Search

This critical review provides an overview of current research activities that focused on the synthesis and application of multi-functional gold and iron oxide (Au–FexOy) hybrid nanoparticles and nanocomposites. An introduction of synthetic strategies that have been developed for generating Au–FexOy nanocomposites with different nanostructures is presented. Surface functionalisation and bioconjugation of these hybrid nanoparticles and nanocomposites are also reviewed. A variety of applications such as theranostics, gene delivery, biosensing, cell sorting, bio-separation, and catalysis is discussed and highlighted. Finally, future trends and perspectives of these sophisticated nanocomposites are outlined. Underpinning the fundamental requirements for effectively forming Au–FexOy hybrid nanocomposite materials would shed light on future development of nanotheranostics, nanomedicines, and chemical technologies. It would be interesting to investigate such multi-component composite nanomaterials with different novel morphologies in the near future to advance chemistry, biology, medicine, and engineering multi-disciplinary research (120 references).

This critical review provides an overview of current research activities that focused on the synthesis and application of multi-functional gold and iron oxide (Au-Fe(x)O(y)) hybrid nanoparticles and nanocomposites. An introduction of synthetic strategies that have been developed for generating Au-Fe(x)O(y) nanocomposites with different nanostructures is presented. Surface functionalisation and bioconjugation of these hybrid nanoparticles and nanocomposites are also reviewed. A variety of applications such as theranostics, gene delivery, biosensing, cell sorting, bio-separation, and catalysis is discussed and highlighted. Finally, future trends and perspectives of these sophisticated nanocomposites are outlined. Underpinning the fundamental requirements for effectively forming Au-Fe(x)O(y) hybrid nanocomposite materials would shed light on future development of nanotheranostics, nanomedicines, and chemical technologies. It would be interesting to investigate such multi-component composite nanomaterials with different novel morphologies in the near future to advance chemistry, biology, medicine, and engineering multi-disciplinary research (120 references).

Summary Circular dichroism (CD) was utilized to probe two discrete loci of the ionophore lasalocid A during conformational perturbations associated with complex formation. This required deconvolution of the observed CD spectrum into components arising from the terminal aromatic ring and the medial ketonic carbonyl. The resultant data allowed us to distinguish between formation of true inclusion complexes with small alkali ions and peripheral ion pairing with alkylammonium ions too bulky to enter and perturb the molecular interior.

Monensin, a carboxylic ionophore, produces strong pressor, positive chronotropic effects and elevates the blood glucose level when injected intravenously (100 μg/kg) into pentobarbital anesthetized dogs or administered orally (2 mg/kg) to conscious dogs. Intravenously administered monensin disappeared from the blood rapidly with a t 1 2 of ca. 2.5 min and, in the conscious dogs, ingested monensin showed a peak plasma level 90 min after feeding; this coincided with the time of maximum increase in arterial blood pressure and blood glucose. In conscious rabbits, although higher doses of monensin were administered, 200 μg/kg intravenously and 10 mg/kg orally, its cardiovascular effects were less than observed in the dog and were slower in onset. This correlated with slower clearing of injected monensin from the blood ( t 1 2 = 8 min ) and slower entry of ingested monensin from the gut into the blood. Rabbit plasma and tissue levels were higher 17 hr after oral ingestion of monensin than six hr after ingestion.

The partition of alkali cations and anions between an aqueous and an immiscible organic phase has been studied in the absence and presence of neutral and carboxylic ionophores of the valinomycin and nigericin types, respectively. Cation extraction into the organic phase was augmented considerably by the ionophores, and a cation specificity of K+≧Rb+>Cs+≫Na+ was found for all the neutral ionophores tested. Evidence is given that the actual values of ion specificity are a function of the solvent polarity, especially for valinomycin where an inversion of the K+/Rb+ specificity was observed. The ionophores examined have the following rank order of effectiveness for K+ extraction into a standard organic phase consisting of 70% toluene-30%n-butanol: valinomycin>18-crown-6≫trinactin>enniatin B≈dinactin>monactin>nonactin. The ion affinity and selectivity data thus obtained have been compared with data previously reported.

The turnover numbers of the ionophores valinomycin, the macrolide actins, enniatin B and dicyclohexyl 18-crown-6 for translocation of cations through the mitochondrial membrane have been compared quantitatively. The rank order of decreasing maximum turnover number for K+ transport calculated on the basis of the ionophore concentration within the membrane is trinactin>dinactin∼monactin∼ valinomycin>nonactin>18-crown-6>enniatin B. The strength of binding of the ionophores to the mitochondria has the following rank order: valinomycin>macrolide actins>enniatin B>18-crown-6.

The significance of intracellular Na+ concentration in catecholamine secretion of cultured bovine adrenal chromaffin cells was investigated using the monovalent carboxylic ionophore monensin. This ionophore, which is known to mediate a one-for-one exchange of intracellular K+ for extracellular Na+, induces a slow, prolonged release of catecholamines which, at 6 h, amounts of 75-90% of the total catecholamines; carbachol induces a rapid pulse of catecholamine secretion of 25-35%. Although secretory granule numbers appear to be qualitatively reduced after carbachol, multiple carbachol, or Ba2+ stimulation, overall granule distribution remains similar to that in untreated cells. Monensin-stimulated catecholamine release requires extracellular Na+ but not Ca2+ whereas carbachol-stimulated catecholamine release requires extracellular Ca2+ and is partially dependent on extracellular Na+. Despite its high selectivity for monovalent ions, monensin is considerably more effective in promoting catecholamine secretion than the divalent ionophores, A23187 and ionomycin, which mediate a more direct entry of extracellular Ca2+ into the cell. We propose that the monensin-stimulated increase in intracellular Na+ levels causes an increase in the availability of intracellular Ca2+ which, in turn, stimulates exocytosis. This hypothesis is supported by the comparable stimulation of catecholamine release by ouabain which inhibits the outwardly directed Na+ pump and thus permits intracellular Na+ to accumulate. The relative magnitudes of the secretion elicited by monensin, carbachol, and the calcium ionophores, are most consistent with the hypothesis that, under normal physiological conditions, Na+ acts by decreasing the propensity of Ca2+-sequestering sites to bind the Ca2+ that enters the cell as a result of acetylcholine stimulation.

Previously, we have shown that multiple drug resistant (MDR) Friend leukemia cells (FLC) are cross-resistant to the positively-charged dye, Rhodamine 123 (Rho 123), and that this resistance can be reversed by verapamil (VER). In the present study we used two zwitterionic rhodamine analogs, Rhodamine 116 and Rhodamine 110, and another positively-charged analog, Rhodamine 6G, to determine whether drug accumulation, resistance and modulation were affected by changes in the charge of these compounds. While there was no differential sensitivity between sensitive and resistant FLC to zwitterionic rhodamines, there was marked differential toxicity between these cell types for the positively-charged analogs. The IC50 values were 1000- and 100-fold greater in resistant than in sensitive cells for Rho 123 and Rho 6G respectively. Intracellular drug accumulation was significantly higher in sensitive as compared to resistant cells for both Rho 123 and Rho 6G, but little difference in drug uptake between these two cell types was observed for Rho 110 and Rho 116. It was also found that the intracellular to extracellular ratio of the positively-charged compounds was greater than unity in both sensitive and resistant cells whereas for the zwitterionic analogs this ratio was less than 1. Furthermore, this ratio of drug uptake was found to be significantly higher for Rho 6G than for Rho 123, which correlated with the high oil:water partition coefficient of Rho 6G (115.6). In MDR cells, verapamil increased Rho 123 and Rho 6G accumulation by 9.4- and 8.6-fold respectively. In addition, IC50 values in resistant cells were reduced greater than 100-fold for Rho 6G and greater than 1000-fold for Rho 123 in the presence of 10 micrograms/ml of verapamil. In contrast, less than 2-fold reduction of IC50 values for both of the zwitterionic analogs could be obtained under the same conditions. These results indicate that the chemical charge of rhodamines plays an important role in their differential accumulation, cytotoxicity and sensitivity to modulators such as verapamil, in sensitive and multi-drug resistant cells. The data also suggest that increased lipophilicity of the positively-charged rhodamines may increase their ability to accumulate in, and subsequently kill, MDR cells.

Alkali ion complex formation by the cyclic depsipeptide, valinomycin, has been studied by means of nuclear magnetic resonance spectroscopy. Characteristic perturbations of the positions and fine structures of the resonances of the protons of specific groups were observed upon complextion of the valinomycin with KCNS and CsCNS in CDCI3. Small amounts of water did not affect the structure of the complexes as judged by nuclear magnetic resonance spectral criteria. Preliminary studies were carried out on the rates of complexation and decomplexation using the broadening of the lactate methyl proton resonances. A measurable rate of transfer of the complexed cation between valinomycin molecules in 80% CH3OH, 20% CDCl3, was observed while no measurable exchange rate was observed in pure CDCl3. These results are consistent with valinomycin functioning as a mobile ion carrier in biological membrane systems.

A number of recent, independent investigations have shown that the binding of Mg++ ( Brierley et al. , 1962 ), Ca++ ( Vasington and Murphy, 1962 ) and Mn++ ( Chappell et al. , 1962 ) to mitochondria share in common a characteristic concomitant binding of phosphate. It has been suggested by Brierley et al. (1962) that the known affinities of these metal ions for phosphate may be involved in the binding and indeed that actual intramitochondrial precipitation of insoluble phosphates is an integral part of the process. We wish to call attention to the similarity of these binding phenomena to that obtainable with guanidine and alkylguanidines ( Pressman, 1963 ). The present communication will extend the similarity by showing that the binding of guanidine shows a common pattern with that of Mg++ in response to various inhibitors, and furthermore, that guanidine and Mg++ are mutually competetive for mitochondrial binding sites.

The glucose-induced increase in light scattered by Ehrlich ascites cells has been ascribed by Packer and Golder to a decrease in mitochondrial volume. They further suggest that the parallel inhibition of respiration and glycolysis arises from the inability of ATP to escape from such contracted mitochondria. We find that the glucose-induced light scattering changes occur not only in the presence of oligomycin, which prevents ATP formation (exclusive of substrate level phosphorylation), but that the same scattering changes are induced by this antibiotic alone. Other mitochondrial inhibitors, such as antimycin A and uncoupling agents, increase light scattering of the tumor cells. Valinomycin, which induces a specific volume change of mitochondria associated with the uptake of K+, causes a large decrease of scattering signal from ascites cells, even in the presence of glucose. As is the case with isolated mitochondria, this swelling is reversed by depletion of mitochondrial energy reserves, by uncoupling, or by anaerobiosis. It is concluded that the light scattering changes obtained with ascites cells during altered metabolic states indeed reflect mitochondrial changes, but the glucose-induced increase in light scattering is not a consequence of the inability of ATP to escape from the mitochondria. The only point in common for a wide variety of agents including glucose which induce a mitochondrial contraction is their ability to lower the mitochondrial energy reserves.

Ethion is identified chemically as O,O,O’,O’-tetraethyl S,S’- methylene diphospholodithioate and has been known under the name of Niagara 1240. Results from 2 years of field tests demonstrated that the compound was effective against two species of aphids on apples, two species of foliage mites and the codling moth. On apples 2 pounds of 25% wettable powder per 100 gallons, applied when trees were either in dormant or delayed-dormant stage of development gave good control of the apple grain aphid, Rhopalosiphum fitchii (Sand.) and the rosy apple aphid, Anuraphis roseus Baker. One pound of ethion wettable powder per 100 gallons applied in pre-pink or in the early postbloom period provided control of the European red mite, Panonychus ulmi (Koch). When applied in mid-season ethion at the rate of 1 pound of 25% wettable powder was also effective for the control of the two-spotted spider mite, Tetranychus telarius (L.). The use of ethion at the 2-pound rate in all post-bloom sprays resulted in control of the codling moth, Carpocapsa pomonella (L.), equal to such standard control chemicals as lead arsenate, DDT and parathion. Lower concentrations were not commercially effective.

Added K+ is necessary for maximum rates of respiration and phosphorylation by rat liver mitochondria when a phosphate accepting system such as glucose and hexokinase is present. K+ is also necessary for maximum rates of respiration and phosphorylation when microsomes, or an acetone-soluble fraction therefrom, are present. When respiration is maximally stimulated by 2,4-dinitrophenol, added K+ does not enhance respiration except that in the 3rd and 4th hour of the experiment it prevents deterioration of the mitochondria. It is concluded that K+ participates in transphosphorylation reactions but does not directly influence the oxidative reactions.

The rate of exchange of the endogenous Mg 2+ of isolated rat liver mitochondria was measured by means of the radioisotope 28 Mg. Two different centrifugation techniques yielding similar results, were utilized for separating the mitochondria from suspenpension media for analysis. Mitochondrial Mg 2+ exchanges very slowly with externally added Mg 2+ . The influx of labeled Mg 2+ observed in the presence of substrate is blocked by treatment with the respiratory inhibitor, rotenone, or the uncoupler, p -trifluoromethoxy(carbonylcyanide) phenylhydrazone. The observed rates of Mg 2+ influx are similar in order of magnitude to rates of K + influx measured under comparable conditions. Parathyroid hormone stimulates Mg 2+ influx, but only when metabolic energy is available to support net Mg 2+ uptake.

In experiments with surviving rat heart slices it has been shown that Persantin and 2,6- 14 C-Persantin permeate the myocardial cell. Under our conditions, 50% or more of the drug in the medium was passed to the intracellular space and distributed among the intracellular compartments. The labeled drug was found in all the particles and in the cytoplasm. Fixation studies in vitro with sarcosomes or with the microsomal fraction, in amounts comparable to those found in the myocardial cell, showed that about 30% of labeled Persantin in the medium was fixed to the sarcosomes; 60% of this appeared to be tightly bound, whereas the remainder could be washed out with isotonic sucrose. The microsomal fraction in vitro fixed about 6% of labeled Persantin in the medium. These differences largely reflect the fact that there is about ten times more nitrogen in the cell attributable to sarcosomes than to the microsomal fraction. From paper Chromatographic studies of the labeled lipid-soluble fraction combined from all intracellular compartments, it can be stated that most of the 14 C-Persantin is not chemically altered under the conditions employed for these studies.

The effects of the cardiotonic ionophore monensin on coronary blood flow, cardiac output, left ventricular (LV) O2 consumption, LV contractility and arterial blood pressure were measured in greyhounds and compared with those in mongrel dogs under pentobarbital anaesthesia. Following intravenous injection of 100 micrograms/kg monensin, all parameters rose, the greyhounds showing a relatively greater response in peak values of coronary blood flow, LV contractility and cardiac output. The increase in the calculated LV mechanical work was proportionately greater than the corresponding increase in LV O2 consumption, hence the derived index, external mechanical efficiency, rose rapidly from an initial value of 0.165 to 0.285 (73%) in greyhounds; mongrel dogs showed a more modest rise from 0.094 to 0.120 (28%) during the same interval. The difference in the responses in the two breeds of dogs are attributed mainly to larger heart size and exercise training in greyhounds as compared with untrained mongrel dogs.

Formidable quantities of microbial metabolites belonging to the class of ionophores are currently being fed to poultry and cattle raised for human consumption. Most fryers and broilers in the United States received 120 ppm monensin in their feed to control coccidiosisl while beef cattle are fed 30 ppm monensin which, possibly by exerting a favorable effect on the flora of the rumen,2 promotes a more efficient fermentation of cellulose and subsequent conversion of feed to meat.3

Publisher Summary The release of endogenous mitochondrial K + caused by the ionophore valinomycin can be coupled to net adenosine triphosphate (ATP) synthesis. Thus, the free energy of a K + concentration gradient can drive an endergonic process, such as phosphorylation; preloading mitochondria with K+ augments the yield of ATP considerably. The use of ionophores to generate a cation diffusion potential to drive various endergonic processes has been applied to a broad spectrum of energy transductions. Exergonic valinomycin-induced K + transport has been shown to drive (1) ATP synthesis by chloroplasts, (2) ATP synthesis by submitochondrial particles, (3) reversed mitochondrial electron transport, (4) Ca 2+ transport by mitochondria, (5) energy-linked spectral changes in bacterial carotenoids, and (6) energy-dependent fluorescence shifts of various membrane probes. These and related phenomena are explained in terms of membrane potential generation coupled to H + transport. The reproducible demonstration of ATP synthesis via mitochondrial K + gradients and includes a description of related processes. Factors important for investigating the relationships between ion gradients and various energy transformations are discussed in the chapter.

The carboxylic ionophore monensin has a biphasic effect on antibody-induced Thy-1 cap formation. At higher concentrations, 5 X 10(-6)-5 X 10(-5) M monesin causes a significant inhibition of receptor capping similar to that previously found with the Ca2+ selective ionophore A23187. At lower concentrations, 5 X 10(-8)-5 X 10(-7) M capping is stimulated. It is concluded that capping at lower ionophore concentrations is a specific response to the ability of monensin to induce a rise in intracellular Na+, which indirectly elevates intracellular Ca2+ activity. This in turn activates the contractile machinery required for the aggregation of surface receptors into capped structures. At higher concentrations monensin acts as a nonspecific detergent, which causes detrimental structural alterations in some of the membrane components involved in the capping process.

The conformational dynamics of lasalocid A have been studied in a series of solvents of graded polarity by means of circular dichroism (CD) and computer-generated molecular models. In high polarity solvents, the uncomplexed anionic ionophore assumes as acyclic conformation minimizing intrinsic molecular strain energy. In this state, the dipoles of the liganding oxygens in the carbon backbone and the terminal carboxylate are stabilized by a high degree of solvent association. As the solvent polarity decreases, the dynamic conformational equilibrium progressively shifts toward a cyclic conformation which predominates at low polarity. Cyclization proceeds by rotation about three carbon-carbon hinge bonds. The resulting twist of the backbone introduces torsional strain which is offset at low polarity by electrostatic stabilization gained through intramolecular hydrogen bonding. Formation of a cation inclusion complex also stabilizes the cyclic conformer, even in relatively polar solvents. These observations suggest a scenario for carboxylic ionophore mediated transmembrane monovalent cation transport at the molecular level. The cation encounters an acyclic ionophore at the membrane interface where it ion pairs to the terminal carboxylate moiety, initiating formation of a lipophilic, cyclic cation inclusion complex. The complex, no longer constrained to the polar interface, diffuses across the membrane interior to the opposite face. There it reequilibrates with the polar environment, the ionophore reassuming the low energy, acyclic conformation and concomitantly releasing the enclosed cation. The free, acyclic ionophore is now confined to the opposite polar interface where it awaits the capture of a new cation to complete its catalytic transport cycle.

MPTP selectively destroys the nigrostriatal dopaminergic neurons in primates and the mouse and produces a neuropathological lesion in the primate similar to that seen in idiopathic Parkinson’s disease (Heikkila, 1984: Langston, 1985; Langston et al., 1983). These findings led to the speculation that environmental toxins may cause idopathic Parkinson’s disease. According to this hypothesis, exposure to a neurotoxin during a short period of time may reduce the number of dopaminergic neurons, so that an age-related slow degeneration of these cells will reduce, at an earlier time, the number of surviving cells to a level insufficient to support normal function. Alternatively, it is possible that long-term exposure to small quantities of a neurotoxin might produce similar effects as short-term exposure to high levels and gradually result in the appearance of clinical symptoms. At present, no chemical has been identified which could be responsible for causing idiopathic Parkinson’s disease. However, in the modern industrial society, humans are exposed to a vast number of xenobiotics, which are novel chemical structures and to which no defense mechanism has developed during evolution. The discovery of MPTP illustrates that a simple chemical which was used as intermediate in chemical syntheses can turn out to be a major neurotoxin. It is therefore important to study xenobiotics on a large scale for their potential neurotoxicity. We have developed culture systems to study the mechanism of action by which MPTP destroys dopaminergic neurons and to search for neurotoxins which, similar to MPTP, produce toxic effects on dopaminergic neurons. Culture systems offer the advantage that relatively large number of compounds can be tested and that their effects can be assessed in absence of interference by the blood brain barrier or metabolizing enzymes.

The conformation of the carboxylic ionophore salinomycin has been examined in a variety of solvents by circular dichroism. The solution conformation of the ionophore has been correlated with Na + and K + affinity and selectivity. Cation selectivity appears to be a function of both the initial conformation of the free anionic ionophore and the ability of the latter to reorient about the cation. An analogy to the induced fit model of enzyme-substrate interaction is proposed.

The monovalention-selective ionophore monensin (0.4 to 33 μ m ) produced significant positive inotropic responses in isolated driven rabbit and cat left atria. These responses were accompanied by an increase of d F d t max and a decrease of time to peak force, but no change of resting force. Monensin concentrations greater than 33 μ m produced negative inotropy and contracture. Reserpine pretreatment in vivo or β-blockade in vitro attenuated the positive inotropic responses in an apparently non-competitive fashion, but had no effect on negative inotropy or the development of contracture. Only the positive inotropic responses to monensin were completely blocked by LaCl3, ruthenium red or low extracellular Ca2+. The ionophore reversed the negative inotropic response to 3 μ m tetrodotoxin. Monensin (1 to 100 μ m ) inhibited Ca2+ accumulation by isolated cardiac microsomes. Addition of monensin to microsomes that had accumulated Ca2+ in the absence of ionophore caused rapid release of a fraction of accumulated Ca2+.

The monovalent carboxylic ionophores form lipid-soluble complexes with alkali cations which transport these ions across membranes. In a biological setting, they promote an electrically neutral exchange of intracellular potassium for extracellular sodium (Pressman, 1976). The ionophore monensin, which has a complexation preference for sodium (Pressman, 1968), has very little capability for complexing or transporting calcium or catecholamines (Pressman, Painter & Fahim, 1980). However, monensin produces a strong positive inotropic effect (Pressman, Harris, Jagger & Johnson, 1967; Sutko et al. 1977; Shlafer, Somani, Pressman & Palmer, 1978; Saini, Hester, Somani & Pressman, 1979) which is attenuated, but not abolished, by β-adrenergic blockade (Sutko et al. 1977; Shlafer et al. 1978; Saini et al. 1979). This indicates that the inotropic effect is partially mediated through an indirect release of catecholamines and, also, through a more direct mechanism, presumably by an alteration of transcellular cation gradients. Studies with cardiac Purkinje fibres show that monensin produces a shortening of the action potential, without an accompanying alteration in the resting potential (Sutko et al. 1977; Shlafer et al. 1978). Inasmuch as monensin should decrease the transcellular gradients of both sodium and potassium, one would expect that not only the action potential, dominated by the sodium diffusion potential, but also the resting potential, dominated by the potassium diffusion potential, should decrease. The present investigation was directed toward resolving the paradox of why monensin fails to alter the resting potential of cardiac muscle segments under the same conditions which alter the action potential. Our results show that depolarization of the resting potential, induced by monensin, is normally masked by the activity of the electrogenic sodium pump, and if the pump is inhibited by ouabain, then the expected depolarization is observed.

The structure and physical chemical properties are reviewed of various ionophores, small molecules which transport cations across biological membranes. The subclass of carboxylic ionophores has dramatic pharmacological properties, dilating the coronary arteries and increasing cardiac contractility, which make them especially interesting. They share related structures containing a terminal carboxyl group hydrogen bonded to the opposite end of the molecule in a cyclic conformation. A variety of oxygen atoms, in heterocyclic rings or linear ether configurations, hydroxyls, and/or ketonic carbonyls as well as carboxyls, focus upon a sphere which can ligand to appropriately fitting cations via ion-dipole interaction. Extreme dependency of liganding energy on fit confers striking ion selectivity upon ionophores. It is speculated that carboxylic ionophores originated as prosthetic groups of ion carriers and represent a natural experiment by the streptomyces genus in the construction of non-proteinaceous binding sites. Ultimately production of these prosthetic groups hypertrophied and subsequently they were elaborated as antibiotics to provide their producers with a survival advantage over competitors. Analogous ionophores have now been synthesized. Unique applications for these compounds are emerging, based either on their selective toxicity or ability to modify physiological processes by altering membrane gradients.

Publisher Summary This chapter examines the biological applications of ion-specific glass electrodes. In biological systems, the permeant ions most likely to be encountered by the electrodes are H + , Na + , and K + . Ion-specific glass electrodes have been employed extensively for measurement of the translocation of K + , Na + , and H + across the mitochondrial membrane. After the equilibration period, rapid ion movement is initiated by addition of minute amounts (10 -8 to 10 -1° M ) of certain antibiotics, for example, valinomycin, to the isolated mitochondria in an appropriate medium. The total ion concentration of the system as a whole remains constant; however, the concentration of an ionic species within the intramitochondrial phase lowers its activity in the extramitochondrial phase that is the phase sensed by the electrodes. Other interesting applications of the electrodes include the simultaneous monitoring in vivo of brain surface, cortex, and blood. For many biological applications, it is more convenient to calibrate electrode responses empirically by addition of standards to the test solution than compute ion concentrations from precise measurements of the electrode potentials. The chapter also discusses the special characteristics of electrodes.

A method is described for the determination of uric acid in serum which is sufficiently precise and reliable to follow small changes at low levels. The advantage of the use in the acetate form of the anion exchange resin Dowex 2-X8 is pointed out. The influence of protein and other anion components of serum on the absorbance of uric acid and on its elution from the resin is noted. Normal values of serum uric acid in dogs as well as man, with sampling deviations, and reproducibility of the method at both levels is demonstrated.

Washed, starved preparations of Escherichia coli B metabolize glucose with concomitant uptake of K + and O 2 and extrusion of H + . Abrupt changes in the rates of these processes are correlated with apparent sequential shifting in time of the control sites of aerobic glycolysis from the glyceraldehyde-3-phosphate dehydrogenase-3-phosphoglyceric phosphokinase couple to phosphofructokinase to hexikinase. It is suggested that the principal inhibitors of phosphofructokinase activity are citrate and ATP. The rate of K + uptake is neither a function of the rate of glucose dissimilation nor of the concentration of ATP in these short-term experiments, and the energy available for the active transport of ions appears to vary as the intracellular chemical environment changes.