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G C Ford

Publications and source records attributed to G C Ford.

At least 55 records · Page 3Linked to original sources
PubMed1985

Calculation of turnover rates in stable-isotope studies.

Tserng and Kalhan have raised the question of the appropriate equation to use for the measurement of turnover rate in a stable-isotope study. In a comparison of glucose turnover measured with 2H-glucose and with 13C-glucose they used five apparently different equations and obtained conflicting answers. There is, however, no difference in principle between the use of a stable isotope as a tracer and the use of a radioactive isotope, and the rate of appearance of tracee in a steady-state system (the turnover) can therefore be shown to be proportional to the equilibrium dilution of the infused tracer. Because the sensitivity of measurement of this dilution made using a gas chromatograph-mass spectrometer is lower than that made by radioactivity measurement, the contribution to the measured turnover rate due to the infusate cannot be neglected, as it usually is in radioisotope work. A convenient calibration curve to establish this dilution is the mole ratio of the pure infusate against the area ratio for the relevant ions. Tserng and Kalhan's apparently conflicting results for glucose turnover using 13C-glucose as the tracer can all be shown to amount to approximately 11.6 mumol min-1 kg-1. This value is only slightly lower (0.05 less than P less than 0.1) than that obtained using 2H-glucose as the tracer and supports the use of 13C-glucose as an alternative.

Blood Glucose↗
PubMed1985

Direct determination of leucine metabolism and protein breakdown in humans using L-[1-13C, 15N]-leucine and the forearm model.

We have used the forearm model to study protein metabolism in six normal healthy subjects in the fed state using L-[1-13C, 15N]-leucine as the substrate tracer. Deep venous and arterialized venous blood samples from the forearm were collected at 10-min intervals 2.5 h into a primed-continuous infusion of the dilabelled tracer. Arterialized venous blood was obtained using a 'hot-box' technique and forearm blood flow was measured by mercury strain-gauge plethysmography. The concentration and isotope enrichment of leucine and its metabolites, alpha-ketoisocaproic acid and CO2, in deep venous and arterialized venous blood were measured by gas chromatography-mass spectrometry and isotope ratio-mass spectrometry. The rates of leucine deamination and reamination were 388 +/- 24 (mean +/- SEM) and 330 +/- 23 nmol (100 ml)-1 min-1 respectively, whilst protein synthesis and breakdown rates were 127 +/- 11 and 87 +/- 10 nmol (100 ml)-1 min-1 respectively across the forearm in the fed state. We have demonstrated that the use of doubly labelled leucine as tracer and application of the mathematical model developed in this study, permits the comprehensive quantification of leucine kinetics including protein breakdown.

Adult↗
PubMed1984

Mechanism of action of aspartate aminotransferase proposed on the basis of its spatial structure.

Aspartate aminotransferase is a pyridoxal phosphate-dependent enzyme that catalyses the transamination reaction: L-aspartate + 2-oxoglutarate----oxaloacetate + L-glutamate. The enzyme shuttles between its pyridoxal and pyridoxamine forms in a double-displacement process. This paper proposes a mechanism of action that delineates the dynamic role of the protein moiety of this enzyme. It is based on crystallographically determined spatial structures (at 2.8 A resolution) of the mitochondrial isoenzyme in its unliganded forms and in complexes with substrate analogues, as well as on model building studies. The enzyme is composed of two identical subunits, which consist of two domains. The coenzyme is bound to the larger domain and is situated in a pocket near the subunit interface. The proximal and distal carboxylate group of dicarboxylic substrates are bound to Arg386 and Arg292 , respectively, the latter residue belonging to the adjacent subunit. These interactions largely determine the substrate specificity of the enzyme. They not only position the substrate efficient catalysis but also bring about a bulk movement of the small domain that closes the active site crevice and moves Arg386 about 3 A closer to the coenzyme. The replacement of the epsilon-amino group of Lys258 by the alpha-amino group of the substrate in the aldimine bond to pyridoxal phosphate is accompanied by a tilting of the coenzyme by approximately 30 degrees. The released epsilon-amino group of Lys258 serves as a proton acceptor/donor in the 1,3- prototropic shift producing the ketimine intermediate. At this stage, or after hydrolysis of the ketimine bond, the coenzyme rotates back to an orientation between that in the "external" aldimine intermediate and that in the pyridoxal form. Throughout this process, the protonated pyridine nitrogen atom maintains a hydrogen bond to the beta-carboxylate group of Asp222 . Upon formation of the pyridoxamine form, the small domain moves back to its original position. The proposed mechanism is compatible with the known kinetic and stereochemical features of enzymic transamination.

Aspartate Aminotransferases↗
PubMed1984

Ferritin: design and formation of an iron-storage molecule.

Although essential for most forms of life, too much iron is harmful. To cope with these antagonistic phenomena an iron-storage molecule, ferritin, has evolved. The structure of horse spleen apoferritin, which has recently been refined, consists of 24 symmetrically related subunits forming a near-spherical hollow shell. In ferritin the central cavity is occupied by an iron core of 'ferrihydrite', a geologically ephemeral mineral found in hot or cold springs and in mine workings, or produced in the laboratory by heating solutions of ferric salts. Ferritin itself forms most readily from apoferritin, in the presence of dioxygen, from FeII, not FeIII. Access to its interior is through small intersubunit channels, and the protein influences both the rate of FeII-oxidation and the form of oxide produced.

Animals↗
PubMed1980

A stable isotope dilution assay for disopyramide and its [13C, 15N]labelled analogue in biological fluids.

The mass spectra of disopyramide phosphate and two stable isotopically labelled analogues have been obtained using electron impact and chemical ionization. The low isotopic purity of [13C, 15N)disopyramide phosphate was shown to be due to the low isotopic purity of the 15N label. A stable isotope dilution assay for disopyramide and [13C, 15N]disopyramide in biological fluids has been developed using [2H14]disopyramide phosphate as the internal standard. This assay will be used to analyse samples obtained after the co-administration of disopyramide phosphate intravenously and [13C, 15N]disopyramide phosphate orally to several animal species.

Animals↗
PubMed1980

The absorption of disopyramide in animals determined using a stable isotope co-administration technique.

The stable isotope co-administration technique for estimating the bioavailability of drugs has been investigated in a series of experiments using rhesus monkeys. The compound chosen for study was disopyramide phosphate. A cross-over study was designed whereby the animals received disopyramide phosphate (administered intravenously at 5 mg kg-1) and [13C, 15N]disopyramide phosphate (administered orally at 5 mg kg-1), with a wash-out period between doses. A co-administration study was carried out whereby both the oral and intravenous doses were administered together. The co-administration study was repeated. The results from the cross-over study showed [13C, 15N]disopyramide to have an oral availability of 4.9 +/- 0.9% (by comparing areas under the plasma concentration versus time curves). The bioavailability was estimated to be 5.7 +/- 0.3% comparing totals excreted in urine over 48 h. The bioavailability of the oral dose was calculated as 8.2 +/- 2.5% (comparing areas under plasma concentration versus time curves) and 9.3 +/- 3.0% (comparing totals excreted in urine) after co-administration. The differences between these results and the cross-over results were examined in a further study, using oral administration only. The animals were dosed orally with a solution containing both disopyramide phosphate (5 mg kg-1) and [13C, 15N]disopyramide phosphate (5 mg kg-1). No differences were observed between the plasma concentration versus time curves or urinary excretion for either isotope. It is unlikely that the discrepancy in bioavailability is due to absorption, metabolism or exretion of the oral dose. It is probable that the high concentration of disopyramide obtained after the intravenous dosage affects the disposition of the oral dose, and this gives the higher figure.

Administration, Oral↗
PubMed1980

Three-dimensional structure of a pyridoxal-phosphate-dependent enzyme, mitochondrial aspartate aminotransferase.

X-ray diffraction studies to 2.8-A resolution have yielded the three-dimensional structure of mitochondrial aspartate aminotransferase (L-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1), an isologous alpha 2 dimer (Mr = 2 x 45,000). The subunits are rich in secondary structure and contain two domains, one of which anchors the coenzyme, pyridoxal 5'-phosphate. Each active site lies between the subunits and is composed of residues from both of them.

Animals↗
PubMed1980

Metabolism of ethynodiol diacetate in the rhesus monkey before and after administration of rifampicin.

1. On administration of a single oral dose of [4-(14)C]ethynodiol diacetate (0.15 mg/kg) to rhesus monkey, plasma concn. of total 14C peaked after about 4 h. About 60% of the plasma radioactivity was present as glucuronide conjugates and no unchanged drug was detected. 2. Some 67 +/- 6% (mean +/- S.D.) of the dose of 14C was excreted in 4 days, 50 +/- 6% in urine and 18 +/- 2% in faeces. Most of the urinary excretion occurred within 24 h of dosage. 3. Glucuronide conjugates accounted for 60% of the urinary 14C, and 46% of the faecal 14C was free steroids. 4. Norethisterone and its tetrahydro metabolites were identified in the free, glucuronide and sulphate fractions of plasma and urine. Keto-4,5-dihydronorethisterones and trihydroxy metabolites were identified in the conjugated fractions of urine, and a complex mixture of polar metabolites was detected in faeces. 5. Rifampicin treatment (7.5 mg/kg/day, orally) for 8 days decreased the half-life of total 14C in plasma following a single oral dose of 4-[14C]ethynodiol diacetate (0.15 mg/kg) from 44 +/- to 24 +/- 2 h. 6. Faecal elimination of total 14C was significantly increased to 29 +/- 5% of the dose following rifampicin treatment, but urinary excretion was unchanged. 7. Rifampicin treatment increased the amount of polar metabolites and decreased the amount of norethisterone in the free and conjugated fractions of plasma and urine. The amounts of sulphate and non-hydrolysed conjugates in faeces were increased after rifampicin treatment.

Animals↗
PubMed1978

A "carrier effect" observed in assay of antidiarrhoeal drug compounds.

A carrier effect has been shown to exist in a stable isotope dilution assay for diphenoxylate. When a tetradeuterated analogue is used as a carrier and internal standard a sevenfold increase in sensitivity is observed for the unlabelled compound. Examination of a related pharmacologically active compound (SC-27166) showed a much smaller effect.

Antidiarrheals↗
PubMed1978

Electron impact and chemical ionization mass spectrometry of steroidal spirolactones.

Electron impact and chemical ionization mass spectra are reported for several steroidal spirolactones and their TMS ethers. The electron impact spectra were characterized generally by low abundance molecular ions and large numbers of fragment ions. Methane chemical ionization spectra exhibited high intensity [M+H]+ and/or [M+H-H2O]+ or [M+H-TMSOH]+ ions with few other fragment ions. Ammonia chemical ionization spectra had intense [M+H]+ and/or [M+NH4]+ ions with a few fragment ions generally formed by loss of H2O or TMSOH from these parent ions. Ammonia chemical ionization gave intense parent ions even for polyhydroxy compounds and their TMS ethers in contrast to methane chemical ionization. The results of this study suggest that a combination of electron impact with ammonia chemical ionization mass spectrometry would offer the best techniques for detection and identification of these compounds in biological fluids.

Ammonia↗
PubMed1977

The measurements of propantheline ion in biological fluids after administering propantheline bromide to man.

The measurement of propantheline ion has been accomplished in urine and plasma following administration of propantheline bromide to man. Trideuteropropantheline bromide is added to the biological fluid to act as a carrier and internal standard to quantify the propantheline ion using multiple ion monitoring. Methane was used as reactant gas but following the discovery of exchange of the trideuteromethyl group when using methane, ammonia was used for later analyses. The determination of propantheline ion in urine and plasma after administration of propantheline bromide to man is described.

Deuterium↗