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H B Dixon

Publications and source records attributed to H B Dixon.

At least 19 recordsLinked to original sources
PubMed1995

The transport of acidic amino acids and their analogues across monolayers of human intestinal absorptive (Caco-2) cells in vitro.

The X-AG system, a sodium-dependent, acidic amino-acid transport system has been implicated in the transport of L-aspartate and L-glutamate across monolayers of human Caco-2 cells, an in vitro model of intestinal absorption. This system, which shares many properties with the L-glutamate carrier present in the human jejunum, is highly saturable (> 95% at 50 microM), vectorial (apical-to-basolateral >> basolateral-to-apical) and sodium-, pH- and temperature-dependent. L-Aspartate was also transported against a 10-fold reverse concentration gradient. These data are consistent with a major (saturable) carrier-mediated pathway superimposed onto a minor non-saturable (diffusional) pathway. The carrier has an absolute sodium-dependence and the Michaelis constants for the sodium-dependent transport component (Km) for L-aspartate and L-glutamate were 56 +/- 3 microM and 65 +/- 6 microM, respectively. Cross-inhibition studies showed that strong interaction with the carrier was limited to close analogues of the natural substrates. Potent inhibitors included L-aspartate, D-aspartate (Ki, 70 microM), L-glutamate (Ki 180 microM) and threo-beta-hydroxy-DL-aspartate (Ki, 55 microM), while partial inhibitors included alpha-methyl-DL-aspartate, D-glutamate, L-asparagine, L-proline and L-alanine. Replacement of the side-chain -COO- group (aspartate) with -SO-3 (L-cysteate, Ki, 65 microM) or -(H)P(O)O- (DL-3-(hydroxyphosphoryl)alanine, Ki, 60 microM) maintained strong interaction with the carrier while -As(O)(OH)O- (DL-3-arsonoalanine, Ki, 1100 microM) and -P(O)(OH)O- (DL-3-phosphonoalanine, Ki, 3270 microM) were much more weakly bound, with the larger, but probably less ionised, arsono analogue being more tightly bound than the phosphono compound. The corresponding analogues of glutamate (homologous extension of the methylene chain) showed negligible interaction. We conclude that Caco-2 monolayers are a relevant experimental model for the study of the transport of acidic amino acids and their analogues in man.

Amino Acids, Dicarboxylic↗
PubMed1995

The removal of 2-oxoacyl residues from the N-terminus of peptides and cystatin in non-denaturing conditions.

The N-terminal residue of a protein or peptide may be converted into a 2-oxoacyl group by non-enzymic transamination. This group may then be removed, to obtain the peptide chain shortened by one residue, by treatment with phenylene-1,2-diamine. Hitherto this scission has required a pH of 4-5, but we find that the reaction will proceed well at pH 7 in the presence of concentrated phosphate buffer. We describe a method using reverse-phase HPLC for determining the extent of scission in model peptides; this method also allows products to be isolated and identified. The new scission conditions have been tested by removing the N-terminal residue from cystatin, an inhibitor of cysteine peptidases; electrospray mass spectrometry was used to assess how this protein reacted.

Amino Acid Sequence↗
PubMed1995

Arsenite release on enzymic transformation of arsonomethyl substrate analogues: a potentially lethal synthesis by glycerol-3-phosphate dehydrogenase.

The isosteric arsenical analogue of glycerol 3-phosphate, 3,4-dihydroxybutylarsonic acid, is a good substrate for rabbit muscle glycerol-3-phosphate dehydrogenase. Its oxidation is accompanied by release of arsenite. This release seems to be due to a spontaneous elimination of arsenite by 3-oxoalkylarsonic acids, as it is also observed in (1) the oxidation of 3-hydroxypropylarsonic acid by yeast alcohol dehydrogenase, (2) treatment of 3,4-dihydroxybutylarsonic acid with periodate and (3) nonenzymic transamination of the glutamate analogue 2-amino-4-arsonobutyric acid. Enzymic formation of 3-oxoalkylarsonic acids in cells can therefore be lethal, as arsenite is poisonous to most organisms because of its high affinity for dithiols such as dihydrolipoyl groups.

Alcohol Dehydrogenase↗
PubMed1995

Enolase and the arsonomethyl analogue of 2-phosphoglycerate.

(RS)-3-Arsono-2-(hydroxymethyl)propionic acid was synthesized by the action of alkaline arsenite on 3-bromo-2-(bromomethyl)propionic acid. It is a substrate for yeast enolase (EC 4.2.1.11) with a Km of 6.5 mM (for 2-phospho-D-glycerate Km = 0.08 mM). The catalytic constant of the enzyme with the arsonomethyl analogue is 230 times lower than with 2-phosphoglycerate.

Arsenicals↗
PubMed1992

Utilization of 2-aminoethylarsonic acid in Pseudomonas aeruginosa.

This paper describes the metabolism, transport and growth inhibition effects of 2-aminoethylarsonic acid (AEA) and 3-aminopropylarsonic acid (APrA). The former compound supported growth of Pseudomonas aeruginosa, as sole nitrogen source. The two arsonates inhibited the growth of this bacterium when 2-aminoethylphosphonic acid (AEP) but not alanine or NH4Cl, was supplied as the only other nitrogen source. The analogy between AEA and the natural compound AEP led us to examine the in vitro and in vivo interaction of AEA with the enzymes of AEP metabolism. The uptake system for AEP (Km 6 microM) was found to be competitively inhibited by AEA and APrA (Ki 18 microM for each). AEP-aminotransferase was found to act on AEA with a Km of 4 mM (3.85 mM for AEP). Alanine and 2-arsonoacetaldehyde was generated concomitantly, in a stoichiometric reaction. In vivo, AEA was catabolized by the AEP-aminotransferase since it was able to first induce this enzyme, then to be an efficient substrate. The lower growth observed may have been due to the slowness with which the permease and the aminotransferase were induced, and hence to a poor supply of alanine by transamination.

Aminoethylphosphonic Acid↗
PubMed1991

The origin of multiply sigmoid curves of pH-dependence. The partitioning of groups among titration pK values.

An acid, HnA, with n ionizing groups is known to have the same titration curve as an equimolar mixture of n hypothetical monobasic acids, whose dissociation constants are known as the 'titration constants' of the real acid. We show that the pH-dependence of any property of HnA is also represented by the sum of one-site titration curves, characterized by these same titration constants. Since one such property is the degree of dissociation of one of the dissociating groups, a fraction of each group shows each of the various titration pK values, so that the group partitions among them. The n groups therefore share the same n titration pK values but differ in the fractions belonging to each. The one H+ ion per molecule that titrates with each pK is thus made up of the fractions, one from each group, that share this pK value. A group may possess a single pK value, in that it contributes virtually all of this pK and almost nothing to the others, only if either (1) in titrates in a different pH range from the other groups or (2) its affinity for H+ is unaffected by their ionization state.

Acids↗
PubMed1991

Effect of magnesium ions on the inhibition of S-adenosylmethionine decarboxylase from Escherichia coli by [2-(amino-oxy)ethyl](5'-deoxyadenosin-5'-yl)(methyl)sulphonium .

[2-(Amino-oxy)ethyl](5'-deoxyadenosin-5'-yl)(methyl)sulphonium+ ++, the amino-oxy analogue of decarboxylated S-adenosylmethionine, is a potent irreversible inhibitor of Escherichia coli S-adenosylmethionine decarboxylase [Khomutov, Zavalova, Syrku, Artamonova & Khomutov (1983) Bioorg. Khim. 9, 130-131; Artamonova, Zavalova, Khomutov & Khomutov (1986) Bioorg. Khim. 12, 206-212]. We have shown that Mg2+ ions are required for the irreversible inhibition of the decarboxylase, and that S-adenosylmethionine protects against this inhibition.

Adenosylmethionine Decarboxylase↗
PubMed1991

The preparation and properties of bromoacetylphosphonic acid.

Bromoacetylphosphonic acid, Br-CH2-CO-PO3H2, was made by brominating dimethyl acetylphosphonate and de-esterifying with HBr. It proves to be a powerful alkylating agent, reacting rapidly with GSH, with a rate constant of about 6M(-1).s(-1) at pH6.

Alkylating Agents↗
PubMed1991

A synthesis of acylphosphonic acids and of 1-aminoalkylphosphonic acids: the action of pyruvate dehydrogenase and lactate dehydrogenase on acetylphosphonic acid.

Acylphosphonic acids, R-CO-PO(OH)2, have been synthesized by the steps [formula: see text] of which the last is new and provides a mild method for de-esterifying acylphosphonic acids. Their reductive amination gives a simple way of making 1-aminoalkylphosphonic acids. Acetylphosphonic acid inhibited NAD+ reduction by pyruvate with the pyruvate dehydrogenases from Escherichia coli and Bacillus stearothermophilus. The inhibition was competitive with pyruvate, with Ki of 6 microM for the E. coli enzyme (pyruvate Km 0.5 mM) and one of 0.4 mM of the B. stearothermophilus enzyme (pyruvate Km 0.1 mM). Acetylphosphonate and its monomethyl ester are substates for pig heart lactate dehydrogenase, with Km values of 15 mM and 10 mM respectively (pyruvate Km 0.05 mM) and specificity constants one thousandth that for pyruvate.

Acetaldehyde↗
PubMed1990

The synthesis of 3-phosphonoalanine, phosphonopyruvic acid and phosphonolactic acid. Scission of the C-P bond during diazotization of phosphonoalanine.

3-Phosphonoalanine has been made by the Strecker synthesis from phosphonoacetaldehyde, which is easily prepared from vinyl acetate. It gives phosphonopyruvate by transamination when treated with glyoxylate. Phosphonolactate, an analogue of phosphoglycerate, is prepared by reducing phosphonopyruvate. Diazotization of phosphonoalanine was investigated as a route for making phosphonolactate: addition of NaNO2 to the isoelectric form of phosphonoalanine gave much scission of the C-P bond with release of phosphate; addition of HBr prevented this release and gave largely the bromo acid. The supplement reports the synthesis of arsonolactate, a similar analogue, by treating chlorolactate with alkaline arsenite.

Chemical Phenomena↗
PubMed1989

Synthesis of 2-aminoethylarsonic acid. A new synthesis of primary.

2-Aminoethylarsonic acid was prepared from 2-choloethylarsonic acid. The route constitutes a new procedure for making primary amines from haloalkanes; chloride was displaced by treatment with 2-aminoethanol at 70 degrees C, and the product was converted into the required primary amine by treatment with periodate.

Amines↗
PubMed1989

Application of the synthetic method to other amines.

Taurine and 2-aminoethylphosphonic acid were synthesized by the method of the main paper [Geoghegan & Dixon (1989) Biochem. J. 260, 295-296], i.e. by treating the corresponding halo compound with 2-aminoethanol and then with periodate.

Aminoethylphosphonic Acid↗
PubMed1989

2-Aminoethylarsonic acid as an analogue of ethanolamine phosphate. Endowment of ethanolamine-phosphate cytidylyltransferase with CTP pyrophosphatase activity.

2-Aminoethylarsonic acid was tested for its ability to act as a substrate for ethanolamine-phosphate cytidylytransferase as a cytidylyl acceptor in place of ethanolamine phosphate. The expected product, like all mixed anhydrides of arsonic acids, should hydrolyse spontaneously with regeneration of the substrate analogue and CMP formation; such CMP production was observed. The limiting velocity with aminoethylarsonic acid is about 90% that with ethanolamine phosphate, and the Michaelis constant is below 20 mM.

Arsenicals↗
PubMed1988

Relations between the dissociation constants of dibasic acids.

The relationships between the molecular pK values of a dibasic acid, its titration pK values and the pH values at which the concentration of monohydronated species is half-maximal are presented. These enable any pair of the values to be found from any other pair.

Carboxylic Acids↗