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Biomedical subjects

K F Tipton

Publications and source records attributed to K F Tipton.

At least 37 records · Page 2Linked to original sources
PubMed2000

Cytochrome P450-dependent N-dealkylation of L-deprenyl in C57BL mouse liver microsomes: effects of in vivo pretreatment with ethanol, phenobarbital, beta-naphthoflavone and L-deprenyl.

The monoamine oxidase inhibitor L-deprenyl [(-)-deprenyl, selegiline] is an effective therapeutic agent for improving early symptoms of idiopathic Parkinson's disease. It appears to exert this action independently of its inhibition of monoamine oxidase B (MAO-B) and some of its metabolites are thought to contribute. Cytochrome P450 (CYP) activities are known to give rise to L-deprenyl metabolites that may affect the dopaminergic system. In order to clarify the interactions of L-deprenyl with these enzymes, C57BL mice were treated with L-deprenyl, ethanol, phenobarbital or beta-naphthoflavone to induce different CYP isozymes. After preincubation of L-deprenyl with liver microsomes from control or treated mice, the metabolites were analysed by a GLC method. L-deprenyl (10 mg/kg i.p. for 3 days) caused a significant decrease in total CYP levels (0.315+/-0.019, L-deprenyl; 0.786+/-0.124, control, nmol/mg protein) and CYP2E1-associated p-nitrophenol hydroxylase activity (0.92+/-0.04 vs. 1.17+/-0.06 nmol/min/mg). Both phenobarbital and ethanol increased the N-depropynylation activity towards L-deprenyl that leads to the formation of methamphetamine (4. 11+/-0.64, phenobarbital; 4.77+/-1.15, ethanol; 1.77+/-0.34, control, nmol/min/mg). Ethanol alone increased the N-demethylation rate of L-deprenyl, that results in formation of nordeprenyl (3.99+/-0.68, ethanol; 1.41+/-0.31, control, nmol/min/mg). Moreover, the N-dealkylation pathways of deprenyl are inhibited by 4-methylpyrazole and disulfiram, two CYP2E1 inhibitors. None of the other treatments modified L-deprenyl metabolism. These findings indicate that mainly CYP2E1 and to a lesser extent CYP2B isozymes are involved in L-deprenyl metabolism. They also suggest that, by reducing CYP content, L-deprenyl treatment may impair the metabolic disposition of other drugs given in combination regimens.

Animals↗
PubMed2000

Tyrosine-hydroxylase immunoreactive cells in the rat striatum following treatment with MPP+.

Tyrosine Hydroxylase is the rate-limiting enzyme in the synthesis of dopamine, and as such, it is widely used as a marker of dopaminergic cells. Within the basal ganglia, the dopaminergic cells are located in the substantia nigra pars compacta, and project to the striatum. It is this pathway which degenerates during Parkinson's disease. The data presented here illustrate examples of tyrosine-hydroxylase immunoreactive cells in striatum following intrastriatal injection with the neurotoxin MPP+. We further show by electron microscopy that these cells are, in fact, neurons and that they possess ultrastructural features of interneurons.

1-Methyl-4-phenylpyridinium↗
PubMed2000

The mitochondrial permeability transition and taurine.

Perturbed cellular calcium homeostasis has been implicated in both apoptosis and necrosis, but the role of altered mitochondrial calcium handling in the cell death process is unclear. Recently we found that taurine, a naturally occurring amino acid potentiates Ca2+ sequestration by rat liver mitochondria. These data, which accounted for the taurine antagonism on Ca2+ release induced by the neurotoxins 1-methyl-4-phenylpyridinium plus 6-hydroxy dopamine previously reported, prompted us to investigate the effects of taurine on the permeability transition (PT) induced experimentally by high Ca2+ plus phosphate concentrations. The parameters used to measure the PT were, mitochondrial swelling, cytochrome c release and membrane potential changes. The results showed that, whereas taurine failed to reverse changes of these parameters, cyclosporin A completely reversed them. Even though these results exclude a role in PT regulation under such gross insult conditions, they cannot exclude an important role for taurine in controlling pore-opening under milder more physiological PT-inducing conditions.

Animals↗
PubMed2000

Taurine-induced attenuation of MPP+ neurotoxicity in vitro: a possible role for the GABA(A) subclass of GABA receptors.

Taurine is a sulphur-containing beta-amino acid found in high (millimolar) concentrations in excitable tissues such as brain and heart. Its suggested roles include osmoregulator, thermoregulator, neuromodulator, and potential neurotransmitter. This amino acid has also been shown to be released in large concentrations during ischaemia and excitotoxin-induced neuronal damage. Here we report a protective effect of taurine against MPP(+)-induced neurotoxicity in coronal slices from rat brain. Significant protective effects were observed at taurine concentrations of 20 and 1 mM, suggesting a potential role for taurine in cases of neuronal insult. Studies with the synthetic taurine analogues taurine phosphonate, guanidinoethane sulphonate, and trimethyltaurine suggested the observed effect to be mediated via an extracellular mechanism. The use of GABA receptor ligands muscimol and bicuculline indicated the effect to be mediated through activation of GABA(A) receptors.

1-Methyl-4-phenylpyridinium↗
PubMed1999

Development of a continuous, fluorometric coupled enzyme assay for thyrotropin-releasing hormone-degrading ectoenzyme.

Thyrotropin-releasing hormone degrading-ectoenzyme (TRH-DE) (EC 3.4. 19.6), removes the N-terminal pyroglutamyl residue of thyrotropin-releasing hormone (TRH). Discontinuous assays have been used to measure TRH-DE activity; however, a continuous assay is needed to make reliable measurements of initial rates and facilitate kinetic studies. Presented is a continuous, coupled enzyme assay for TRH-DE in which TRH-DE hydrolyzed the substrate, pyroglutamyl-histidyl-prolylamido-4-methyl coumarin (TRHMCA), to give His-ProMCA, which was then cleaved by dipeptidyl peptidase IV (EC 3.4.14.5) to give 7-amino-4-methyl coumarin (MCA). Reaction progress was monitored continuously by measuring the increase in MCA fluorescence. This assay should be especially useful for rapid screening of potential TRH-DE inhibitors. A previously reported discontinuous assay, where nonenzymatic cyclization at 80 degrees C was used to liberate MCA from His-ProMCA, was found to underestimate the amount of product formed. A modified procedure that avoids this is presented. Initial rates and kinetic parameters for TRHMCA hydrolysis by TRH-DE determined using this modified assay correspond with those determined by the continuous assay. Discontinuous and continuous assays gave K(m) values for TRHMCA of 3.4 +/- 0.7 microM (n = 5) and 3.8 +/- 0.5 microM (n = 5), respectively. K(i) values determined by the discontinuous assay for TRH and TRH-OH were 35 +/- 4 microM (n = 3) and 311 +/- 31 microM (n = 5), respectively.

Aminopeptidases↗
PubMed1999

Potentiation of mitochondrial Ca2+ sequestration by taurine.

The effects of taurine (2-aminoethanesulphonic acid) and its analogues, 2-aminoethylarsonic acid, 2-hydroxyethanesulphonic (isethionic) acid, 3-aminopropanesulphonic acid, 2-aminoethylphosphonic acid, and N,N-dimethyltaurine, were studied on the transport of Ca2+ by mitochondria isolated from rat liver. Taurine enhanced Ca2+ uptake in an apparently saturable process, with a Km value of about 2.63 mM. Taurine behaved as an uncompetitive activator of Ca2+ uptake, increasing both the apparent Km and Vmax values of the process. This effect was not modified in the presence of cyclosporin A (CsA). N,N-Dimethyltaurine also stimulated Ca2+ uptake at higher concentrations, but there was no evidence that the process was saturable over the concentration range used (1-10 mM). Aminoethylarsonate was a weak inhibitor of basal Ca2+ uptake, but inhibited that stimulated by taurine in an apparently competitive fashion (Ki = 0.05 mM). The other analogues had no significant effects on this process. Taurine either in the presence or the absence of CsA had no effect on Ca2+ release induced by 200 nM ruthenium red. Thus, the mechanism of taurine-enhanced Ca2+ accumulation appears to involve stimulation of Ca2+ uptake via the uniport system rather than inhibition of Ca2+ release via the ion (Na+/Ca2+ and/or H+/Ca2+) exchangers or by taurine modulating the permeability transition of the mitochondrial inner membrane. Overall, these findings indicate an interaction of taurine with an as yet unidentified mitochondrial site which might regulate the activity of the uniporter. The unique role of taurine in modulating mitochondrial Ca2+ homeostasis might be of particular importance under pathological conditions that are characterised by cell Ca2+ overload, such as ischaemia and oxidative stress.

Animals↗
PubMed1999

Simultaneous determination of basal and evoked output levels of aspartate, glutamate, taurine and 4-aminobutyric acid during microdialysis and from superfused brain slices.

A HPLC method, involving pre-column derivatisation with o-phthalaldehyde and fluorescence detection, is described. It allows the resolution of aspartate, glutamate, taurine and GABA, in a single run with detection limits of 3.2, 1.7, 1.4 and 2 fmol/microl of perfusate, respectively. It is sufficiently sensitive and rapid (15 min) for the determination "on line" of the four amino acids in perfusates obtained during in vivo microdialysis experiments. The procedure has been used to determine basal, K+ - or veratridine-stimulated release of these amino acids in different brain areas during microdialysis and from perfused tissue slices.

Animals↗
PubMed1999

Indirect measurement of mitochondrial proton leak and its application.

In mitochondria, ATP synthesis is coupled to oxygen consumption by the proton electrochemical gradient established across the mitochondrial inner membrane in a process termed oxidative phosphorylation. It has long been known from stoichiometric studies that ATP synthesis is not perfectly coupled to oxygen consumption. The major inefficiency in the system is leakage of protons across the mitochondrial inner membrane driven by the proton electrochemical gradient. The kinetics of the proton leak can be determined indirectly, by measuring the oxygen consumption of mitochondria under non-phosphorylating conditions (plus oligomycin) as a function of the proton electrochemical gradient. This experimental system provides a convenient means to investigate inner membrane permeability to protons and the effect of factors that may effect that permeability. In this paper we review some results from our laboratory of indirect measurement of mitochondrial proton leak and how it has been applied to investigate the effect of aging, obesity and thyroid status on proton leak. The results show that (i) proton leak in isolated liver mitochondria is not significantly different in a comparison of young and old rats, in contrast (ii) there is an apparent increase in proton leak in in situ mitochondria in hepatocytes from old rats when compared to those from young rats, (iii) proton leak in neuronal mitochondria in situ in synaptosomes is not significantly different in young and old rats, (iv) proton leak is greater in isolated liver mitochondria from ob/ob mice compared to lean controls, (v) acute leptin (OB protein) administration restores the increased leak rate in isolated liver mitochondria from ob/ob mice to that of lean controls, (vi) administration of thyroid hormone (T3) increases proton leak in rat muscle mitochondria, and (vii) proton leak in muscle mitochondria is insensitive to the presence of GDP. It is proposed that the experimental system described here for measuring proton leak, is an ideal functional assay for determining whether the novel uncoupling proteins increase inner membrane permeability to protons.

Aging↗
PubMed1999

Determination of monoamine oxidase activity by HPLC with fluorimetric detection.

The aldehyde produced from the oxidative deamination of primary and secondary amines by the monoamine oxidases (EC 1.4.3.4; MAO) or the semicarbazide-sensitive amine oxidases (EC 1.4.3.6; SSAO) may be determined followed reaction at elevated temperatures with 2-diphenylacetyl-1,3-indandione-1-hydrazone (DIH), separation by high-performance chromatography liquid (C-8 column, eluting isocratically with acetonitrile, ammonium acetate, water) and fluorimetric detection (excitation and emission wavelengths 430 and 525 nm). The detection limits for benzaldehyde, p-hydroxybenzaldehyde and 2-phenylacetaldehyde were 125 nM, 150 and 62.3 nM, respectively. Thus the assay is appropriate for determination of amine oxidase activities towards benzylamine, 2-phenylethylamine and tyramine. The fluorescence of the DIH adduct with indole-3-aldehyde was strongly quenched, giving a relatively high detection limit (17.5 microM). The detection limit was lower (3.8 microM) when the absorbance at 430 nm was monitored. Enzyme activities determined by this procedure were shown to be linear with enzyme-protein concentration (rat liver mitochondria). The presence of 1-2 mM semicarbazide, necessary for determining MAO activities in samples also containing SSAO, did not adversely affect the derivatization reaction. The DIH-aldehyde adducts were sufficiently stable to permit their storage at low temperatures prior to assay. The product produced by reaction of 5-hydroxyindole acetaldehyde with DIH had no significant fluorescence and too low an absorbance at 430 nm to allow its determination for assay of activities towards 5-HT. This procedure can also measure succinic semialdehyde (detection limit 240 nM) and thus would be applicable to the determination of GABA transaminase activity.

Aldehydes↗
PubMed1999

Involvement of monooxygenases and amine oxidases in hydroxyl radical generation in vivo.

The levels of hydroxyl radicals in vivo were measured in rat blood plasma by determining the formation of 2,3-dihydroxybenzoate (2,3-DHB) resulting from the attack of hydroxyl radicals on injected salicylate. This approach was used to study the effects of alterations in the activities of cytochrome P-540 (CYP)-dependent hydroxylases (monooxygenases) and monoamine oxidase (MAO), two groups of enzymes that can produce reactive oxygen species in the reactions they catalyse. Pretreatment with inducers of the cytochrome P540 (CYP) hydroxylases, such as phenobarbital and dexamethasone, resulted in substantial increases in the plasma oxygen radical formation, suggesting that the action of these enzymes on endogenous substrates, or on exogenous substrates such as salicylate, may contribute to oxygen radical formation in vivo. In contrast, pretreatment with the monoamine oxidase (MAO) inhibitors clorgyline and deprenyl did not have any significant effect on the plasma 2,3-DHB levels, perhaps reflecting the different intracellular locations of MAO and the CYP-dependent hydroxylases. Injection of pentylamine, a substrate of MAO-B and semicarbazide-sensitive amine oxidase (SSAO) was also without significant effect.

Animals↗
PubMed1999

Some peculiar aspects of monoamine oxidase inhibition.

CN- ions enhance the inhibition of monoamine oxidase by the hydrazine derivatives, phenelzine [2-phenylethylhydrazine] and pheniprazine [(1-methyl-2-phenylethyl)hydrazine]. This involves partial competitive activation of the initial noncovalent enzyme-inhibitor complex with no significant effect on the subsequent reaction to give the irreversibly inhibited species. Whereas the maximum effects on pheniprazine inhibition of rat liver MAO-B occurred at about 5 microM cyanide, concentrations of 5 mM were necessary for maximum stimulation of MAO-A inhibition. A comparison of the behaviour of rat and ox MAO revealed considerable differences in their sensitivities to pheniprazine and the potentiating effects of cyanide. Species differences were also evident in the interactions derivatives of milacemide [2-n-pentylaminoacetamide] as substrates and mechanism-based inhibitors of MAO-B. In one case there was evidence for apparently large difference in inhibitor sensitivities between human brain MAO-B from different individuals.

Animals↗
PubMed1999

Eukaryotic aldehyde dehydrogenase (ALDH) genes: human polymorphisms, and recommended nomenclature based on divergent evolution and chromosomal mapping.

As currently being performed with an increasing number of superfamilies, a standardized gene nomenclature system is proposed here, based on divergent evolution, using multiple alignment analysis of all 86 eukaryotic aldehyde dehydrogenase (ALDH) amino-acid sequences known at this time. The ALDHs represent a superfamily of NAD(P)(+)-dependent enzymes having similar primary structures that oxidize a wide spectrum of endogenous and exogenous aliphatic and aromatic aldehydes. To date, a total of 54 animal, 15 plant, 14 yeast, and three fungal ALDH genes or cDNAs have been sequenced. These ALDHs can be divided into a total of 18 families (comprising 37 subfamilies), and all nonhuman ALDH genes are named here after the established human ALDH genes, when possible. An ALDH protein from one gene family is defined as having approximately < or = 40% amino-acid identity to that from another family. Two members of the same subfamily exhibit approximately > or = 60% amino-acid identity and are expected to be located at the same subchromosomal site. For naming each gene, it is proposed that the root symbol 'ALDH' denoting 'aldehyde dehydrogenase' be followed by an Arabic number representing the family and, when needed, a letter designating the subfamily and an Arabic number denoting the individual gene within the subfamily; all letters are capitalized in all mammals except mouse and fruit fly, e.g. 'human ALDH3A1 (mouse, Drosophila Aldh3a1).' It is suggested that the Human Gene Nomenclature Guidelines (http://++www.gene.ucl.ac.uk/nomenclature/guidelines.h tml) be used for all species other than mouse and Drosophila. Following these guidelines, the gene is italicized, whereas the corresponding cDNA, mRNA, protein or enzyme activity is written with upper-case letters and without italics, e.g. 'human, mouse or Drosophila ALDH3A1 cDNA, mRNA, or activity'. If an orthologous gene between species cannot be identified with certainty, sequential naming of these genes will be carried out in chronological order as they are reported to us. In addition, 20 human ALDH variant alleles that have been reported to date are listed herein and are recommended to be given numbers (or a number plus a capital letter) following an asterisk (e.g. 'ALDH3A2*2, ALDH2*4C'). It is anticipated that this eukaryotic ALDH gene nomenclature system will be extended to include bacterial genes within the next 2 years and that this nomenclature system will require updating on a regular basis; an ALDH Web site has been established for this purpose (http://++www.uchsc.edu/sp./sp./alcdbase/a ldhcov.html) and will serve as a medium for interaction amongst colleagues in this field.

Aldehyde Dehydrogenase↗
PubMed1998

Mechanism of inhibition of mitochondrial respiratory complex I by 6-hydroxydopamine and its prevention by desferrioxamine.

Inhibition of mitochondrial complex I by 6-hydroxydopamine was studied in brain and liver preparations. NADH-quinone reductase activity of this complex from rat brain was inhibited by 6-hydroxydopamine partially uncompetitively with respect to NADH with a value of Ki 0.051 +/- 0.014 mM. The inhibition patterns for liver NADH-quinone reductase were more complicated than those obtained with the brain enzyme. Desferrioxamine behaved as a 'competitive' activator of complex I from both liver and brain (Ka = 2 mM and 0.02 mM, respectively). It also protected brain complex I against the inhibition by increasing Ki value about 10-fold. Furthermore, in the presence of desferrioxamine the residual activity of enzyme-substrate-inhibitor complex was increased. The data suggest that desferrioxamine does not compete directly with 6-hydroxydopamine for binding to the inhibitory site, but induces a conformation which is unfavorable for the binding of the inhibitor to the protein. The qualitative and quantitative differences between the behavior of the liver and brain enzyme complexes indicate that the assumption that the behavior of liver mitochondria can be used as a model for the situation in brain should be reconsidered.

Animals↗
PubMed1998

Purification and characterization of membrane-bound semicarbazide-sensitive amine oxidase (SSAO) from bovine lung.

Semicarbazide-sensitive amine oxidase (SSAO) has been purified from bovine lung microsomes in a form which is catalytically active and stable to storage. The enzyme, an integral membrane protein, was solubilized with Triton X-100 and purification was achieved, in the presence of detergent, by chromatography with Cibacron Blue 3GA-agarose, hydroxylapatite, Lens culinaris-agarose, Resource Q-FPLC and gel filtration on Superdex 200 HR-FPLC. This is the first reported procedure for the extensive purification of a membrane-bound SSAO. The purified enzyme had an apparent Mr of 400000 but exhibited microheterogeneity with SDS/PAGE and isoelectric focusing, probably as a result of its glycoprotein nature. It behaved as a tetramer with subunits with apparent Mr values of 100. Antibodies raised towards the purified enzyme cross-reacted with the enzymes from human lung and bovine plasma. Redox-cycling staining and reaction with carbonyl reagents were consistent with the presence of a quinone cofactor, possibly topa quinone. The enzyme was also shown to contain two mol of Cu/mol of enzyme and removal of half of this bound copper resulted essentially in complete inhibition of enzyme activity. In contrast to the reported behaviour of the SSAO enzymes from plasma, the bovine lung enzyme was relatively insensitive to inhibition by cyanide, copper-chelating agents and amiloride. The specificity of the bovine lung enzyme was also narrower than reported for soluble SSAO. It catalysed the oxidative deamination of benzylamine, methylamine, 2-phenylethylamine and histamine but had no significant activity towards dopamine, 5-hydroxytryptamine, tryptamine or tyramine.

Amine Oxidase (Copper-Containing)↗
PubMed1998

Conversion of taurine into N-chlorotaurine (taurine chloramine) and sulphoacetaldehyde in response to oxidative stress.

N-Chlorotaurine (taurine chloramine), formed by treating taurine with hypochlorous acid, was shown to decompose to sulphoacetaldehyde with a first-order rate constant of 9.9+/-0.5 x 10(-4).h-1 at 37 degrees C in 0.1 M phosphate buffer, pH 7.4. Rat liver homogenates accelerated this decay in a process that was proportional to tissue-protein concentration and saturable, with maximum velocity (Vmax) and Km values of 0.28+/-0.01 nmol/min per mg of protein and 37+/-9 microM respectively. This activity was found to be lost on heat denaturation, but retained after dialysis. There was no detectable formation of sulphoacetaldehyde when taurine itself was incubated with the tissue homogenates under the same conditions. Activation of human neutrophils (1.67 x 10(6) cells/ml) with latex beads resulted in a respiratory burst of oxygen-radical production, the products of which were partially sequestered by 12.5 mM taurine. Under these conditions sulphoacetaldehyde was generated at a constant rate of 637+/-18 pmol/h per ml for over 7 h. A non-activated neutrophil suspension contained constant levels of 1.42+/-0.02 nmol/ml sulphoacetaldehyde, as did activated cells incubated in the absence of taurine, a basal level which may indicate a steady turnover of taurine in these cells. Such formation of chlorotaurine and its decay to the aldehyde may be the first steps in the metabolism of taurine to isethionate (2-hydroxyethanesulphonate) that has been demonstrated by various authors to occur in vivo.

Acetaldehyde↗