Phosphoenolpyruvate carboxylase of Escherichia coli. The role of lysyl residues in the catalytic and regulatory functions

Phosphoenolpyruvate (PEP) carboxylase [EC 4.1.1.31] of E. coli was inactivated by 2,4,6-trinitrobenzene sulfonate (TNBS), a reagent known to attack amino groups in polypeptides. When the modified enzyme was hydrolyzed with acid, epsilon-trinitrophenyl lysine (TNP-lysine) was identified as a product. Close similarity of the absorption spectrum of the modified enzyme to that of TNP-alpha-acetyl lysine and other observations indicated that most of the amino acid residues modified were lysyl residues. Spectrophotometric determination suggested that five lysyl residues out of 37 residues per subunit were modified concomitant with the complete inactivation of the enzyme. DL-Phospholactate (P-lactate), a potent competitive inhibitor of the enzyme, protected the enzyme from TNBS inactivation. The concentration of P-lactate required for half-maximal protection was 3 mM in the presence of Mg2+ and acetyl-CoA (CoASAc), which is one of the allosteric activators of the enzyme. About 1.3 lysyl residues per subunit were protected from modification by 10 mM P-lactate, indicating that one or two lysyl residues are essential for the catalytic activity and are located at or near the active site. The Km values of the partially inactivated enzyme for PEP and Mg2+ were essentially unchanged, though Vmax was decreased. The partially inactivated enzyme showed no sensitivity to the allosteric activators, i.e., fructose 1,6-bisphosphate (Fru-1,6-P2) and GTP, or to the allosteric inhibitor, i.e., L-aspartate (or L-malate), but retained sensitivities to other activators, i.e., CoASAc and long-chain fatty acids. P-lactate, in the presence of Mg2+ and CoASAc, protected the enzyme from inactivation, but did not protect it from desensitization to Fru-1,6-P2, GTP, and L-aspartate. However, when the modification was carried out in the presence of L-malate, the enzyme was protected from desensitization to L-aspartate (or L-malate), but was not protected from desensitization to Fru-1,6-P2 and GTP. These results indicate that the lysyl residues involved in the catalytic and regulatory functions are different from each other, and that lysyl residues involved in the regulation by L-aspartate (or L-malate) are also different from those involved in the regulation by Fru-1,6-P2 and GTP.

Orexin neurons in hypothalamic slice cultures are vulnerable to endoplasmic reticulum stress

Narcolepsy results from disruption of orexin neurons in the hypothalamus that play a key role in maintenance of the arousal state. Underlying mechanisms leading to selective loss of orexin neurons remain unknown. On the other hand, endoplasmic reticulum stress, namely, conditions associated with impairment of endoplasmic reticulum functions such as proper folding and sorting of newly synthesized proteins, is implicated in pathogenesis of several types of neurodegenerative disorders. Here we found that application of endoplasmic reticulum stress inducers such as tunicamycin (that prevents protein N-glycosylation) and thapsigargin (that inhibits Ca²⁺-ATPase) to organotypic slice cultures of the hypothalamus caused preferential loss of orexin-immunoreactive neurons, as compared to melanin-concentrating hormone- or calcitonin gene-related peptide-immunoreactive neurons. The decrease in orexin-immunoreactive neurons at early time points (6-24 h) was not accompanied by induction of cell death as indicated by the absence of caspase-3 activation and no significant change in the number of NeuN-positive cells, whereas sustained treatment with tunicamycin for 72 h induced cell death. At 24-h treatment, tunicamycin and thapsigargin did not decrease expression of prepro-orexin mRNA, suggesting that post-transcriptional mechanisms were responsible for depletion of orexin peptides. In addition, inhibition of axonal transport by colchicine and inhibition of proteasomal activity by MG132 significantly prevented the decrease in orexin immunoreactivity by tunicamycin. Comparative examinations of expression of unfolded protein response-related proteins revealed that C/EBP-homologous protein (a transcription factor that promotes induction of apoptosis) as well as phosphorylated form of RNA-dependent protein kinase-like endoplasmic reticulum kinase (a protein kinase that mediates inhibition of protein translation) was expressed more prominently in orexin neurons than in melanin-concentrating hormone neurons, in response to tunicamycin. These results indicate that orexin neurons are particularly sensitive to endoplasmic reticulum stress, which may be relevant to pathogenic events in narcolepsy.

Stringent control of intermediary metabolism in Escherichia coli: pyruvate excretion by cells grown on succinate

A large amount of pyruvate was excreted into the medium by CP78 (rel+) cells grown on succinate when they were starved for amino acids. In contrast, no such excretion was observed with CP79 (rel-) cells. This phenomenon was also seen with two other isogenic pairs of strains: NF161 (rel+) and NF162 (rel-), and 10B601 (rel+) and 10B602 (rel-). Besides succinate, L-malate, and fumarate were effective carbon sources for the excretion, but glucose, glycerol, and acetate were not. When DL-lactate was used, not only CP78 but also CP79 cells excreted pyruvate. Experiments using [1,4-14C]succinate as a carbon source revealed that pyruvate was formed by decarboxylation of one carboxyl group of succinate and that the pyruvate excretion amounted to about 40% of the total succinate degraded. Experiments designed to elucidate the mechanism of the excretion yielded the following observations. (i) The concentration of pyruvate in CP78 cells grown on the C4-dicarboxylic acids mentioned above was not significantly changed upon amino acid starvation. (ii) Guanosine 5'-diphosphate-3'-diphosphate exerted no effect on the activities of several enzymes thought to be involved in pyruvate-related metabolism. It is suggested firstly that the excretion was not due to some impairment in the biosynthetic pathway of a particular amino acid, but was due to the stringent control of central amphibolic metabolism, and secondly that no de novo protein synthesis was involved in the excretion.

Characterization of a Saccharomyces cerevisiae mutant, N22, defective in ergosterol synthesis and preparation of [28-14C]ergosta-5,7-dien-3 beta-ol with the mutant

Analysis of sterols was made with a Saccharomyces cerevisiae mutant, N22, which was resistant to nystatin and defective in ergosterol synthesis, and with its parent strain, M10 (haploid type, methionineless, petite). The main sterol of M10 was ergosterol, whereas that of N22 was ergosta-5,7-dien-3 beta-ol. A small amount of ergosterol was found also in N22. This indicates that the delta 22-desaturation reaction in N22 is blocked, though not completely. Ergosta-5,8-dien-3 beta-ol, an unusual sterol, was detected in N22 but not in M10. Variations in the amounts and compositions of free and esterified sterols of both strains were examined during cultivation and the subsequent aerobic adaptation. The contents of free sterols, which were mostly composed of the respective main sterols of both strains, did not change markedly. In contrast, the contents and compositions of esterified sterols of both strains varied depending on the growth conditions. When the cells of both strains were aerobically adapted, the total contents of esterified sterols increase, as did, as the intermediate sterols. Upon aerobic adaptation in the presence of [methyl-14C]methionine, [28-14C]ergosta-5,7-dien-3 beta-ol accumulated markedly in N22 cells. Using this mutant, we devised a convenient method for preparation of the radioactive sterol in high yield.

Enzymatic formation of nerolidol in cell-free extract of Rhodotorula glutinis

Enzymatic formation of nerolidol was demonstrated by incubation of [14C]farnesyl pyrophosphate with the ultracentrifugal supernatant of cell-free extract of Rhodotorula glutinis. Farnesol was also formed concomitantly with the formation of nerolidol and the ratios of formation of both alcohols were from 1:3 to 1:4. Divalent cation was necessary for the reaction and Mn2+ was much more active than Mg2+ for nerolidol formation. No nerolidol was formed when farnesyl monophosphate or farnesol was used instead of farnesyl pyrophosphate as a substrate. Nerolidyl pyrophosphate or nerolidyl monophosphate could not be detected as an intermediate in the reaction. Based on these observations, nerolidol was presumed to be formed not via nerolidyl pyrophosphate or nerolidyl monophosphate but via a carbonium ion intermediate which was formed by cleavage of the carbon-oxygen bond of farnesyl pyrophosphate. This reaction seems to proceed in a similar manner to the acid hydrolysis of farnesyl pyrophosphate to form nerolidol and farnesol.

Occurrence of thermolabile and regulatory NAD-linked glutamate dehydrogenase in Pseudomonas fluorescens

NAD-linked glutamate dehydrogeanse [EC 1.4.1.2] was detected together with NADP-linked glutamate dehydrogenase [EC 1.4.1.4] and aspartase [EC 4.3.1.1] in Pseudomonas fluorescens cells. The three enzymes were distinctly separated by DEAE-Sephadex column chromatography. The NAD-linked enzyme was extremely thermolabile and was rapidly inactivated even at temperatures as low as 35--40 degrees C. The combined addition of NAD+ and glutamate, however, effectively stabilized the enzyme. The glutamate saturation profile of the NAD-linked enzyme exhibited cooperativity with a Hill coefficient (n) of 1.4. ATP inhibited the enzyme in an allosteric manner, increasing the n value to 2.2. These results suggest a novel type of metabolic regulation shared by the three enzymes in the biosynthesis and catabolism of amino acids.

Accumulation of zymosterol in yeast grown in the presence of ethionine

In order to identify the methyl acceptor for the methylation of sterol side-chains in ergosterol biosynthesis, Saccharomyces cerevisiae (wild type) was grown in the presence and absence of ethionine which was expected to be an inhibitor of the methylation. Gas-liquid chromatographic analyses of the sterols in the cells grown in the absence of ethionine showed that ergosterol was the most abundant sterol. On the other hand, a sterol, named sterol Z, accounted for more than 50% of the total sterols in the cells grown in the presence of ethionine. As a result of experiments to raise the yield of sterol Z, the best concentration of DL-ethionine for the production was found to be 1.0 mM. The use of the methionine-less mutant was less effective for the production of sterol Z. Sterol Z was isolated by repeated TLC and was identified as zymosterol from its melting point, GLC and mass spectrometry. The role of zymosterol and other sterols as the methyl-acceptor sterol in ergosterol biosynthesis is also discussed.

Characterization of nystatin-resistant mutants of Saccharomyces cerevisiae and preparation of sterol intermediates using the mutants

Analysis of sterols of Saccharomyces cerevisiae mutants N3, N15, N26, and N3H, defective in sterol biosynthesis, was performed. Strains N3, N15, and N26 were isolated from their mother strain, M10, by screening with nystatin (Nagai et al. (1980) Mie Med. J. 30, 215-224), and strain N3H was isolated from N3 as a doubly-mutated strain. The main sterols of N3, N15, N26, and N3H were ergosta-7,22-dienol, ergost-8-enol, cholesta-5,7,24-trienol, and ergosta-7,22,24(28)-trienol, respectively. The former three strains were characterized as defective in delta 5-desaturation, delta 8--delta 7 isomerization, and C-24 transmethylation. Strain N3H was found to be defective in delta 5-desaturation as well as in delta 24(28)-reduction. However, the defect of N26 and N3H was suggested to be leaky, since small amounts of ergosterol and ergosta-7,22-dienol were found in these mutants, respectively. In N15, an accumulation (2% in total sterols) of the compound likely to be hydroxylated sterol was found. By aerobic adaptation of these strains, the accumulation of these strains, the accumulations of ergosta-7,22-dienol (22 mg/g dry cells), ergosta-7,22,24(28)-trienol (24 mg), ergosta-8,24(28)-dienol (18 mg), and cholesta-8,24-dienol (22 mg) reached a maximum in N3, N3H, N15, and N26 after 20, 20, 30, and 30 h, respectively. These strains appear to be useful for making 14C-labeled and non-labeled preparations of the above sterols.

YM-14673, a new thyrotropin-releasing hormone analog, augments long-term potentiation in the mossy fiber-CA3 system of guinea pig hippocampal slices

Effects of a new thyrotropin-releasing hormone (TRH) analog, N alpha-[[(S)-4-oxo-2-azetidinyl]carbonyl]L-histidyl-L-prolinamide dihydrate (YM-14673), which improves experimentally induced memory dysfunction, on long-term potentiation (LTP) in the mossy fiber-CA3 system, were investigated using guinea pig hippocampal slices. At concentrations of 10(-7) M and 10(-6) M, YM-14673 significantly augmented LTP in a concentration dependent manner. The magnitude of effect of 10(-6) M YM-14673 was similar to that of 10(-6) M TRH. As LTP in the hippocampus is regarded as an elementary process of memory and learning, the augmenting effect of YM-14673 on LTP in the present study may contribute to this drug's ability to remedy memory dysfunction.

Furosemide-sensitive calcium rise induced by GABAA-receptor stimulation in cultures of embryonic rat striatal neurons

Changes in [Ca2+]i induced by gamma-aminobutyric acid (GABA) were investigated in primary cultured neurons obtained from fetal rat striatum. GABA and muscimol induced [Ca2+]i rise, and bicuculline blocked the effect of GABA. The [Ca2+]i elevating effect of GABA was also abolished by removal of extracellular Ca2+ or by application of nicardipine. Furthermore, furosemide, an inhibitor of Na+/K+/ 2Cl- co-transport, reversibly inhibited the GABA-induced [Ca2+]i rise. These results suggest that due to the elevated level of intracellular Cl- maintained by Na+/K+/2Cl- transport activity in these neurons, opening of GABAA-receptor-associated Cl- channels results in Cl- efflux, leading to membrane depolarization and activation of L-type voltage-dependent Ca2+ channels.

Possible physiological roles of aspartase, NAD- and NADP-requiring glutamate dehydrogenases of Pseudomonas fluorescens

The levels of aspartase, NADP- and NAD-requiring glutamate dehydrogenases (GDHs) in Pseudomonas fluorescens grown under various nutritional conditions were determined. NADP-GDH showed the highest value on glucose-ammonium sulfate medium and markedly lower values on amino-acid and casamino-acids media, while the reverse was found for the NAD-GDH, as in the case of other microorganisms with two GDHs. Aspartase did not show a marked variation between the media examined. Glucose nutritionally induced NADP-GDH but suppressed NAD-GDH; and it had no effect on aspartase, which was slightly induced by casamino acids. Transfer of the cells grown on glucose-ammonium sulfate medium to casamino-acids medium clearly increased the levels of NAD-GDH and aspartase, while addition of chloramphenicol to the media abolished the increases, suggesting that the increases were due to de novo synthesis of the enzyme proteins. These results indicate that the aspartase of this microorganism has a different function from those in others, including Escherichia coli.

Phosphoenolpyruvate carboxylase of Escherichia coli. Hydrophobic chromatography using specific elution with allosteric inhibitor

The adsorption of Escherichia coli phosphoenolpyruvate carboxylase [EC 4.1.1.31] to butyl-, hexyl-, and octyl-Sepharose gels was investigated. The enzyme was nearly completely adsorbed to the latter two gels both in the absence and presence of high concentrations of ammonium sulfate. At intermediate concentrations--0.1 M in the case of hexyl-Sepharose--virtually no adsorption was observed. Upon application of an increasing or decreasing concentration gradient of the salt, the enzyme was eluted at various concentrations of the salt depending on chain length of the immobilized alkyl groups. The adsorption to hexyl-Sepharose at 0.7 M ammonium sulfate was markedly decreased by L-aspartate, the allosteric inhibitor, whereas it was increased by acetyl-CoA, one of the allosteric activators. Evidence was obtained suggesting that these changes in adsorption were due to conformational alterations of the enzyme elicited by these effectors. The enzyme seemed to have been adsorbed at its hydrophobic regions which were distinct from the allosteric site for long-chain fatty acids. The specific elution with L-aspartate in the presence of 0.82 M ammonium sulfate could successfully be applied to purification of the enzyme. By this hydrophobic interaction chromatography, the enzyme was purified about 55-fold over its partially purified preparation with a recovery of 73%. The obtained enzyme preparation was almost homogeneous as judged from sodium dodecylsulfate-polyacrylamide gel electrophoresis.

A potential role of Ras-mediated signal transduction for the enhancement of depolarization-induced Ca2+ responses in hippocampal neurons by basic fibroblast growth factor

Chronic treatment with basic fibroblast growth factor (bFGF) increases the expression of functional L-type voltage-dependent Ca2+ channels (VDCCs) in fetal rat hippocampal neurons. We investigated the intracellular signaling mechanisms involved in this effect, using high K+ depolarization-induced elevation of intracellular Ca2+ concentrations as a measure. Genistein, a protein tyrosine kinase inhibitor, significantly attenuated the effect of bFGF. The effect of bFGF was also diminished by concurrent application of a Ras inactivator, N-acetyl-S-farnesyl-l-cysteine. In contrast, a phospholipase C inhibitor U73122, a phosphatidylinositol-3 kinase inhibitor wortmannin, Li+ which inhibits inositol phospholipid turnover, or a protein kinase inhibitor calphostin C did not inhibit the effect of bFGF. Phorbol 12-myristate 13-acetate, a protein kinase C activator, did not mimic the effect of bFGF. On the other hand, an adenylyl cyclase activator forskolin and a cyclic AMP analog 8-Br-cyclic AMP markedly attenuated the effect of bFGF, which indicates the presence of a cyclic AMP-mediated negative regulatory mechanism, possibly the interference of Ras-Raf interaction. These results suggest that Ras-mediated signal transduction is required for the enhancement by bFGF of VDCC responses in hippocampal neurons.

Deprenyl rescues dopaminergic neurons in organotypic slice cultures of neonatal rat mesencephalon from N-methyl-D-aspartate toxicity

The potential neuroprotective effect of (-)-deprenyl (R-N,alpha-dimethyl-N-2-propynylbenzeneethanamine) against N-methyl-D-aspartate (NMDA) excitotoxicity was investigated on rat mesencephalic dopaminergic neurons in organotypic slice cultures. While 24 h application of NMDA (100 microM) caused a marked decrease in the number of surviving dopaminergic neurons, simultaneous application of (-)-deprenyl significantly attenuated the cytotoxic effect of NMDA. (+)-Deprenyl showed a less potent but still significant protective effect against NMDA insult. Pre-treatment of cultures with (-)-deprenyl conferred no protection against subsequent NMDA insult, suggesting that the protective effect of (-)-deprenyl may be independent of its irreversible inhibitory action on monoamine oxidase B. (-)-Deprenyl was also ineffective in preventing cell death induced by H2O2. These results indicated that (-)-deprenyl protects dopaminergic neurons from NMDA excitotoxicity through a mechanism distinct from monoamine oxidase inhibition or detoxification of reaction oxygen species.

Primary culture of postnatal rat suprachiasmatic neurons in serum-free supplemented medium

We have previously reported that postnatal hypothalamic neurons can be maintained in low density culture using astrocyte conditioned medium. The present study was designed to establish a method for the culture of postnatal hypothalamic neurons in a chemically defined medium. Neurons were dissociated from the suprachiasmatic nucleus (SCN) of the hypothalamus of 21-day-old rats and plated on plastic dishes. First, the effects of several factors which have been known to exert trophic effects on neuronal cells were examined in culture medium containing 10% fetal bovine serum. We have found that platelet-derived growth factor, interleukin-1 beta and vitronectin in combination markedly increased the number of surviving neurons bearing processes. Next we tested such effects in serum-free minimum essential medium. When these factors were added together the SCN neurons could be maintained in culture for up to 3 weeks without medium change. In this supplemented medium, SCN neurons gradually extended processes from 3-5 days after plating, and the cell number with processes reached maximal at days 8-11. The cells were identified as SCN neurons by the immunocytochemical staining for microtubule-associated protein 2 (MAP2) and vasoactive intestinal polypeptide. This culture method may be valuable for investigating the electrophysiological properties and the mechanisms of regeneration of mature central neurons.

Graded, irreversible changes in crayfish giant axon as manifestations of lidocaine neurotoxicity in vitro

High concentrations of lidocaine induce irreversible conduction block with little effect on resting membrane potential (Em). We assumed the mechanism of persistent neurologic deficit caused by local anesthetics may result from neural death, as represented by the loss of Em. We investigated the effects of lidocaine on Em and action potential (AP) in single crayfish giant axons in vitro. Axons were perfused with two doses of lidocaine for either 15 or 30 min, and they were continuously washed. No axons exposed to 80 mM lidocaine for 30 min showed recovery of AP and Em. Those exposed to 40 mM for 30 min and 80 mM for 15 min showed a return to baseline for Em, but no recovery of AP. Those exposed to 40 mM lidocaine for 15 min showed full recovery of Em and AP immediately after washing. The membrane depolarization was significantly greater during exposure to 80 mM lidocaine for 30 min than in other groups. We conclude that lidocaine has a direct neurotoxic effect on crayfish giant axons and that the generation of AP is more vulnerable than the maintenance of Em. The irreversibility of AP and Em is dose- and time-dependent.

IMPLICATIONS

Highly concentrated lidocaine induced an irreversible conduction block and a complete loss of resting membrane potential in crayfish giant axons in vitro. Our results may represent a possible explanation for various grades of local anesthetic-induced neurotoxicity in clinical cases if the same toxicity occurs in mammalian nerves in vivo.