Mechanisms of pheromone inactivation in the gut of Periplaneta americana

The sex attractant of the cockroach, Periplaneta americana, has been shown in earlier work to be largely inactivated by dissected midgut from males and from mated females. It is only slightly inactivated by the midgut from virgin females. In this paper, the sex pheromone inactivating system is further studied and shown to be active in late instar larvae of both sexes. In the male this pheromone inactivation is inhibited by piperonyl butoxide, a microsomal oxidase inhibitor. This compound appears to act on the midgut tissue directly. In the mated female, piperonyl butoxide has little effect. When the pheromone inactivating capacity is partitioned into soluble and tissue components, it appears that the soluble component is most active in the male, whereas the tissue component is most active in the female. Evidence from heat inactivation, trichloracetic acid precipitation, and the use of soy bean trypsin inhibitor, as well as the time course of the reaction, suggest that the factor or factors inactivating pheromone are proteins, probably enzymes. Evidence that at least part of the pheromone inactivating capacity is due to microsomal oxidases is considered. It is also observed that both pheromone and piperonyl butoxide absorb to membranes.     

Effects of a nitrate reductase inactivating enzyme and NAD(P)H on the nitrate reductase from higher plants and Neurospora.

Evidence is presented which suggests that the NAD(P)H-cytochrome c reductase component of nitrate reductase is the main site of action of the inactivating enzyme. When tested on the nitrate reductase (NADH) from the maize root and scutella, the NADH-cytochrome c reductase was inactivated at a greater rate than was the FADH2-nitrate reductase component. With the Neurospora nitrate reductase (NADPH) only the NADPH-cytochrome c reductase was inactivated. p-Chloromercuribenzoate at 50 muM, which gave almost complete inhibition of the NADH-cytochrome c reductase fraction of the maize nitrate reductase, had no marked effect on the action of the inactivating enzyme. A reversible inactivation of the maize nitrate reductase has been shown to occur during incubation with NAD(P)H. In contrast to the action of the inactivating enzyme, it is the FADH2-nitrate reductase alone which is inactivated. No inactivation of the Neurospora nitrate reductase was produced by NAD(P)H alone and also in the presence of FAD. The lack of effect of the inactivating enzyme and NAD(P)H on the FADH2-nitrate reductase of Neurospora suggests some differences in its structure or conformation from that of the maize enzyme. A low level of cyanide (0.4 mu M) markedly enhanced the action of NAD(P)H on the maize enzyme; Cyanide at a higher level (6 mu M) did give inactivation of the Neurospora nitrate reductase in the presence of NADPH and FAD. The maize nitrate reductase, when partially inactivated by NADH and cyanide, was not altered as a substrate for the inactivating enzyme. The maize root inactivating enzyme was also shown to inactivate the nitrate reductase (NADH) in the pea leaf. It had no effect on the nitrate reductase from either Pseudomonas denitrificans or Nitrobacter agilis.     

Voltage gated ionic channels in rat cultured astrocytes, reactive astrocytes and an astrocyte-oligodendrocyte progenitor cell

Astrocytes (both type 1 and type 2), cultured from the central nervous system of newborn or 7 day old rats show voltage gated sodium and potassium channels that are activated when the membrane is depolarized to greater than -40 mV. The sodium channels in these cells have an h-infinity curve similar to that of nodal membranes but the activation (peak current-voltage) curves are shifted along the voltage axis by about +30 mV. These sodium currents are blocked only by high concentrations of tetrodotoxin. The voltage activated potassium currents in both types of astrocyte show at least two components; an inactivating component that is suppressed at holding potentials of greater than -40 mV and a persistent, non-inactivating current. Several types of single channel currents were observed in outside-out membrane patches from type 2 astrocytes. One type of potassium channel showed inactivation on depolarization and may contribute to the whole-cell inactivating current. In contrast, oligodendrocytes showed no obvious voltage gated membrane channels. The properties of the type 2 astrocyte-oligodendrocyte progenitor cell were investigated in two ways: 1) by examination of cells just beginning to differentiate along the "electrically silent" oligodendrocyte pathway or 2) by recording from progenitor cells cultured for 24 hours in the presence of cycloheximide to block the appearance of new membrane channels. In both cases, voltage gated inward (sodium) and outward (potassium) currents were noted. The outward current response showed both an inactivating and a non-inactivating component. Similar voltage activated inward and outward membrane currents were noted in reactive astrocytes freshly isolated (3-6 hours) from lesioned areas of adult rat brains.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inactivation of maize leaf phosphoenolpyruvate carboxylase by the binding to chloroplast membranes

Phosphoenolpyruvate carboxylase (PEPC) purified from maize (Zea mays L.) leaves associates with maize leaf chloroplast membrane in vitro. The binding of PEPC to the membrane results in enzyme inactivation. A protein isolated from a maize leaf chloroplast membrane preparation inactivated PEPC. Treatment with membrane preparation or with partially purified inactivating protein accelerates PEPC inactivation at low temperature (4°C). Interaction of PEPC with chloroplast membrane or inactivating protein may inactivate the enzyme by influencing dissociation of the enzyme active tetramer.     

On the voltage-dependent action of tetrodotoxin.

The use of the maximum rate-of-rise of the action potential (Vmax) as a measure of the sodium conductance in excitable membranes is invalid. In the case of membrane action potentials, Vmax depends on the total ionic current across the membrane; drugs or conditions that alter the potassium or leak conductances will also affect Vmax. Likewise, long-term depolarization of the membrane lessens the fraction of total ionic current that passes through the sodium channels by increasing potassium conductance and inactivating the sodium conductance, and thereby reduces the effect of Vmax of drugs that specifically block sodium channels. The resultant artifact, an apparent voltage-dependent potency of such drugs, is theoretically simulated for the effects of tetrodotoxin on the Hodgkin-Huxley squid axon.     

Interactions between quaternary lidocaine, the sodium channel gates, and tetrodotoxin.

A voltage clamp technique was used to study sodium currents and gating currents in squid axons internally perfused with the membrane impermeant sodium channel blocker, QX-314. Block by QX-314 is strongly and reversibly enhanced if a train of depolarizing pulses precedes the measurement. The depolarization-induced block is antagonized by external sodium. This antagonism provides evidence that the blocking site for the drug lies inside the channel. Depolarization-induced block of sodium current by QX-314 is accompanied by nearly twofold reduction in gating charge movement. This reduction does not add to a depolarization-induced immobilization of gating charge normally present and believed to be associated with inactivation of sodium channels. Failure to act additively suggests that both, inactivation and QX-314, affect the same component of gating charge movement. Judged from gating current measurement, a drug-blocked channel is an inactivated channel. In the presence of external tetrodotoxin and internal QX-314, gating charge movement is always half its normal size regardless of conditioning, as it QX-314 is then permanently present in the channel.     

Sodium channel inactivation in the crayfish giant axon. Must channels open before inactivating?

Experiments on sodium channel inactivation kinetics were performed on voltage-clamped crayfish giant axons. The primary goal was to investigate whether channels must open before inactivating. Voltage-clamp artifacts were minimized by the use of low-sodium solutions and full series resistance compensation, and the spatial uniformity of the currents was checked with a closely spaced pair of electrodes used to measure local current densities. For membrane potentials between -40 and +40 mV, sodium currents decay to zero with a single exponential time-course. The time constant for decay is a steep function of membrane potential. The time-course of inactivation measured with the double-pulse method is very similar to the decay of current at the same potential. Steady-state inactivation curves measured with different test pulses are identical. The time-course of double pulse inactivation shows a lag that roughly correlates with the opening of sodium channels, but detailed comparisons with the time course of the prepulse current suggest that it is not strictly necessary for channels to open before inactivating. Measurements of the potential dependence of the integral of sodium conductance area also inconsistent with the simplest cases of models in which channels must open before inactivating.     

机械分离的果蝇幼虫中枢神经元全细胞钾电流的特性

培养的果蝇胚胎及幼虫中枢神经元已被广泛用于细胞膜离子通道,突触传递和胞内信使调节等电生理学研究,在本实验中,利用机械震荡分离方法获得了大量的果蝇幼虫中枢神经元,其中大部分为Ⅱ型神经元,运用膜片钳技术,鉴定了Ⅱ型神经元上五种具有不同动力学特性的全细胞钾电流,其中E型电流表型表现出与其它四种电流完全不同的“钟形”激活特性,进一步的研究还表明该类型电流具有明显的钙依赖性,而且它具有与其它四种电流不同的衰减特性。     

Inactivation of enteroviruses by ascorbic acid and sodium bisulfite.

Poliovirus type 1, coxsackievirus type A9, and echovirus type 7 were inactivated by sodium bisulfite and ascorbic acid. Inactivation rates depended upon concentration, temperature, and pH. RNA infectivity was lost during inactivation; the capsid was also altered by these inactivating agents, as determined by enzyme sensitivity assays and by tests of adsorption to cells. Structural modifications of the virus particles were not identical, suggesting that the mechanism of inactivation by ascorbic acid differs from that of sodium bisulfite.     

Inactivation of enteroviruses by ascorbic acid and sodium bisulfite.

Poliovirus type 1, coxsackievirus type A9, and echovirus type 7 were inactivated by sodium bisulfite and ascorbic acid. Inactivation rates depended upon concentration, temperature, and pH. RNA infectivity was lost during inactivation; the capsid was also altered by these inactivating agents, as determined by enzyme sensitivity assays and by tests of adsorption to cells. Structural modifications of the virus particles were not identical, suggesting that the mechanism of inactivation by ascorbic acid differs from that of sodium bisulfite.     

Phentolamine selectively affects the fast sodium component of sensory adaptation in an insect mechanoreceptor

Phentolamine and related compounds have several different actions on nervous tissues in vertebrates and invertebrates, including a local anesthetic effect. However, recent work suggests that phentolamine can interfere with sensory transduction in insect mechanoreceptors at significantly lower concentrations than are required for conduction block. We tested the actions of phentolamine on sensory transduction and encoding in an insect mechanoreceptor, the cockroach tactile spine neuron and found that 500 microM phentolamine increased the action potential threshold by 50%. The passive membrane properties of the neuron were not affected, but one component of dynamic threshold change was strongly and selectively reduced. This component has previously been attributed to slowly inactivating sodium channels in the action potential initiating region, suggesting that these channels are the most phentolamine-sensitive sites.     

The effect of temperature on the outward currents in the soma of molluscan neurones in voltage-clamp conditions.

The delayed outward current in snail neurones was separated into two components with different temperature sensitivity: (i) a persistent component and (ii) a transient (inactivating) component. The effect of cooling on the value of the transient current is strongly dependent upon the value of the conditioning potential. It was supposed that cooling causes a decrease in the negative surface potential in the vicinity of the potassium pathways and removes their inactivation. Simultaneously cooling depresses the potassium conductance. The effect on surface potential is more distinct with conditioning potentials at which a significant fraction of the transient outward current is inactivated. The effect of cooling on the transient component of the fast outward current was similar to that on the transient component of the delayed outward current.     

Tail currents in the myelinated axon of Xenopus laevis suggest a two-open-state Na channel.

Na tail currents in the myelinated axon of Xenopus laevis were measured at -70 mV after steps to -10 mV. The tail currents were biexponential, comprising a fast and a slow component. The time constant of the slow tail component, analyzed in the time window 0.35-0.50 ms, was independent of step duration, and had a value of 0.23 ms. The amplitude, extrapolated back to time 0, varied, however, with step duration. It reached a peak after 0.7 ms and inactivated relatively slowly (at 2.1 ms the absolute value was reduced by approximately 30%). The amplitude of the fast component, estimated by subtracting the amplitude of the slow component from the calculated total tail current amplitude, reached a peak (three times larger than that of the slow component) after 0.5 ms and inactivated relatively fast (at 2.1 ms it was reduced by approximately 65%). The results were explained by a novel Na channel model, comprising two open states bifurcating from a common closed state and with separate inactivating pathways. A voltage-regulated use of the two pathways explains a number of findings reported in the literature.     

The relationship between the inactivating fraction of the asymmetry current and gating of the sodium channel in the squid giant axon

A quantitative comparison between the voltage dependence of the inactivating component of the asymmetrical charge transfer in the squid giant axon and that of the sodium conductance indicates that activation of the sodium system involves either three subunits operating in parallel or a three-step series mechanism. This is confirmed by an examination of the relative timing of the flow of asymmetry and ionic currents during the opening and closing of the sodium channels. In agreement with previous suggestions, inactivation is coupled sequentially to activation. The evidence appears to argue against a trimeric system with three wholly independent subunits and favours a monomeric system that undergoes a complex sequence of conformational changes.     

Blockade of persistent sodium currents contributes to the riluzole-induced inhibition of spontaneous activity and oscillations in injured DRG neurons

In addition to a fast activating and immediately inactivating inward sodium current, many types of excitable cells possess a noninactivating or slowly inactivating component: the persistent sodium current (I(NaP)). The I(NaP) is found in normal primary sensory neurons where it is mediated by tetrodotoxin-sensitive sodium channels. The dorsal root ganglion (DRG) is the gateway for ectopic impulses that originate in pathological pain signals from the periphery. However, the role of I(NaP) in DRG neurons remains unclear, particularly in neuropathic pain states. Using in vivo recordings from single medium- and large-diameter fibers isolated from the compressed DRG in Sprague-Dawley rats, we show that local application of riluzole, which blocks the I(NaP), also inhibits the spontaneous activity of A-type DRG neurons in a dose-dependent manner. Significantly, riluzole also abolished subthreshold membrane potential oscillations (SMPOs), although DRG neurons still responded to intracellular current injection with a single full-sized spike. In addition, the I(NaP) was enhanced in medium- and large-sized neurons of the compressed DRG, while bath-applied riluzole significantly inhibited the I(NaP) without affecting the transient sodium current (I(NaT)). Taken together, these results demonstrate for the first time that the I(NaP) blocker riluzole selectively inhibits I(NaP) and thereby blocks SMPOs and the ectopic spontaneous activity of injured A-type DRG neurons. This suggests that the I(NaP) of DRG neurons is a potential target for treating neuropathic pain at the peripheral level.     

Affinity labeling of the cytosolic and membrane components of the respiratory burst oxidase by the 2',3'-dialdehyde derivative of NADPH. Evidence for a cytosolic location of the nucleotide-binding site in the resting cell

The 2',3'-dialdehyde of NADPH (NADPH dialdehyde) appears to act as an affinity label toward the respiratory burst oxidase of human neutrophils, inactivating the enzyme by attaching covalently to a residue at its NADPH-binding site. Although the oxidase in activated neutrophils is known to reside in the plasma membrane, our studies showed that in resting neutrophils the NADPH dialdehyde-sensitive component of the enzyme was located in the cytosol. These findings suggest that one of the steps in the activation of the respiratory burst oxidase is the transfer of its NADPH-binding component from the cytosol to the plasma membrane of the cell.     

Regulation of kinase reactions in pig heart pyruvate dehydrogenase complex.

1. Pig heart pyruvate dehydrogenase complex is inactivated by phosphorylation (MgATP2-) of an alpha-chain of the decarboxylase component. Three serine residues may be phosphorylated, one of which (site 1) is the major inactivating site. 2. The relative rates of phosphorylation are site 1 greater than 2 greater than site 3. 3. The kinetics of the inactivating phosphorylation were investigated by measuring inactivation of the complex with MgATP2-. The apparent Km for the Mg complex of ATP was 25.5 microM; ADP was a competitive inhibitor (Ki 69.8 microM) and sodium pyruvate an uncompetitive inhibitor (Ki 2.8 microM). Inactivation was accelerated by increasing concentration ratios of NADH/NAD+ and of acetyl-CoA/CoA. 4. The kinetics of additional phosphorylations (predominantly site 2 under these conditions) were investigated by measurement of 32P incorporation into non-radioactive pyruvate dehydrogenase phosphate containing 3-6% of active complex, and assumed from parrallel experiments with 32P labelling to contain 91% of protein-bound phosphate in site 1 and 9% in site 2. 5. The apparent Km for the Mg complex of ATP was 10.1 microM; ADP was a competitive inhibitor (Ki 31.5 microM) and sodium pyruvate an uncompetitive inhibitor (Ki 1.1 mM). 6. Incorporation was accelerated by increasing concentration ratios of NADH/NAD+ and of acetyl-CoA/CoA, although it was less marked at the highest ratios.     

A nitrate reductase inactivating enzyme from the maize root

The nitrate reductase in the mature root extract of 3-day maize (Zea mays) seedlings was relatively labile in vitro. Insoluble polyvinylpyrrolidone used in the extraction medium produced only a slight increase in the stability of the enzyme. Mixing the mature root extract with that of the root tip promoted the inactivation of nitrate reductase in the latter. The inactivating factor in the mature root was separated from nitrate reductase by (NH₄)₂SO₄ precipitation. Nitrate reductase was found in the 40% (NH₄)₂SO₄ precipitate, while the inactivating factor was largely precipitated by 40 to 55% (NH₄)₂SO₄. The latter fraction of the mature root inactivated the nitrate reductase isolated from the root tip, mature root, and scutellum. The inactivating factor, which has a Q₁₀ 15 to 25 C of 2.2, was heat labile, and hence has been designated as a nitrate reductase inactivating enzyme. The reduced flavin mononucleotide nitrate reductase was also inactivated, while an NADH cytochrome c reductase in nitrate-grown seedlings was inactivated but at a slower rate. The inactivating enzyme had no influence on the activity of nitrite reductase, glutamate dehydrogenase, xanthine oxidase, and isocitrate lyase. The activity of the nitrate reductase inactivating enzyme was not influenced by nitrate and was also found in the mature root of minus nitrate-grown seedlings.     

Isomorphism on a physical system of the Hodgkin-Huxley equations for sodium conductance: toxin allosteric effects and gating currents

An isomorphism on a physical system of the Hodgkin-Huxley equations for sodium ion conductance in the nerve membrane is derived. The physical system consists of 8 states. It shows that the voltage dependence of the sodium conductance arises from a change in ionization of the molecule that provides the ion-selective conductance channels. It associates reversibly with singly charged (H+?) and doubly charged (Ca++?) ions. The inactivation process is the result of the associating of an ionized particle by half of the states. The effect of toxins and narcotics in blocking or inactivating sodium conductance can be understood as an enzyme or allosteric change of the standard free energy difference of the molecule that provides the sodium channels. The effect of changing pH and Ca++ substrate concentration on the sodium conductance is predicted. The gating charge current is predicted. The time constant predicted is in agreement with experiment.     

Gating of the squid sodium channel at positive potentials. I. Macroscopic ionic and gating currents.

Macroscopic ionic sodium currents and gating currents were studied in voltage-clamped, dialyzed giant axons of the squid Loligo pealei under conditions of regular and inverse sodium gradients. Sodium currents showed regular kinetics but inactivation was incomplete, showing a maintained current for depolarizations lasting 18 ms. The ratio of the maintained current to the peak current increased with depolarization and it did not depend on the direction of the current flow or the sodium gradient. The time constant of inactivation was not affected by the sodium gradient. Double-pulse experiments allowed the separation of a normal inactivating component and a noninactivating component of the sodium currents. In gating current experiments, the results from double-pulse protocols showed that the charge was decreased by the prepulse and that the slow component of the 'on' gating current was preferentially depressed. As expected, charge immobilization was established faster at higher depolarizations than at low depolarizations, however, the amount of immobilized charge was unaffected by the pulse amplitude. This indicates that the incomplete sodium inactivation observed at high depolarizations is not the result of decreased charge immobilization; the maintained current must be due to a conductance that appears after normal charge immobilization and fast inactivation.