Search PubMed⌕ Search

Biomedical subjects

M C Walker

Publications and source records attributed to M C Walker.

At least 19 recordsLinked to original sources
PubMed1995

Ascorbate and glutamate release in the rat hippocampus after perforant path stimulation: a "dialysis electrode" study.

Excitatory amino acids have been proposed to play a critical role in the development and maintenance of epileptic seizures and in the development of neuronal damage. Previous animal studies of glutamate during seizures, however, have often failed to measure any rise in glutamate. We have overcome many of the problems of these studies by using an animal model in which epileptic afterdischarges are induced by stimulation of the perforant path, and glutamate and ascorbate are measured using a newly developed microdialysis electrode that combines the advantages of microdialysis and in vivo electrochemistry. We have successfully shown (1) a rise in glutamate after an epileptic afterdischarge, (2) a concomitant initial fall and then a later rise in ascorbate, and (3) progressive dwindling of this effect when afterdischarges are repeated within minutes, despite similar electroencephalographic responses. The possible mechanisms of these effects are discussed and include ascorbate/glutamate heteroexchange, reversal of the glutamate uptake mechanism, and augmentation of glutamate uptake after a seizure.

Animals↗
PubMed1995

The intensive care treatment of convulsive status epilepticus in the UK. Results of a national survey and recommendations.

Six hundred and ninety-four members of the Intensive Care Society working in the UK were surveyed by postal questionnaire between May and November 1993 to determine their management of convulsive status epilepticus resistant to initial therapy with intravenous diazepam and phenytoin. Four hundred and eight forms were completed and returned (58.8%). The survey revealed that, following failure of initial management, a benzodiazepine infusion (35%) or anaesthetic induction agent (32%) were the preferred second lines of treatment in intensive care units. In paediatric intensive care units, phenobarbitone (31%) was the agent of choice. Most respondents (57%) gave anaesthetic induction agents within 60 min of the start of status epilepticus, the majority choosing thiopentone (82%). Patients were usually monitored using clinical assessment only (45%), except in paediatric intensive care units and specialist neurological or neurosurgical units where the majority used a cerebral function monitor. Only 12% of the respondents were aware of a protocol for status epilepticus in their intensive care units. The most frequently used therapeutic and monitoring strategies in the management of refractory status epilepticus in the UK are insufficient and need re-evaluation.

Anesthetics, General↗
PubMed1995

Case of simple partial status epilepticus in occipital lobe epilepsy misdiagnosed as migraine: clinical, electrophysiological, and magnetic resonance imaging characteristics.

An 31-year-old man had a unique form of occipital lobe epilepsy. Since age 13 years, he has had episodes of simple partial status epilepticus (SE) occurring twice a month. These typically consisted of elementary visual hallucinations of flashing lights obscuring his left visual field for a period of 2 days, associated with a severe frontal headache initially diagnosed as migraine. These episodes of simple partial SE then evolved to a complex partial seizure (CPS) or secondarily generalized seizure. There were unique EEG features, including: (a) the perception of a flash of light in the left visual field with a single sharp/slow wave discharge over the right occipital lobe, (b) right occipital lobe epileptiform activity during the prolonged aura, and (c) an abnormal response to photic stimulation, with occipital lobe discharges during low rates of stimulation (3-5 Hz), time-locked to the stimulus. High-resolution magnetic resonance imaging (MRI) with quantitative morphometry demonstrated that the right hemisphere and right caudate nucleus were smaller than those on the left. An abnormal gyral pattern was also noted over the right parietal region. Occasionally, distinguishing occipital lobe epilepsy from migraine may be difficult.

Adult↗
PubMed1994

Structure-activity relationships in the oxidation of benzylamine analogues by bovine liver mitochondrial monoamine oxidase B.

The influence of para and meta substitution of benzylamine on its interaction with bovine liver mitochondrial monoamine oxidase B (MAO B) has been investigated by steady-state and reductive half-reaction anaerobic stopped-flow kinetic approaches. Steady-state kinetic properties of each benzylamine analogue suggest that para or meta substitution does not alter the mechanistic pathway of catalysis [Husain, M., et al. (1982) Biochemistry 21, 595-600]. All analogues tested exhibited Dkcat values ranging from 5.5 to 8.9 and D[kcat/Km(amine)] values ranging from 3.3 to 8.1 D[kcat/Km(O2)] values of approximately 1 are observed for all substrate analogues. Values for Kd were calculated from steady-state isotope effect data [Klinman, J.P., & Matthews, R.G. (1985) J. Am. Chem. Soc. 107, 1058-1060] and are in good agreement with Ks values determined from analysis of the rate of MAO B reduction as a function of benzylamine analogue concentration in reductive half-reaction experiments. A linear correlation of benzylamine analogue Kd values with the hydrophobicity parameter (phi) is observed for the para-substituted analogues where the binding affinity increases with increasing hydrophobicity of the substituent. Statistical treatment of the correlation shows a small negative contribution to binding by the van der Waals volume (VW) of the para substituent. meta-Substituted benzylamine analogues show a decreased binding affinity with the VW of the substituent and no correlation with the hydrophobicity value of the substituents tested. No spectral evidence was found for any flavin radical intermediates during the time course of MAO B flavin reduction in anaerobic reductive half-reduction stopped-flow experiments with any of the alpha,alpha-diprotio- or alpha,alpha-dideuteriobenzylamine analogues tested. The limiting rates of enzyme reduction exhibit large Dk values (6.5-14.1) for all of the analogues tested. para-Substituted benzylamine analogues reduce MAO B with limiting rates that correlate with the steric influence (Es value) of the substituent. Statistical analysis shows the rate of MAO B reduction by para-substituted analogues to be retarded by increased values of Es and, with a smaller contribution, by the hydrophobicity value of the substituent. The rate of MAO B reduction by meta-substituted benzylamine analogues is essentially independent of the nature of the substituent. No evidence was found for any electronic contribution to the rate of MAO B flavin reduction by any of the analogues tested. These data demonstrate the steric orientation of the substrate to be important in the rate of amine oxidation by MAO B and that ring meta substituents favor this orientation.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Functionalized 3,5-dihydroxybenzoates as potent novel inhibitors of EPSP synthase.

Aromatic analogues of the EPSP synthase enzyme substrate (S3P), reaction intermediate (1), and product (EPSP) were synthesized from 3,5-dihydroxybenzoic acid and were evaluated as inhibitors of E. coli EPSP synthase. These simple, synthetically accessible aromatic analogues are highly effective competitive inhibitors versus S3P with an apparent Ki for the tetrahedral intermediate analogue 4 of 160 +/- 40 nM. This demonstrates that a simple benzene ring is a quite suitable substitute for the complex shikimate ring in the design of EPSP synthase inhibitors.

3-Phosphoshikimate 1-Carboxyvinyltransferase↗
PubMed1994

Clozapine in the treatment of a young adolescent with schizophrenia.

Although recent reports have suggested that clozapine may be efficacious in the treatment of adolescents with schizophrenia, few studies have examined the use of clozapine in patients younger than 17 years of age. We describe highly successful trials of clozapine conducted in a 13-year-old girl and her cousin, both of whom developed severe symptoms of schizophrenia, which were refractory to neuroleptic medication, at the ages of 11 and 13 years, respectively. The pharmacology of and clinical experience with clozapine are reviewed.

Adolescent↗
PubMed1994

Developments in antiepileptic drug therapy.

Development of new antiepileptic drugs has continued to progress, with both the evolution of new drugs, as well as new formulations of old drugs. It is hoped that, with advances in the understanding of the molecular basis of epilepsy, novel antiepileptic drugs will come about through design rather than serendipity. This article concentrates on a selection of the more promising antiepileptic drugs and reviews their recent progress. Felbamate, lamotrigine, vigabatrin, zonisamide, and gabapentin are already in clinical use in certain countries. Stiripentol and other potential antiepileptic drugs are also reviewed. In addition, there have been strategies to improve the pharmacokinetics of currently used antiepileptic drugs and we have concentrated on controlled-release carbamazepine and oxcarbazepine, both available in certain countries. It is, however, increasingly apparent that good comparative studies are needed before definitive advice can be given in the drug treatment of epilepsy.

Anticonvulsants↗
PubMed1994

Complex partial status epilepticus: a recurrent problem.

Twenty patients with complex partial status epilepticus were identified retrospectively from a specialist neurology hospital. Seventeen patients experienced recurrent episodes of complex partial status epilepticus, often occurring at regular intervals, usually over many years, and while being treated with effective anti-epileptic drugs. No unifying cause for the recurrences, and no common epilepsy aetiologies, were identified. In spite of the frequency of recurrence and length of history, none of the patients showed any marked evidence of cognitive or neurological deterioration. Complex partial status epilepticus is more common than is generally recognised, should be differentiated from other forms of non-convulsive status, and is often difficult to treat.

Adult↗
PubMed1994

Muscle fructose-2,6-bisphosphate and glucose-1,6-bisphosphate during insulin-induced hypoglycemia.

Glucose production during insulin-induced hypoglycemia in the fasted state is heavily dependent on the process of hepatic gluconeogenesis. Skeletal muscle glycogen is one possible source of lactate for hepatic gluconeogenesis. Fructose 2,6-bisphosphate (F-2,6-P2) and glucose 1,6-bisphosphate (G-1,6-P2) are two allosteric activators of muscle glycolysis. To investigate their putative role in the control of muscle lactate production during hypoglycemia, fasted rats were infused via jugular catheters with insulin in 0.9% NaCl or with 0.9% NaCl alone for 60 or 120 min. Muscles were removed and clamp frozen in liquid nitrogen. The insulin infusion produced plasma insulin values of 97 +/- 13 microU/ml after 1 h and 100 +/- 9 microU/ml after 2 h. Blood glucose in the saline-infused rats was 4.6 +/- 0.2 mM after 1 h and 5.1 +/- 0.1 mM after 2 h compared with 1.5 +/- 0.01 and 1.0 +/- 0.1 mM after 1 and 2 h, respectively, in the insulin-infused rats. The hypoglycemic rats had significantly elevated plasma epinephrine and blood lactate levels compared with the saline-infused rats. F-2,6-P2 and G-1,6-P2 were increased two- to five-fold in white quadriceps of hypoglycemic rats compared with that of saline-infused rats. The results are consistent with F-2,6-P2 and G-1,6-P2 playing a role in stimulating muscle lactate production as a source of gluconeogenic substrate during insulin-induced hypoglycemia.

Animals↗
PubMed1993

Spectral and kinetic studies of imine product formation in the oxidation of p-(N,N-dimethylamino)benzylamine analogues by monoamine oxidase B.

The oxidative deamination of p-(N,N-dimethylamino)benzylamine and N-methyl-p-(N,N-dimethylamino)benzylamine by bovine liver monoamine oxidase B has been investigated by absorption spectral, steady-state, and stopped-flow kinetic studies. An absorbing intermediate with a maximum at 390 nm is observed with either analogue in turnover experiments at neutral pH and is identified as due to the formation of protonated imine as the initial product. p-(N,N-Dimethylamino)benzaldehyde is the final product formed from either substrate analogue. Anaerobic stopped-flow measurements show N-methyl-p-(N,N-dimethylamino)benzylamine to reduce enzyme-bound flavin with a limiting rate of 1.8 s-1 concurrent with the appearance of a 390-nm absorption due to protonated imine product with a limiting rate of 1.7 s-1. Both observed rates are somewhat faster than catalytic turnover (1.5 s-1). Under anaerobic conditions, the decay of protonated N-methyl-p-(N,N-dimethylamino)benzenimine is much slower than turnover (k = 4.8 x 10(4) s-1). p-(N,N-Dimethylamino)benzylamine reduces the enzyme with a limiting rate of 2.1 s-1, which is faster than catalytic turnover (1.2 s-1). Protonated imine formation is also observed with this substrate with an apparent limiting rate of 1.3 s-1. The decay of the protonated p-(N,N-dimethylamino)benzenimine absorbance is slower than catalytic turnover but faster than the rate of aldehyde formation under anaerobic conditions. Deuterium kinetic isotope effect values of approximately 10 are observed both for flavin reduction and for protonated imine formation. No isotope effect is observed for the rate of imine decay.(ABSTRACT TRUNCATED AT 250 WORDS)

Aniline Compounds↗
PubMed1993

Differential scanning calorimetric study of 5-enolpyruvoyl shikimate-3-phosphate synthase and its complexes with shikimate-3-phosphate and glyphosate: irreversible thermal transitions.

The thermal denaturation of native Escherichia coli 5-enolpyruvoyl shikimate-3-phosphate (EPSP) synthase, its binary complex with shikimate-3-phosphate (S3P) and its ternary complex with S3P and glyphosate have been studied using highly-sensitive differential scanning calorimetry (DSC). All observed transitions are strongly scanning-rate-dependent and irreversible. Consistent with these observations, the data were better fit by a simple irreversible model than by the controversial reversible model more commonly employed. The results obtained provide additional support for the application of irreversible models to the thermal denaturation of proteins. The calculated parameters, activation energy (Ea), enthalpy of denaturation (delta H) and transition temperature (Tm), obtained from fitting to an irreversible model agree well with values obtained from approximation techniques. Further, the results show that the formation of the ternary complex greatly enhances the thermal stability of the enzyme (delta Tm = 10.6 degrees C), while the binding of S3P alone increases the transition temperature only slightly (delta Tm = 3 degrees C). The heat of binding calculated at the transition temperature also demonstrates the greater stability of the ternary complex (delta H = -70 kcal/mol) versus the binary complex (delta H = -10 kcal/mol).

3-Phosphoshikimate 1-Carboxyvinyltransferase↗
PubMed1993

Hypersensitivity of human testis-tumour cell lines to chemotherapeutic drugs.

Metastatic testis tumours, in contrast to most other types of cancer, can be cured by drugs. To investigate which classes of chemotherapeutic drug are differentially toxic to testis-tumour cells, we compared the in vitro dose-response curves of 5 human testis and 5 bladder-cancer cell lines to 12 compounds. The testis cells were hypersensitive to drugs that interact directly with DNA (m-amsa, bleomycin, cisplatin, doxorubicin, methylnitrosourea, mitozolomide, etoposide, mitomycin-C), but little or no difference between the 2 cell types was seen following exposure to drugs whose mechanisms of action do not involve direct interaction with DNA (methotrexate, 5-fluorouracil, colchicine, vinblastine). We conclude that testis tumour cells are either less tolerant of, or have a reduced capacity to repair, DNA damage.

Antineoplastic Agents↗
PubMed1993

Summary report: workshop on the potential risks of antibody-dependent enhancement in human HIV vaccine trials.

Concern that ADE of HIV infection could occur in vivo, as a result of HIV immunization, has arisen for several reasons. Immune-mediated disease enhancement occurs in several human and animal viral diseases, including lentiviral diseases. Tropism for host M/M cells is a common characteristic in these diseases. Sera from naturally infected, and possibly HIV-immunized, individuals have been shown to contain infection enhancing antibodies in vitro. Finally, there is considerable genetic, and potentially antigenic, diversity among HIV-1 isolates. This workshop was convened to evaluate these concerns regarding ADE of HIV infection in human HIV vaccine trials and to propose studies that would address this potential risk. Although there is currently no evidence that immune-mediated enhancement of disease occurs in HIV, there is clearly a need for carefully designed experiments to further evaluate this issue. As there are several notable diseases for which in vitro ADE does not correlate with ADE in vivo, in vitro data are insufficient to deter development of current HIV-1 vaccine candidates. In vivo correlates of protection/enhancement are necessary to evaluate the ADE risk accurately. The development of an HIV animal model that would allow testing of vaccine candidates is of primary importance.

AIDS Vaccines↗
PubMed1992

Substrate synergism and the steady-state kinetic reaction mechanism for EPSP synthase from Escherichia coli.

Previous studies of Escherichia coli 5-enolpyruvoylshikimate-3-phosphate synthase (EPSPS, EC 2.5.1.19) have suggested that the kinetic reaction mechanism for this enzyme in the forward direction is equilibrium ordered with shikimate 3-phosphate (S3P) binding first followed by phosphoenolpyruvate (PEP). Recent results from this laboratory, however, measuring direct binding of PEP and PEP analogues to free EPSPS suggest more random character to the enzyme. Steady-state kinetic and spectroscopic studies presented here indicate that E. coli EPSPS does indeed follow a random kinetic mechanism. Initial velocity studies with S3P and PEP show competitive substrate inhibition by PEP added to a normal intersecting pattern. Substrate inhibition is proposed to occur by competitive binding of PEP at the S3P site [Ki(PEP) = 6-8 mM]. To test for a productive EPSPS.PEP binary complex, the reaction order of EPSPS was evaluated with shikimic acid and PEP as substrates. The mechanism for this reaction is equilibrium ordered with PEP binding first giving a Kia value for PEP in agreement with the independently measured Kd of 0.39 mM (shikimate Km = 25 mM). Results from this study also show that the 3-phosphate moiety of S3P offers 8.7 kcal/mol in binding energy versus a hydroxyl in this position. Over 60% of this binding energy is expressed in binding of substrate to enzyme rather than toward increasing kcat. Glyphosate inhibition of shikimate turnover was poor with approximately 8 x 10(4) loss in binding capacity compared to the normal reaction, consistent with the independently measured Kd of 12 mM for the EPSPS.glyphosate binary complex. The EPSPS.glyphosate complex induces shikimate binding, however, by a factor of 7 greater than EPSPS.PEP. Carboxyallenyl phosphate and (Z)-3-fluoro-PEP were found to be strong inhibitors of the enzyme that have surprising affinity for the S3P binding domain in addition to the PEP site as measured both kinetically and by direct observation with 31P NMR. The collective data indicate that the true kinetic mechanism for EPSPS in the forward direction is random with synergistic binding occurring between substrates and inhibitors. The synergism explains how the mechanism can be random with S3P and PEP, but yet equilibrium ordered with PEP binding first for shikimate turnover. Synergism also accounts for how glyphosate can be a strong inhibitor of the normal reaction, but poor versus shikimate turnover.

3-Phosphoshikimate 1-Carboxyvinyltransferase↗