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Biomedical subjects

K F Tipton

Publications and source records attributed to K F Tipton.

At least 91 records · Page 5Linked to original sources
PubMed1993

The effect of side chain substitution at positions 2 and 3 of the heterocyclic ring of N-acetylenic analogues of tryptamine as monoamine oxidase inhibitors.

N-Acetylenic analogues of tryptamine in which the side chain is located at position 2 of the indole ring are compared with those in which the side chain is located at position 3, in terms of their actions as inhibitors of monoamine oxidases A and B. IC50 values at 0 and 30 min of pre-incubation were determined. Time-dependence and irreversible inhibition confirmed that all of them behave as mechanism-based inhibitors. The kinetic constants of each inhibition step were determined for both monoamine oxidase forms and compared between them. In all cases the first-order rate constants for the covalent adduct formation were similar to inhibitor selectivity which is derived solely from differences in affinities for non-covalent binding to the A and B enzymes. Those compounds where the acetylenic side chain was substituted at position 2 of the heterocyclic ring and selective inhibitors of monoamine oxidase A were more potent than those with the side chain in position 3.

Acetylene↗
PubMed1993

Reduction of striatal N-methyl-D-aspartate toxicity by inhibition of nitric oxide synthase.

Coronal slices of rat brain were incubated for 40 min in 300 microM kainate (KA) or 500 microM N-methyl-D-aspartate (NMDA). Histological examination showed neuronal degeneration accompanied by significant losses in the activity of neuron-specific enolase (NSE; EC 4.2.1.11) (-23% KA; -26% NMDA). The activity of the glial enzyme glutamine synthetase (GS; EC 6.3.1.2) was also reduced (-32% KA; -27% NMDA). Pre-incubation with 100 microM L-NG-nitroarginine (L-N-ARG), an inhibitor of nitric oxide (NO) synthase (EC 1.14.23.-), for 20 min attenuated the toxicity of toxicity of NMDA, but not KA. NSE levels after successive incubation in L-N-ARG and NMDA were 95% of controls incubated in Krebs bicarbonate medium only (GS activity 89% of controls). In contrast, pre-incubation with L-N-ARG prior to the addition of KA resulted in neuronal degeneration and significant reductions in NSE levels and GS activities. These observations suggest that the unrestricted function of NO synthase is significant in mediating NMDA neurotoxicity whereas KA toxicity is associated with alternative mechanisms not linked to NO production.

Amino Acid Oxidoreductases↗
PubMed1993

Monoamine oxidase inhibitors and the cheese effect.

The behavior of inhibitors of monoamine oxidase-A (MAO-A) is considered in terms of the possibility of having an effective antidepressant that does not give rise to hypertensive interactions with dietary tyramine. Studies with punch-biopsy samples of human intestine and rat intestinal samples show MAO-A to be the predominant form of the enzyme in both species. Transport studies with everted rat intestinal preparations indicate that tyramine is extensively metabolized during transport through the intestine. Selective inhibition of MAO-A by clorgyline results in a large increase in the amount of unchanged tyramine transported, whereas selective inhibition of MAO-B with L-deprenyl (selegiline) has no significant effect. The behavior of reversible MAO-A inhibitors can significantly reduce, but not entirely eliminate, these effects on the intestinal metabolism of tyramine, but only if the inhibition is competitive in nature.

Animals↗
PubMed1993

Effect of orthotopic liver transplantation and chemical denervation of the liver on the activities of hepatic monoamine oxidase and catechol-O-methyltransferase.

The denervation of some tissue is associated with a fall in the activities of monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT). Here we report on the effect of orthotopic liver transplantation and chemical denervation of the liver on the enzymes. Liver transplantation was performed on Lewis rats (n = 7). Denervation (n = 8) was by intraportal injection of 6-hydroxydopamine (75 mg/kg). A control group (n = 8) was also included. The norepinephrine content of the transplanted and denervated livers was reduced by greater than 99% (P < 0.001) and 95% (P < 0.001), respectively. The activity of hepatic COMT (substrate: catechol [5 mM] was not affected by transplantation or denervation. The activity of MAO with 0.1 mM 5-hydroxytryptamine (5-HT) (substrate for MAO-A) and with 0.01 mM 2-phenylethylamine (substrate for MAO-B) were not affected by denervation. In the transplanted liver, the activity of MAO with 5-HT and 2-phenylethylamine was increased by 26% (P < 0.05) and by 53% (P < 0.001), respectively. The ratios of the activities of the A to B forms of MAO (approximately 70% A to 30% B) was not affected by either procedure. Enzyme sensitivity for MAO inhibitors clorgyline and deprenyl were not significantly altered by transplantation. The concentration of plasma norepinephrine in the transplantation group was significantly lower than either the control (P < 0.001) or denervation groups (P < 0.05). We conclude from our results that the metabolism of circulating catecholamines by the liver is unlikely to be impaired after liver transplantation.

Animals↗
PubMed1993

Use of an electrode selective for 1-methyl-4-phenylpyridinium (MPP+) to measure its uptake and accumulation by mitochondria.

An ion-selective electrode specific for the 1-methyl-4-phenylpyridinium ion (MPP+) was developed which allowed the measurement of the initial rate of uptake and the extent of accumulation of MPP+ by rat liver mitochondria. Using this electrode we demonstrated that the initial rate of uptake of MPP+ was not saturable and that the distribution of MPP+ across the mitochondrial inner membrane did not equilibrate with the membrane potential, in contrast to other lipophilic cations such as the methyltriphenyl phosphonium ion (TPMP+). Furthermore, incubation of mitochondria respiring on succinate with MPP+ decreases the membrane potential. The possibility that the interaction of MPP+ with mitochondria may be more complex than accumulation followed by inhibition of site 1, suggested by these and other data, is discussed.

1-Methyl-4-phenylpyridinium↗
PubMed1992

Uptake and accumulation of 1-methyl-4-phenylpyridinium by rat liver mitochondria measured using an ion-selective electrode.

The compound 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes selective destruction of nigrostriatal dopaminergic neurons in primates, giving rise to a condition resembling Parkinson's disease. The toxicity of MPTP is believed to be due to its metabolite 1-methyl-4-phenylpyridinium (MPP+). MPP+ is an inhibitor of mitochondrial respiration at the NADH-ubiquinone oxidoreductase site and this, together with its selective transport into dopaminergic nerve terminals, accounts for its neurotoxicity. In this paper an electrode selective for MPP+ was developed and used to measure the rate of uptake and the steady-state accumulation of MPP+ in rat liver mitochondria. The initial rates of MPP+ uptake were not saturable, confirming previous work that the transport of MPP+ is not carrier-mediated. The membrane potential of mitochondria respiring on succinate was decreased by MPP+ and the steady-state accumulation ratio of MPP+ did not come to equilibrium with the mitochondrial transmembrane potential gradient (delta psi). The effect of the cation exchanger tetraphenylboron (5 microM) was to increase the initial rate of MPP+ uptake by about 20-fold and the steady-state accumulation by about 2-fold. This suggests that there may be a mechanism of efflux of MPP+ from mitochondria which allows MPP+ to cycle across the membrane and thus decrease delta psi. These data indicate that MPP+ interacts with mitochondria independently of its inhibition of NADH-ubiquinone oxidoreductase, and these alternative interactions may be of relevance for its mechanism of neurotoxicity.

1-Methyl-4-phenylpyridinium↗
PubMed1992

Steady-state kinetic analysis of aldehyde dehydrogenase from human erythrocytes.

The steady-state kinetics of purified cytoplasmic aldehyde dehydrogenase (EC 1.2.1.3) from human erythrocytes have been studied at 37 degrees C. Previous studies of the enzyme from several mammalian sources, which used a lower assay temperature, have been difficult to interpret because of the substrate activation by acetaldehyde which led to complex kinetic behaviour. At 37 degrees C the initial-rate data do not depart significantly from Michaelis-Menten kinetics. Studies of the variation of initial rates as a function of the concentrations of both substrates and studies of the inhibition by NADH were consistent with a sequential mechanism being followed. High-substrate inhibition by acetaldehyde was competitive with respect to NAD+. The enzyme was not inhibited by the product acetate and thus the results of these studies, although consistent with an ordered mechanism in which NAD+ was the first substrate to bind, were inconclusive. That such a mechanism was followed was confirmed by determination of the initial-rate behaviour in the presence of acetaldehyde and glycolaldehyde as alternative substrates. When the reciprocal of the initial rate of NADH formation was plotted against the acetaldehyde concentration at a series of fixed ratios between that substrate and glycolaldehyde, a linear 'mixed inhibition' pattern was obtained, confirming the mechanism to be ordered with NAD+ being the leading substrate and with kinetically significant ternary complex-formation.

Acetaldehyde↗
PubMed1992

Kinetic and inhibition studies on catechol-O-methyltransferase affinity labelling by N-(3,4-dihydroxyphenyl)maleimide.

Initial velocity and product inhibition studies have been performed on soluble catechol-O-methyltransferase which has been partially purified from pig liver. The results are consistent with an ordered reaction mechanism, in which S-adenosyl-L-methionine (AdoMet) is the leading substrate. The enzyme is irreversibly inhibited by maleimide derivatives in a biphasic manner, which suggests a differential reaction with two thiol groups. N-(3,4-Dihydroxyphenyl)maleimide, which has a reactive moiety (maleimide ring) and an affinity moiety (catechol ring), acts as an affinity labelling compound on the more reactive SH group; AdoMet and Mg2+ protect against this modification. Total protection of this SH group results in a pseudo-first-order inhibition of the enzyme, with the apparent rate constant being proportional to the inhibitor concentration. All the other maleimide derivatives studied inhibited the enzyme by reacting with one of the two SH groups in a non-specific manner. The reaction of the other, more reactive, SH group was either specific (active-site-directed) or non-specific, depending on the substituent present in the affinity moiety and also on the length of an intermediate chain of methylene groups present between this moiety and the reactive maleimide ring. In the presence of both AdoMet and Mg2+, 3,5-dinitrocatechol, a reversible inhibitor of the enzyme which is competitive with respect to the catechol substrate, protects the enzyme from inactivation by any of the maleimide derivatives. The adducts of these maleimide derivatives formed with dithiothreitol inhibit the enzyme reversibly, showing inhibition patterns that are consistent with the mechanism deduced from the initial velocity and product inhibition studies.

Affinity Labels↗
PubMed1992

Effects of chronic ethanol feeding on rat liver mitochondrial energy metabolism.

Chronic alcohol consumption is known to decrease hepatic mitochondrial respiration rate. It was shown here that the proton leak through the mitochondrial inner membrane was unaffected by chronic ethanol treatment. This indicates that changes in proton leak are not responsible for the alterations in respiration found in mitochondria isolated from ethanol-treated rats. Therefore, the lowered coupled respiration rate is solely due to a decrease in the activity of the electron transport chain. However, this alteration was only evident in coupled respiration (i.e. state 4) and was not apparent in uncoupled respiration. Thus, chronic ethanol treatment decreases the activity of the mitochondrial electron transport chain components which have control over coupled, but not uncoupled, respiration. Mitochondrial energy metabolism is regulated by thyroid hormone status. It was shown that the chronic alcohol treatment did not affect the circulating levels of thyroxine. Furthermore, the activity of mitochondrial alpha-glycerophosphate dehydrogenase, which is strongly affected by thyroid hormones, was unaltered by alcohol treatment. Thus, the effects of ethanol treatment on mitochondria occur independently of changes in circulating thyroid hormone levels.

Animals↗
PubMed1992

Oxidative ring-coupling of tyrosine and its derivatives by purified rat intestinal peroxidase.

Intestinal peroxidase was shown to catalyse the oxidative ring-coupling of tyrosine, alpha-methyltyrosine, tyramine and morphine whereas amphetamine was not oxidized to any detectable extent. The oxidative ring-coupling reaction can be monitored by changes in absorbance spectra and the dimers formed in this way with morphine and alpha-methyltyrosine were identified by mass spectrometry. Intestinal peroxidase also catalysed the peroxidatic oxidation of L-DOPA and alpha-methyl-L-DOPA, but in this case the reaction would be expected to be more complicated and to yield a variety of possible products. The kinetic parameters for the oxidation of each of these substrates were determined. Since the products of the oxidative ring-coupling reactions may have different pharmacological properties to those of the parent compounds, these studies suggest that, in the presence of an adequate supply of metabolically produced hydrogen peroxide, the action of intestinal peroxidase may affect the behaviour and pharmacokinetics of these compounds after oral administration.

Amphetamine↗
PubMed1992

Interactions of the neurotoxin MPTP and its demethylated derivative (PTP) with monoamine oxidase-B.

The kinetics of the interactions of MPTP and its N-des-methyl-derivative (PTP) have been studied. Both were mechanism-based inhibitors as well as substrates for the enzyme. Analysis of the reaction progress-curves for the formation of the corresponding dihydropyridine derivatives allowed the kinetic parameters for the process and the partition ratio, which corresponds to the number of mol. of product formed per mol. of enzyme inactivated, to be determined for both compounds. The conversion of MPTP to its corresponding pyridinium-ion derivative through the action of MAO-B is known to be essential for its neurotoxicity. PTP has been reported not to be neurotoxic, although it appears to be a relatively good substrate for MAO-B as well as acting as a mechanism-based inhibitor. Studies of the changes in absorbance spectra during the MAO-B catalysed oxidation were consistent with the formation of the corresponding pyridinium-ion derivative (MPP+), which is known to be the effective neurotoxin, as the end-product when MPTP was oxidized. In contrast the oxidation of PTP appeared to stop at the dihydropyridine stage with no significant further oxidation to the corresponding pyridine-derivative.

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