Calcium efflux from human erythrocyte ghosts

Summary The passive Ca efflux from human red cell ghosts was studied in media of differing ion compositions and compared to the ATP-dependent Ca efflux. Cells were loaded with⁴⁵Ca during reversible hemolysis, and the loss of radioactivity into the non-radioactive incubation medium was measured, usually for 3 hr at 37°C. Analysis of the efflux curves revealed that⁴⁵Ca efflux followed the kinetics of a simple two-compartment system. In the concentration range between 0 and 1mm Ca in the external solution ([Ca⁺⁺] _o ), the rate constant of passive Ca efflux (k min^–1, fraction of⁴⁵Ca lost per minute into the medium) increased from 0.00732 to 0.0150 min^–1. There was no further increase at higher [Ca⁺⁺] _o . The relation between the rate constant of Ca efflux and [Ca⁺⁺] _o is thus characterized by saturation kinetics. The passive transfer system for Ca could also be activated by Sr. The alkali metal ions Na, K and Li did not seem to have any significant influence on passive Ca transfer. The passive Ca efflux was slightly inhibited by Mg and strongly inhibited by Pb. Under most experimental conditions, a fraction of 15 to 50% of the intracellular Ca seemed to be inexchangeable. The inexchangeable fraction decreased with increasing [Ca⁺⁺] _o and increased with increasing [Ca⁺⁺] _i . It was not influenced by alkali metal ions, CN or Pb, but it could be completely removed from the cells by the addition of 0.1mm Mersalyl to the incubation medium or by hemolysis with addition of a detergent. The active ATP-dependent Ca transport differed characteristically from passive transfer; the rate constant decreased with increasing [Ca⁺⁺] _o , and the inexchangeable Ca fraction increased with increasing [Ca⁺⁺] _o . The experimental results suggest that there exists a carrier-mediated Ca–Ca exchange diffusion in the erythrocyte membrane and that only a fraction of the ghost cell population participates in the Ca exchange diffusion.     

Potassium channels in plasmalemma ofNitella cells at rest

Summary Cation channels of passive transport in the plasmalemma ofNitella flexilis cells at rest were studied by the voltageclamp technique using microelectrodes. Two types of potassium channels have been identified. They are activated at different voltages: over –100 to –80 mV (D-channels) and below –100 mV (H-channels). The zero-current potential of instantaneous voltage-current curves (IVCC's) for both types of channels shifts by 50 to 55 mV in response to a 10-fold increase of K⁺ concentration in the solution. Ion movement in D-channels follows the free diffusion mechanism; in H-channels the independence principle is violated. The channel selectivity (in the order of decreasing permeability) is: K⁺>Rb⁺>NH ₄ ⁺ >Na⁺Li⁺>Cs⁺>TEA⁺ choline⁺. It has been found that D-channel Cs⁺ block is potential dependent while tetraethylammonium (TEA⁺) blocks H-channels in a potential-independent manner, but H⁺ ions do not affect the inward potassium current of the channels. Two types of potassium channels appear to be located in different parts of the membrane and their entrance parts are of different structure.     

Carbon and nitrogen cycling in intertidal sediments near Doel,Scheldt Estuary

Carbon and nitrogen cycling in intertidal mud flat sediments in the Scheldt Estuary was studied using measurements of carbon dioxide, methane and nitrous oxide emission rates and pore-water profiles of CO₂, ammonium and nitrate. A comparison between chamber measured carbon dioxide fluxes and those based on CO₂ pore-water gradients using Fick's First law indicates that apparent diffusion coefficients are 2 to 28 times higher than bulk sediment diffusion coefficients based on molecular diffusion. Seasonal changes in gaseous carbon fluxes or CO₂ pore water concentrations cannot be used directly, or in a simple way, to determine seasonal rates of mineralization, because of marked seasonal changes in pore-water storage and exchange parameters.The annual amount of carbon delivered to the sediment is 42 mol m^–2, of which about 42% becomes buried, the remaining being emitted as methane (7%) or carbon dioxide (50%). Each year about 2.6 mol N m^–2 of particulate nitrogen reaches the sediment; 1.1 mol m^–2 is buried and 1.6 mol m^–2 is mineralized to ammonium. Only 0.42 mol m^–2 yr^–1 of the ammonium produced escapes from the sediments, the remaining being first nitrified (1.2 mol m^–2 yr^–1) and then denitrified (1.7 mol m^–2 yr^–1). Simple calculations indicate that intertidal sediments may account for about 14% and 30% of the total estuarine retention of nitrogen and carbon, respectively.     

Cell-to-cell diffusion selectivity in staminal hairs ofSetcreasea purpurea

Summary Diffusion coefficients for FITC-molecular probes in intercellular pores (D) and rate of molecular probe loss into the vacuole (k₁) have been obtained for FITC molecular probes in staminal hairs ofSetcreasea purpurea. The kinetic curves of FITC-Gly, -Ala, -Leu,-Ser, -Thr, -Cys, -Met, -Tyr, -Asp, -Glu, -Asn, -Gln, -Lys, -His,-Arg, -(Asp)₂, -(Glu)₂, -(Lys)₂, -(Asp)₃, -(Glu)₃, -(Gln)₂, -(Gln)₃, -(Gln)₄, and carboxyfluorescein (group I probes) matched the curves calculated for simple diffusion through a chain of cells, while the majority of kinetic curves of FITC-Phe, and -Try (group II probes) did not. None of the kinetic curves for FITC-(Met)₂ and -(His)₂ (group III probes) matched. Average Ds for group I probes ranged from 0.77× 10^–8cm²/s to 3.75× 10^–8cm²/s and for group II probes were 0.50× 10^–8cm²/s. A meaningful average D for group III probes could not be calculated. Average k₁ for group I probes ranged from 1.62× 10^–7/m²/s to 13.21× 10^–7/m²/s, and for group II probes were 5.42 and 11.54× 10^–7/m²/s. Average k₁s for group III probes could not be calculated. Symplastic transport occurred by cell-to-cell diffusion for most of the probes (e.g., group I probes) but not always for some (e.g., group II probes) and never for others (group III probes). The rate of cell-to-cell diffusion and loss within the vacuole depended upon the molecule's specific structure, molecular weight and charge. We concluded that plasmodesmata select for molecules that are hydrophilic, small and have a charge of from — 2 to — 4, and against molecules that contain either Phe, Try, Met or His groups.Abbreviations CF carboxyfluorescein - D diffusion coefficients for FITC-molecular probes in intercellular pores - k₁ rate of FITC-molecular probe loss     

Pressure effect on the dynamics of an isolated α-helix studied by 15N-1H NMR relaxation

Dynamics and structure of (1–36)bacteriorhodopsin solubilized in chloroform/methanol mixture (1:1) were investigated by ¹H-¹⁵N NMR spectroscopy under a hydrostatic pressure of 2000 bar. It was shown that the peptide retains its spatial structure at high pressure. ¹⁵N transverse and longitudinal relaxation times, ¹⁵N{¹H} nuclear Overhauser effects, chemical shifts and the translation diffusion rate of the peptide at 2000 bar were compared with the respective data at ambient pressure [Orekhov et al. (1999) J. Biomol. NMR, 14, 345–356]. The model free analysis of the relaxation data for the helical 9–31 fragment revealed that the high pressure decreases the overall rotation and translation diffusion, as well as apparent order parameters of fast picosecond internal motions (S² _f) but has no effect on internal nanosecond motions (S² _s and _s) of the peptide. The decrease of translation and overall rotation diffusion was attributed to the increase in solvent viscosity and the decrease of apparent order parameters S² _f to a compression of hydrogen bonds. It is suggested that this compression causes an elongation of H-N bonds and a decrease of absolute values of chemical shift anisotropy (CSA). In particular, the observed decrease of S² _f at 2000 bar can be explained by 0.001 nm increase of N-H bond lengths and 10 ppm decrease of ¹⁵N CSA values.     

Chloride distribution in the proximal convoluted tubule ofNecturus kidney

Summary To assess the mechanism(s) by which intraluminal chloride concentration is raised above equilibrium values, intracellular Cl^– activity ( _i ^Cl ) was studied in the proximal tubule ofNecturus kidney. Paired measurements of cell membrane PD (V _BL) and Cl-selective electrode PD (V _BL ^Cl ) were performed in single tubules, during reversible shifts of peritubular or luminal fluid composition. Steadystate _i ^Cl was estimated at 14.6±0.6 mmol/liter, a figure substantially higher than that predicted for passive distribution. To determine the site of the uphill Cl^– transport into the cell, an inhibitor of anion transport (SITS) was added to the perfusion fluid. Introduction of SITS in peritubular perfusate decreased _i ^Cl , whereas addition of the drug in luminal fluid slightly increased _i ^Cl ; both results are consistent with basolateral membrane uphill Cl^– transport from interstitium to the cell. TMA⁺ for Na⁺ substitutions in either luminal or peritubular perfusate had no effect on _i ^Cl . Removal of bicarbonate from peritubular fluid, at constant pH (a situation increasing HCO ₃ ^– outflux), resulted in an increase of _i ^Cl , presumably related to enhanced Cl^– cell influx: we infer that Cl^– is exchanged against HCO ₃ ^– at the basolateral membrane. The following mechanism is suggested to account for the rise in luminal Cl^– concentration above equilibrium values: intracellular CO₂ hydration gives rise to cell HCO ₃ ^– concentrations above equilibrium. The passive exit of HCO ₃ ^– at the basolateral membrane energizes an uphill entry of Cl^– into the cell. The resulting increase of _i ^Cl , above equilibrium, generates downhill Cl^– diffusion from cell to lumen. As a result, luminal Cl^– concentration also increases.C.N.R.S. Greco 24. Part of this work was presented at the 12^th annual meeting of the American Society of Nephrology, Boston, Mass. (Edelman et al., 1979).     

Effect of amiloride,ouabain and Ba++ on the nonsteady-state Na−K pump flux and short-circuit current in isolated frog skin epithelia

Summary Effect of amiloride, ouabain, and Ba⁺⁺ on the nonsteady-state Na–K pump flux and short-circuit current in isolated frog skin epithelia.The active Na⁺ transport across isolated frog skin occurs in two steps: passive diffusion across the apical membrane of the cells followed by an active extrusion from the cells via the Na⁺–K⁺ pump at the basolateral membrane. In isolated epithelia with a very small Na⁺ efflux, the appearing Na⁺-flux in the basolateral solution is equal to the rate of the pump, whereas the short-circuit current (SCC) is equal to the active transepithelial Na⁺ transport. It was found that blocking the passive diffusion of Na⁺ across the apical membrane (addition of amiloride) resulted in an instantaneous inhibition of the SCC (the transepithelial Na⁺ transport, whereas the appearing flux (the rate of the Na⁺–K⁺ pump) decreased with a halftime of 1.9 min. Addition of the Na⁺–K⁺ pump inhibitor ouabain (0.1mm) resulted in a faster and bigger inhibition of the appearing flux than of the SCC. Thus, by simultaneous measurement of the SCC and the appearing Na⁺ flux one can elucidate whether an inhibitor exerts its effect by inhibiting the pump or by decreasing the passive permeability. Addition of the K⁺ channel inhibitor Ba⁺⁺, in a concentration which gave maximum inhibition of the SCC, had no effect on the appearing flux (the rate of the Na–K pump) in the first 2 min, although the inhibition of the SCC was already at its maximum.It is argued that in the short period, where the Ba⁺⁺-induced inhibition of SCC is at its maximum and the appearing flux in unchanged, the decrease in the SCC (SCC) is equal to the net K⁺ flux via the Na⁺–K⁺ pump, and the coupling ratio () of the Na⁺–K⁺ pump can be calculated from the following equation =SCC _t=0/SCC where SCC _t=0 is the steady-state SCC before the addition of Ba⁺⁺.     

Cellular mechanism of HCO 3 − and Cl− transport in insect salt gland

Summary Active HCO ₃ ^t- secretion in the anterior rectal salt gland of the mosquito larva,Aedes dorsalis, is mediated by a 11 Cl^–/HCO ₃ ^– exchanger. The cellular mechanisms of HCO ₃ ^– and Cl^– transport are examined using ion- and voltage-sensitive microelectrodes in conjunction with a microperfused preparation which allowed rapid saline changes. Addition of DIDS or acetazolamide to, or removal of CO₂ and HCO ₃ ^– from, the serosal bath caused large (20 to 50 mV) hyperpolarizations of apical membrane potential (V _a) and had little effect on basolateral potential (V _bl). Changes in luminal Cl^– concentration alteredV _a in a repid, linear manner with a slope of 42.2 mV/decaloga _Cl ^l –. Intracellular Cl^– activity was 23.5mm and was approximately 10mm lower than that predicted for a passive distribution across the apical membrane. Changes in serosal Cl^– concentration had no effect onV _bl, indicating an electrically silent basolateral Cl^– exit step. Intracellular pH in anterior rectal cells was 7.67 and the calculated was 14.4mm. These results show that under control conditions HCO₃ enters the anterior rectal cell by an active mechanism against an electrochemical gradient of 77.1 mV and exits the cell at the apical membrane down a favorable electrochemical gradient of 27.6 mV. A tentative cellular model is proposed in which Cl enters the apical membrane of the anterior rectal cells by passive, electrodiffusive movement through a Cl^–-selective channel, and HCO ₃ ^– exits the cell by an active or passive electrogenic transport mechanism. The electrically silent nature of basolateral Cl^– exit and HCO₃ entry, and the effects of serosal addition of the Cl^–/HCO₃ exchange inhibitor, DIDS, on and transepithelial potential (V _ic) suggest strongly that the basolateral membrane is the site of a direct coupling between Cl^– and HCO ₃ ^– movements.     

Transport of malic acid in Leuconostoc oenos strains defective in malolactic fermentation: a model to evaluate the kinetic parameters

Two Leuconostoc oenos mutant strains unable to metabolize malic acid were differentiated by [U-¹⁴C]-labelled L-malate transport assays into a malolactic-enzyme-deficient mutant and a malate-transport-defective mutant. A mathematical analysis of the data from L-malic acid uptake at three pH values (5.2, 4.5, and 3.2) in the malolactic-enzyme-deficient strains suggest two simultaneous uptake mechanisms, presumably a carrier-mediated transport and a passive diffusion for the anionic and the undissociated forms of the acid, respectively. The apparent affinity constant (K _m t) and the maximal rate (V _m t) values for L-malate active transport were, 12 mM and 43 mol L-malate·mg^–1·s^–1, respectively. Active transport was constitutive and strongly inhibited by protonophores and by ATPase inhibitors. L-Lactic acid appeared to inhibit L-malic acid transport, suggesting an L-lactate/L-malate exchange. At pH values of 4.5 or above, the passive diffusion of L-malic acid was negligible. However, at pH 3.2, the mean pH of wine, the permeability of the cells to the undissociated acid by simple diffusion could represent more than 50% of total L-malic acid uptake, with a diffusion constant (K _D) of 0.1 s^–1. Correspondence to: C. Divies     

Effects of anaerobic conditions on water and solute relations,and on active transport in roots of maize (Zea mays L.)

The effects of anoxia on water and solute transport across excised roots of young maize plants (Zea mays L. cv. Tanker) grown hydroponically have been studied. With the aid of the root pressure probe, root pressure (P_r), root hydraulic conductivity (Lp_r), and root permeability (P_sr), and reflection ( _sr) coefficients were measured using potassium nitrate (a typical nutrient salt) and sodium nitrate (an atypical nutrient salt) as solutes. During a period of 10–15 h, anaerobic treatment (0.0–0.2 g O₂·m^-3 in root medium) caused a decrease of root pressure by 0.01–0.28 MPa (by 10–80% of original root pressure) after a short transient increase. For a time period of 5 h, the decrease in the stationary root pressure was not reversible. Under anaerobic conditions, roots still behaved like osmometers and were not leaky. The root hydraulic conductivity measured in osmotic experiments (osmotic solute: NaNO₃) was smaller by one to two orders of magnitude than that measured in the presence of hydrostatic gradients. Both the osmotic and hydrostatic hydraulic conductivity decreased during anaerobic treatment by 28 and 44%, respectively, at a constant reflection coefficient of the solutes ( _sr=0.3–1.0). As with root pressure, changes in root permeability to water and solutes were not reversible within 5 h. Under aerobic conditions and at low external concentrations (31–59 mOsmol·kg^-1), osmotic response curves were monophasic for KNO₃, i.e. there was no passive uptake of solutes. Response curves became biphasic at higher concentrations (100–150 mOsmol·kg^-1)- For NaNO₃, response curves were biphasic at all concentrations. Presumably, this pattern was a consequence of the fact that potassium had already accumulated in the xylem. During anoxia, accumulation of potassium in the xylem was reduced, and biphasic responses were also obtained at lower potassium concentrations applied to the medium. The results are discussed in terms of a pump/leak model of the root in which anoxia affects both the active ion pumping and the permeability of the root to nutrient salts (leakage). The effects of anaerobiosis on the passive transport properties of the root (Lp_r, P_sr, _sr) are in line with the recently proposed composite transport model of the root.Abbreviations and Symbols A_r root surface area - Lp_r root hydraulic conductivity - Lp_rh hydrostatic hydraulic conductivity of root - Lp_ro osmotic hydraulic conductivity of root - P_r root pressure - P_sr permeability coefficient of root - _sr reflection coefficient of root The authors thank Mr. Walter Melchior for the curve-fitting program used to work out Lp_rh values from root pressure relaxations and Mr. Mohammad Hajirezai (Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth) for making the ATP measurements. The assistance of Mrs. Libuse Badewitz in making the drawings and the technical help of Mr. Burkhard Stumpf are also gratefully acknowledged.     

Analog circuit of theAcetabularia membrane

Summary The high membrane potential ofAcetabularia (E _m=–170 mV) is due to an electrogenic pump in parallel with the passive diffusion system (E _d=–80 mV) which could be studied separately in the cold, when the pump is blocked. Electrical measurements under normal conditions show that the pump pathway consists of its electromotive forceE _p with two elementsP ₁ andP ₂ in series;P ₂ is shunted by a large capacitance (C _p=3 mF cm^–2). The nonlinear current-voltage relationship ofP ₁ (light- and temperature-sensitive) could be determined separately; it reflects the properties of a carrier-mediated electrogenic pump. The value ofE _p (–190 mV) indicates a stoichiometry of 21 between electrogenically transported charges and ATP. The electrical energy, normally stored inC _p, compares well with the metabolic energy, stored in the ATP pool. The nonlinear current-voltage relationship ofP ₂ (attributed to phosphorylating reactions) is also sensitive to light and temperature and is responsible for the region of negative conductance of the overall current-voltage relationship. The power of the pump (1 W cm^–2) amounts to some percent of the total energy turnover. The high Cl^– fluxes (1 nmol cm^–2 sec^–1) and the electrical properties of the plasmalemma are not as closely related as assumed previously. For kinetic reasons, a direct and specific Cl^– pathway between the vacuole and outside is postulated to exist.     

Thiol-dependent passive K+Cl− transport in sheep red blood cells: VI. Functional heterogeneity and immunologic identity with volume-stimulated K+(Rb+) fluxes

Summary Ouabain-resistant (OR), volume-or N-ethylmaleimide (NEM)-stimulated K⁺(Rb⁺)Cl^– fluxes were measured in low-K⁺ sheep red cells and found to be functionally separate but immunologically similar. In anisosmotic solutions both K⁺ effluxes and Rb⁺ influxes of NEM-treated and control cells were additive. In contrast to the NEM-stimulated K⁺Cl^– flux, metabolic depletion did not reduce K⁺Cl^– flux of normal or swollen cells. The anion preference of OR K⁺ efflux in NEM-treated cells was Br^–>Cl^–>HCO ₃ ^– =F^–I^–=NO ₃ ^– =CNS^–, and hence consistent with a reported Br^–>Cl^–>NO ₃ ^– sequence of the volume-dependent K⁺Cl^– transport. Alloimmune anti-L_l antibodies known to decrease passive K⁺ fluxes in low K⁺ cells reduced by 51% both volume-and NEM-stimulated, furosemidesensitive Rb⁺Cl^– fluxes suggesting their immunologic identity, a conclusion also supported by anti-L₁ absorption studies. Since pretreatment with anti-L₁ prevented the activation of Rb⁺ influx by NEM, and the impermeant glutathionmaleimide-I did not stimulate Rb⁺Cl^– influx, the NEM reactive SH groups must be located apart from the L₁ antigen either within the membrane or on its cytoplasmic face. A model is proposed consisting of a K⁺Cl^– transport path(s) regulated by a protein with two functional subunits or domains; a chemically (C _s) and a volume (V _s)-stimulated domain, both interfacing with the L₁ surface antigen. Attachment of alloanti-L₁ from the outside reduces K⁺Cl^– transport stimulated throughC _s by NEM orV _s by cell swelling.     

Two dimensional diffusion of small molecules on protein surfaces: an EPR study of the restricted translational diffusion of protein-bound spin labels

Heisenberg spin exchange rates and dipole-dipole spin lattice relaxation rates for deuterated ¹⁴N- and ¹⁵N-spin labels bound selectively to the histidine His15 and to the lysines Lys13, 96, 97 of the lysozyme molecule have been determined with the aid of electron spin resonance spectroscopy. The results can be interpreted in terms of a two dimensional translational diffusion of the nitroxide tips of the spin labels along the protein surface within restricted surface areas. The spin labels are regarded as models for long amino acid side chains and as probes for the dynamics of protein and water in the vicinity of the protein surface. The translational diffusion coefficient DP_II is reduced by a factor of between six and thirty compared to the value of D found for the spin labels in bulk water, its value for T = 295 K is given by (1.3±0.6)·10^–10m²s^–1 D (2.4±0.3) 10^–11 m²s^–1. Offprint requests to: H.-J. Steinhoff     

Asymmetric diffusion into the postsynaptic neuron: An extremely efficient mechanism for removing excess GABA from synaptic clefts on the Deiters' neurone plasma membrane

Microdissected Deiters' neuron plasma membranes have been used for studying the passage of GABA through the membrane both in the inward and outward direction. Working with 0.2 mM GABA in the compartment simulating the outside of the neurone and with 2.0 mM GABA in the one simulating the inside we found a net transport of GABA towards the inside. This mechanism does not require a Na⁺ ion gradient across the membrane. The nature of the transport process involved was studied by determining the rate of [³H]-GABA inward passage as a function of GABA concentration (1 nM–800 M) on the outward side of the membrane. The results have shown that until 50 M a diffusion process (v=D₁×C, where D₁=3.1×10^–11 1/m²×sec) is the sole mechanism involved. Above 50 M a second diffusion process is activated v=D₂×(C–50×10^–6), where D₂=2.8×10^–11 1/m²×sec. Taking in account both inward and outward directed diffusion, one can calculate 16 M as the equilibrium concentration of GABA on the outward side of the membrane. From a kinetic point of view, these diffusion processes are able to reduce GABA concentration in a synaptic cleft from 3 mM to 20 M within 3 sec. These diffusion systems are discussed as extremely efficient in removing the excess of released GABA in the synaptic cleft.     

Effects of Pb<Superscript>2+</Superscript> ions on Na<Superscript>+</Superscript> transport in the isolated skin of the toad <Emphasis Type="Italic">Pleurodema thaul</Emphasis>

The effects induced by lead ions on the short-circuit current (SCC) and on the potential difference (V) of the toad Pleurodema thaul skin were investigated. Pb²⁺ applied to the outer (mucosal) surface increased SCC and V and when applied to the inner (serosal) surface decreased both parameters. The stimulatory effect, but not the inhibitory action, was reversible after washout of the metal ion. The amiloride test showed that the increase was due principally to stimulation of the driving potential for Na⁺ (V-E_Na+) and that inhibition was accompanied by a reduction in the V-E_Na+ and also by a significant decrease in skin resistance indicating possible disruption of membrane and/or cell integrity. The effect of noradrenaline was increased by outer and decreased by inner administration of Pb²⁺. The results suggest that mucosal Pb²⁺ activates toad skin ion transport by stimulating the V-E_Na+ and that serosal Pb²⁺, with easier access to membrane and cellular constituents, inactivates this mechanism, revealing greater toxicity when applied to the inner surface of the skin. Abbreviations: SCC – short-circuit current; V – potential difference; V-E_Na+– driving potential for Na⁺; ENaC – epithelial sodium channel; R_Na+– active sodium resistance; RS – passive or shunt resistance; G_Na– active sodium conductance; GS – passive or shunt conductance; Gmax – total conductance; EC₅₀– half-maximal excitatory concentration; IC₅₀– half maximal inhibitory concentration; NA – noradrenaline.     

Biosorption of chromium to fungi

Eighteen fungal strains were isolated from water and soil samples and tested for their ability to enrich chromium. The microorganism with the highest enrichment capacity, a zygomycete (Mucor hiemalis MP/92/3/4), was chosen for detailed investigations. Some basic tests such as the pH-dependence, the kinetics of the enrichment and the metal selectivity were carried out with the two most frequent oxidation states of chromium, the trivalent cation (Cr³⁺) and the hexavalent anion (CrO₄ ^2–). With Cr³⁺ the enrichment showed a saturation kinetic reaching 70% of the maximum capacity after about 30 min, whereas with CrO₄ ^2– a linear time course with a much lower metal enrichment was observed. The highest level of enrichment for Cr³⁺ was observed at pH 5.5 (21.4 mg/g dry wt), and for CrO₄ ^2– at pH 1 (4.3 mg/g dry wt). Investigations concerning the metal enrichment selectivity resulted in the following series of decreasing ion uptake: Cr³⁺ > Cu²⁺ > Pb²⁺ > Ag⁺ > Al³⁺ > Co²⁺ > Zn²⁺ > Ni²⁺ > Fe²⁺ > Mo⁵⁺ > Cd²⁺ > ^2– > CrO₄ ^2– > VO^3–, calculated on a molar basis. Trivalent chromium caused a staining of the outer cell wall region in transmission electron microscopy. The localization of chromium in the stained outer layers of the cell wall could be verified by electron energy loss spectroscopy. The enrichment of Cr³⁺ by M. hiemalis seemed to be mainly a passive biosorption to the cell wall, whereas for the uptake of CrO₄ ^2– intracellular accumulation as well as biosorption is possible.     

Sulfur cycling in the water column of Little Rock Lake, Wisconsin

The S cycle in the water column of a small, soft-water lake was studied for 9 years as part of an experimental study of the effects of acid rain on lakes. The two basins of the lake were artificially separated, and one basin was experimentally acidified with sulfuric acid while the other served as a reference or control. Spatial and seasonal patterns of sulfate uptake by plankton (53–70 mmol m^–2 yr^–1), deposition of sulfur to sediments in settling seston (53 mmol m^–2 yr^–1), and sulfate diffusion (0–39 mmol m^–2 yr^–1) into sediments were examined. Measurements of inputs (12–108 mmol m^–2 yr^–1) and outputs (5.5–25 mmol m^–2 yr^–1) allowed construction of a mass balance that was then compared with rates of S accumulation in sediments cores (10–28 mmol m^–2 yr^–1) and measured fluxes of S into the sediments. Because of the low SO₄ ^2– concentrations (µmole L^–1) in the lake, annual uptake by plankton (53–70 mmol m^–2 yr^–1) represented a large fraction (>50%) of the SO₄ ^2– inventory in the lake. Despite this large flux through the plankton, only small seasonal fluctuations in SO₄ ^2– concentrations (µmole L^–1) were observed; rapid mineralization of organic matter (half-life <3 months) prevented sulfate depletion in the water column. The turnover time for sulfate in the water column is only 1.4 yr; much less than the 11-yr turnover time of a conservative ion in this seepage lake. Sulfate diffusion into and reduction in the sediments (0–160 µmole m^–2 d^–1) caused SO₄ ^2– depletion in the hypolimnion. Modeling of seasonal changes in lake-water SO₄ ^2– concentrations indicated that only 30–50% of the diffusive flux of sulfate to the sediments was permanently incorporated in solid phases, and about 15% of sulfur in settling seston was buried in the sediments. The utility of sulfur mass balances for seepage lakes would be enhanced if uncertainty about the deposition velocity for both sulfate aerosols and SO₂, uncertainty in calculation of a lake-wide rate of S accumulation in sediments, and uncertainty in the measured diffusive fluxes could be further constrained.     

The basal permeability to water of human red blood cells evaluated by a nuclear magnetic resonance technique

The characteristics of water diffusional permeability (P) of human red blood cells were studied on isolated erythrocytes by a doping nuclear magnetic resonance technique. In order to estimate the basal permeability the maximal inhibition of water diffusion was induced by exposure of red blood cells to p-chloromercuribenzene sulfonate (PCMBS) under various conditions (concentration, duration, temperature). The lowest values of P were around 0.7×10^–3 cm s^–1 at 10°C, 1.2×10^–3 cm s^–1 at 15°C, 1.4×10^–3 cm s^–1 at 20°C, 1.8×10^–3 cm s^–1 at 25°C, 2.1×10^–3 cm s^–1 at 30°C and 3.5×10^–3 cm s^–1 at 37°C. The mean value of the activation energy of water diffusion (E_a,d) was 25 kJ/mol for control and 43.7 kJ/mol for PCMBS-inhibited erythrocytes. The values of P and E_a,d obtained after induction of maximal inhibition of water diffusion by PCMBS can be taken as references for the basal permeability to water of the human red blood cell membrane.     

Photon correlation spectroscopy of human IgG

The translational diffusion coefficient D _20,w ⁰ , of monomeric human immunoglobulin G (IgG) has been studied by photon-correlation spectroscopy as a function of pH and protein concentration. At pH 7.6, we find D _20,w ⁰ =3.89×10^–7±0.02 cm²/sec, in good agreement with the value determined by classic mehods. This value corresponds to an effective hydrodynamic radius R, of 55.1±0.3 Å. As pH is increased to 8.9; with the same ionic strength, the molecule appears to expand slightly (3.5% increase in hydrodynamic radius). The concentration dependence of the IgG diffusion constant is interpreted in terms of solution electrostatic effects and shows that long-range repulsive interactions are negligible in the buffer used. The diffusion coefficient for dimeric IgG has also been determined to be D_20,w=2.81×10^–7±0.04 cm²/sec at 1.6 mg/ml, which corresponds to a hydrodynamic radius of 75 Å. For light-scattering studies of protein molecules in the dimension range of 5–10 nm (M_r=10⁵–10⁷) we find monomeric horse spleen ferritin well suited as a reference standard. Ferritin is a spherical molecule with a hydrodynamic radius R of 6.9±0.1 nm and is stable for years in our standard Tris-HCl-NaCl buffer even at room temperature.     

Fluxes of methane and carbon dioxide from a small productive lake to the atmosphere

The fluxes of CH₄ and CO₂ to the atmosphere, and the relative contributions of ebullition and molecular diffusion, were determined for a small hypertrophic freshwater lake (Priest Pot, UK) over the period May to October 1997. The average total flux of CH₄ and CO₂ (estimated from 7 sites on the lake) was approximately 52 mmol m^–2 d^–1 and was apportioned 12 and 40 mmol m^–2 d^–1 toCH₄ and CO₂ respectively. Diffusion across the air-water interface accounted for the loss of 0.4and 40 mmol m^–2 d^–1 of CH₄ and CO₂ respectively whilst the corresponding figures for ebullition losses were 12.0 (CH₄) and 0.23 (CO₂) mmol m^–2 d^–1. Most CH₄ (96%) was lost by ebullition, and most CO₂ (99%) by diffusive processes. The ebullition of gas, measured at weekly intervals along a transect of the lake, showed high spatial and temporal variation. The CH₄ content of the trapped gas varied between 44 and 88% (by volume) and was highest at the deepest points. Pulses of gas ebullition were detected during periods of rapidly falling barometric pressure. Therelevance of the measurements to global estimates ofcarbon emission from freshwaters are discussed.