Rate-limiting step in electron transport. Osmotically sensitive diffusion of quinones through voids in the bilayer

A Note on the Consequences of a Hot Mitochondrion: Some Recent Developments and Open Questions

Background: Chrétien and co-workers (PLOS Biology. 2018;16(1):e2003992) recently suggested that the mitochondrion might possibly be hotter than its surrounding (by as much as 10°C). Objectives: To examine the validity of this claim and review the possible implications and repercussion of such a claim – if true – on some aspects of mitochondrial biochemistry and biophysics. Results: Both the chemical gradient and the electrical gradient Gibbs energy terms in the central equation of chemiosmotic theory are temperature dependent, the first explicitly and the second implicitly. A hotter mitochondrion – as claimed – would imply a 3% correction in the chemical gradient term, but we cannot estimate the corresponding effect on the electrical term at this time since the functional dependence of the voltage on the temperature is not known to the best of the authors’ knowledge. Further, if this claim is true and to the extent claimed (10°C), this may imply some heat-engine character for mitochondrial thermodynamic operation albeit this may only represent 4% at most. Conclusions: Doubts and criticisms regarding the suggestion of a hotter mitochondrion have been raised and are briefly discussed. These doubts are contrasted with some data and considerations that support the claim of a hotter mitochondrion. It is concluded that the mitochondrion is probably hotter than its environment but not to the extent claimed by Chrétien et al. and that the thermodynamic efficiency and the mode of operation of the mitochondrion as an electrochemical battery are very slightly perturbed by even the maximum claimed revision of the temperature of its operation.

Analysis of 2009⁻2012 Nutrition Health and Examination Survey (NHANES) Data to Estimate the Median Water Intake Associated with Meeting Hydration Criteria for Individuals Aged 12⁻80 in the US Population

In 2005, US water intake recommendations were based on analyses of Nutrition Health and Examination Surveys (NHANES) III data that examined if hydration classification varied by water intake and estimated the median water intake associated with hydration in persons aged 19–30. Given the upcoming 2020–2025 Dietary Guidelines review, this analysis addressed the same two aims with 2009–2012 NHANES data. Methods were updated by defining hydration criteria in terms of multiple measures (serum sodium 135–144 mmol/L and urine osmolality < 500 mmol/kg), expressing water intake as ml/kg, distinguishing plain water intake (PWI) from total water intake (TWI), using weighted age- and sex-specific multivariable models to control for determinants of water intake requirements, and selecting two study samples (the non-acutely ill US population and a sub-group without selected chronic disease risk factors). In the US population and sub-group, the relative risk (RR) of meeting the hydration criteria was significantly greater for individuals with TWI ≥ 45 mL/kg or PWI ≥ 20 mL/kg (for the US population 19–50 years of age: adjusted RR = 1.36, 95% CI: 1.10–1.68 for males; adjusted RR = 1.70, 95% CI: 1.49–1.95 for females. For the sub-group 51–70 years of age: adjusted RR = 2.20, 95% CI: 1.15–4.18 for males; adjusted RR = 2.00, 95% CI: 1.18–3.40 for females). The median (SE) TWI and PWI associated with meeting the hydration criteria for males and females 19–50 years of age were 42 (2) mL/kg and 14 (1) mL/kg and 43 (2) mL/kg and 16 (1) mL/kg, respectively. The significant association between water intake and hydration classification differs from the null association underlying the 2005 water intake recommendations and may lead to different reasoning and inferences for the 2020–2025 Dietary Guidelines.

Observability analysis of biochemical process models as a valuable tool for the development of mechanistic soft sensors

By enabling the estimation of difficult-to-measure target variables using available indirect measurements, mechanistic soft sensors have become important tools for various bioprocess monitoring and control scenarios. Despite promising higher process efficiencies and increased process understanding, widespread application of soft sensors has been stalled by uncertainty about the feasibility and reliability of their estimations given present process analytical constraints. Observability analysis can provide an indication of the possibility and reliability of soft sensor estimations by analyzing the structural properties of first-principle (mechanistic) models. In addition, it can provide a criteria for selection of suitable measurement methods with respect to their information content; thereby leading to successful implementation of soft sensors in bioprocess development and manufacturing environments. We demonstrate the utility of observability analysis for two classes of upstream bioprocesses: the processes involving growth and ethanol formation by Saccharomyces cerevisiae and the process of penicillin production by Penicillium chrysogenum. Results obtained from laboratory-scale cultivations in addition to in-silico experiments enable a comparison of theoretical aspects of observability analysis and the real-life performance of soft sensors. By taking the expected error of measurements provided to the soft sensor into account, an innovative scaling approach facilitates a higher degree of comparability of observability results among various measurement configurations and process conditions.

Respiration hastens maturation and lowers yield in rice

Role of respiration in plant growth remains an enigma. Growth of meristematic cells, which are not photosynthetic, is entirely driven by endogenous respiration. Does respiration determine growth and size or does it merely burn off the carbon depleting the biomass? We show here that respiration of the germinating rice seed, which is contributed largely by the meristematic cells of the embryo, quantitatively correlates with the dynamics of much of plant growth, starting with the time for germination to the time for flowering and yield. Seed respiration appears to define the quantitative phenotype that contributes to yield via growth dynamics that could be discerned even in commercial varieties, which are biased towards higher yield, despite considerable susceptibility of the dynamics to environmental perturbations. Intrinsic variation, irreducible despite stringent growth conditions, required independent validation of relevant physiological variables both by critical sampling design and by constructing dendrograms for the interrelationships between variables that yield high consensus. More importantly, seed respiration, by mediating the generation clock time via variable time for maturation as seen in rice, directly offers the plausible basis for the phenotypic variation, a major ecological stratagem in a variable environment with uncertain water availability. Faster respiring rice plants appear to complete growth dynamics sooner, mature faster, resulting in a smaller plant with lower yield. Counter to the common allometric views, respiration appears to determine size in the rice plant, and offers a valid physiological means, within the limits of intrinsic variation, to help parental selection in breeding.

The alternative oxidase mediated respiration contributes to growth, resistance to hyperosmotic media and accumulation of secondary metabolites in three species

Plant respiration, similar to respiration in animal mitochondria, exhibits both osmosensitive and insensitive components with the clear distinction that the insensitive respiration in plants is quantitatively better described as 'less' sensitive rather than 'insensitive'. Salicylic hydroxamic acid (SHAM)-sensitive respiration was compared with the respiration sensitive to other inhibitors in rice, yeast and Dunaliella salina. The influence of SHAM was largely in the osmotically less sensitive component and enhanced with external osmotic pressure unlike other inhibitors that inhibited the osmotically sensitive component. SHAM inhibited germination and root growth but not shoot growth. Osmotic remediation of respiration that developed in due course of time with rice seedlings was abolished by SHAM and was not due to water and ionic uptake mechanisms. Yeast and Dunaliella also showed susceptibility of growth and respiration to SHAM. Glycerol retention was influenced by all inhibitors, while growth was inhibited demonstrably by SHAM in Dunaliella. Respiration in plants needs to be seen as a positive contribution to overall growth and not merely for burning away of the biomass.

Mitochondrial lipid abnormality and electron transport chain impairment in mice lacking alpha-synuclein

The presynaptic protein alpha-synuclein, implicated in Parkinson disease (PD), binds phospholipids and has a role in brain fatty acid (FA) metabolism. In mice lacking alpha-synuclein (Snca-/-), total brain steady-state mass of the mitochondria-specific phospholipid, cardiolipin, is reduced 22% and its acyl side chains show a 51% increase in saturated FAs and a 25% reduction in essential n-6, but not n-3, polyunsaturated FAs. Additionally, 23% reduction in phosphatidylglycerol content, the immediate biosynthetic precursor of cardiolipin, was observed without alterations in the content of other brain phospholipids. Consistent with these changes, more ordered lipid head group and acyl chain packing with enhanced rotational motion of diphenylhexatriene (DPH) about its long axis were demonstrated in time-resolved DPH fluorescence lifetime experiments. These abnormalities in mitochondrial membrane properties were associated with a 15% reduction in linked complex I/III activity of the electron transport chain, without reductions in mitochondrial number, complex II/III activity, or individual complex I, II, III, or IV activity. Reduced complex I activity is thought to be a critical factor in the development of PD. Thus, altered membrane composition and structure and impaired complex I/III function in Snca-/- brain suggest a relationship between alpha-synuclein's role in brain lipid metabolism, mitochondrial function, and PD.

Mitochondrial reactive oxygen species contribute to high NaCl-induced activation of the transcription factor TonEBP/OREBP

Hypertonicity activates the transcription factor tonicity-responsive enhancer/osmotic response element binding protein (TonEBP/OREBP), resulting in increased expression of genes involved in osmoprotective accumulation of organic osmolytes, including glycine betaine, and in increased expression of osmoprotective heat shock proteins. Our previous studies showed that high NaCl increases reactive oxygen species (ROS), which contribute to activation of TonEBP/OREBP. Mitochondria are a major source of ROS. The purpose of the present study was to examine whether mitochondria produce the ROS that contribute to activation of TonEBP/OREBP. We inhibited mitochondrial ROS production in HEK293 cells with rotenone and myxothiazol, which inhibit mitochondrial complexes I and III, respectively. Rotenone (250 nM) and myxothiazol (12 nM) reduce high NaCl-induced ROS over 40%, whereas apocynin (100 microM), an inhibitor of NADPH oxidase, and allopurinol (100 microM), an inhibitor of xanthine oxidase, have no significant effect. Rotenone and myxothiazol reduce high NaCl-induced increases in TonEBP/OREBP transcriptional activity (ORE/TonE reporter assay) and BGT1 (betaine transporter) mRNA abundance ranging from 53 to 69%. They inhibit high NaCl-induced TonEBP/OREBP transactivating activity, but not its nuclear translocation. Release of ATP into the medium on hypertonic stress has been proposed to be a signal that triggers cellular osmotic responses. However, we do not detect release of ATP into the medium or inhibition of high NaCl-induced ORE/TonE reporter activity by an ATPase, apyrase (20 U/ml), indicating that high NaCl-induced activation of TonEBP/OREBP is not mediated by release of ATP. We conclude that high NaCl increases mitochondrial ROS production, which contributes to the activation of TonEBP/OREBP by increasing its transactivating activity.

Impact of cholesterol on voids in phospholipid membranes

Free volume pockets or voids are important to many biological processes in cell membranes. Free volume fluctuations are a prerequisite for diffusion of lipids and other macromolecules in lipid bilayers. Permeation of small solutes across a membrane, as well as diffusion of solutes in the membrane interior are further examples of phenomena where voids and their properties play a central role. Cholesterol has been suggested to change the structure and function of membranes by altering their free volume properties. We study the effect of cholesterol on the properties of voids in dipalmitoylphosphatidylcholine (DPPC) bilayers by means of atomistic molecular dynamics simulations. We find that an increasing cholesterol concentration reduces the total amount of free volume in a bilayer. The effect of cholesterol on individual voids is most prominent in the region where the steroid ring structures of cholesterol molecules are located. Here a growing cholesterol content reduces the number of voids, completely removing voids of the size of a cholesterol molecule. The voids also become more elongated. The broad orientational distribution of voids observed in pure DPPC is, with a 30% molar concentration of cholesterol, replaced by a distribution where orientation along the bilayer normal is favored. Our results suggest that instead of being uniformly distributed to the whole bilayer, these effects are localized to the close vicinity of cholesterol molecules.

How did cells get their size?

Cells exercise size homeostasis, and the origins of their ability to do so is the topic of this essay. Before there were cells, there were protocells. The most basic questions about protocells as objects are: What were they made of, and how big were they? Asking how big they were implies that the answer to the first part includes a boundary. The best candidate for that boundary is a self-assembling lipid bilayer. Therefore, protocells are defined here as Darwinian liposomes-bilayer vesicles with mutable on-board replicases linked to phenotypes. Because liposomes undergo spontaneous fission and fusion, and are subject to osmotic forces, size regulation in the earliest protocells would essentially have been liposome physics. For successful protocells, averting osmotic lysis would have been the first order of business. However, from the outset size mattered too, because of sex and reproduction (i.e., genome mixing and genome copying in entities with phenotypes). Protocell fission and fusion would have blended seamlessly into protocell sex and reproduction, making any gene product that furnished control over protocell size changes doubly adaptive. A recurrent theme is the feedback role of bilayer tension in protocell size control. Ways in which primitive peptides and their aggregates (e.g., channels) might have allowed liposomes to gain improved volume and surface area homeostasis are suggested. Domain-swapped proteins that polymerize as filaments are discussed as the origin of cytoskeleton structures that diversify and stabilize liposome shapes and sizes. Throughout, attention is paid to the question of set points for cell size.

Mitochondrial dysfunction is an early event in high-NaCl-induced apoptosis of mIMCD3 cells

Raising osmolality to 700 mosmol/kgH(2)O by the addition of NaCl rapidly kills most murine inner renal medullary collecting duct cells (mIMCD3), but they survive at 500 mosmol/kgH(2)O. At 300 and 500 mosmol/kgH(2)O, NADH autofluorescence is present in a mitochondria-associated, punctate perinuclear pattern. Within 45 s to 30 min at 700 mosmol/kgH(2)O, the autofluorescence spreads diffusely throughout the cell. This correlates with mitochondrial membrane depolarization, measured as decreased tetramethylrhodamine methyl ester perchlorate (TMRM) fluorescence. Mitochondrial dysfunction should increase the cellular ADP/ATP ratio. In agreement, this ratio increases within 1-6 h. Mitochondrial morphology (transmission electron microscopy) is unaffected, but nuclear hypercondensation becomes evident. Progressive apoptosis occurs beginning 1 h after osmolality is raised to 700, but not to 500, mosmol/kgH(2)O. General caspase activity and caspase-9 activity increase only after 6 h at 700 mosmol/kgH(2)O. The mitochondrial Bcl-2/Bax ratio decreases within 1-3 h, but no cytochrome c release is evident. The mitochondria contain little p53 at any osmolality. Adding urea to 700 mosmol/kgH(2)O does not change NADH or TMRM fluorescence. We conclude that extreme acute hypertonicity causes a mitochondrial dysfunction involved in the initiation of apoptosis.

Large solutes induce structural perturbations in proteins and membranes

Structural perturbations in biopolymers with hydrophobic interiors i.e. specific proteins and dimyristoylphosphatidylcholine (DMPC) vesicles were investigated as a function of solute concentrations in the medium. 1,6-diphenyl-1,3,5-hexatriene (DPH) was used as fluorescent probe. Response of DPH was comparable to that of intrinsic tryptophan in BSA in terms of steady state and time resolved fluorescence. The solutes induced a decrease in steady state anisotropy as well as rotational correlation time (computed from lifetime measurements) for DPH in both proteins and membranes. Enhanced access of the quencher potassium iodide to tryptophan in bovine serum albumin (BSA) and ovalbumin, and enhanced terbium leakage in DMPC vesicles induced by various solutes concomitant with decreased anisotropy/correlation time were consistent with structural perturbations of the nature of defects or voids in these polymers.

Solvent interconnectedness permits measurement of proximal as well as distant phase transitions in polymer mixtures by fluorescence

We monitored the fluorescence intensity and anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated in bovine serum albumin (BSA) and dimyristoylphosphatidylcholine (DMPC) vesicle membranes, which in turn were embedded in optically clear gelatin solutions, as a function of temperature. DPH in BSA gave unanticipated large changes in fluorescence intensity and anisotropy at the instant of gelatin gel melting. Both steady state anisotropy and fluorescence intensity reported the gel-sol transition point in gelatin unambiguously, which was independently confirmed as physical-pour point of the gel. In the case of DMPC vesicles, fluorescence intensity indicated the gelatin transition, while the anisotropy indicated DMPC phase transition. This fluorescence methodology uniquely offered a common probe for two distinct transitions in two distinct domains interconnected by the solvent, water.

What does a common channel for electrolytes and non-electrolytes in the sperm mean?

Cell volume is central to osmoregulation in intact cells. Bovine spermatozoa, as also other mammalian spermatozoa, exhibit a very rapid regulatory volume decrease (RVD) when exposed to hypotonic saline media. This response, fastest known in animal cells, is mediated by a putative potassium channel which the pharmacological properties of a conventional channel and yet admits both electrolytes and non-electrolytes. The evolutionary basis and functional role of this conserved quinine-sensitive channel in mammalian sperm could offer hitherto unexplored facets of the link(s) between ecology and reproduction.

The positive role of voids in the plasma membrane in growth and energetics of Escherichia coli

Bacterial respiration, endogenous as well as induced respiration by glucose, lactose and glycine betaine, was found to be sensitive to external solute concentration. Permeability of hydrogen peroxide, a non-electrolyte of molecular size between water and urea, through the bacterial membranes changed directly with the rate of respiration (an activity residing in the bacterial plasma membrane) in E. coli and the enhanced permeability and respiratory activity were highly correlated. Hydrogen peroxide permeability and induction of voids (spaces in the matrix of the bilayer into which hydrophobic fluorescent probes partition, which in turn were used to assess the modulation of these cavities) were shown to be a direct and excellent measure of leak conductance. Fluorescence intensity and anisotropy of the extrinsic fluorescent probes (incorporated by growing bacteria in their presence) decreased with increased respiration in bacteria, consistent with lowered molecular restriction and enhanced hydration in the membrane phase for these probes as seen in dimyristoylphosphatidylcholine bilayers due to phase transition. The physical basis of osmotic phenomena, as a relevant (thermodynamic) volume, could relate to water exchange or compression, depending on the osmotic domain. In the domain of compression in bacteria, i.e. well above the isotonic range, the computed activation volume was consistent with voids in the membrane. This study emphasises a major role of leak conductance in bacterial physiology and growth.

Role of the mitochondrial Hsp70s, Ssc1 and Ssq1, in the maturation of Yfh1

The mitochondrial matrix of the yeast Saccharomyces cerevisiae contains two molecular chaperones of the Hsp70 class, Ssc1 and Ssq1. We report that Ssc1 and Ssq1 play sequential roles in the import and maturation of the yeast frataxin homologue (Yfh1). In vitro, radiolabeled Yfh1 was not imported into ssc1-3 mutant mitochondria, remaining in a protease-sensitive precursor form. As reported earlier, the Yfh1 intermediate form was only slowly processed to the mature form in Deltassq1 mitochondria (S. A. B. Knight, N. B. V. Sepuri, D. Pain, and A. Dancis, J. Biol. Chem. 273:18389-18393, 1998). However, the intermediate form in both wild-type and Deltassq1 mitochondria was entirely within the inner membrane, as it was resistant to digestion with protease after disruption of the outer membrane. Therefore, we conclude that Ssc1, which is present in mitochondria in approximately a 1,000-fold excess over Ssq1, is required for Yfh1 import into the matrix, while Ssq1 is necessary for the efficient processing of the intermediate to the mature form in isolated mitochondria. However, the steady-state level of mature Yfh1 in Deltassq1 mitochondria is approximately 75% of that found in wild-type mitochondria, indicating that this retardation in processing does not dramatically affect cellular concentrations. Therefore, Ssq1 likely has roles in addition to facilitating the processing of Yfh1. Twofold overexpression of Ssc1 partially suppresses the cold-sensitive growth phenotype of Deltassq1 cells, as well as the accumulation of mitochondrial iron and the defects in Fe/S enzyme activities normally found in Deltassq1 mitochondria. Deltassq1 mitochondria containing twofold-more Ssc1 efficiently converted the intermediate form of Yfh1 to the mature form. This correlation between the observed processing defect and suppression of in vivo phenotypes suggests that Ssc1 is able to carry out the functions of Ssq1, but only when present in approximately a 2,000-fold excess over normal levels of Ssq1.

Effect of protein hydration on receptor conformation: decreased levels of bound water promote metarhodopsin II formation

Neutral solutes were used to investigate the effects of osmotic stress both on the ability of rhodopsin to undergo its activating conformation change and on acyl chain packing in the rod outer segment (ROS) disk membrane. The equilibrium concentration of metarhodopsin II (MII), the conformation of photoactivated rhodopsin, which binds and activates transducin, was increased by glycerol, sucrose, and stachyose in a manner which was linear with osmolality. Analysis of this shift in equilibrium in terms of the dependence of ln(Keq) on osmolality revealed that 20 +/- 1 water molecules are released during the MI-to-MII transition at 20 degrees C, and at 35 degrees C 13 +/- 1 waters are released. At 35 degrees C the average time constant for MII formation was increased from 1.20 +/- 0.09 ms to 1.63 +/- 0.09 ms by addition of 1 osmolal sucrose or glycerol. The effect of the neutral solutes on acyl chain packing in the ROS disk membrane was assessed via measurements of the fluorescence lifetime and anisotropy decay of 1,6-diphenyl-1,3,5-hexatriene (DPH). Analysis of the anisotropy decay of DPH in terms of the rotational diffusion model showed that the angular width of the equilibrium orientational distribution of DPH about the membrane normal was progressively narrowed by increased osmolality. The parameter fv, which is proportional to the overlap between the DPH orientational probability distribution and a random orientational distribution, was reduced by the osmolytes in a manner which was linear with osmolality. This study highlights the potentially opposing interplay between the effect of membrane surface hydration on both the lipid bilayer and integral membrane protein structure. Our results further demonstrate that the binding and release of water molecules play an important role in modulating functional conformational changes for integral membrane proteins, as well as for soluble globular proteins.

Volume regulation of spermatozoa by quinine-sensitive channels

Bovine spermatozoa were shown to exhibit rapid regulatory volume decrease (RVD) when exposed to hypotonic saline media. This quinine- and quinidine-sensitive regulatory volume decrease was coincident with K+ release due to stretch-activation of inhibitor-specific presumptive K+ channels. The regulatory volume decrease response was much faster than a similar phenomenon observed in human peripheral blood lymphocytes. Studies on volume changes in different electrolyte and nonelectrolyte media suggested that: (1) this inhibitor-specific channel could also be a nonspecific pore in the spermatozoal membrane for nonelectrolytes below 150 daltons; (2) subpopulations (of nearly equal size) of the spermatozoa differ in the expression of the pore; (3) capacitation abolishes this distinction between subpopulations of spermatozoa; and (4) the general case of RVD for other mammalian spermatozoa was also established.

Assessment of molecular sieving across bacterial outer membrane of Pseudomonas

The role of the permeability barrier of the outer membrane of Pseudomonas was re-evaluated based on the physical theory of molecular sieving in view of its intrinsic antibiotic resistance. We developed a set of analytical procedures based on parametric and non-parametric statistical tests to evaluate, validate and adopt the better among a set of competing non-linear models of diffusion. The molecular mass dependence of uptake of non-electrolytes in bacteria yielded a quantitative measure to distinguish between sieving mechanisms and specific uptake/efflux mechanisms. The experimental data, supported by the physical model of DEAE-Sephadex and various analytical models and extensive simulation of the errors, both in measurement and models, yielded evidence consistent with the relaxation of the outer membrane matrix barrier in Pseudomonas.

Dependence of flux size and efficiency of oxidative phosphorylation on external osmolarity in isolated rat liver mitochondria: role of adenine nucleotide carrier

The aim of this work was a thermodynamic and kinetic study of the influence of varying external osmolarity on overall oxidative phosphorylations in isolated rat liver mitochondria. When external osmolarity is increased from 100 to 400 mosM by using a non-penetrant sugar: (i) matrix volume diminishes, (ii) state 3 respiratory rate decreases when state 4 slightly varies, (iii) states 3 and 4 protonmotive force and NAD(P)H level increase, whereas oxidative phosphorylation efficiency (ATP/O) decreases. Indeed, respiratory flux versus protonmotive force relationships depend on the osmolarity considered: the lower the external osmolarity, the higher the span of overall driving force necessary for the same respiratory rate. To further investigate the mechanism of the decrease in respiratory and ATP synthesis flux leading to a lowering in oxidative phosphorylation efficiency, we determined the adenine nucleotide carrier control coefficient on respiratory and ATP synthesis rates respectively. The main result is that the adenine nucleotide carrier control coefficient on respiratory rate decreases, and conversely that adenine nucleotide carrier control on ATP synthesis rate increases, from iso- to hyperosmolarity. Furthermore, whatever the osmolarity, when state 3 respiratory rate is titrated with carboxyatractyloside, the same relationship is observed between ATP/O ratio and respiratory flux. From many previous studies, it has been shown that an increase in external osmolarity and a consequent decrease in matrix volume inhibits almost all mitochondrial proton pumps (coupling site 1 and 2 of respiratory chain, ATPase) in different ways. In this work, we show that in phosphorylating mitochondria, the adenine nucleotide carrier plays a key role: its inhibition as the external osmolarity increases lowers the state 3 respiration close to state 4 level and consequently leads to a decrease in oxidative phosphorylation efficiency.

Mutation of the Ser2 codon of the light-harvesting B870 alpha polypeptide of Rhodobacter capsulatus partially suppresses the pufX phenotype

The exact function of the pufX gene product of Rhodobacter capsulatus is uncertain, but deletion of the pufX gene renders cells incapable of phototrophic growth on a minimal medium, and photosynthetic electron transfer is impaired in vitro. However, suppressor mutants that are able to grow phototropically are readily isolated. Two such suppressor mutants were characterized as to their phototrophic growth properties, their fluorescence at different incident light intensities, the integrity of their chromatophores, and their abilities to generate a transmembrane potential. We found that the photosynthetic apparatus in the suppressor mutants was less stable than that of the pseudo-wild-type and primary mutant strains and that the suppressor mutants used light energy less efficiently than the pseudo-wild-type strain. Therefore, the suppressor strains are more precisely designated partial suppressor mutants. The locations and sequences of the suppressor mutations were determined, and both were found to change the second codon of the pufA gene. It is hypothesized that the serine residue specified by this codon is important in interactions between the B870 alpha protein and other membrane-bound polypeptides and that suppressor mutations at this position partially compensate for loss of the PufX protein. A model is proposed for the function of the PufX protein.

Redox control of beta-oxidation in rat liver mitochondria

Coupled rat liver mitochondria were incubated with [U-14C]hexadecanoate and carnitine which resulted in the formation of acyl-, 2-enoyl- and 3-hydroxyacyl-CoA and carnitine esters. The production of 2-enoyl-CoA and 3-hydroxyacyl-CoA esters was associated with a significant lowering of the NAD+/NADH ratio, in contrast to rat muscle mitochondria [Eaton, S., Bhuiyan, A. K. M. J., Kler, R. S., Turnbull, D. M. & Bartlett, K. (1993) Biochem. J. 289, 161-172], suggesting that control by the respiratory chain is important under normal conditions. When NAD+/NADH ratios were held low by succinate-induced reverse electron flow, 3-enoyl-CoA esters were also detected, probably formed by the action of 3,2-enoyl-CoA isomerase. Measurement of the flux of beta-oxidation at different osmolalities showed that flux was strongly dependent on osmolality changes in the physiological range. Measurement of the CoA and carnitine esters resulting from incubations made at different osmolalities showed that there was an increase in the amounts of the saturated acyl-CoA esters with respect to 2-enoyl-CoA and 3-hydroxyacyl-CoA esters, consistent with control by the electron-transfer flavoprotein-ubiquinone segment [Halestrap, A. P. & Dunlop, J. L. (1986) Biochem. J. 239, 559-565]. This however could not be the only factor operating as indicated by the continued presence of 2-enoyl-CoA and 3-hydroxyacyl-CoA esters at high osmolalities.