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C F Bryce

Publications and source records attributed to C F Bryce.

15 recordsLinked to original sources
PubMed1994

Steady-state modelling of metabolic flux between the tricarboxylic acid cycle and the glyoxylate bypass in Escherichia coli.

In this study, mathematical modelling, using the computer package MetaModel, was employed to calculate the steady-state fluxes and the concentration of various metabolites of the central pathways during growth of Escherichia coli on acetate. This package also enabled us to formulate the matrices of the elasticity coefficients and the control and response coefficients under different steady states. In this paper, we have assessed the relative contribution of the competing enzymes at the metabolic junction of isocitrate, i.e. isocitrate dehydrogenase (ICDH) and isocitrate lyase (ICL), to the overall distribution of carbon flux among the enzymes of the central pathways thus extending the pioneering work of Walsh and Koshland on the partition of carbon flux between the two metabolic cycles of the tricarboxylic acid and the glyoxylate bypass. This study revealed that ICDH is not 'rate limiting' during growth on acetate and that flux through ICL is essential not only to replenish the central pathways with biosynthetic precursors but also to sustain a high intracellular level of isocitrate. Furthermore, above certain threshold concentration of ICL, the Krebs cycle and the glyoxylate bypass work in concert and the partition of carbon flux between ICDH and ICL is no longer a problem.

Acetates↗
PubMed1976

The anomalous kinetics of coupled aspartate aminotransferase and malate dehydrogenase. Evidence for compartmentation of oxaloacetate.

Cytoplasmic aspartate aminotransferase and malate dehydrogenase were purified from pig heart. Kinetic parameters were determined for the separate reaction catalysed by each enzyme and used to predict the course of the coupled reaction: (see article). Although a lag phase should have been easily seen, none was detected. The same coupled reaction was also carried out by using radioactive aspartate in the presence of unlabelled oxaloacetate. The reaction was quenched with HClO4 after 70 ms and the specific radioactivity of the malate produced in this system was found to be essentially the same as that of the original aspartate. These results show that oxaloacetate produced by the aspartate aminotransferase is converted into malate by malate dehydrogenase before it equilibrates with the pool of unlabelled oxaloacetate and are consistent with a proposal that the enzymes are associated in a complex. However, no physical evidence of the existence of a complex could be found. An alternative means of compartmentation of the intermediate as an unstable isomer is considered.

Animals↗
PubMed1973

The organ-specificity of ferritin in human and horse liver and spleen.

1. Ferritin was isolated from human and horse spleen and liver, and apoferritin prepared therefrom. 2. The electrophoretic mobilities of the four apoferritins were determined on polyacrylamide gels and on cellulose acetate strips, and all found to be equal. 3. Homologous ferritins share reactions of identity in immunodiffusion experiments, whereas heterologous ferritins show only partial identity. 4. The subunit molecular weight of each of the apoferritins was determined by polyacrylamide-gel electrophoresis in sodium dodecyl sulphate and by chromatography on agarose columns in 6m-guanidine-HCl. A value of approx. 18500 was found in all cases. The proteins all had sedimentation coefficients of 17-18S. It thus seems that they have identical quaternary structures. 5. The amino acid compositions of the proteins revealed distinct differences both between organs and between species. This was confirmed by analysis of the tryptic peptide patterns, where it was found that about one-third of the peptides were common to the four proteins and the other two-thirds varied from protein to protein. 6. It is concluded that the apoferritins present in the liver and spleen of human and horse are both organ- and species-specific. 7. The apoferritin isolated from the liver of a patient with idiopathic haemochromatosis was identical with normal human liver apoferritin by the criteria described above.

Aged↗
PubMed1973

Subunit interactions in horse spleen apoferritin. Dissociation by extremes of pH.

1. The dissociation of horse spleen apoferritin as a function of pH was analysed by sedimentation-velocity techniques. The oligomer is stable in the range pH2.8-10.6. Between pH2.8 and 1.6 and 10.6 and 13.0 both oligomer and subunits can be detected. At pH values between 1.6 and 1.0 the subunit is the only species observed, although below pH1.0 aggregation of the subunits to a particle sedimenting much faster than the oligomer occurs. 2. When apoferritin is first dissociated into subunits at low pH values and then dialysed into buffers of pH1.5-5.0, the subunit reassociates to oligomer in the pH range 3.1-4.3. 3. U.v.-difference spectroscopy was used to study conformational changes occurring during the dissociation process. The difference spectrum in acid can be accounted for by the transfer of four to five tyrosine residues/subunit from the interior of the protein into the solvent. This process is reversed on reassociation, but shows the same hysteresis as found by sedimentation techniques. The difference spectrum in alkali is more complex, but is consistent with the deprotonation of tyrosine residues, which appear to have rather high pK values. 4. In addition to the involvement of tyrosine residues in the conformational change at low pH values, spectral evidence is presented that one tryptophan residue/subunit also changes its environment before dissociation and subsequent to reassociation. 5. Analysis of the dissociation and reassociation of apoferritin at low pH values suggests that this is a co-operative process involving protonation and deprotonation of at least two carboxyl functions of rather low intrinsic pK. The dissociation at alkaline pH values does not appear to be co-operative. 6. Of the five tyrosine residues/subunit only one can be nitrated with tetranitromethane. Guanidination of lysine residues results in the modification of seven out of a total of nine residues/subunit. Nine out of the ten arginine residues/subunit react with cyclohexanedione.

Amino Acids↗
PubMed1973

The catalytic activity of horse spleen apoferritin. Preliminary kinetic studies and the effect of chemical modification.

1. Horse spleen apoferritin catalyses the oxidation of Fe(2+) to Fe(3+) with molecular O(2) as electron acceptor under conditions where a number of other proteins have no such effect. The product is similar to ferritin by a number of criteria. 2. The progress curve is hyperbolic and the increase in initial velocity is linear with increasing apoferritin concentration. With respect to Fe(2+) the reaction follows Michaelis-Menten kinetics. The pH-dependence of the reaction was determined between pH4.3 and 6.0. 3. Modification of both tryptophan residues/apoferritin subunit with 2-nitrophenylsulphenyl chloride does not affect either k(cat.) or K(m) for the oxidation. Neither does the guanidination of seven out of nine lysine residues/subunit, the modification of nine out of ten arginine residues/subunit with cyclohexanedione, or the nitration of one out of five tyrosine residues/subunit with tetranitromethane. 4. The carboxymethylation of two out of three cysteine residues/subunit and of one out of six histidine residues/subunit can be achieved with iodoacetic acid. This carboxymethylated apoferritin is completely inactive in Fe(2+) oxidation. 5. Apoferritin does not take up Fe(3+). It appears from these results that Fe(2+) is the form in which iron is taken up by ferritin in a reaction where the protein acts as an enzyme which traps the product in the interior of the protein shell.

Amino Acids↗