Purification, crystallization and preliminary X-ray diffraction analysis of haemorrhagin IV from the snake venom of Agkistrodon acutus

Haemorrhagin IV, a medium molecular weight haemorrhagin from the snake venom of Agkistrodon acutus (AaHIV), has been purified and crystallized. The molecular weight and isoelectric point of AaHIV are 44 kDa and pI 5.0, respectively. The crystal belongs to space group C222(1) with unit-cell dimensions of a = 124.2, b = 114.5, c = 98.4 A, and could o diffract X-rays to 3.0 A, resolution. There are one or two molecules in the crystallographic asymmetric unit.

Synthesis and photochemical properties of a kainate precursor and activation of kainate and AMPA receptor channels on a microsecond time scale

Kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolylpropionate (AMPA) receptors are transmembrane proteins that can form ion channels upon binding a specific ligand. The receptors are located at major excitatory synapses in the mammalian central nervous system. Kainate and AMPA receptors participate in many physiological activities of the brain, including learning and memory, and are involved in many neurological disorders. Elucidation of the mechanisms of receptor transmembrane channel formation, inhibition, and regulation is important in understanding fundamental central nervous system function and in designing potential therapeutic agents. Kainate can activate both kainate and AMPA receptors, leading to channel opening in the microsecond to millisecond time region. A newly developed laser pulse photolysis technique, with a microsecond time resolution, has been successfully used to study the chemical reactions of receptor proteins in the microsecond to millisecond time region. To apply the technique to kainate and AMPA receptors, a photolabile kainate precursor in which a caging group, the alpha-carboxy-2-nitrobenzyl group, is attached to the gamma-carboxyl group of kainic acid has been synthesized. The photolytic release of free kainate from the caged kainate on the microsecond time scale, initiated by a pulse of laser light at 308 nm, was measured. The quantum yield is 0.34 at pH 6.8 and room temperature. The half-life of the major component (approximately 86%) of the photolytic reaction is 45 microseconds, while that of the minor component (approximately 14%) is 0.7 ms. The effects of the caged kainate on kainate and AMPA receptors endogenously expressed in rat hippocampal neurons were also evaluated. Caged kainate (750 microM) did not activate the receptor channels, nor did it potentiate or inhibit the kainate response. Photolysis of the caged kainate by a pulse of 333-nm laser light resulted in a rapid rise (with a t1/2 of 0.4 ms) in the whole-cell current due to the opening of kainate-activated receptor channels. The results presented demonstrate that this kainate precursor is suitable for rapid chemical kinetic investigations of the kainate and AMPA receptors in the microsecond to millisecond time region.

Crystallization and preliminary X-ray diffraction studies of haemorrhagin I from the snake venom of Agkistrodon acutus

Haemorrhagin I from the snake venom of Agkistrodon acutus (AaHI) has been crystallized using the hanging-drop vapour diffusion method. The crystals belong to space group P4(1)2(1)2 or P4(3)2(1)2 with unit-cell dimensions a = b = 63.61 and c = 95.69 A. There is one molecule in the asymmetric unit. Data to 2.35 A resolution have been collected using a single-crystal.

Cocaine: mechanism of inhibition of a muscle acetylcholine receptor studied by a laser-pulse photolysis technique

Effects of cocaine on the muscle nicotinic acetylcholine receptor were investigated by using a chemical kinetic technique with a microsecond time resolution. This membrane-bound receptor regulates signal transmission between nerve and muscle cells, initiates muscle contraction, and is inhibited by cocaine, an abused drug. The inhibition mechanism is not well understood because of the lack of chemical kinetic techniques with the appropriate (microsecond) time resolution. Such a technique, utilizing laser-pulse photolysis, was recently developed; by using it the following results were obtained. (i) The apparent cocaine dissociation constant of the closed-channel receptor form is approximately 50 microM. High carbamoylcholine concentration and, therefore, increased concentrations of the open-channel receptor form, decrease receptor affinity for cocaine approximately 6-fold. (ii) The rate of the receptor reaction with cocaine is at least approximately 30-fold slower than the channel-opening rate, resulting in a cocaine-induced decrease in the concentration of open receptor channels without a concomitant decrease in the channel-opening or -closing rates. (iii) The channel-closing rate increases approximately 1.5-fold as the cocaine concentration is increased from 20 to 60 microM but then remains constant as the concentration is increased further. The results are consistent with a mechanism in which cocaine first binds rapidly to a regulatory site of the receptor, which can still form transmembrane channels. Subsequently, a slow step (t1/2 approximately 70 ms) leads to a receptor form that cannot form transmembrane channels, and acetylcholine receptor-mediated signal transmission is, therefore, blocked. Implications for the search for therapeutic agents that alleviate cocaine poisoning are mentioned.

Phonons Localized at Step Edges: A Route to Understanding Forces at Extended Surface Defects

Inelastic helium atom scattering has been used to measure the phonons on a stepped metallic crystalline surface, Ni(977). When the scattering plane is oriented parallel to the step edges and perpendicular to the terraces, two branches of step-induced phonons are observed. These branches are identified as transversely polarized, step-localized modes that propagate along the step edge. Analysis reveals significant anisotropy in the force field near the step edge, with all forces near the step edge being substantially smaller than in the bulk. Such measurements provide valuable information on metallic bonding and interface stability near extended surface defects.

Comparative observations on the types of atrophic gastritis and biopsy pathology of gastric mucosa

300 cases of chronic atrophic gastritis were classified into 3 types of qi-deficiency of spleen and stomach, yang-deficiency of spleen and stomach, and yin-deficiency of spleen and stomach according to syndrome differentiation of traditional Chinese medicine. The biopsy specimens of gastric mucosa were examined histopathologically and for the presence of pyloric spirobacteria. The results showed that the type of yin-deficiency was the most serious in the degree of pathologic changes, or the glandular atrophy and the interstitial inflammatory cellular infiltration, while the rate of pyloric spirobacteria presence was comparable in all types.

Characterization of the mouse Rep-3 gene: sequence similarities to bacterial and yeast mismatch-repair proteins

The mouse Rep-3 gene is transcribed divergently from the same promoter region as the dihydrofolate reductase-encoding gene and has a deduced amino-acid sequence that shares identity with the bacterial protein, MutS, which is involved in DNA mismatch repair. We have cloned Rep-3, mapped it and sequenced all of the known exons and their intron junction sequences. We find that the open reading frame is considerably larger than initially reported and that the most abundant form of Rep-3 mRNA encodes a protein of 123 kDa. The gene spans at least 134 kb and consists of 26 exons, including several alternatively spliced exons. All of the exon/intron junctions match the expected consensus sequences with the exception of the splice junctions for intron 6, which has AT and AC dinucleotides instead of the usual GT and AG bordering the exon sequences. The junction sequences for this intron share consensus sequences with three intron sequences from other genes, thereby helping to establish an alternative consensus sequence.

Photolabile precursors of glutamate: synthesis, photochemical properties, and activation of glutamate receptors on a microsecond time scale

Newly synthesized photolabile derivatives of glutamate, caged glutamate, that release free glutamate on a microsecond time scale after a pulse of UV laser light are described. 2-Nitrobenzyl derivatives were attached to the amino or carboxyl groups of glutamate. Substitution with a -CO2- group at the benzylic carbon accelerates the photolysis reaction when compared to -H and -CH3 substituents. gamma-O-(alpha-Carboxy-2-nitrobenzyl)glutamate is stable at neutral pH. In 100 mM phosphate buffer at pH 7.0, the compound is photolyzed at 308 nm with a quantum product yield of 0.14. The half-life of the major component of the photolytic reaction, as judged by the transient absorbance change at 430 nm, is 21 microseconds (approximately 90%); the half-life of a minor component (approximately 10%) is 0.2 ms. The amino-linked derivatives have half-lives in the millisecond region and a 4-fold lower quantum yield. The potential of the newly synthesized compound for use in rapid chemical kinetic investigations of glutamate receptors is demonstrated. (i) The caged glutamate at 1 mM concentration does not desensitize glutamate receptors in rat hippocampal neurons. (ii) Caged glutamate (1 mM) does not inhibit activation of the receptors by 50 microM glutamate. (iii) Photolysis of the compound induces rapid onset of transmembrane currents in rat hippocampal neurons.

Structure of the allosteric regulatory enzyme of purine biosynthesis

Multi-wavelength anomalous diffraction (MAD) has been used to determine the structure of the regulatory enzyme of de novo synthesis of purine nucleotides, glutamine 5-phosphoribosyl-1-pyrophosphate (PRPP) amidotransferase, from Bacillus subtilis. This allosteric enzyme, a 200-kilodalton tetramer, is subject to end product regulation by purine nucleotides. The metalloenzyme from B. subtilis is a paradigm for the higher eukaryotic enzymes, which have been refractory to isolation in stable form. The two folding domains of the polypeptide are correlated with functional domains for glutamine binding and for transfer of ammonia to the substrate PRPP. Eight molecules of the feedback inhibitor adenosine monophosphate (AMP) are bound to the tetrameric enzyme in two types of binding sites: the PRPP catalytic site of each subunit and an unusual regulatory site that is immediately adjacent to each active site but is between subunits. An oxygen-sensitive [4Fe-4S] cluster in each subunit is proposed to regulate protein turnover in vivo and is distant from the catalytic site. Oxygen sensitivity of the cluster is diminished by AMP, which blocks a channel through the protein to the cluster. The structure is representative of both glutamine amidotransferases and phosphoribosyltransferases.

Photolysis of a protecting group for the carboxyl function of neurotransmitters within 3 microseconds and with product quantum yield of 0.2

The synthesis of a photosensitive blocking group for the carboxyl function of neurotransmitters, in this case glycine, is reported. The compound, 2-methoxy-5-nitrophenyl glycine ester (caged glycine), is photolyzed by a laser pulse at 308 or 337 nm within 3 microseconds and with a product quantum yield of 0.2. The compound is hydrolyzed in water with a time constant tau of 6.1 min at pH 7.1 and 3 hr at pH 4.0. Mouse cerebral cortical neurons containing glycine receptors were used in biological assays. A cell-flow device, in which solutions of caged glycine at pH 4.0 were mixed with buffer to give a final pH of 7.1, was used to equilibrate the compound with receptors on the cell surface. Neither the caged compound nor the 2-methoxy-5-nitrophenol photolysis product affected the glycine receptors or modified their response to glycine. When cells equilibrated with caged glycine are irradiated by a laser pulse at 337 nm, glycine receptor channels are opened, as detected in whole-cell current recordings. The approach described may be used in the synthesis and characterization of photolabile precursors of neurotransmitters and other compounds that contain carboxyl groups and for kinetic investigations of neurotransmitter receptors in central nervous system cells in the microsecond time domain.

An acetylcholine receptor regulatory site in BC3H1 cells: characterized by laser-pulse photolysis in the microsecond-to-millisecond time region

When a neurotransmitter binds to its specific receptor, the protein forms transmembrane channels through which ions flow, leading to changes in transmembrane voltage that trigger signal transmission between neurons. How do inhibitors affect this process? Interesting and extensive information comes from investigations of the acetylcholine receptor, the best known of these proteins. This receptor is inhibited by cationic inhibitors, including local anesthetics, and acetylcholine at high concentrations. The accepted mechanism, elegant in its simplicity, is that these compounds enter the receptor-channel after it opens and block inorganic ion flux. This mechanism requires that the inhibitors affect only the apparent rate constant for channel closing (k'cl). An alternative mechanism invokes a specific regulatory (inhibitory) site to which inhibitors bind before the channel opens and the signal is transmitted. This mechanism requires that the inhibitors affect the apparent rate constants for both channel opening (k'op) and closing. The effect of inhibitors on k'op has not been determined previously. This report describes the use of a newly developed laser-pulse photolysis technique with a dead time of approximately 120 microseconds to determine the effect of a local anesthetic, procaine, one of the best studied cationic inhibitors of the acetylcholine receptor, on both k'op and k'cl. Both k'op and k'cl were found to decrease with increasing procaine concentration. This effect of the inhibitor of k'op cannot be explained by the open-channel-blocking mechanism but is consistent with the existence of a regulatory (inhibitory) receptor site.

Exon mapping by PCR

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