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Biomedical subjects

G L Turner

Publications and source records attributed to G L Turner.

At least 19 recordsLinked to original sources
PubMed1992

Supply of O2 regulates O2 demand during utilization of reserves of poly-beta-hydroxybutyrate in N2-fixing soybean bacteroids.

A liquid reaction medium containing dissolved air and oxyleghaemoglobin, but no energy-yielding substrate, was supplied to bacteroids confined in a stirred flow reaction chamber. The relative oxygenation of the leghaemoglobin in the chamber was determined automatically by spectrophotometry of the effluent solution, and the concentrations of free, dissolved O2 ([O2]) and rates of O2 consumption were calculated. Dissolved CO2 and NH3 from N2 fixation were determined in fractions of the effluent solution. Bacteroids utilized endogenous reserves of poly-beta-hydroxybutyrate (PHB), which were depleted by 9.2% during a typical 5 h-long experiment. Stepwise increases in flow rate (increasing supply of O2) initially produced a drop in O2 demand and resulted in a rise in [O2] and a decline in N2 fixation. Subsequently, O2 demand rose (presumably because of increased mobilization of substrate from PHB) and [O2] declined to a low level. N2 fixation was fully restored, or even enhanced, within 15-20 min of establishment of a new, steady [O2]. This pattern of regulation by O2 supply was completely eliminated by adding low concentrations (20-50 microM) of oxidizable substrate (succinate, malate, ethanol) to the reaction medium. During endogenous activity, rates of CO2 evolution were proportional to, but less than, rates of O2 consumption up to 5.4 nmol O2 min-1 mg-1, above which CO2 evolution exceeded O2 consumption. These and other features of endogenous activity are discussed in relation to sustaining N2 fixation by nodules in vivo.

Hydroxybutyrates↗
PubMed1984

Synthesis and activity of nitrogenase in Klebsiella pneumoniae exposed to low concentrations of oxygen.

Effects of very low concentrations of dissolved O2 on nitrogenase activity in Klebsiella pneumoniae were studied in a stirred chamber system which enabled simultaneous measurements of steady-state O2 concentrations, O2 consumption and C2H2 reduction. A strain carrying a chromosomal nifH::lac fusion as well as the Nif+ plasmid pRD1, expressed nitrogenase activity with 80 nM-O2, a concentration known to inhibit nifH::lac expression by about 50% Thus nitrogenase activity in vivo was no more sensitive to O2 than expression of nifH::lac. When compared with anaerobic treatments, dissolved O2 near 30 nM apparently stimulated nitrogenase derepression and enhanced the activity of nitrogenase synthesized anaerobically. Thus, in this organism, N2 fixation occurs in microaerobic as well as anaerobic conditions.

Enzyme Repression↗
PubMed1982

Thallium-205 nuclear magnetic resonance study of the thallium(I)-gramicidin A association in trifluoroethanol.

Association of the thallous ion with gramicidin in 2,2,2-trifluoroethanol has been investigated by thallium-205 NMR spectroscopy. The data obtained suggest that the gramicidin dimer has two strong binding sites and one or more weak binding sites. Association constants for the strong binding sites were found to have the same value. From the temperature dependence of the strong binding site association constants, values for the association enthalpy and entropy of -2.13 +/- 0.12 kcal/mol and +5.45 +/- 0.04 eu, respectively, were obtained.

Binding Sites↗
PubMed1978

Activity of nitrogenase and glutamine synthetase in relation to availability of oxygen in continuous cultures of a strain of cowpea Rhizobium sp. supplied with excess ammonium.

In samples from nitrogen-fixing continuous cultures of strain CB756 of the cowpea type rhizobia (Rhizobium sp.), newly fixed NH+4 is in equiblibrium with the medium, from where it is assimilated by the glutamine synthetase/glutamate synthase pathway. In samples from steady state cultures with different degrees of oxygen-limitation, nitrogenase activity was positively correlated with the biosynthetic of glutamine synthetase in cell free extracts. Also, activities in biosynthetic assays were positively correlated with activities in gamma-glutamyl transferase assays containing 60 mM Mg2+. Relative adenylylation of glutamine synthetase was conveniently measured in cell free extracts as the ratio of gamma-glutamyl transferase activities without and with addition of 60 mM Mg2+. Automatic control of oxygen supply was used to facilitate the study of transitions between steady-state continuous cultures with high and low nitrogenase activities. Adenylylation of glutamine synthetase and repression of nitrogenase activity in the presence of excess NH+4, were masked when oxygen strongly limited culture yield. Partial relief of the limitation in cultures supplied with 10 mM NH+4 produced early decline in nitrogenase activity and increase in relative adenylylation of glutamine synthetase. Decreased oxygen supply produced a rapid decline in relative adenylylation, followed by increased nitrogenase activity, supporting the concept that control of nitrogenase synthesis is modulated by glutamine synthetase adenylylation in these bacteria.

Ammonium Sulfate↗
PubMed1976

Nitrogenase activity and respiration of cultures of Rhizobium spp. with special reference to concentrations of dissolved oxygen.

Studies of nitrogenase in cultures of the cowpea rhizobia (Rhizobium spp.) strains 32H1 and CB756 are reported. Preliminary experiments established that, even when agar cultures were grown in air, suspensions of bacteria prepared anaerobically from them were most active at low concentrations of free dissolved O2. Consequently, assays for activity used low concentrations of O2, stabilized by adding the nodule pigment leghaemoglobin. In continuous, glutamine-limited cultures of 32H1, nitrogenase activity appeared only when the concentration of dissolved O2 in the cultures approached 1 muM. Lowering the glutamine concentration in the medium supplied to the culture from 2 to 1 mM halved the cell yield and nitrogenase activity was also diminished. Omitting succinate from the medium caused the concentration of dissolved O2 to rise and nitrogenase activity was lost. Upon restoration of the succinate supply, the O2 concentration immediately fell and nitrogenase was restored. The activity doubled in about 8 h, whereas the doubling time of this culture was 14 h. Sonic extracts of 32H1 cells from continuous cultures with active nitrogenase contained components reacting with antiserum against nitrogenase Mo-Fe protein from soybean bacteroids. Continuous cultures grown at higher O2 concentration, with only a trace of active nitrogenase, contained less of these antigens and they were not detected in highly aerobic cultures. Nitrogenase activity of a continuous culture was repressed by NH+4; the apparent half-life was about 90 min. Cells of 32H1 from a continuous culture growing at between 30 and 100 muM dissolved O2 possessed a protective mechanism which permitted respiration to increase following exposure to a rapid increase in O2 concentration from low levels (O2 shock). This effect disappeared as the O2 concentration for growth was reduced towards 1 muM.

Ammonium Sulfate↗
PubMed1976

Nitrogen fixation by bacteria from the hindgut of termites.

Anaerobically grown bacteria isolated from the hindgut contents of the termites Coptotermes lacteus (Froggatt), Mastotermes darwiniensis Froggatt and Nasutitermes exitiosus (Hill) were nitrogenase-positive as assayed by acetylene reduction. Nitrogen fixation, confirmed with 15N2, was highest in the isolate from M. darwiniensis. All isolates were identified as Citrobacter freundii (Braak) Werkman & Gillen.

Anaerobiosis↗
PubMed1976

Acquisition of concepts by TMR children as a function of type of modeling, rule verbalization, and observer gender.

A four-factor repeated measures design was established in order to determine (a) whether or not TMR children could acquire a novel rule-governed concept via modeling and (b) the effects of type of modeling demonstration, verbal rule provision, and observer gender on concept matching and transfer by TMR children. Twenty-four male and 24 female TMR children (mean IQ=44 and mean MA=5.67 years) were divided into four groups each equated by stratified assignment on IQ and MA. One male and one female group were randomly selected to observe a live female model present (a) massed-demonstration trials while verbalizing the concept, (b) massed trials without the concept being verbalized, (c) distributed-demonstration trials with the conceptual verbalization, and (d) distributed trials without conceptual verbalization. Subjects were then presented with three transfer tasks, each subsequently requiring a greater degree of generalization from the demonstration task. It was found that TMR children could acquire and transfer the complex concept via modeling. Distributed demonstrations yielded better initial response matching, while massed demonstrations led to better transfer. Rule provision facilitated transfer for massed-demonstration groups and facilitated matching for the male distributed-trials groups.

Adolescent↗
PubMed1975

Involvement of oxyleghaemoglobin and cytochrome P-450 in an efficient oxidative phosphorylation pathway which supports nitrogen fixation in Rhizobium.

Cellular ATP level, ATP/ADP ratio and nitrogenase activity rise when oxyleghaemoglobin is added to respiring suspensions of Rhizobium japonicum bacteroids from soybean root nodules. Increased gaseous O2 tension is much less efficient than oxyleghaemoglobin in stimulation of bacteroid ATP production. Studies with the inhibitor carbonyl cyanide m-chlorophenylhydrazone show this ATP to be generated as a consequence of oxidative phosphorylation. N-Phenylimidazole, a specific cytochrome P-450 inhibitor, also lowers the efficiency of bacteroid oxidative phosphorylation. An approximately linear relationship is observed between ATP/ADP ratio and nitrogenase activity as N-phenylimidazole concentration is lowered. It is suggested that cytochrome P-450 is a component of the leghaemoglobin-facilitated respiration pathway and that it may act as intracellular O2 carrier rather than terminal oxidase. A less efficient oxidase appears to function when cytochrome P-450 is inhibited.

Adenosine Diphosphate↗
PubMed1975

Leghaemoglobin and the supply of O2 to nitrogen-fixing root nodule bacteroids: presence of two oxidase systems and ATP production at low free O2 concentration.

Studies of rates of consumption of dissolved O2 by suspensions of bacteroids (Rhizobium japonicum, strain CB1809) from soybean root nodules showed the presence of two different terminal oxidase systems. A high-affinity system, sensitive to inhibition by N-phenylimidazole and by carbon monoxide, was most active when the dissolved O2 was between 0-01 and 0-1 muM. At 1 muM-O2 or higher, this oxidase system had little activity and O2 was consumed largely by a low-affinity system insensitive to these inhibitors. At low concentrations of dissolved O2, bacteroid respiration rates appeared to be diffusion-limited. When purified oxyleghaemoglobin was added to such systems, this restriction was relieved and respiration was maintained to much lower concentrations of free dissolved O2, where nitrogenase activity was greatest. Analysis of reactions which were terminated at various stages during the depletion of O2 from oxyleghaemoglobin showed that at low free O2 concentration, the high-affinity pathway produced up to five times greater bacteroid ATP concentrations than the low-affinity oxidase pathway operating about 1 muM free O2 in the absence of leghaemoglobin. At intermediate free O2 concentrations, occurring during the later stages of deoxygenation of oxymyoglobin, intermediate concentrations of ATP were found in the bacteroids.

Adenosine Triphosphate↗
PubMed1973

Kinetic studies of nitrogenase from soya-bean root-nodule bacteroids.

The apparent Michaelis constants [K'(N(2)) and K'(C(2)H(2))] and the corresponding apparent maximum velocity values (V') for soya-bean bacteroid nitrogenase increased concomitantly in response to increases in nitrogenase Fe-protein concentration and ATP concentration in cell-free assays and in response to O(2) pressure in intact nodules and bacteroid suspensions. K'(C(2)H(2)) in cell-free assays was also affected by pH and by Na(2)S(2)O(4) concentration. Nitrogenase Fe-protein behaved as a catalytic effector reacting at interacting sites on the nitrogenase Fe-Mo-protein. The results indicated that the Fe-Mo-protein probably bears the catalytic sites for N(2) and C(2)H(2) reduction. It is concluded that reduction of N(2) or C(2)H(2) by this nitrogenase involves a reaction mechanism with a sequence of unknown order. The sequence in which substrate, enzyme, effector, ATP and reductant react determines which of the various rate-constants are involved in the apparent Michaelis constant, whose true kinetic meaning was thus unresolved.

Adenosine Triphosphate↗