrolysis by quisqualate were best fit to two sites. The EC50 values were 0.43 and 44 microM. The high-affinity sites were 70% of the total. A number of EAA analogues were tested for inhibition of PI metabolism. One of these, L-aspartate-beta-hydroxamate (L-A beta HA), was identified as a novel inhibitor of this response. L-A beta HA was equipotent as an inhibitor of PI metabolism stimulated by ibotenate, quisqualate, and trans-ACPD. The data for this inhibition were best fit to two sites. Between 32 and 48% of the total sites had high affinity with IC50 values in the range of 1.2-6.3 microM. The low-affinity sites had IC50 values between 610 and 2,700 microM. DL-2-Amino-3-phosphonopropionate (DL-AP3) was also equipotent as an inhibitor of PI hydrolysis stimulated by ibotenate, quisqualate, and trans-ACPD (IC50 values were 480-850 microM). In contrast to the data for L-A beta HA, the data for DL-AP3 were best fit to a single site. Both of these inhibitors reduced the maximal response caused by the agonists, consistent with noncompetitive mechanisms of action. Several experiments were designed to examine potential mechanisms for these noncompetitive effects. These studies suggest that either L-A beta HA and DL-AP3 bind to a site on the receptor and irreversibly block activation of the receptor, or that these inhibitors act via a distinct site that specifically regulates EAA receptors coupled to PI hydrolysis.

L-aspartate-beta-hydroxamate exhibits mixed agonist/antagonist activity at the glutamate metabotropic receptor in rat neonatal cerebrocortial slices

L-aspartate-beta-hydroxamate, a glutamate uptake inhibitor, was investigated for activity at a glutamate metabotropic receptor (mGluR) in neonatal rat cerebral cortical slices. Stimulation of phosphatidylinositol hydrolysis by 100 microM (1S,3R)-ACPD was inhibited only very weakly, to a maximal extent of 28%, L-aspartate-beta-hydroxamate did however exhibit agonist activity (EC50 = 760 microM) and, although much less potent than (1S,3R)-ACPD (EC50 = 20 microM), its efficacy was approximately 70% of the latter. These results indicate that, at least in this preparation, offspartate-beta-hydroxamate is of little value as an antagonist at the mGluR receptor.

Binding of protein S to C4b-binding protein. Mutagenesis of protein S

Protein S and C4b-binding protein (C4BP) form a tight complex (Kd approximately 0.6 nM) the physiologic purpose of which is unknown. The participation of protein S in this complex was investigated using site-specific mutagenesis. Normal recombinant human protein S (rHPS) and five specifically mutated protein S analogs were expressed in transformed human kidney 293 cells and the following properties were characterized: solution-phase C4BP binding, ability to be cleaved by thrombin, ability to act as a cofactor in the activated protein C-catalyzed inactivation of factor Va, and gamma-carboxyglutamic acid content. In some cases, beta-hydroxyaspartic acid plus beta-hydroxyasparagine content was also determined. Binding studies indicated that while clearly important for a high affinity interaction, the amino acid sequence Gly605-Ile614 identified by Walker (Walker, F J. (1989) J. Biol. Chem. 264, 17645-17648) does not account for all the binding energy of the HPS-C4BP interaction. All mutants perturbed in this region or lacking it altogether displayed reduced C4BP binding, and some retained anticoagulant cofactor function. Neither human factor X nor human steroid-binding protein had any measurable ability to compete with plasma HPS for C4BP binding. Furthermore, bovine protein S and a rHPS analog with bovine sequence from Gly597-Trp629 bound to human C4BP with the same affinity as did HPS, and both proteins substituted effectively for HPS as a cofactor for activated protein C in an otherwise human anticoagulation system. Together these results suggest that optimal binding of protein S to C4BP requires the putative alpha-helix Gly605-Ile614, as well as other undetermined regions of protein S, and that the regions of HPS responsible for C4BP binding and activated protein C cofactor function are structurally isolated.

Inhibition of excitatory amino acid-stimulated phosphoinositide hydrolysis in rat hippocampus by l-aspartate-β-hydroxamate

The effect of a series of glutamate uptake inhibitors was tested on ibotenate-stimulated phosphoinositide hydrolysis. The pharmacological profile of the inhibitory effect of these compounds on the ibotenate response was quite different from that on glutamate uptake. Aspartate-beta-hydroxamate was the most potent compound with the L-isomer (IC50 11 +/- 2 microM) being considerably more potent than the D-isomer (IC50 104 +/- 12 microM). The effect of the L-aspartate-beta-hydroxamate was found to be specific for ibotenate and quisqualate-stimulated phosphoinositide hydrolysis; this compound did not affect hydrolysis stimulated by carbachol, K+ or sodium fluoride. The inhibition of the ibotenate response was found to involve a non-competitive and irreversible mechanism.

Deamidation and succinimide formation by gamma-N-methylasparagine: potential pitfalls of amino acid analysis

The biological function of the post-translationally methylated amino acid gamma-N-methylasparagine (gamma-NMA) in proteins is unknown. We are examining the premise that amide methylation protects against deamidation. The free amino acids Asn, gamma-NMA, Gln, and delta-N-methylglutamine (delta-NMG) were incubated at elevated temperature and a variety of pH conditions to assay for deamidation. Gln disappears 12- to 14-fold more rapidly than delta-NMG, and Asn hydrolyzes to Asp and NH3 as expected. However, the gamma-NMA deamidation rate is severely overestimated by simply measuring the disappearance of starting material because gamma-NMA undergoes a cyclization reaction in preference to deamidation. At pH 1 the predominant gamma-NMA reaction is formation of stable 3-amino-N-methylsuccinimide (NMS) and this occurs greater than 10-fold faster than Asn deamidation. At pH 4.0, 7.4, and 9.0 NMS is readily formed but it is unstable and partitions between the parent compound, gamma-NMA, and a second species, alpha-N-methylasparagine. At pH 7.4 and 9.0 gamma-NMA disappears 4-fold slower than Asn but the methyl amide hydrolysis rate is diminished by as much as 13-fold. The Asn incubations over the pH range 1-9 yield scant evidence of a succinimide intermediate. It is concluded that the amide methylation provides a unique reaction pathway and stabilization for the N-methylsuccinimide species. Amino acid analysis by o-phthalaldehyde postcolumn reaction fails to detect isoasparagine, alpha-N-methylasparagine, and NMS. Amino acid analysis by precolumn derivatization with phenyl isothiocyanate destroys NMS and therefore cannot quantitate this compound. The ninhydrin postcolumn derivatization method is able to detect and quantitate all of these amino acid species.

Comparison of some arthropod toxins and toxin fragments as antagonists of excitatory amino acid-induced excitation of rat spinal neurones

Wasp and spider venom toxins, which block glutamatergic transmission at invertebrate neuromuscular junctions, have recently been shown to block transmission at glutamate-operated synapses in mammalian central nervous system. Using the technique of iontophoresis on spinal neurones in anaesthetised rats, we have compared the action of five arthropod toxins and two toxin fragments, on responses to excitatory amino acids including alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, kainate and N-methyl-D-aspartate (NMDA). All toxins caused greater than 70% mean reduction of non-NMDA responses. Only argiotoxin636 significantly reduced responses to NMDA. This blockade, like that induced by philanthotoxin-433 and -343, was readily reversible whereas blockade induced by Joro Spider toxin or Nephila Spider toxin was less readily reversible. Neither 2,4-dihydroxyphenylacetate nor 2,4-dihydroxyphenylacetylasparagine blocked NMDA or non-NMDA responses. It appears, therefore that small structural differences in the polyamine part of these toxin molecules give rise to different activity profiles with respect to selectivity and reversibility.

beta-Hydroxyaspartic acid and beta-hydroxyasparagine residues in recombinant human protein S are not required for anticoagulant cofactor activity or for binding to C4b-binding protein

Among the vitamin K-dependent plasma proteins, only protein S contains the post-translationally modified amino acid erythro-beta-hydroxyasparagine (Hyn). Protein S also contains erythro-beta-hydroxyaspartic acid (Hya). The function of these unusual amino acids, located in the epidermal growth factor-like domains, is unknown. To determine if these post-translational modifications contribute to the functional integrity of human protein S (HPS), recombinant human protein S lacking Hya and Hyn (rHPSdesHya/Hyn) was purified from the medium of human kidney 293 cells that were transfected with HPS cDNA and grown in the presence of the hydroxylase inhibitor 2,2'-dipyridyl. Solution-phase equilibrium binding studies revealed that rHPSdesHya/Hyn binds C4b-binding protein (C4BP) in a manner indistinguishable from recombinant HPS and plasma-derived HPS, exhibiting a Kd in the presence of 2 mM CaCl2 of approximately 0.7 nM and a Kd in the presence of 4 mM EDTA approximately 10-fold higher. In a purified component system, rHPSdesHya/Hyn displayed normal anticoagulant cofactor activity in the activated protein C-catalyzed inactivation of coagulation factor Va bound in the prothrombinase complex. In addition, digestion of rHPSdesHya/Hyn with thrombin in the presence of EDTA appeared normal, and 2 mM CaCl2 prevented the cleavage. Together these results suggest that the post-translational modifications of Asn and Asp residues are not necessary for the macromolecular or Ca2+ interactions associated with the anticoagulant and C4BP binding characteristics of HPS.

Anti-tumoral activity of L and D isomers of aspartic acid beta-hydroxamate on L5178Y leukemia

D and L isomers of aspartic acid beta-hydroxamate (respectively DAH and LAH) were compared for their in vitro and in vivo activity against the murine leukemia L5178Y and their tolerance in vivo in DBA/2 mice. DAH and LAH displayed comparable cytotoxic activity against L5178Y leukemia in vitro. Death of leukemia cells was observed at concentrations above 1.2 mM for both DAH and LAH. High concentrations of L-asparagine partially reversed the growth-inhibitory effects of DAH and LAH on L5178Y cells for concentrations of DAH and LAH lower than 0.6 mM. Intraperitoneal administration of DAH and LAH to mice showed that the LD10, LD50 and LD90 of DAH was 3- to 4-fold greater for DAH than for LAH. DAH was able to eradicate L5178Y tumors in mice without inducing toxic deaths, whereas LAH at comparable doses killed all the animals treated.

[The effect of early visual deprivation on the level of N-acetyl-1-asparaginic acid and the activity of phosphate-activated glutaminase in visual analyzer structures and various regions of the dog cerebral cortex and cerebellum]

Under conditions of 45-days visual deprivation content of N-acetyl-L-aspartic acid was considerably increased in visual cortex (field 17) tissues and decreased in the superior colliculus and exterior geniculate body. At the same time, content of N-acetyl-L-aspartic acid was not altered in the visual cortex mitochondrial fraction; in the outer structures studied level of this amino acid was distinctly decreased both in tissues and mitochondrial fractions. After prolongation of the visual deprivation up to 90 days content of N-acetyl-L-aspartic acid was sharply decreased in tissues and mitochondrial fractions of all the formations studied of dog brain visual analyzer. The maximal 4.5-fold decrease was found in mitochondrial fractions of the visual analyzer (field 17), in the exterior geniculate body and superior colliculus content of the amino acid was decreased 2.4-3.9-fold as compared with controls. In 90 days-long visual deprivation total activity of phosphate-dependent glutaminase was considerably increased (1.18-3.71-fold as compared with controls) in myelinic (A), membraneous (B), light (C), and heavy synaptosomal and synaptic mitochondrial (E) subfractions of the visual cortex (field 17). The rate of the enzyme activation was distinctly higher in these subfractions as compared with that of the motor and parietal cortical subfractions. After 90 days-long visual deprivation the similar alterations were found in A, B, D and E cerebellum subfractions but these alterations were less distinct as compared with the visual, motor and parietal subfractions.