Homology modeling of cephalopod lens S-crystallin: a natural mutant of sigma-class glutathione transferase with diminished endogenous activity

The soluble S-crystallin constitutes the major lens protein in cephalopods. The primary amino acid sequence of S-crystallin shows an overall 41% identity with the digestive gland sigma-class glutathione transferase (GST) of cephalopod. However, the lens S-crystallin fails to bind to the S-hexylglutathione affinity column and shows very little GST activity in the nucleophilic aromatic substitution reaction between GSH and 1-chloro-2,4-dinitrobenzene. When compared with other classes of GST, the S-crystallin has an 11-amino acid residues insertion between the conserved alpha4 and alpha5 helices. Based on the crystal structure of squid sigma-class GST, a tertiary structure model for the octopus lens S-crystallin is constructed. The modeled S-crystallin structure has an overall topology similar to the squid sigma-class GST, albeit with longer alpha4 and alpha5 helical chains, corresponding to the long insertion. This insertion, however, makes the active center region of S-crystallin to be in a more closed conformation than the sigma-class GST. The active center region of S-crystallin is even more shielded and buried after dimerization, which may explain for the failure of S-crystallin to bind to the immobilized-glutathione in affinity chromatography. In the active site region, the electrostatic potential surface calculated from the modeled structure is quite different from that of squid GST. The positively charged environment, which contributes to stabilize the negatively charged Meisenheimer complex, is altered in S-crystallin probably because of mutation of Asn99 in GST to Asp101 in S-crystallin. Furthermore, the important Phe106 in authentic GST is changed to His108 in S-crystallin. Combining the topological differences as revealed by computer graphics and sequence variation at these structurally relevant residues provide strong structural evidences to account for the much decreased GST activity of S-crystallin as compared with the authentic GST of the digestive gland.

Synaptic excitation in the dorsal nucleus of the lateral lemniscus

The dorsal nucleus of the lateral lemniscus (DNLL) is a distinct auditory neuronal group located ventral to the inferior colliculus (IC). It receives excitatory and inhibitory afferent inputs from various structures of the auditory lower brainstem and sends GABAergic inhibitory efferents mainly to the contralateral DNLL and the bilateral IC. The synaptic excitation in DNLL neurons consists of two components, an early fast depolarization and a later long lasting one. Glutamate is the probable excitatory neurotransmitter for DNLL neurons. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors mediate the early part of the excitation while N-Methyl-D-aspartate (NMDA) receptors mediate the long lasting component. The long lasting NMDA receptor-mediated component in the DNLL may contribute to a prolonged inhibition in the IC. The DNLL is thought to be a structure for processing binaural information. Most DNLL neurons in rat and bat are sensitive to interaural intensity differences (IIDs). They are excited by stimulation of the contralateral ear and inhibited by stimulation of the ipsilateral ear, showing an excitatory/inhibitory (EI) binaural response pattern. The EI pattern can be attributed to synaptic inputs that originate from various structures in the lower auditory brainstem and impinge on the DNLL neurons. In cat some DNLL neurons are sensitive to IIDs and some are sensitive to interaural time differences. In addition, DNLL neurons exhibit different temporal response patterns to contralateral tonal stimulation and respond to amplitude modulated tones, implying that DNLL may contribute to processing temporally complex acoustic information. DNLL neurons shape binaural responses in the contralateral inferior colliculus and auditory cortex through their inhibitory brainstem projections and contribute to the accuracy with which animals localize sounds in space.

Identification of free deaminated sialic acid (2-keto-3-deoxy-D-glycero-D-galacto-nononic acid) in human red blood cells and its elevated expression in fetal cord red blood cells and ovarian cancer cells

Chemical studies have shown the occurrence of the deaminated sialic acid 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) in paired samples of blood obtained from mothers and newborns of healthy human individuals. Most of the KDN was found in red blood cells, although low levels were detected in mononuclear cells. No N-glycolylneuraminic acid was detected. Unexpectedly, nearly all of the KDN in fetal cord and matched maternal red blood cells was present as the free sugar and comparatively little occurred conjugated or as cytidine 5'-KDN phosphate. The amount of free KDN in fetal newborn red blood cells was 2.4-fold higher than in red blood cells from the mothers or from healthy nonpregnant women. Free KDN was also identified in normal human ovaries, in ovarian tumors, and in ascites cells obtained from ovarian cancer patients. Importantly, as in fetal cord red blood cells, a distinguishing feature of KDN expression in ovarian tumor cells was an elevated level of free KDN compared with normal controls. A positive correlation was found between an increase in the ratio of free KDN/N-acetylneuraminic acid in ovarian adenocarcinomas and the stage of malignancy. This was particularly evident in tumor cells isolated from the ascites fluid. The central importance of these new findings is 2-fold. First, they show that free KDN is a minor but ubiquitous sialic acid in human red blood cells and that its elevated expression in red blood cells from fetal cord blood compared with maternal red blood cells may be developmentally related to blood cell formation during embryogenesis. Second, the enhanced expression of KDN in ovarian cancer cells suggests that this sialic acid, like the alpha2,8-linked polysialic acid glycotope, may be an oncofetal antigen in these tumors and thus could be an "early warning" signal for onset of disease and/or a marker for detection of recurrence of disease. These new findings highlight the importance of elucidating the role that KDN and KDN-containing glycoconjugates may play in normal development and malignancy.

Characterization of phospholipase A2 (PLA2) from Taiwan Cobra: isoenzymes and their site-directed mutants

Extracellular and secretory phospholipase A2 (PLA2), a class of phospholipid digesting enzyme, is widely distributed in animal venoms of reptiles and insects. Two cDNAs encoding PLA2 isoenzymes from Taiwan Cobra (Naja naja atra) were cloned into pQE-30 plasmid vector and expressed in Escherichia coli. The recombinant products were subjected to refolding using sulfonation under reduction/oxidation conditions with glutathione and enterokinase removal of His-tag, resulting in the active recombinant PLA2 with the same molecular masses of native enzymes as determined by mass spectrometry. The recombinant PLA2 was also shown by circular dichroism to possess a secondary structure similar to native PLA2. The enzymatic activity of the major isoenzyme (PLA2-1) is higher than the other minor isoenzyme (PLA2-2), which shows two amino acid difference from PLA2-1. Site-directed mutagenesis was used to probe the structure/function relationship of two highly conserved residues among all reported PLA2, i.e., His-47 and Asp-93. Replacement of His-47 residue by either Ala or Arg resulted in the complete loss of activity. Similarly, the mutant Asp-93 --> Asn (D93N) also retained little activity. These results suggest that both His-47 and Asp-93 are essential for the catalytic activity of PLA2. Computer graphic study, based on homology modelling, highlights the differences between native PLA2 isoenzymes and their site-directed mutants, which may account for the differences in the observed biological activity.

Glutamate receptors underlying excitatory synaptic transmission in the rat's lateral superior olive studied in vitro

Glutamate receptors underlying synaptic excitation in the rat's lateral superior olive were studied by whole-cell patch clamp recordings in a brain slice preparation. Recordings from two morphological types of cells, bipolar and multipolar, identified by intracellular labeling with biocytin, showed that there were no obvious differences in responses mediated or modulated by ionotropic and metabotropic receptors between these two types of neurons. The excitatory postsynaptic potentials (EPSPs) elicited by ipsilateral stimulation of the trapezoid body consisted of two components. An earlier component, which had faster rise time constant and decay time constant, was mediated by non-NMDA receptors. A later component, which had slower rise time and decay time constants, was mediated by NMDA receptors. Suprathreshold responses (action potentials), which arose from the early component, were always abolished by the non-NMDA antagonist, CNQX, but not by the NMDA antagonist, APV. These results suggest that both non-NMDA and NMDA receptors are present in LSO neurons, and that fast excitatory transmission in LSO is primarily mediated by non-NMDA receptors. The metabotropic glutamate receptor agonists, t-ACPD and L-AP4, reduced the size of EPSPs evoked by stimulation of the ipsilateral trapezoid body in LSO neurons; the reductive action of t-ACPD was reversed by the antagonist, MCPG, indicating that metabotropic glutamate receptors, probably group II and III subtypes, can modulate excitatory synaptic transmission in LSO.

Characterization of three endogenous peptide inhibitors for multiple metalloproteinases with fibrinogenolytic activity from the venom of Taiwan habu (Trimeresurus mucrosquamatus)

Three small peptide components were isolated and purified from the venom of Taiwan habu (Trimeresurus mucrosquamatus), which show specific activity to inhibit the strong proteolytic activity of multiple metalloproteinases present in the crude venom. Using multiple chromatographies coupled with successive ultrafiltrations, three inhibitors, i.e. pyroglutamate-lysine-tryptophan (pyroGlu-Lys-Trp), pyroglutamate-asparagine-tryptophan (pyroGlu-Asn-Trp) and pyroglutamate-glutamine-tryptophan (pyroGlu-Gln-Trp) were obtained in good yields and high homogeneity. The yields of these peptide fractions were estimated to be about 0.65 mg, 0.55 mg and 0.42 mg from 250 mg total lyophilized crude venom, which corresponded to the approximate concentrations of 8.4 mM, 7.3 mM and 5.4 mM respectively in venom secretion. Detailed and unambiguous structural determination was established by amino acid analyses, mass spectrometry and microsequencing of purified peptides. Further functional characterization of these three tripeptides showed that they could weakly inhibit three metalloproteinases previously isolated from the same venom. The inhibitory activities were similar among these tripeptides and their IC50 (concentration for 50% inhibition) were estimated in a range of 0.20-0.95 mM, which is much more effective than citrate, another venom protease inhibitor of low molecular-weight component. Since these tripeptides are the endogenous peptide inhibitors present in the lumen of venom glands, it is conceivable that they may act as a self-defensive mechanism against the auto-digestive deleterious effect of the strong metalloproteinases in vivo, particularly several zinc-dependent metalloproteinases present in crotalid and viperid venoms.

Chemical mechanism of the endogenous argininosuccinate lyase activity of duck lens delta2-crystallin

The endogenous argininosuccinate lyase activity of duck delta2-crystallin was specifically inactivated by the histidine-specific reagent, diethyl pyrocarbonate. The protein was protected by l-citrulline or l-arginine from the diethyl pyrocarbonate inactivation. To characterize further the chemical mechanism of the delta2-crystallin-catalysed reaction, deuterium-labelled argininosuccinate was enzymically synthesized from fumarate and l-arginine with delta2-crystallin in 2H2O. The argininosuccinate synthesized contained about 19% of the anhydride form; however, the deuterium was clearly demonstrated to be incorporated enantioselectively. Only the pro-HR atom at C-9 of the succinate moiety was labelled in the [2H]argininosuccinate-9-d synthesized, which indicates an anti-elimination mechanism for the endogenous argininosuccinate lyase activity of delta2-crystallin. The enzymic activity of duck lens delta2-crystallin in the pH range 5.5-8.5 was investigated using both protium- and deuterium-labelled argininosuccinate as the substrate. From the logkcat versus pH plot, two molecular pKa values of 6.18+/-0.02 and 8.75+/-0.03 were detected in the delta2-crystallin-argininosuccinate binary complex. The former must be dehydronated and the latter hydronated to achieve an optimum reaction rate. The logkcat/Km versus pH plot suggested two molecular pKa values of 5.96+/-0.09 and 8.29+/-0.10 for the free delta2-crystallin to be involved in the substrate binding. Small kinetic isotope effects of 1.17+/-0.02 and 1.05+/-0.09 were found for kcat and kcat/Km respectively. Combining results from labelling and kinetic analysis indicates that the endogenous argininosuccinate lyase activity of duck delta2-crystallin is compatible with a stepwise E1cB mechanism, the rate-limiting step probably at the C-N bond-cleavage step.

High-performance capillary electrophoretic characterization of different types of oligo- and polysialic acid chains

We have carried out comparative structural analysis of novel oligo- and polysialic acid chains from diverse sources. Controlled acid hydrolysates of (a) colominic acid, alpha2-->8-linked homopolymer of N-acetylneuraminic acid (Neu5Ac), (b) alpha2-->8-linked oligo/polyNeu5Gc chains present in rainbow trout egg polysialoglycoprotein, and (c) alpha2-->8-linked oligomers of deaminoneuraminic acid (KDN) residues of KDN-rich glycoprotein derived from rainbow trout vitelline envelope were analyzed by high-performance capillary electrophoresis (HPCE). The results showed that three different types of alpha2-->8-linked oligosialic acids having same degree of polymerization can be separated by HPCE. A partial hydrolysate of colominic acid with mild acid was shown by CE to form intramolecular esters during the controlled hydrolysis and the subsequent workup procedure. In contrast, lactonization of (-->5-Oglycolyl-Neu5Gcalpha2-->)n, alpha2-->5-Oglycolyl-linked homopolymer of N-glycolylneuraminic acid (Neu5Gc) present in the egg jelly coat of sea urchin, did not take place as readily as in (-->8Neu5Acalpha2-->)n.

Physiological role of the association complexes of alpha-crystallin and its substrates on the chaperone activity

Previous reports on the chaperone activity of alpha-crystallin to prevent protein denaturation and thermal aggregation have suggested that partially denatured proteins can bind alpha-crystallin in its central region. Likewise, beta- and gamma-crystallin can also be localized to the central cavity of alpha-crystallin particle in vivo, which provides indirect evidence that alpha-crystallin can function as a chaperone in the intact lens. In this study, we have further demonstrated that the binding of the substrate proteins to alpha-crystallin by short-term preincubation may mimic the in vivo conditions of crystallin association. Preheating of alpha-crystallin with its substrate proteins at 60 degrees C for 20 min resulted in the formation of stable complexes between alpha-crystallin and its substrates (8.0% of insulin or 5.3% of gamma-crystallin was involved in complex formation). Under such conditions, the chaperone activity of alpha-crystallin to inhibit dithiothreitol-, ultraviolet-, or oxidation-induced protein aggregation can be greatly enhanced. Since UV-irradiation and oxidative stress are common insults to eye lenses under normal physiological conditions, the presence of alpha/gamma and alpha/beta complex in vivo may play an important role to maintain the lens in a transparent state.

Characterization, cloning, and expression of porcine alpha B crystallin

alpha-Crystallin is a major lens protein present in the lenses of all vertebrate species. Recent studies have revealed that bovine alpha-crystallins possess genuine chaperone activity similar to small heat-shock proteins. In order to compare this chaperone-like structural protein from the eye lenses of different mammalian species, we have cloned and expressed one of the main alpha-crystallin subunits, i.e., alpha B crystallin, from the porcine lenses in order to facilitate the structure-function evaluation and comparison of this chaperonin protein. cDNA encoding alpha B subunit chain was obtained using a new "Marathon cDNA amplification" protocol of Polymerase Chain Reaction (PCR). PCR-amplified product corresponding to alpha B subunit was then ligated into pGEM-T plasmid and prepared for nucleotide sequencing by the dideoxy-nucleotide chain-termination method. Sequencing several positive clones containing DNA inserts coding for alpha B-crystallin subunit constructed only one complete full-length reading frame of 525 base pairs similar to human and bovine alpha B subunits, covering a deduced protein sequence of 175 amino acids including the universal translation-initiating methionine. The porcine alpha B crystallin shows only 3 and 7 residues difference to bovine and human alpha B crystallins respectively, revealing the close relatedness among mammalian eye lens proteins. The sequence differences between porcine and sub-mammalian species such as chicken and bullfrog are much greater, especially at the N- and C-terminal regions of these alpha B crystallins. Expression of alpha B subunit chain in E. coli vector generated a polypeptide which can cross-react with the antiserum against the native and purified alpha B subunit from the native porcine lenses albeit with a much lower activity.

GABAergic projections from the lateral lemniscus to the inferior colliculus of the rat

The objective of the present study was to provide direct evidence regarding GABAergic projections from the nuclei of the lateral lemniscus to the central nucleus of the inferior colliculus (ICC), and from the ICC to the opposite ICC. Projections of GABAergic neurons in the rat were investigated by a combination of fluorogold (FG) retrograde tracing and GABA immunocytochemistry. FG was first injected into a frequency-defined region (11-13 kHz) in the center of the ICC, and 1-2 weeks was allowed for retrograde transport. Vibratome sections were then cut through the brainstem and stained with GABA antibody. Double-labeling was taken as evidence of GABAergic neurons projecting to the ICC. The results from FG retrograde labeling alone showed that neurons in the dorsal nucleus of the lateral lemniscus (DNLL) bilaterally, in the intermediate and ventral nucleus of the lateral lemniscus (INLL and VNLL) ipsilaterally, and in the ICC contralaterally project to the ICC. GABA immunostaining alone showed substantial numbers of GABA positive neurons in the nuclei of the lateral lemniscus and the inferior colliculus. FG and GABA double-labeled neurons were present in all nuclei of the lateral lemniscus that project to the ICC. The greatest concentration of double-labeled neurons was found bilaterally in the DNLL, suggesting a prominent GABAergic projection from the DNLL to the ICC. The presence of many double-labeled neurons in the ipsilateral INLL and VNLL suggests that there are also GABAergic inputs from the INLL and VNLL to the ICC. No double-labeled neurons were found in the contralateral ICC, which suggests the possibility of a prominent non-GABAergic projection.

Two novel alpha-neurotoxins isolated from Taiwan cobra: sequence characterization and phylogenetic comparison of homologous neurotoxins

Two novel postsynaptic neurotoxins (alpha-neurotoxins) isolated and purified from the Taiwan cobra venom (Naja naja atra) possess distinct primary sequences and different neurotoxicities as compared with the most abundant and lethal component in the venom, i.e., cobrotoxin characterized before from the same venom. The complete sequences of two neurotoxin analogues were determined by N-terminal Edman degradation and comparison of amino acid compositions of proteolytic toxin fragments with other homologous toxins of known sequences. The short-chain neurotoxin consists of 61 amino acid residues with eight conserved cysteine residues and is found to show 78% sequence identity with cobrotoxin. The other toxin, consisting of 65 residues with ten cysteines, belongs to the family of long-chain neurotoxins. It is the first long-chain alpha-neurotoxin reported from the Taiwan cobra. The lethal toxicities of these two novel neurotoxins were much lower than cobrotoxin, albeit with close structural homology among the three toxins in terms of their primary sequences and tertiary structure predicted by homology modeling. Multiple sequence alignment and comparison coupled with construction of a phylogenetic tree for various alpha-neurotoxins of Naja and closely related genuses have established that all nicotinic alpha-neurotoxins present in the snake family of Elapidae are closely related to each other, presumably derived from an ancestral polypeptide by gene duplication and subsequent multiple mutational substitutions.

Rumen-stable delivery systems

Ruminants have a distinct digestive system which serves a unique symbiotic relationship between the host animal and predominantly anaerobic rumen bacteria and protozoa. Rumen fermentation can be both beneficial by enabling utilization of cellulose and non-protein nitrogen and detrimental by reducing the nutritive value of some carbohydrates, high biological value proteins and by hydrogenating unsaturated lipids. In addition it can also result in the modification and inactivation of many pharmacologically active ingredients administered to the host animal via the oral route. The advances in ruminant nutrition and health demand a rumen-stable delivery system which can deliver the active ingredient post-ruminally while simultaneously meet efficacy, safety and cost criteria. In contrast to drug delivery systems for humans, the demand for low-cost has hindered the development of effective rumen-stable delivery systems. Historically, heat and chemical treatment of feed components, low solubility analogues or lipid-based formulations have been used to achieve some degree of rumen-stability, and products have been developed accordingly. Recently, a polymeric pH-dependent rumen-stable delivery system has been developed and commercialized. The rationale of this delivery system is based on the pH difference between ruminal and abomasal fluids. The delivery system is composed of a basic polymer, a hydrophobic substance and a pigment material. It can be applied as a coating to solid particles via a common encapsulation method such as air-suspension coating. In the future, the delivery system could be used to deliver micronutrients and pharmaceuticals post-ruminally to ruminant animals. A further possible application of the delivery system is that it could also be combined with other controlled delivery devices/systems in order to enhance slow release or to achieve targeted delivery needs for ruminants. This paper discusses the rumen protection and the abomasal release mechanism of the polymeric coating. It also reviews other rumen stable delivery systems and methods for evaluating their in vitro and in vivo performance.

Hyperpolarization-activated inward current in neurons of the rat's dorsal nucleus of the lateral lemniscus in vitro

Hyperpolarization-activated inward current in neurons of the rat's dorsal nucleus of the lateral lemniscus in vitro. J. Neurophysiol. 78: 2235-2245, 1997. The hyperpolarization-activated current (Ih) underlying inward rectification in neurons of the rat's dorsal nucleus of the lateral lemniscus (DNLL) was investigated using whole cell patch-clamp techniques. Patch recordings were made from DNLL neurons of young rats (21-30 days old) in 400 micro;m tissue slices. Under current clamp, injection of negative current produced a graded hyperpolarization of the cell membrane, often with a gradual sag in the membrane potential toward the resting value. The rate and magnitude of the sag depended on the amount of hyperpolarizing current. Larger current resulted in a larger and faster decay of the voltage. Under voltage clamp, hyperpolarizing voltage steps elicited a slowly activating inward current that was presumably responsible for the sag observed in the voltage response to a steady hyperpolarizing current recorded under current clamp. Activation of the inward current (Ih) was voltage and time dependent. The current just was seen at a membrane potential of -70 mV and was activated fully at -140 mV. The voltage value of half-maximal activation of Ih was -78.0 +/- 6.0 (SE) mV. The rate of Ih activation was best approximated by a single exponential function with a time constant that was voltage dependent, ranging from 276 +/- 27 ms at -100 mV to 186 +/- 11 ms at -140 mV. Reversal potential (Eh) of Ih current was more positive than the resting potential. Raising the extracellular potassium concentration shifted Eh to a more depolarized value, whereas lowering the extracellular sodium concentration shifted Eh in a more negative direction. Ih was sensitive to extracellular cesium but relatively insensitive to extracellular barium. The current amplitude near maximal-activation (about -140 mV) was reduced to 40% of control by 1 mM cesium but was reduced to only 71% of control by 2 mM barium. When the membrane potential was near the resting potential (about -60 mV), cesium had no effect on the membrane potential, current-evoked firing rate and input resistance but reduced the spontaneous firing. When the membrane potential was more negative than -70 mV, cesium hyperpolarized the cell, decreased current-evoked firing and increased the input resistance. Ih in DNLL neurons does not contribute to the normal resting potential but may enhance the extent of excitation, thereby making the DNLL a consistently powerful inhibitory source to upper levels of the auditory system.

Phycocyanobilin is the natural precursor of the phytochrome chromophore in the green alga Mesotaenium caldariorum

Compared with phytochromes isolated from etiolated higher plant tissues and a number of lower plant species, the absorption spectrum of phytochrome isolated from the unicellular green alga Mesotaenium caldariorum is blue-shifted (Kidd, D. G., and Lagarias, J. C. (1990) J. Biol. Chem. 265, 7029-7035). The present studies were undertaken to determine whether this blue shift is due to a chromophore other than phytochromobilin or reflects a different protein environment for the phytochromobilin prosthetic group. Using reversed phase high performance liquid chromatography, we show that soluble protein extracts prepared from algal chloroplasts contain the enzyme activities for ferredoxin-dependent conversions of biliverdin IXalpha to (3Z)-phytochromobilin and (3Z)-phytochromobilin to (3Z)-phycocyanobilin. In vitro assembly of recombinant algal apophytochrome was undertaken with (3E)-phytochromobilin and (3E)-phycocyanobilin. The difference spectrum of the (3E)-phycocyanobilin adduct was indistinguishable from that of phytochrome isolated from dark-adapted algal cells, while the (3E)-phytochromobilin adduct displayed red-shifted absorption maxima relative to purified algal phytochrome. These studies indicate that phycocyanobilin is the immediate precursor of the green algal phytochrome chromophore and that phytochromobilin is an intermediate in its biosynthesis in Mesotaenium.

A cyanobacterial phytochrome two-component light sensory system

The biliprotein phytochrome regulates plant growth and developmental responses to the ambient light environment through an unknown mechanism. Biochemical analyses demonstrate that phytochrome is an ancient molecule that evolved from a more compact light sensor in cyanobacteria. The cyanobacterial phytochrome Cph1 is a light-regulated histidine kinase that mediates red, far-red reversible phosphorylation of a small response regulator, Rcp1 (response regulator for cyanobacterial phytochrome), encoded by the adjacent gene, thus implicating protein phosphorylation-dephosphorylation in the initial step of light signal transduction by phytochrome.

Effect of heat-induced structural perturbation of secondary and tertiary structures on the chaperone activity of alpha-crystallin

alpha-Crystallin, a major protein of the lens, is known to have chaperone activity to protect other proteins against thermal aggregation. Heat-induced structural change of alpha-crystallin was previously shown to increase its chaperone activity. In this report, we studied the thermal reversibility of alpha-crystallin and the effect of change in secondary structure on its chaperone function in vitro. The heat-induced conformational changes in the aromatic region of near-UV CD spectra showed only a small degree of reversibility. The structural transitions from 50 to 70 degrees C were largely reversible if the incubation time was short. However, the protective ability to inhibit thermal aggregation of alcohol dehydrogenase by alpha-crystallin was essentially similar at 48 and 70 degrees C. Under long-term heating at high temperatures, there was a time-dependent irreversibility of structural change in alpha-crystallin as revealed by CD spectroscopy. Such denatured alpha-crystallin by long-term heating can still preserve its ability to prevent UV-induced aggregation of gamma-crystallin at room temperature, indicating relatively little effect of heat-induced changes in secondary structure on the chaperone activity of alpha-crystallin.

Synaptic excitation in the dorsal nucleus of the lateral lemniscus: whole-cell patch-clamp recordings from rat brain slice

The synaptic events underlying the excitation of neurons in the rat's dorsal nucleus of the lateral lemniscus were studied by whole-cell patch-clamp recordings in a brain slice preparation of the auditory midbrain. Both current-clamp and voltage-clamp data were obtained with the brain slice submerged in artificial cerebrospinal fluid. The rats were between 21 and 35 days of age at the time the recordings were made. Synaptic responses were evoked by a bipolar stimulating electrode placed on the lateral lemniscus just ventral to the dorsal nucleus. To eliminate glycinergic inhibitory responses, all physiological data were gathered with 0.5 microM strychnine added to the saline bath. Under current-clamp conditions, excitatory postsynaptic potentials could be subdivided into early and late components. The early component produced a single, highly reliable, short-latency spike and the later component produced a more variable, long-latency spike or train of spikes. The non-N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, completely blocked the early excitatory postsynaptic potential and its associated action potential. The N-methyl-D-aspartate antagonist, D,L-2-amino-5-phosphonovaleric acid, blocked the later excitatory postsynaptic potential and its action potentials. Typically, both early and late excitatory postsynaptic potentials could be recorded from the same cell, but the early excitatory postsynaptic potential was evoked at lower stimulus levels and had a larger amplitude than the later excitatory postsynaptic potential. Under voltage-clamp conditions, dorsal nucleus of the lateral lemniscus neurons responded to stimulation of the lateral lemniscus with excitatory postsynaptic currents. Outward excitatory postsynaptic currents were recorded with holding potentials that depolarized the cell membrane and inward currents were seen when the cell was hyperpolarized. The current-voltage (I-V) relation of the early peak portion of the excitatory postsynaptic current was nearly linear, whereas the I-V relation of the later excitatory postsynaptic current (12 ms after the peak) was non-linear over the range between -50 and - 100 mV. The outward excitatory postsynaptic current consisted of an early current that was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later current that was blocked by D,L-2-amino-5-phosphonovaleric acid. In artificial cerebrospinal fluid with normal concentrations of Mg2+, the inward excitatory postsynaptic current was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, but was not affected by D,L-2-amino-5-phosphonovaleric acid. In Mg2+-free artificial cerebrospinal fluid. however, the early component of the inward excitatory postsynaptic current was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later component was blocked by D,L-2-amino-5-phosphonovaleric acid. The results indicate that both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated synaptic responses are present in dorsal nucleus of the lateral lemniscus neurons of rats at 21-35 days of age. The N-methyl-D-aspartate component had a longer time-course and a higher threshold than the non-N-methyl-D-aspartate component, and was subject to a voltage-dependent Mg2+ block when the cell's membrane was hyperpolarized. The long-duration N-methyl-D-aspartate component is probably responsible for the prolonged inhibitory effect of dorsal nucleus of the lateral lemniscus neurons on physiological responses in the rat's inferior colliculus.

alpha-Crystallin acting as a molecular chaperonin against photodamage by UV irradiation

alpha-Crystallin, a major protein of the eye lens, is known to have chaperone activity in preventing heat-induced aggregation of enzymes and other crystallins. In this study, we investigate the ability of alpha-crystallin to inhibit UV-light-induced aggregation of other lens proteins and the effect of exposure of alpha-crystallin to UV irradiation on its chaperone activity. The chaperone activities of alpha-crystallin preincubated at different temperatures were found to be different and could be correlated with its change in quaternary structure as determined by the fluorescence probe ANS (8-anilo-1-naphthalene sulfonate). alpha-Crystallin can inhibit the aggregation of gamma-crystallin from UV irradiation at room temperature, and the preheated alpha-crystallins provide more protection than the native one. Upon irradiation by UV light, alpha-crystallin gradually lost its ability to protect beta-crystallin against thermal aggregation. The loss of the chaperone efficacy of alpha-crystallin to protect other lens proteins may shed light on human cataract formation induced by long-term exposure to UV irradiation.

Study of structure-activity correlation in destruxins, a class of cyclodepsipeptides possessing suppressive effect on the generation of hepatitis B virus surface antigen in human hepatoma cells

A new destruxin [destruxin E2 chlorohydrin] was isolated from the culture medium of Metarrhizium anisopliae and its structure was determined by NMR spectroscopy and mass spectrometry. As compared with other destruxins, the new destruxin showed a lower suppressive activity on the production of hepatitis B virus surface antigen in human hepatoma Hep3B cells. NMR study coupled with molecular modeling by computer graphics has revealed that the hydrophobicity nature of the convex surface characteristic of all destruxin molecules plays an important role in their biological activity.

Protein engineering of venom toxins by synthetic approach and NMR dynamic simulation: status of basic amino acid residues in waglerin I

The tertiary structure of waglerin I has been determined by NMR and dynamic simulated annealing [Chuang et al., Biochim. Biophys. Acta 1292, 145-155 (1996)]. It is believed that the peptide basicity of waglerin may play an important role for its activity due to its high content of basic amino acids. In order to investigate the active site of the toxin, seven analogues of waglerin, [Ala3]-waglerin, [Ala7]-waglerin, [Ala10]-waglerin, [Ala14]-waglerin, [Ala18]-waglerin, [Ala20]-waglerin and [Ala22]-waglerin have been synthesized chemically by single replacement of basic amino acid residues one by one with Ala. By correlation of structures for each analogue with LD50 toxicity bioassays, it is found that the [Ala10]-waglerin exhibits no toxicity and the active site of the native toxin seems to reside in the proximity of the disulfide loop, which is spatially close to His10. Furthermore, the closer is the disulfide loop to the basic amino acid in waglerin, the more influential is the basic amino acid on the toxicity of waglerin. Based on the tertiary structure of waglerin, the structures of all synthetic analogues were derived based on computer-simulated modelling. By the pair-wise structural comparison, the disulfide loop in [Ala10]-waglerin analogue is found to be twisted as compared to the native form, in agreement with the lack of toxicity for this synthetic analogue.

Isolation of a venom factor devoid of proteolytic activity from Taiwan habu (Trimeresurus mucrosquamatus): N-terminal sequence homology and no functional similarity to factors IX/X-binding proteins and botrocetin

One novel venom factor was isolated and purified from the venom of Taiwan habu (Trimeresurus mucrosquamatus) using two consecutive anion-exchange and gel-filtration chromatographies followed by cation-exchange HPLC. Further characterization of the purified protein indicated that it lacks the proteolytic activity toward fibrinogen molecules, suggesting that this protein factor does not belong to the familes of metalloproteinases and thrombin-like serine proteases commonly found in the crude venoms of various crotalid snakes. The purified protein exists as a native dimeric protein of 26 kDa, consisting of two closely similar subunits of 16 and 13 kDa, held together by disulfide linkage. N-Terminal sequence analysis revealed that both chains are homologous to each other at the N-terminal fragment and also similar to the factors IX/X-binding protein isolated from Trimeresurus flavoviridis and botrocetin from Bothrops jararaca. This study points to the existence of one new two-chain venom factor without fibrinogenase activity from Taiwan habu, which, in contrast to botrocetin, promotes platelet agglutination even in the absence of von Willebrand factor. Unlike factors IX/X-binding proteins, it did not show affinity to coagulation factors IX and X in the presence of Ca2+ ion. It also shows no inhibition on thrombin, in contrast with bothrojaracin, a thrombin inhibitor isolated from Bothrops jararaca venom. We have therefore named this novel venom factor trimecetin to distinguish it from some structurally related venom factors present in various crotalid and viperid snakes.

Effects of dietary nucleoside-nucleotide mixture on memory in aged and young memory deficient mice

Intestinal mucosa, bone marrow hematopoietic cells and brain have limited capacity for the de novo synthesis of nucleosides (NSs) and nucleotides (NTs). Whereas the role of dietary NS and NT in the former two tissues is known, it is not known in the brain. Therefore we studied the effect of dietary NS and NT mixture on memory in aged mice (Experiment 1) and young memory deficient mice (Experiment 2). Memory retention was studied by step-through type passive avoidance performance (maximum 180 seconds). In Experiment 1 aged (7 month old) senescence accelerated mice (SAM) were fed 20% casein diet (control) or this diet supplemented with 0.5% NS/NT mixture for 12 weeks. Memory was studied 1, 2 and 3 days after the electric shock (punishment). In Experiment 2, young (1 month old) memory deficient mice (Dull mice) and normal mice (ddY mice) were fed the same diets as those in Experiment 1 for 12 weeks. Memory retention was studied 1 and 3 days after the punishment. In the aged SAM the average time of avoidance and also the percentages of successful memory 2 and 3 days after the punishment were significantly higher in the NS/NT diet group than the control diet group (P < 0.05). In the Dull mice percentage of successful memory was higher in the NS/NT diet group than in the control group 3 days after the punishment, however, such an effect was not observed in the normal mice. These results suggest that insufficient endogenous supply of NSs and NTs may be responsible for the factor of memory deficiency with aging or of genetical memory deficiency, which can be improved by the dietary administration of NSs and NTs.

The methylotrophic yeast Pichia pastoris synthesizes a functionally active chromophore precursor of the plant photoreceptor phytochrome

Induction of the expression of an algal phytochrome cDNA in the methylotrophic yeast Pichia pastoris led to time-dependent formation of photoactive holophytochrome without the addition of exogenous bilins. Both in vivo and in vitro difference spectra of this phytochromic species are very similar to those of higher plant phytochrome A, supporting the conclusion that this species possesses a phytochromobilin prosthetic group. Zinc blot analyses confirm that a bilin chromophore is covalently bound to the algal phytochrome apoprotein. The hypothesis that P. pastoris contains phytochromobilin synthase, the enzyme that converts biliverdin IX alpha to phytochromobilin, was also addressed in this study. Soluble extracts from P. pastoris were able to convert biliverdin to a bilin pigment, which produced a native difference spectrum upon assembly with oat apophytochrome A. HPLC analyses confirm that biliverdin is converted to both 3E- and 3Z-isomers of phytochromobilin. These investigations demonstrate that the ability to synthesize phytochromobilin is not restricted to photosynthetic organisms and support the hypothesis of a more widespread distribution of the phytochrome photoreceptor.

Potassium currents and membrane excitability of neurons in the rat's dorsal nucleus of the lateral lemniscus

1. The contribution of voltage-activated outward potassium currents to membrane excitability of neurons in the rat's dorsal nucleus of the lateral lemniscus (DNLL) was studied in a brain slice preparation using whole cell patch-clamp and intracellular recordings. Voltage-clamp methods and pharmacological manipulations were used to examine the currents regulating membrane dynamics in DNLL. 2. A delayed sustained outward current was evoked by applying depolarizing voltage steps across the cell membrane from a holding potential of -50 mV. An additional transient outward current was evoked when the depolarizing steps were preceded by a hyperpolarizing prepulse of -110 or -120 mV. 3. The transient outward current peaked within 6.8 ms of the onset of a depolarizing pulse. It decayed with a time constant of 12.3 ms for a 60-mV depolarizing voltage shift. Half-inactivation of this current occurred at -81.3 mV. The time constant for removal of the inactivation was 17.4 ms. The transient current had a high sensitivity to 4-aminopyridine (4-AP). 4. The sustained current was activated more slowly and was more sensitive to tetraethylammonium (TEA) than the transient current. The sustained current had both Ca2+-dependent and Ca2+-independent components. The Ca2+-dependent portion emerged at potentials of about -35 mV and was activated fully at +10 mV. The Ca2+-independent component was activated at potentials more positive than -40 mV and increased in magnitude with further depolarization. Inactivation of the Ca2+-independent component was voltage dependent. Also, TEA suppressed the Ca2+-independent compound. 5. The transient current in DNLL neurons closely resembled the A current (IA) described for hippocampal and other neurons in both kinetics and pharmacology. The Ca2+-independent component of the sustained current resembled the K current (IK) described for other neurons in both its properties of activation and inactivation and its pharmacology. 6. The outward current of some DNLL neurons was found to contain a dendrotoxin-sensitive component. This component reached its peak at 6.8 ms and had voltage-sensitive time constants of decay of 25.5 and 8.5 ms with voltage steps of 40 and 60 mV, respectively. 7. Application of 4-AP and TEA markedly prolonged the spike width, abolished the fast component of the after hyperpolarization and depolarized the cell membrane. Also, the number of action potentials produced by positive current injection increased under the influence of 4-AP and TEA. Membrane excitability and spike repolarization were dependent on both 4-AP-sensitive transient and TEA-sensitive sustained currents. 8. Neurons in DNLL typically exhibit a steady discharge of action potentials in response to sustained membrane depolarization. The rate and temporal pattern of production of action potentials in these cells are determined by the combination of transient and sustained potassium channels.