Note on the mathematical theory of oxygen consumption at low oxygen pressures

Summary A mathematical analysis of the diffusion of oxygen into a cell is made, under the assumption, that the true rate of consumption of oxygen by protoplasm is constant and independent on the oxygen pressure. The results are compared with experimental data. For unfertilizedArbacia eggs and for bacteria the experimental curves cannot be represented by the theory. Some additional assumptions, like those proposed byGerard, seem unavoidable. For fertilizedArbacia eggs a good agreement may however be obtained, by taking for the diffusion coefficient of oxygen trough the protoplasm 7×10⁻⁷ cm²·min⁻¹, and for the permeability of the cell surface to oxygen 4.25×10⁻⁴cm·min⁻¹. With 1 Text-figure     

Production of organic acids by Corynebacterium glutamicum under oxygen deprivation

Under oxygen deprivation, aerobic Corynebacterium glutamicum produce organic acids from glucose at high yields in mineral medium even though their proliferation is arrested. To develop a new, high-productivity bioprocess based on these unique features, characteristics of organic acid production by C. glutamicum under oxygen deprivation were investigated. The main organic acids produced from glucose under these conditions were lactic acid and succinic acid. Addition of bicarbonate, which is a co-substrate for anaplerotic enzymes, increased the glucose consumption rate, leading to increased organic acid production rates. With increasing concentration of bicarbonate, the yield of succinic acid increased, whereas that of lactic acid decreased. There was a direct correlation between cell concentration and organic acid production rates even at elevated cell densities, and productivities of lactic acid and succinic acid were 42.9 g l⁻¹ h⁻¹ and 11.7 g l⁻¹ h⁻¹, respectively, at a cell concentration of 60 g dry cell l⁻¹. This cell-recycling continuous reaction demonstrated that rates of organic acid production by C. glutamicum could be maintained for at least 360 h.     

Modulation of photosynthesis and respiration in guard and mesophyll cell protoplasts by oxygen concentration

Stomatal movement is an energetic oxygen-requiring process. In the present study, the effect of oxygen concentration on mitochondrial respiratory activity and red-light-dependent photosynthetic oxygen evolution by Vicia faba and Brassica napus guard cell protoplasts was examined. Comparative measurements were made with mesophyll cell protoplasts isolated from the same species. At air saturated levels of dissolved oxygen in the protoplast suspension media, respiration rates by mesophyll protoplasts ranged from 6 to 10μmoles O₂ mg^?1 chl h^?1, while guard cell protoplasts respired at rates of 200–300 μmoles O₂ mg chl^?1 h^?1, depending on the species. Lowering the oxygen concentration below 50–60 mmol m^?3 resulted in a decrease in guard cell respiration rates, while rates by mesophyll cell protoplasts were reduced only at much lower concentrations of dissolved oxygen. Rates of photosynthesis in mesophyll cell protoplasts isolated from both species showed only a minor reduction in activity at low oxygen concentrations. In contrast, photosynthesis by guard cell protoplasts isolated from V. faba and B. napus decreased concomitantly with respiration. Oligomycin, an inhibitor of oxidative phos-phorylation, reduced photosynthesis in mesophyll cell protoplasts by 27–46% and in guard cell protoplasts by 51–58%. The reduction in both guard cell photosynthesis and respiration following exposure to low oxygen concentrations suggest close metabolic coupling between the two activities, possibly mediated by the availability of substrate for respiration associated with photosynthetic electron transport activity and subsequent export of redox equivalents.     

Role of reactive oxygen species in the response of barley to necrotrophic pathogens

Summary.  The interactions between Hordeum vulgare (barley) and two fungal necrotrophs, Rhynchosporium secalis and Pyrenophora teres (causal agents of barley leaf scald and net blotch), were investigated in a detached-leaf system. An early oxidative burst specific to epidermal cells was observed in both the susceptible and resistant responses to R. secalis, and later on, a second susceptible-specific burst was observed. Time points of the first and the second burst correlated closely with pathogen contact to the plasma membrane and subsequent cell death, respectively. HO₂ ^•/O₂ ⁻ levels in resistant and susceptible responses to P. teres were limited in comparison. During later stages, HO₂ ^•/O₂ ⁻ was only detected in 2 to 3 epidermal cells immediately adjacent to phenolic browning and cell death observed during the susceptible response. However, H₂O₂ was detected in the majority of mesophyll cells adjacent to the observed lesion caused by P. teres. In contrast to observations during challenge with R. secalis, no direct contact between P. teres and the plasma membrane at sites of reactive oxygen species production was evident. Preinfiltration of leaves with antioxidants prior to challenge with either pathogen had no effect on resistance responses but did limit the growth of the pathogens and inhibit the extent of cell death during susceptible responses. These results suggest a possible role for reactive oxygen species in the induction of cell death during the challenge of a susceptible plant cell with a necrotrophic fungal leaf pathogen. Received May 2, 2002; accepted July 26, 2002; published online May 21, 2003 RID="*" ID="*" Correspondence and reprints: Department of Plant Science, Waite Campus PMB1, University of Adelaide, Glen Osmond, South Australia 5064, Australia.     

Enhancement of oxygen absorption into sodium sulfite solutions

Oxygen absorption enhancement in a sodium sulfite solution was studied in the absence and presence of copper catalyst both for absorption across the liquid surface in a stirred cell and for absorption from individual bubbles rising through a turbulent liquid. The enhancement factor was determined from the ratio of oxygen and argon mass transfer coefficients, measured under identical experimental conditions in the same batch of liquid. It has been found that the oxygen absorption is not chemically enhanced, as long as the mass transfer coefficient, k_L⁰, is high enough, i.e., higher than the value 1.4 × 10^?4 m sec^?1 for the sulfite solution we used. An analysis of our data as well as literature data indicates that the sulfite system is poorly suited for studies of the volumetric mass transfer coefficient of physical absorption (k_L⁰a) in fermentors, inasmuch as oxygen absorption can be chemically enhanced while the degree of enhancement depends on the operating conditions of batch aeration, as well as on the concentration of trace impurities with catalytic effects upon the sulfite solution used.     

2,3-Butanediol production by Enterobacter aerogenes in continuous culture: role of oxygen supply

Summary The influence of oxygen on growth and production of 2,3-butanediol and acetoin by Enterobacter aerogenes was studied in continuous culture. At all dilution rates (D) studied cell mass increased steadily with increasing oxygen uptake rate (OUR), hence the micro-aerobic cultivation was energy-limited. The biomass yield on oxygen increased with D which suggests that cells need more energy for maintenance functions at lower D. At each D an optimal OUR giving highest volumetric productivity for the sum of butanediol and acetoin was found. The optimal OUR increased with D. The occurrence of optimal OURs results from the various effects of O₂ on growth and specific productivity. The latter was found to be a linear function of the specific OUR irrespective of D. At optimal OUR the cells proved to have equal specific OURs and equal specific productivities representing a fixed relationship between fermentative and respiratory metabolism. The product yield based on glucose and corrected for biomass formation was 80%. A product concentration as high as 43 g/l was obtained at D =0.1 h^–1 while the volumetric productivity was the highest at D =0.28 h^–1 (5.6 g/l and hour). The findings further indicate that growth and product generation are obviously non-associated phenomena. Hence, high productivities may be achievable by cell recycling and cell immobilisation systems. Offprint requests to: W.-D. Deckwer     

Effects of oxygen on Pseudomonas nautica growth on n-alkane with or without nitrate

The denitrifying marine bacterium, Pseudomonas nautica 617, can grow on lactate aerobically or anaerobically in presence of nitrate with generation times of 1.5 and 3 h respectively. The growth on heptadecane occurs only in presence of oxygen whatever its concentration with a genrration time of 8.5 h. The influence of oxygen, carbon sources (lactate or heptadecane) and nitrate was examined on O₂, NO₃ ^-, NO₂ ^- consumption, on nitrate and nitrite reductases activities, on cell yields, and on the ratio of CO₂ produced per unit of biomass. Pseudomonas nautica metabolizes hydrocarbons under denitrifying conditions in the presence of oxygen. Nitrate and nitrite are used during growth on lactate and heptadecane up to oxygen concentrations corresponding to 50 and 30% of air-saturation, respectively. When growth on n-alkane was not oxygen-limited (above 50% of air-saturation) the catabolism decreases in favour of carbon incorporation into the cell. Nitrate and nitrite reductases were strongly inhibited after 20% of airsaturation in the presence of lactate as growth substrate. With n-alkane, only the nitrate reductase activity was greatly reduced.     

Effect of oxygen on batch yogurt cultures

A yogurt culture (Streptococcus thermophilus 15HA + Lactobacillus delbrueckii subsp. bulgaricus 2-11) was studied in conditions of aerobic batch fermentation (10–40% dissolved oxygen in milk). The growth and acidification of S. thermophilus 15HA were stimulated at 20% oxygen concentration and the lactic acid process in a mixed culture was shortened by 1 h (2.5 h for the aerobic culture and 3.5 h for the anaerobic mixed culture). Streptococcus thermophilus 15HA oxygen tolerance was significantly impaired at oxygen concentrations in the milk above 30%. Though S. thermophilus 15HA was able to overcome to some extent the impact of high oxygen concentration (40%), the lactic acid produced was insufficient to coagulate the milk casein (4.0 g lactic acid l⁻¹ in the mixed culture and 3.8 g lactic acid l⁻¹ in the pure culture). A dramatic decrease in the viable cell count of L. delbrueckii subsp. bulgaricus 2-11 in the pure and mixed cultures was recorded at 30% dissolved oxygen. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enhancement of docosahexaenoic acid production by Schizochytrium sp. using a two-stage oxygen supply control strategy based on oxygen transfer coefficient

Aims: To improve the yield and productivity of docosahexaenoic acid (DHA) by Schizochytrium sp. in terms of the analysis of microbial physiology. Methods and Results: A two‐stage oxygen supply control strategy, aimed at achieving high concentration and high productivity of DHA, was proposed. At the first 40 h, K_La was controlled at 150·1 h^?1 to obtain high μ for cell growth, subsequently K_La was controlled at 88·5 h^?1 to maintain high q_p for high DHA accumulation. Finally, the maximum lipid, DHA content and DHA productivity reached 46·6, 17·7 g l^?1 and 111 mg l^?1 h^?1, which were 43·83%, 63·88% and 32·14% over the best results controlled by constant K_La. Conclusions: This paper described a two‐stage oxygen supply control strategy based on the kinetic analysis for efficient DHA fermentation by Schizochytrium sp. Significance and Impact of the study: This study showed the advantage of two‐stage control strategy in terms of microbial physiology. As K_La is a scaling‐up parameter, the idea developed in this paper could be scaled‐up to industrial process and applied to other industrial biotechnological processes to achieve both high product concentration and high productivity.     

Endoplasmic reticulum‐mediated unfolded protein response is an integral part of singlet oxygen signalling in plants

Singlet oxygen (¹O₂) is a by‐product of photosynthesis that triggers a signalling pathway leading to stress acclimation or to cell death. By analyzing gene expressions in a ¹O₂‐overproducing Arabidopsis mutant (ch1) under different light regimes, we show here that the ¹O₂ signalling pathway involves the endoplasmic reticulum (ER)‐mediated unfolded protein response (UPR). ch1 plants in low light exhibited a moderate activation of UPR genes, in particular bZIP60, and low concentrations of the UPR‐inducer tunicamycin enhanced tolerance to photooxidative stress, together suggesting a role for UPR in plant acclimation to low ¹O₂ levels. Exposure of ch1 to high light stress ultimately leading to cell death resulted in a marked upregulation of the two UPR branches (bZIP60/IRE1 and bZIP28/bZIP17). Accordingly, mutational suppression of bZIP60 and bZIP28 increased plant phototolerance, and a strong UPR activation by high tunicamycin concentrations promoted high light‐induced cell death. Conversely, light acclimation of ch1 to ¹O₂ stress put a limitation in the high light‐induced expression of UPR genes, except for the gene encoding the BIP3 chaperone, which was selectively upregulated. BIP3 deletion enhanced Arabidopsis photosensitivity while plants treated with a chemical chaperone exhibited enhanced phototolerance. In conclusion, ¹O₂ induces the ER‐mediated UPR response that fulfils a dual role in high light stress: a moderate UPR, with selective induction of BIP3, is part of the acclimatory response to ¹O₂, and a strong activation of the whole UPR is associated with cell death.     

Quantification of oxygen production and respiration rates in mixotrophic cultivation of microalgae in nonstirred photobioreactors

Online monitoring and controlling of different cellular parameters are key issues in aerobic bioprocesses. Since mixotrophic cultivation, in which we observe a mixture of cellular respiration and oxygen production has gained more popularity, there is a need for an on‐process quantification of these parameters. The presented and adapted double gassing‐out method applied to a mixotrophic cultivation of Galdieria sulphuraria , will be a tool for monitoring and further optimization of algal fermentation in nonstirred photobioreactors (PBR). We measured the highest net specific oxygen production rate (opr _net) as 5.73 · 10^?3 mol_O2 g^?1 h^?1 at the lowest oxygen uptake rate (OUR) of 1.00 · 10^?4 mol_O2 L^?1 h^?1. Due to higher cell densities, we also demonstrated the increasing shading effect by a decrease of opr _net, reaching the lowest value of 1.25 10^?5 mol_O2 g^?1 h^?1. Nevertheless, with this on process measurement, we can predict the relation between the zone in which oxygen is net produced to the area where cell respiration dominates in a PBR, which has a major impact to optimize cell growth along with the formation of different products of interest such as pigments.     

Effect of external oxygen demand on radial oxygen loss by Juncus roots in titanium citrate solutions

Radial oxygen loss (ROL) from the roots of two semiaquatic rushes, Juncus effusus L. and Juncus inflexus L., was studied in reducing titanium citrate buffer, using both closed incubations and a flow-through, titrimetric system. In closed experiments, roots released oxygen at a constant rate over a wide range of external oxygen demands, with the ROL rate only depending on sink strength at low demands, and no oxygen release into oxidized solutions. In the titrimetric experiments, roots continued to release oxygen at constant rates when provided with a constant external oxygen demand. ROL was higher in J. effusus (9·5 ± 1 × 10^?7 mol O₂ h^?1 root^?1) than in J. inflexus (4·5 ± 0·5 × 10^?7 mol O₂ h^?1 root^?1). Light and dark changes around the shoots did not affect the ROL rate in J. inflexus, whereas in J. effusus ROL was ≈ 1·75 times higher in the light than in the dark, presumably due to changes in stomatal aperture. These results suggest that ROL is controlled by the external oxygen demand at low to moderate reducing intensities, but that structural limitations to oxygen diffusion rates prevent ROL from continuing to increase at higher external oxygen demands.     

A critical role of the transient receptor potential melastatin 2 channel in a positive feedback mechanism for reactive oxygen species-induced delayed cell death

Transient receptor potential melastatin 2 (TRPM2) channel activation by reactive oxygen species (ROS) plays a critical role in delayed neuronal cell death, responsible for postischemia brain damage via altering intracellular Zn²⁺ homeostasis, but a mechanistic understanding is still lacking. Here, we showed that H₂O₂ induced neuroblastoma SH-SY5Y cell death with a significant delay, dependently of the TRPM2 channel and increased [Zn²⁺]_i, and therefore used this cell model to investigate the mechanisms underlying ROS-induced TRPM2-mediated delayed cell death. H₂O₂ increased concentration-dependently the [Zn²⁺]_i and caused lysosomal dysfunction and Zn²⁺ loss and, furthermore, mitochondrial Zn²⁺ accumulation, fragmentation, and ROS generation. Such effects were suppressed by preventing poly(adenosine diphosphate ribose, ADPR) polymerase-1-dependent TRPM2 channel activation with PJ34 and 3,3′,5,5′-tetra-tert-butyldiphenoquinone, inhibiting the TRPM2 channel with 2-aminoethoxydiphenyl borate (2-APB) and N-(p-amylcinnamoyl)anthranilic acid, or chelating Zn²⁺ with N,N,N,N-tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN). Bafilomycin-induced lysosomal dysfunction also resulted in mitochondrial Zn²⁺ accumulation, fragmentation, and ROS generation that were inhibited by PJ34 or 2-APB, suggesting that these mitochondrial events are TRPM2 dependent and sequela of lysosomal dysfunction. Mitochondrial TRPM2 expression was detected and exposure to ADPR-induced Zn²⁺ uptake in isolated mitochondria, which was prevented by TPEN. H₂O₂-induced delayed cell death was inhibited by apocynin and diphenyleneiodonium, nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) oxidase (NOX) inhibitors, GKT137831, an NOX1/4-specific inhibitor, or Gö6983, a protein kinase C (PKC) inhibitor. Moreover, inhibition of PKC/NOX prevented H₂O₂-induced ROS generation, lysosomal dysfunction and Zn²⁺ release, and mitochondrial Zn²⁺ accumulation, fragmentation and ROS generation. Collectively, these results support a critical role for the TRPM2 channel in coupling PKC/NOX-mediated ROS generation, lysosomal Zn²⁺ release, and mitochondrial Zn²⁺ accumulation, and ROS generation to form a vicious positive feedback signaling mechanism for ROS-induced delayed cell death.     

Protein synthesis and oxygen consumption in fish cells

Oxygen consumption and protein synthesis were measured concurently in four fish cell types: BF-2 and TRG-2 cell lines, rainbow trout macrophages and scale cells. The fractional rates of protein synthesis (percentage of the protein mass synthesised per day) were ranked: BF-2 cells > macrophages > RTG-2 cells > scale cells. Oxygen consumption rates were ranked BF-2 cells = macrophages = RTG-2 cells > scale cells. Within three of the cell types (BF-2, RTG-2 and scale cells) oxygen consumption and protein synthesis were linearly correlated, whereas comparison between the four cell types gave rise to an exponential relationship between fractional rates of protein synthesis and oxygen consumption. Inhibition of protein synthesis with cycloheximide by 41–65% resulted in a 62–89% reduction in oxygen consumption depending on cell type. Calculations of the aerobic cost of protein synthesis using the cycloheximide-sensitive protein synthesis and oxygen consumption rates resulted in estimates ranging from 11 to 217 mol O₂·mg protein^-1 synthesised depending on the cell type. The lowest net protein synthesis costs, which are close to theoretical values for peptide bond formation, were associated with the most rapid rates of protein synthesis.Abbreviations ANOV A analysis of variance - BDH British Drug Housing - BOC British Oxygen Company - ADT A ethylenediaminetetra-acetic acid - FCS foetal calf serum - HEPES N-[2-hydroxyethyl] piperazine-N-[2-ethane-sulphonic acid] - LCD least square differnece - MEM munimum essential media - PBS phosphate-buffered saline - PCA perchloric acid - SIS spectral index of the sample - tSIE transformed spectral index of the external standard spectrum     

Induction of astaxanthin formation by reactive oxygen species in mixotrophic culture of Chlorococcum sp.

The induction of astaxanthin formation by reactive oxygen species in mixotrophic culture of Chlorococcum sp. was investigated. H₂O₂ (0.1 mM) enhanced the total astaxanthin formation from 5.8 to 6.5 mg g^–1 cell dry wt. Fe²⁺ (0.5 mM) added to the medium with H₂O₂ (0.1 mM) further promoted astaxanthin formation to 7.1 mg g^–1 cell dry wt. Similarly, Fe²⁺ (0.5 mM) together with methyl viologen (0.01 mM) promoted astaxanthin formation to 6.3 mg g^–1 cell dry wt. In contrast, an addition of KI (1 mM), a specific scavenger for hydroxyl radicals (OH), together with H₂O₂ (0.1 mM) and Fe²⁺ (0.5 mM), to the medium decreased astaxanthin formation to 1.8 mg g^–1 cell dry wt. KI (1 mM) also inhibited the enhancement of carotenogenesis by superoxide anion radicals (O₂ ^–), with a decrease of astaxanthin formation to 1.7 mg g^–1 cell dry wt. This suggested that O₂ ^– might be transformed to OH before promoting carotenogenesis in Chlorococcum sp.     

Lactobacillus sanfranciscensis CB1: manganese, oxygen, superoxide dismutase and metabolism

The latency phase, growth rate, cell yield and end-products of Lactobacillus sanfranciscensis CB1 were affected by oxygen and the supply of 225 μM Mn²⁺. Mn²⁺ was especially related to the highest substrate consumption. Aerobiosis and Mn²⁺ supply were responsible for the highest superoxide dismutase activity. An auto-inhibitory accumulation of H₂O₂ meant that the survival of air-grown cells supplied with Mn²⁺ rapidly decreased during the stationary phase. As shown by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, Mn²⁺ supply influenced protein expression. As shown by non-denaturating zymograms, Lb. sanfranciscensis CB1 expressed an approximately 12.5-kDa superoxide dismutase, which is probably Mn-dependent. The enzyme was insensitive to H₂O₂ treatment, was not induced by the presence of paraquat under aerobic conditions and was relatively stable at pH 4.0. Sourdoughs that contained high levels of oxygen enhanced cell growth, acidification and acetic acid production by Lb. sanfranciscensis CB1. Received: 24 July 1998 / Received last revision: 11 November 1998 / Accepted: 13 November 1998     

1Alpha,25-dihydroxyvitamin D3 modulation of adipocyte reactive oxygen species production

Objective: We have previously shown 1α,25‐dihydroxyvitamin D₃ [1α,25‐(OH)₂D₃] to inhibit mitochondrial uncoupling protein 2 (UCP2) expression in adipocytes and that in vivo suppression of calcitriol levels with calcium‐rich diets increases UCP2 expression. Because UCP2 plays a significant role in the clearance of reactive oxygen species (ROS), we studied the effect of calcitriol on ROS production and ROS‐induced adipocyte proliferation. Research Methods and Procedures: ROS production in human and murine adipocytes was stimulated by high glucose (30 mM) or H₂O₂ (100 nM). Results: Both approaches resulted in increased ROS production by 27% to 100% (p < 0.05) and increased cell proliferation by 15% to 39% (p < 0.03). These effects were augmented by the addition of mitochondrial uncoupling inhibitor guanosine 5′‐diphosphate (GDP; 100 μM) or 1α,25‐(OH)₂D₃ (10 nM) and attenuated by UCP2 overexpression, suggesting that inhibition of mitochondrial uncoupling suppresses clearance of ROS and increases adipocyte proliferation. The addition of α ± tocopherol (1 μM) inhibited cell proliferation in adipocytes treated with either H₂O₂ or high glucose, indicating that ROS plays a major role in the regulation of cell proliferation in adipocytes. Moreover, stimulation of ROS with high glucose and H₂O₂ resulted in a 2‐ to 5‐fold increase in adipocyte intracellular calcium ([Ca²⁺]i; p < 0.001), and calcium channel antagonism (nifedipine, 10 μM) suppressed ROS induced calcium influx and cell proliferation, indicating that [Ca²⁺]i may also regulate ROS production and exert a mitogenic effect in adipocytes. Discussion: These data support a role of 1α,25‐(OH)₂D₃, UCP2, and [Ca²⁺]i in the regulation of adipocyte ROS production.     

Energetics of Acidianus ambivalens growth in response to oxygen availability

All life requires energy to drive metabolic reactions such as growth and cell maintenance; therefore, fluctuations in energy availability can alter microbial activity. There is a gap in our knowledge concerning how energy availability affects the growth of extreme chemolithoautotrophs. Toward this end, we investigated the growth of thermoacidophile Acidianus ambivalens during sulfur oxidation under aerobic to microaerophilic conditions. Calorimetry was used to measure enthalpy (ΔH_inc) of microbial activity, and chemical changes in growth media were measured to calculate Gibbs energy change (ΔG_inc) during incubation. In all experiments, Gibbs energy was primarily dissipated through the release of heat, which suggests enthalpy‐driven growth. In microaerophilic conditions, growth was significantly more efficient in terms of biomass yield (defined as C‐mol biomass per mole sulfur consumed) and resulted in lower ΔG_inc and ΔH_inc. ΔG_inc in oxygen‐limited (OL) and oxygen‐ and CO₂‐limited (OCL) microaerophilic growth conditions resulted in averages of ?1.44 × 10³ kJ/C‐mol and ?7.56 × 10² kJ/C‐mol, respectively, and average ΔH_inc values of ?1.11 × 10⁵ kJ/C‐mol and ?4.43 × 10⁴ kJ/C‐mol, respectively. High‐oxygen experiments resulted in lower biomass yield values, an increase in ΔG_inc to ?1.71 × 10⁴ kJ/C‐mol, and more exothermic ΔH_inc values of ?4.71 × 10⁵ kJ/C‐mol. The observed inefficiency in high‐oxygen conditions may suggest larger maintenance energy demands due to oxidative stresses and a preference for growth in microaerophilic environments.     

Regulation of glutamine and pyruvate oxidation in cultured adrenocortical cells by cortisol,antioxidants, and oxygen: Effects on cell proliferation

The regulation of CO₂ production from [U-¹⁴C]glutamine and C₂ of [2-¹⁴C]pyruvate was investigated in cultured bovine adrenocortical cells, and the effect of alterations in the relative rates of oxidation of these substrates on cell proliferation, particularly in the presence of an inhibitor of transamination reactions, was examined. ¹⁴CO₂ production from 2 mM [U-¹⁴C]glutamine and 2 mM [2-¹⁴C]pyruvate was measured in the presence of 100 μM 2,4-dinitrophenol, an uncoupler of oxidative phosphorylation. Treatment of primary cultures for 24 h with 50 μM cortisol increased the oxidation of [¹⁴C]glutamine relative to that of [¹⁴C]pyruvate, an effect dependent on prior low cell density. Cortisol treatment also resulted in a prolonged delay in the onset of proliferation from low density, and completely inhibited growth in the presence of 2 mM aminooxyacetate, which reduces mitochondrial utilization of glutamine. The effects on glutamine and pyruvate metabolism and on cell growth, with or without aminooxyacetate, were prevented by simultaneous treatment with the antioxidants dimethyl sulfoxide (10 mM) and butylated hydroxyanisole (100 μM), suggesting the involvement of lipid peroxidation in the action of cortisol, as previously demonstrated for its action on 11β-hydroxylase. During continued proliferation of adrenocortical cells in the absence of cortisol there was also a slower increase in the oxidation of [¹⁴C]glutamine relative to that of [¹⁴C]pyruvate as a function of population doubling level. The rate of this increase was slowed by growth of cells in 2% O₂ rather than the standard 19% O₂, and accelerated by continued growth of cells in the presence of cortisol. The rate of increase in the oxidation of [¹⁴C]glutamine relative to that of [¹⁴C]pyruvate under these three conditions correlated with inhibition of cell growth by aminooxyacetate. In contrast to the complete inhibition of growth in aminooxyacetate demonstrated by cortisol-treated cells, control cells (19% O₂) did proliferate, although growth was limited, whereas cells at 2% O₂ proliferated to a much greater extent. In the absence of aminooxyacetate the rate of growth in primary adrenocortical cell cultures under these three conditions was similar. Lipid peroxidation appears to make cultured adrenocortical cells dependent on glutamine for mitochondrial function and proliferation by inhibiting the utilization of the normal substrate, pyruvate.     

Photosynthetic energy storage efficiency,oxygen evolution and chloroplast movement

When there is a saturating supply of dissolved carbon available, photosynthetic energy storage efficiency (ES) varies linearly with light fluence rate (I) for both Vallisneria americana and Pisum sativum leaves. The frequently reported hyperbolic relationship between ES and I occurs only when low levels of dissolved carbon are present in the medium. The linear relationship has its origin in intracellular events and implies that two heat-producing processes limit the value of ES. The rate of one process varies as I and the other varies as I². The rates of both processes were changed after a 2 hour exposure to 400 μmol photons m⁻² s⁻¹ of red light, speeding up the process that depends linearly on I and slowing the other. Illumination for 1 hour with 100 μmol photons m⁻² s⁻¹ of blue (but not red) light moves many chloroplasts from the periclinal to the anticlinal cell walls [Inoue and Shibata (1973) Planta 114: 341–358]. Blue light exposure of V. americana leaf sections (a) reduced the rate of oxygen evolution under light-limiting conditions by about 22%; (b) increased the value of ES by an amount dependent on the light fluence rate; and (c) decreased the slope of (ES v I). The slope change indicated that light absorption had fallen by 26% after blue light exposure. The rate of oxygen evolution (V) was measured under light-limiting conditions with leaf sections in which the chloroplasts had been immobilised after blue or red light exposure. With both red and blue-exposed leaf sections, V fell by about 50% after exposure to 1 hour of 1250 μmol photons m⁻² s⁻¹ of white light. Thus accumulation of chloroplasts on anticlinal walls did not protect the leaf from photoinactivation by a high light fluence rate. This revised version was published online in August 2006 with corrections to the Cover Date.