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Biomedical subjects

T K Smith

Publications and source records attributed to T K Smith.

At least 19 recordsLinked to original sources

Substrate specificity of the dolichol phosphate mannose: glucosaminyl phosphatidylinositol alpha1-4-mannosyltransferase of the glycosylphosphatidylinositol biosynthetic pathway of African trypanosomes.

The biosynthesis of glycosylphosphatidylinositol (GPI) precursors in Trypanosoma brucei involves the D-mannosylation of D-GlcN alpha 1-6-D-myo-inositol-1-PO4-sn-1,2-diacylglycerol (GlcN-PI). An assay for the first mannosyltransferase of the pathway, Dol-P-Man:GlcN-PI alpha 1-4-mannosyltransferase, is described. Analysis of the acceptor specificity revealed (a) that the enzyme requires the myo-inositol residue of the GlcN-PI substrate have the D configuration; (b) that the enzyme requires the presence of the NH2 group of the D-GlcN residue; (c) that GlcNAc-PI is more efficiently presented to the enzyme than GlcN-PI, suggesting a degree of substrate channelling via the preceding GlcNAc-PI de-N-acetylase enzyme; (d) that the fatty acid and phosphoglycerol components of the phosphatidyl moiety are important for enhancing substrate presentation and substrate recognition, respectively; and (e) that D-GlcN alpha 1-6-D-myo-inositol is the minimum structure that can support detectable acceptor activity. Analysis of the donor specificity revealed that short chain (C5 and C15) analogues of dolichol phosphate can act as substrates for the trypanosomal dolichol-phosphomannose synthetase, whereas the corresponding mannopyranosides cannot act as donors for the Dol-P-Man:GlcN-PI alpha 1-4-mannosyltransferase.

Animals

Morphology and projections of neurons in Remak's nerve of the domestic fowl revealed by intracellular injection of biocytin.

Micro-injections of biocytin were made into neurons in whole-mount preparations of Remak's nerve of the domestic fowl to visualise the morphology and projections of Remak's neurons. Remak's neurons were classified into four distinct morphological types. Remak type-I and -II neurons had a morphology resembling that of mammalian sympathetic neurons. Type-I neurons (found only in juxta-jejunal ganglia) had numerous primary dendritic processes (8-14) with large dendritic fields and extensive dendritic arborizations. Type-II neurons had 2-9 primary dendritic processes, large dendritic fields and sparse dendritic branching. These were found in similar numbers in juxta-rectal and juxta-jejunal ganglia. Remak type-III neurons were the most numerous cell type of juxta-rectal ganglia. They had small cell somata and short dendritic processes that branched infrequently. Remak type-IV neurons (found only in juxta-jejunal ganglia) had a morphology resembling that of invertebrate neurons in that they possessed a prominent long tapering axon from which most of the numerous long dendritic processes emerged. In juxta-jejunal ganglia, all type-IV and most type-I, -II and -III neurons projected orally, whereas axons of juxta-rectal neurons (types II and III) projected either orally or aborally, or projected directly into a lateral nerve bundle supplying the gut. These regional differences in neuron types and axonal projections suggest that different neural circuits exist between Remak's nerve and the small and large intestine.

Animals

Effect of feeding raw soybeans on polyamine metabolism in chicks and the therapeutic effect of exogenous putrescine.

The inclusion of isolated soy protein in milk replacer diets for calves and neonatal pigs inhibits development of intestinal mucosal cells. Simultaneous administration of putrescine partially overcomes this effect. We therefore conducted experiments to determine the potential for dietary putrescine to overcome the toxicity of raw soybeans in chicks. In the first experiment, week-old chicks were fed either an isolated soy protein-based control diet or an isoenergetic and isonitrogenous diet containing 52% raw, ground soybeans for 14 d. The feeding of raw soybeans depressed (P < .001) growth and feed consumption, caused enlargement (P < .001) of the duodenum and pancreas, depressed (P < .001) activities of polyamine synthetic enzymes in the duodenum, and reduced (P < .01) duodenal tissue concentrations of putrescine. In the second experiment, the diet containing raw soybeans was fed with and without .2, .3, and .4% supplemental putrescine. The feeding of supplemental putrescine largely overcame the inhibition of growth due to the feeding of raw soybeans and increased intestinal putrescine concentrations. Putrescine supplementation had no effect, however, on pancreatic and intestinal enlargement in birds fed raw soybeans and tended to depress the activity of polyamine synthetic enzymes. The beneficial effects of putrescine supplementation were confirmed in the third experiment when up to 1.0% supplemental putrescine was fed. We conclude that the toxicity of raw soybeans to chicks can be overcome by feeding putrescine. These effects are likely due to improved nutrient uptake by overcoming the adverse effects of lectins in the intestinal tract and are not likely due to alleviation of the pancreatic enlargement caused by protease inhibitors.

Adenosylmethionine Decarboxylase

Chemical modification of active site residues in gamma-glutamyl transpeptidase. Aspartate 422 and cysteine 453.

gamma-Glutamyl transpeptidase, an enzyme of central significance in glutathione metabolism, is inactivated by iodoacetamide, which esterifies an active site carboxyl group identified here as that of Asp-422. Treatment of the inactivated enzyme with hydroxylamine leads to deesterification and to restoration of enzymatic activity. N-Acetylimidazole, which also inactivates the enzyme, acetylates several amino acid residues. Acetylation exposes Cys-453, which is buried in the native enzyme, to reaction with iodoacetamide. Incubation of the acetylated enzyme with glutamine produces a stabilized gamma-glutamyl-enzyme form which is (a) located exclusively on the light subunit, (b) more labile to base than to acid, (c) destabilized by denaturation of the enzyme with guanidinium ions, and (d) reactive with hydroxylamine to form gamma-glutamylhydroxamate. Stabilization of the gamma-glutamyl-enzyme appears to be associated with acetylation of lysine residues (including Lys-99). These and other findings suggest that the alpha-amino group of the gamma-glutamyl substrate is linked electrostatically to Asp-422 so as to facilitate reaction of the gamma-carbonyl of the substrate with an enzyme hydroxyl group to form a gamma-glutamyl-enzyme.

Acetylation

Different sites of acivicin binding and inactivation of gamma-glutamyl transpeptidases.

Acivicin is a potent inhibitor of gamma-glutamyl transpeptidase (EC 2.3.2.2), an enzyme of importance in glutathione metabolism. Acivicin inhibition and binding are prevented by gamma-glutamyl substrates and analogs (e.g., serine plus borate), consistent with the previous postulate that acivicin and substrates bind to the same enzyme site. Inactivation of rat kidney transpeptidase by acivicin leads to its binding as an ester to Thr-523. The pig enzyme, which has Ala-523 in place of Thr-523, is inhibited by acivicin with esterification at Ser-405. The human enzyme has Thr-524 (corresponding to Thr-523 in rat); its inactivation leads to esterification of Ser-406 (corresponding to Ser-405 in rat and pig). Hydroxylamine treatment of the acivicin-inactivated enzymes restores activity and releases the acivicin-derived threo-beta-hydroxyglutamate moiety. The findings indicate that there are significant structural differences between the active site region of the rat enzyme and the active site regions of the human and pig. Human mutant enzymes in which Thr-524 and Ser-406 were replaced by Ala, separately and together, are enzymatically active, indicating that these amino acid residues are not required for catalysis. However, esterification of these residues (and of another near the active site) effectively blocks the active site or hinders its function. Acivicin can bind at enzyme sites that are close to that at which gamma-glutamylation occurs; it may bind at the latter site and then be transesterified to another enzyme site.

Alanine

On the cytotoxicity and genotoxicity of allyl and phenethyl isothiocyanates and their parent glucosinolates sinigrin and gluconasturtiin.

Four compounds commonly found in the human diet, allyl isothiocyanate (AITC), phenethyl isothiocyanate (PEITC) and their parent glucosinolates sinigrin and gluconasturtiin, were tested for cytotoxic and genotoxic effects in a Chinese hamster ovary cell line (CHO). The isothiocyanates were found to be more than one thousand times more cytotoxic than the glucosinolates, showing significant cytotoxic activity at concentrations below 1.0 microgram/ml. AITC was unable to induce either chromosome aberrations or sister chromatid exchanges (SCEs) even at highly cytotoxic doses. In contrast, PEITC was found to induce both aberrations and SCE at concentrations of 0.9-1.2 micrograms/ml whilst sinigrin and gluconasturtiin induced aberrations at concentrations above 2 mg/ml.

Animals

Selective toxicity of compounds naturally present in food toward the transformed phenotype of human colorectal cell line HT29.

It has previously been observed that allyl isothiocyanate, a compound naturally present in the diet, is more cytotoxic toward the human colorectal adenocarcinoma cell line HT29 in its control transformed state than after exposure to sodium butyrate or to dimethylformamide, which slow growth and induce differentiation (detransformation). In the present study, a range of other dietary compounds were assayed for such selective toxicity. These compounds were chosen as constituents of foodstuffs that have been identified from epidemiologic studies as being potentially antitumorigenic and also as having anticarcinogenic activity in experimental models. Benzyl and phenethyl isothiocyanate, benzyl thiocyanate, and quercetin showed decreased toxicity towards HT29 after detransformation of the cells by one or both treatments, whereas no change was observed in the sensitivity to diallyl sulfide or diallyl disulfide. It is proposed that the presence of such selectively toxic compounds in the diet may inhibit the development of tumors by interfering with the growth of preneoplastic lesions while having little effect on normal cells. The cumulative effects of these inhibitions may contribute to the chemopreventive properties of the parent foodstuffs observed in epidemiologic studies.

Adenocarcinoma

Toxicity and growth-promoting potential of spermine when fed to chicks.

Previous studies have shown that the feeding of putrescine, a biogenic amine and the precursor of the mammalian polyamines, can promote whole-body growth of chicks. The current study was undertaken to determine the effect of spermine, also a biogenic amine and the most cationic of the polyamines, under similar conditions. In Exp. 1, 120 week-old chicks were fed purified crystalline amino acid-based diets containing 0, .2, .4, .6, .8, or 1.0% spermine for 14 d. Spermine proved highly toxic and growth rates were reduced compared with controls when even .2% was fed. In Exp. 2, chicks were fed 0, .0375, .0750, or .1000% spermine. These concentrations proved less toxic than those used in Exp. 1. Supplemental dietary cysteine was then provided at 0, .3, .6, and .9% together with 0, .025, .050, or .400% spermine (Exp. 3) because depletion of cellular glutathione has been suggested as contributing to spermine's toxicity. Even high levels of cysteine supplementation did not overcome spermine's toxicity. Subsequent dietary provision of L-2-oxothiazolidine-4-carboxylic acid (OTC, Exp. 4), a cysteine prodrug, showed that depletion of cellular glutathione was not likely a cause of spermine toxicosis. A trend toward increased weight gain and feed efficiency was observed when low concentrations of spermine were fed. It was concluded, however, that dietary spermine was more toxic to chicks than was previously seen for putrescine, that any growth-promoting effects of dietary spermine are small, and that supplements of dietary cysteine or OTC are unlikely to increase these effects by overcoming spermine toxicosis.

Adenosylmethionine Decarboxylase

Altered tissue amino acid metabolism in acute T-2 toxicosis.

T-2 toxin is a Fusarium trichothecene mycotoxin that has been shown to alter brain neurochemistry and eating behavior in animals eating contaminated diets. Experiments were conducted to determine the role of altered tissue amino acid metabolism in the etiology of acute T-2 toxicosis. Fasted weanling rats were orally dosed with 0 or 2.0 mg T-2 toxin/kg body weight. Blood, brain, liver, and muscle tissue were excised 4 and 8 hr after dosing, and amino acid concentrations were determined. Hepatic enlargement coupled with reduced liver concentrations of free small neutral, large neutral, and basic amino acids were seen 4 hr after dosing. Brain and muscle amino acid concentrations were largely refractory to treatment, while the plasma concentrations of tyrosine and lysine, and the sum of the basic amino acids fell. Hepatic amino acid concentrations returned to control levels 8 hr after dosing at which time aminoacidemia was seen. This was due partially to an increase in plasma concentrations of large neutral amino acids including particularly the branched-chain amino acids. A subsequent experiment was conducted to determine the effect of T-2 toxin on 14C-leucine uptake and incorporation into protein in liver slices 4 hr after dosing. Exposure to T-2 toxin reduced total (free + protein-bound) uptake of leucine due primarily to reduced incorporation of leucine into newly-synthesized hepatic protein. It was concluded that reduced amino acid uptake by liver preceded aminoacidemia in acute T-2 toxicosis, although it is not clear how this might influence subsequent changes in brain neurochemistry and behavior.

Amino Acids

Interaction of gamma-glutamyl transpeptidase with acivicin.

Inactivation of gamma-glutamyl transpeptidase by acivicin (L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazole acetic acid) is rapid, thought to be irreversible, and associated with binding of close to 1 mol of inhibitor/mol of enzyme. Previous studies with [3-14C]acivicin indicated binding (prevented by substrate) to a specific hydroxyl group (threonine 523) of the rat kidney enzyme. In the present work, we found that such inactivation can be reversed by treating the inhibited enzyme with hydroxylamine. Reactivation (more than 85% complete) is associated with release from the inactivated enzyme of compounds that exhibit the properties of threo-beta-hydroxy-L-gamma-glutamyl hydroxamate and 3-hydroxypyrrolidone-2-carboxylate. We found that the enzyme acts very slowly on acivicin, at a rate that is about 10(-9) that of its normal catalytic rate with glutathione, to form threo-beta-hydroxy-L-glutamate and hydroxylamine. The findings indicate that inhibition by acivicin involves its transformation on the enzyme to an inhibitory species which is attached, apparently by ester linkage, to a specific hydroxyl group of the enzyme. The very slow rate of release of this intermediate appears to account for the observed inhibition.

Animals

Active deglycosylated mammalian gamma-glutamyl transpeptidase.

gamma-Glutamyl transpeptidase, a highly glycosylated heterodimeric enzyme that is usually attached to the external surface of cell membranes, is of major importance in the metabolism of glutathione. The enzyme, which has been isolated from many animal sources, contains a large amount of carbohydrate, which is linked to both protein subunits. Previous work has not shown whether such carbohydrate is needed for enzyme activity nor indicated its functional role. Notably, gamma-glutamyl transpeptidase isolated from Escherichia coli, which exhibits about 80% amino acid sequence homology with the rat enzyme, has only about 0.1% of its specific enzymatic activity and is not glycosylated. Here we treated the highly glycosylated gamma-glutamyl transpeptidases isolated from rat and pig kidneys with a mixture of glycosidases and then separated two completely active gamma-glutamyl transpeptidase fractions from each species. One fraction was completely devoid of carbohydrate and was fully active as compared with the respective isolated enzymes, but differed in solubility and stability. The other fraction, which contained 10-20% of the initially bound carbohydrate, exhibited a marked increase in susceptibility to proteases. The oligosaccharide chains of gamma-glutamyl transpeptidase may protect against protease action (including self-destruction by the inherent protease activity of the light subunit) during synthesis of the active enzyme from its single chain precursor, as well as after enzyme synthesis.

Animals

Cytokines decrease apolipoprotein accumulation in medium from Hep G2 cells.

Cytokines, important biochemical mediators of inflammation, cause a rapid fall in the plasma concentration of cholesterol in vivo. One mechanism by which cytokines may cause acquired hypocholesterolemia is by decreasing the hepatic synthesis and secretion of apolipoproteins. To test this hypothesis, we incubated Hep G2 cells with human recombinant tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6. Each of the cytokines resulted in a dose-related reduction in the concentrations of apolipoprotein (apo) A-I, apoB, and lecithin:cholesterol acyltransferase (LCAT) activity in the medium after 24 hours of incubation. The effect of cytokines on apolipoprotein accumulation was not affected by preincubation of Hep G2 cells with fatty acids. Cytokines decreased the concentration of cellular apoA-I mRNA in a dose-related fashion but did not affect cellular concentrations of apoB mRNA. The concentrations of triglyceride and cholesterol were also reduced in the medium of cells incubated with cytokines. Total cell sterol synthesis rates were calculated by [14C]acetate incorporation. Cells incubated with interleukin-6 had a 31% increase in sterol synthesis rate but a 41% decrease in sterol secretion. These data suggest that these cytokines can decrease the hepatic synthesis and/or secretion of apolipoproteins and that this may explain, in part, the acquired hypocholesterolemia seen during acute and chronic inflammation.

Apolipoprotein A-I

Mechanism of cyclic AMP-induced hyperpolarization in canine colon.

The mechanism of forskolin (FSK)-induced hyperpolarization was investigated in strips of canine colonic circular muscle. FSK responses were compared to those of the K+ channel opener lemakalim (LEM). Both FSK (10 microM) and LEM (10 microM) hyperpolarized cells near the myenteric border by 10 to 20 mV. Responses to both agents were abolished by 35 mM external K+, indicating a probable mediation by K+ channels. FSK increased the open probability of Ca(++)-activated K+ channels in isolated colonic myocytes. However, in muscle strips charybdotoxin (100 nM) and tetraethylammonium (10 mM) failed to reduce FSK- and LEM-induced hyperpolarizations whereas tetrapentylammonium (50 microM) and 4-aminopyridine (10 mM) blocked both responses. Phencyclidine (100 microM), Ba++ (1 mM) and the antagonist of ATP-sensitive K+ currents glybenclamide (10 microM) blocked LEM- but not FSK-induced hyperpolarizations. Delayed rectifier current in isolated myocytes was activated near -20 mV and was blocked by (order of potency): nifedipine > tetrapentylammonium > phencyclidine > 4-aminopyridine > tetraethylammonium. Charybdotoxin (100 nM), Ba++ (1 mM) and glybenclamide (10 microM) were without effect. Ca(++)-activated K+ current was activated near +30 mV and was blocked by: charybdotoxin > tetraethylammonium > tetrapentylammonium >> phencyclidine = 4-amino-pyridine. These data suggest that LEM induces membrane hyperpolarization by activation of a K+ current with a pharmacology similar to ATP-sensitive K+ current whereas cyclic AMP-induced hyperpolarization appears to involve activation of a current other than delayed rectifier current, Ca(++)-activated K+ current or ATP-sensitive K+ current.

Animals

Methionine toxicity in the rat in relation to hepatic accumulation of S-adenosylmethionine: prevention by dietary stimulation of the hepatic transsulfuration pathway.

Rats were fed toxic levels of methionine with or without simultaneous dietary supplements of glycine and serine. Feed intake, growth rate, and metabolite concentrations in intestine, plasma, liver, skeletal muscle, and kidneys were monitored. Both toxic amounts of methionine and supplemental glycine and serine affected the tissue distribution of several amino acids resulting in similar, opposite, and diet-specific effects on the parameters studied. These changes were considered to be normal responses of amino acid metabolism to diet and to reflect metabolite flows between tissues. The feeding of toxic levels of methionine resulted in the accumulation of methionine, taurine, and glutathione in all tissues measured, but caused marked accumulation of S-adenosylmethionine and its catabolites only in liver. Hepatic accumulation of S-adenosylmethionine was accompanied by 40% stimulation of methionine adenosyltransferase and 40% repression of spermine synthase over a 2-week period. Simultaneous dietary supplements of glycine and serine combined with toxic levels of methionine markedly stimulated hepatic methionine catabolism. As a result, tissue distribution of methionine and glutathione returned close to normal in all tissues measured and accumulation of hepatic S-adenosylmethionine and its catabolites was prevented. Concentrations of taurine in liver, blood, and kidneys were further elevated, suggesting increased conversion of methionine to taurine followed by urinary excretion. These changes were accompanied by normalization of the above enzyme activities and the absence of symptoms of methionine toxicity. It was concluded that methionine toxicity is likely to be linked to hepatic accumulation of S-adenosylmethionine, resulting in liver dysfunction probably due to nonenzymatic methylation of liver macromolecules. Accumulation of tissue glutathione may also contribute to toxicity.

Amino Acids

Beta-adrenergic inhibition of electrical and mechanical activity in canine colon: role of cAMP.

The effects of beta-adrenergic receptor stimulation on the electrical and mechanical activity of canine colonic circular muscles were compared with forskolin (Fsk), a known stimulator of adenylate cyclase. The actions of isoproterenol (Iso) were mediated by beta 2-receptors. Iso and Fsk increased intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels in both the presence and absence of acetylcholine (ACh), whereas ACh (0.3 microM) alone reduced cAMP levels. These agents caused inhibition of spontaneous and ACh-induced contractions. Inhibition was associated with a reduction in the amplitude and duration of electrical slow waves recorded near the submucosal border. Near the myenteric border, Iso and Fsk hyperpolarized the membrane by up to 30 mV and changed the pattern of electrical rhythmicity. These effects were mimicked by 8-bromo-cAMP (1-3 mM). Contractile inhibition with Fsk and Iso was associated with a decrease in the amplitude and duration of Ca2+ transients measured with fura-2 fluorescence. cAMP (10-300 microM) reduced the Ca2+ sensitivity of the contractile apparatus in muscles permeabilized with staphylococcal alpha-toxin. The actions of Iso appear linked to cAMP. We hypothesize that cAMP produces relaxation both by modulation of membrane ionic channels with a consequent decline in the entry of Ca2+ as well as through a decrease in the sensitivity of the contractile apparatus to Ca2+.

8-Bromo Cyclic Adenosine Monophosphate

Effect of interleukin-1 alpha on lipoprotein lipids in cynomolgus monkeys: comparison to tumor necrosis factor.

Acute inflammation is associated with changes in lipoprotein metabolism. Cytokines are thought to mediate the metabolic effects of the inflammatory process. This study was undertaken to compare the effects of interleukin-1 alpha (IL-1 alpha) to tumor necrosis factor (TNF) on lipoprotein metabolism in non-human primates. Recombinant human IL-1 alpha (100 micrograms/kg), TNF alpha (20 micrograms/kg) and lipopolysaccharide (20 micrograms/kg) were injected into cynomolgus monkeys. Lipoprotein concentrations, plasma activities of post-heparin lipase (PHLA) and lecithin:cholesterol acyltransferase (LCAT) were measured prior to and 24 and 48 h after, injection. All three injections caused afebrile response in the animals. Interleukin-1 alpha had no effect on plasma lipoprotein concentrations, composition of lipoproteins or enzyme activity. In contrast, injection of TNF caused significant changes in lipoprotein concentrations. There was a 38% increase in plasma triacylglycerol and 30% decrease in plasma cholesterol at 48 h after injection. Concentrations of apolipoproteins A-I and B were decreased 20% and 44%, respectively, at 48 h. Compositional analyses of lipoprotein particles after TNF injection showed that both the LDL and HDL particles had decreased content of cholesterol ester and increased triacylglycerol after injection, and plasma activities of PHLA and LCAT were decreased. These changes were qualitatively similar to those seen after LPS injection. These data suggest that, unlike TNF, IL-1 alpha is not an important mediator of the inflammatory process on lipoprotein metabolism in cynomolgus monkeys.

Animals

Dexamethasone increases apolipoprotein A-I concentrations in medium and apolipoprotein A-I mRNA abundance from Hep G2 cells.

Glucocorticoid hormones increase high-density lipoprotein (HDL) levels in vivo. However, there is little known about the mechanism by which glucocorticoids alter HDL metabolism. Hep G2 cells were incubated with dexamethasone to determine the effect of glucocorticoids on apolipoprotein (apo) A-I secretion. Dexamethasone increased apo A-I concentration in a dose-dependent fashion. After 24 hours, 5.5 x 10(-5) mol/L dexamethasone increased apo A-I accumulation in culture medium by 54%. Detectable increases in apo A-I concentration were noted in medium by 5 hours of incubation and persisted up to 48 hours. Cellular apo A-I mRNA concentration increased by 28% after incubation with dexamethasone for 24 hours. The increase in apo A-I mRNA concentration was detectable within 3 hours after incubation with dexamethasone. In contrast, incubation with dexamethasone decreased apo B concentration by 43% in culture medium, but it had no effect on cellular apo B mRNA concentrations. Dexamethasone had little effect on cholesterol and triglyceride accumulation in the medium. Incubation with albumin alone did not affect apo A-I concentration, but it decreased apo B concentration by 30% in the medium. Incubation with albumin and dexamethasone had no effect on apo A-I concentration in medium and had no additive effect on apo B concentration. These data suggest dexamethasone increases secretion of apo A-I by Hep G2 cells by increasing mRNA levels.

Albumins