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Biomedical subjects

K F Tipton

Publications and source records attributed to K F Tipton.

At least 109 records · Page 6Linked to original sources
PubMed1992

Developmental aspects of the monoamine-degrading enzyme monoamine oxidase.

In the rat heart, monoamine oxidase (MAO)-B activity was shown to predominate in 2- to 3-week-old animals, whereas MAO-A activity was reported to be very low in newborn rats and to increase considerably with age until it predominates. These results are in contrast with those found in the mouse heart, where an age-dependent increase in MAO-B activity with no changes in 5-hydroxytryptamine deaminating activity was found to occur. There is evidence that the adult values of MAO activity are reached early in development in rat kidney and liver. In the rat lung the adult values of MAO-A activity are reached by day 40, whereas MAO-B activity is still increasing by day 80. Important differences have been reported in the developmental pattern of the two forms of MAO in the rat and mouse brain, with a decrease in the MAO-A/MAO-B ratio during postnatal development. In the human brain, the ontogenetic development of MAO-A and MAO-B appears to parallel that observed in the rodent brain. It is worth noting that most of the available data have to be considered with reservation owing to many methodological problems. Further studies are clearly needed to get reliable information on the ontogenesis of MAO in mammalian tissues.

Animals↗
PubMed1991

The inhibition of glutamine synthetase in rat corpus striatum in vitro by methionine sulfoximine increases the neurotoxic effects of kainate and N-methyl-D-aspartate.

Coronal slices of rat brain were incubated in Krebs bicarbonate medium containing kainate (300 microM), or N-methyl-D-aspartate (500 microM). Degeneration of striatal neurons by both these toxins was apparent after 40 min incubation, and was accompanied by a 33% (kainate) and 21% (N-methyl-D-aspartate) reduction in striatal glutamine synthetase activity. Pre-incubation of the slices with 500 microM L-methionine sulfoximine, an inhibitor of glutamine synthetase, for 20 min prior to the exposure to either kainate or N-methyl-D-aspartate, again showed extensive degeneration of striatal neurons, and a supra-additive reduction in glutamine synthetase activity in the tissue. The activity of the neuronal marker enzyme, neuron-specific enolase, was also reduced by pre-incubation of the slices with L-methionine sulfoximine before the addition of kainate or N-methyl-D-aspartate, but to a much lesser extent than glutamine synthetase. The results are discussed in terms of a possible mechanism of interaction between either kainate or N-methyl-D-aspartate, and glial cell metabolism.

Animals↗
PubMed1991

The effects of assay temperature on the complex kinetics of acetaldehyde oxidation by aldehyde dehydrogenase from human erythrocytes.

Several studies have shown preparations of the cytosolic aldehyde dehydrogenase (EC 1.2.1.3) from sheep and human liver and from human erythrocytes to exhibit complex kinetic behaviour in which the dependence of the initial velocity on the concentration of acetaldehyde gives rise to downwardly curving double-reciprocal plots. This behaviour has often been analysed in terms of a sharp discontinuity in the double-reciprocal plots and its possible implications for the oxidation of acetaldehyde and other pharmacologically important aldehydes has been a subject of speculation. In the present work, it is shown that the purified, apparently homogeneous, enzyme from human erythrocytes exhibits such complex kinetic behaviour when initial rates are determined at 25 degrees, although the double-reciprocal plots describe a smooth curve with no sharp discontinuity. However, when the assays were performed at 37 degrees there was no significant deviation from Michaelis-Menten kinetics over a wide range of acetaldehyde concentrations (0.2-30 mM). At higher concentrations of acetaldehyde inhibition occurred which was competitive with respect to NAD+. These results, which indicate that the complex kinetic behaviour of aldehyde dehydrogenase is not important at physiological temperature, are interpreted in terms of the mechanisms that have been advanced to explain the phenomena.

Acetaldehyde↗
PubMed1991

The inhibition of monoamine oxidase by brofaromine.

The inhibition of rat liver mitochondrial monoamine oxidase-A (MAO-A) by brofaromine was time-dependent at low enzyme and inhibitor concentrations. The apparent sensitivity to inhibition decreased when the concentration of the mitochondrial preparation was increased. After preincubation of the enzyme with brofaromine repeated washing of the preparation, by sedimentation and resuspension, resulted in a gradual recovery of activity. This occurred more slowly than was the case when the reversible inhibitor amphetamine was used. After incubation with radioactively-labeled brofaromine the loss of radioactivity also occurred slowly. After incubation with radioactively-labeled pargyline polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphage (SDS-PAGE) showed the radioactivity to be associated with a peptide of approximate Mr 50,000, corresponding to the subunit of MAO. Pretreatment with unlabeled pargyline depressed this labeling by pargyline, indicating the latter compound to bind to the active-site of the enzyme. Labeling experiments with radioactive brofaromine indicated that there was a high degree of non-specific binding but that no significant radioactivity remained associated with the enzyme on SDS-PAGE. Chromatographic techniques and determination of H2O2 liberation indicated that, in liver there was no appreciable metabolism of brofaromine under the conditions used in the inhibition experiments. These data indicate brofaromine to be a tight-binding, but reversible inhibitor of MAO.

Animals↗
PubMed1991

Is the oxidation of milacemide by monoamine oxidase a major factor in its anticonvulsant actions?

The anticonvulsant drug milacemide (2-n-pentylaminoacetamide) is known to be oxidized by monoamine oxidase-B to yield glycinamide which then breaks-down to give glycine. It has been postulated that it is this liberation of glycine in the brain that accounts for the anticonvulsant effects. In order to test this hypothesis, and since amines bearing a methyl-group in the alpha-position have been shown to be resistant to oxidation by monoamine oxidase, the effects of milacemide were compared with those of alpha-methyl-milacemide. Although the latter compound was found to be toxic at higher concentrations, it was found to antagonize bicuculline-induced convulsions in mice. When milacemide was administered to mice (0.5 mmol/kg, p.o.) there was a substantial increase in urinary glycinamide excretion. No such increase was observed after the administration of the same dose of alpha-methyl-milacemide. Furthermore, alpha-methyl-milacemide was not oxidized by either monoamine oxidase-A or -B in vitro to any detectable extent, although it was a competitive inhibitor of both forms of the enzyme. The findings that alpha-methyl-milacemide has anticonvulsant properties in the bicuculline test but is not a substrate for monoamine oxidase or a source of urinary glycinamide cast doubt on the importance of the oxidation or milacemide to form glycinamide as a major factor in its anticonvulsant action.

Acetamides↗
PubMed1991

The oxidation of dopamine by the semicarbazide-sensitive amine oxidase (SSAO) from rat vas deferens.

The activities of monoamine oxidase A and B and the semicarbazide-sensitive amine oxidase from rat vas deferens were compared towards benzylamine and dopamine. The selective inhibitors (-)-deprenyl and clorgyline were used to allow the contributions of the A and B forms of monoamine oxidase to be determined separately. Comparison of the kinetic constants of the three enzymes towards dopamine indicated that, although each of them had activity towards this substrate, their relative contributions to the total oxidative deamination would depend on the substrate concentration. At all concentrations in the range 1 microM to 10 mM monoamine oxidase-B would contribute about 50% of the total activity. In the range 1 to 10 microM the contributions made by activities of monoamine oxidase-A and the semicarbazide-sensitive enzyme were similar but at higher concentrations the activity of the latter enzyme became more important, its contribution to the total activity rising to some 35% of the total at 500 microM dopamine. The activity of the semicarbazide-sensitive enzyme towards dopamine might thus be important under conditions where either or both the monoamine oxidases were inhibited in pharmacological studies. Its possible relevance to Norrie disease, in which both forms of the human enzyme are deficient, requires further examination.

Amine Oxidase (Copper-Containing)↗
PubMed1991

The sulphydryl groups of ox brain and liver glutamate dehydrogenase preparations and the effects of oxidation on their inhibitor sensitivities.

Glutamate dehydrogenase preparations from several sources have been shown to have suffered limited proteolysis during purification. This proteolysis has been previously shown to involve removal of the N-terminal tetrapeptide and to result in changes in the regulatory properties of the enzyme. In the present work the previously unidentified N-terminal residue of the unproteolysed enzyme from ox brain and liver is shown to be cysteine. The thiol group of this residue is masked in the native enzyme but it becomes accessible after reduction. Exposure of solutions of the unproteolysed enzyme to air oxidation causes large changes in its sensitivity to inhibition by the antipsychotic drug perphenazine, GTP and by high concentrations of NADH. No such changes occurred in the behaviour of preparations of the enzyme that had suffered proteolysis during purification under these conditions.

Amino Acid Sequence↗
PubMed1990

The binding of endogenous and exogenous substance-P in human plasma.

High levels of substance-P are present in the plasma of patients with carcinoid tumours and some thyrotoxic conditions. The majority of the substance-P in the blood plasma was shown, by immunoassay, to be associated with high molecular-weight material in a complex that could be dissociated by repeated gel-filtration. Smaller amounts of an intermediate molecular-weight (about 65,000 Da) complex were also detected. Chemical crosslinking with glutaraldehyde was used to show that the radioactively-labelled derivative [125I]Tyr-8-substance-P was able to bind to the high-Mr fraction of human plasma and also to human serum albumin. Binding to serum albumin was also demonstrated by equilibrium gel-filtration. Substance-P added to human plasma from a thyrotoxic subject, which contained high endogenous levels of the tachykinin (980 pg/mL), was rapidly degraded during incubation at 37 degrees, whereas the endogenous substance-P was considerably more stable. These results suggest that the binding of substance-P to blood plasma components may play an important role in protecting it against degradation. Furthermore, immunoassay techniques involving prior extraction, which fail to detect the bound substance-P, will give inaccurate measurements of the levels of this peptide in plasma.

Chromatography, Gel↗
PubMed1990

Inhibition of ox brain glutamate dehydrogenase by perphenazine.

Factors affecting the inhibition of ox brain glutamate dehydrogenase (GDH) by the antipsychotic drug perphenazine have been studied. Inhibition was found to be of mixed type with respect to 2-oxoglutarate and competitive towards NADH. However, the data indicate that perphenazine binds to a site distinct from the catalytic site to which NADH binds. Perphenazine also enhanced the high-substrate inhibition by these two substrates. Inhibition by perphenazine was not affected by the allosteric effector GTP but it was enhanced by increasing pH, in the range of 6.3 to 7.6, and diminished by increasing ionic strength. Low concentrations of perphenazine relieved the inhibition of GDH by phosphatidylserine and cardiolipin. However, at higher concentrations phosphatidylserine did not interfere with the inhibition by perphenazine whereas cardiolipin relieved it. The possible significance of these interactions in terms of the behaviour of this antipsychotic drug in vivo are discussed.

Animals↗
PubMed1990

The inhibition of glutamate dehydrogenase by some antipsychotic drugs.

The phenothiazines chlorpromazine and perphenazine and the butyrophenone haloperidol were shown to be reversible inhibitors of glutamate dehydrogenase (GDH). Inhibition by chlorpromazine was found to be partial, whereas haloperidol and perphenazine would, apparently, give full inhibition at saturating concentrations. Double-reciprocal plots of the difference between activities in the absence and presence of the inhibitor against the inhibitor concentration were linear with chlorpromazine and perphenazine but parabolic when haloperidol was used as the inhibitor. Comparisons between the responses of commercially available preparations of ox liver GDH, which have been shown to have suffered limited proteolysis during purification, and the ox brain enzyme prepared by a procedure which does not result in such proteolysis, revealed the latter preparation to be more sensitive to inhibition because of a higher apparent affinity for these drugs. The apparent dissociation constants for enzyme-drug interactions were, however considerably higher than the concentrations that have been reported to occur in vivo following chronic administration of chlorpromazine and haloperidol. This casts doubt on earlier claims that inhibition of GDH may be involved in the antipsychotic actions of these drugs, although it might be a factor in the side effects associated with the use of such compounds.

Animals↗
PubMed1990

Determination of amines, amine metabolites and some amine metabolizing enzymes by high performance liquid chromatography.

Some useful high performance liquid chromatographic methods for the determination of amines, amine metabolites and amine metabolizing enzymes are described. These include the separation of tyramine in wines and beers, determination of tryptamine in urine, assay of monoamine oxidase and catechol-O-methyltransferase and analysis of amine-aldehyde condensation products.

Aldehydes↗
PubMed1990

The interactions of monoamine oxidase with some derivatives of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

The interactions of a number of derivatives of the dopaminergic neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) with monoamine oxidase (MAO) have been studied. The desmethyl derivative, 4-phenyl-1,2,3,6-tetrahydropyridine (PTP), was oxidised by MAO-B to form the corresponding dihydropyridine. Initial interaction with both forms of MAO was competitive. However the degree of inhibition of MAO-B, but not MAO-A, increased with time and became irreversible. Substitution of a methyl group at the 6-position of the heterocyclic ring of MPTP prevented it from acting as a substrate for MAO-B and greatly decreased its potency as an inhibitor of that form of MAO, although it remained a good competitive inhibitor of MAO-A. Replacement of the tetrahydropyridine ring of PTP by piperidine apparently abolished the ability to act as a substrate for MAO-B. The compound was however a competitive inhibitor of both forms of MAO. Substitution of a 3-(chlorophenyl)-methoxy-group at the 4'-position of this compound and PTP led to compounds which appeared to be devoid of substrate activity although they were potent, highly-selective, time-dependent inhibitors of MAO-B.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1990

Inhibition of MAO by substituted tryptamine analogues.

Three different acetylenic analogues of tryptamine, in which the side chain is attached at the 2 position of the heterocyclic ring, were studied as inhibitors of MAO-A and MAO-B. IC50 values were determined after 30 min preincubation of the enzyme and inhibitor, at 37 degrees C before assay. Irreversibility and time-dependence of the inhibition were also established in each case. The kinetic parameters defining non-covalent complex formation and covalent adduct formation were calculated for the mechanism-based inhibition of both MAO-A and MAO-B by these compounds.

Animals↗
PubMed1990

Amine oxidase activities in chemically-induced mammary cancer in the rat.

The activities and distributions of monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) were studied in solid breast tumours induced in rat by treatment with 7,12-dimethylbenz(alpha)anthracene (DMBA). It was observed that increasing degree of malignancy was associated with an increase of MAO-A activity an decrease of MAO-B and SSAO activities. The Km values did not change significantly with malignancy but Vmax values for MAO-A increased whereas Vmax for MAO-B and SSAO diminished with malignancy. It was detected in the more malignant tumours the presence of endogenous reversible inhibitor of SSAO activity not removed by dialysis.

9,10-Dimethyl-1,2-benzanthracene↗