Characterization of the passive and active transport mechanisms for bile acid uptake into rat isolated intestinal epithelial cells

The unstirred water layer has been shown to lead to an underestimation of apparent passive permeability coefficients ($\text{P(}\text{app}\text{)}$) and cause a significant overestimation of apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{(K}{}_{\mathrm{<mtext>m</mtext>}}\text{(}\text{app}\text{))}$ values for active transport processes in intestinal whole tissue preparations. Isolated cells offer several potential advantages in the study of transport processes including a decreased diffusion layer of water adjacent to their absorptive membranes. Initial studies in cells isolated from rat intestine involving measurements of CO₂ and lactate production and O₂ consumption showed that overall metabolic pathways were functioning. Next, unidirectional uptake rates of bile acids across the isolated cell membrane were determined following correction for extracellular fluid contamination with a non-absorbable marker. Using epithelial cells isolated from jejunum $\text{P(}\text{app}\text{)}$ for eight bile acid monomers varied from 24.9 (taurocholate) to 1563 (deoxycholate) nmol/min/100 mg protein/mM. From these data the incremental free energy changes for the addition of a hydroxyl, glycine and taurine group to the bile acid molecule were calculated to be 982, 1040 and 1464 cal/mol, respectively, values similar to those obtained after correction for unstirred water layer resistance in whole tissue preparations. Following subtraction of the passive component in isolated ileal cells complete kinetic curves for taurocholate and taurodeoxycholate yielded $\text{V(}\text{app}\text{)}$ values of 109 and 70 nmol/min per 100 mg, respectively. $\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{(}\text{app}\text{)}$ values of 0.24 mM (taurocholate) and 0.10 mM (taurodeoxycholate) are lower than usually recorded in whole tissue. Bile acid uptake into cells from ileum, but not jejunum, was affected by temperature, metabolic and competitive inhibition. These studies indicate that isolated epithelial cells are a metabolically viable, relatively purified intestinal preparation which discriminates between active and passive transport processes for bile acids under conditions where unstirred water layer artifacts are are minimized.     

Thiamine transport in thiamine-deficient rats role of the unstirred water layer

As part or a systematic study of alcoholism and thiamine absorption, the effect of diet-induced thiamine deficiency and the role of the unstirred water layer on thiamine transport were investigated. Using ³H-labeled dextran as a marker of adherent mucosal volume, jejunal uptake of ¹⁴C-labeled thiamine hydrochloride was measured, in vitro, in thiamine-deficient rats and pair-fed controls. Uptake of low thiamine concentrations (0.2 and 0.5 μM) was greater in the thiamine-deficient rats thatn in the controls. In contrast, uptake rates for high thiamine concentrations (20 and 50 μM) were similar in both groups. While $\text{1J}{}_{\max }$ was unaltered, $\text{1K}{}_{\mathrm{m}}$ was decreased in thiamine deficiency, suggesting a decrease in unstirred water layer thickness. Accordingly, the thickness of the water layer was measured in both groups of animals and correlated with $\text{1J}{}_{\max }$ and $\text{1K}{}_{\mathrm{m}}$ under unstirred and st irred conditions. Without stirring, there was no difference in $\text{1J}{}_{\max }$ between the two groups. In contrast, both $\text{1K}{}_{\mathrm{m}}$ and the water layer were reduced in the thiamine-deficient rats. With stirring, $\text{1J}{}_{\max }$ was not affected, but both $\text{1K}{}_{\mathrm{m}}$ and the water layer thickness were reduced to similar values in both groups. Reversal of thiamine deficiency resulted in the return of thiamine uptake and the unstirred water layer thickness to control values. These data support the concept of a dual system of thiamine transport and emphasize the role of the unstirred water layer as an important determinant of transport kinetics not only under physiologic situations but also in diet-induced rat thiamine deficiency, a model for a clinical pathological state. The decrease in the unstirred water layer thickness in thiamine deficiency may be also viewed as a possible adaptive mechanism to facilitate absorption of meager supplies of thiamine.     

Study on models of single populations: an expansion of the logistic and exponential equations

Using the adsorption theory of chemical kinetics, a new equation concerning the growth of single populations is presented:

$\text{d}\text{X}\text{d}\text{t}\text{=}\text{μ}{}_{\mathrm{c}}\text{X(1 ?)}\text{X}\text{X}{}_{\mathrm{m}}\text{1?}\text{X}\text{X}{}_{\mathrm{m}}{}^{\mathrm{\prime }}$

or in its integral form:

$\text{ln}\text{X}\text{X}{}_{\mathrm{o}}\text{?}\text{ln}\text{X}{}_{\mathrm{m}}\text{?X}\text{X}{}_{\mathrm{m}}\text{?X}{}_{\mathrm{o}}\text{+}\text{X}{}_{\mathrm{m}}\text{X}{}^{\mathrm{\prime }}{}_{\mathrm{m}}\text{X}{}_{\mathrm{m}}\text{?X}\text{X}{}_{\mathrm{m}}\text{?X}{}_{\mathrm{o}}\text{=μ}{}_{\mathrm{c}}\text{(t?t}{}_{\mathrm{o}}\text{)}$

This equation attempts to explain the relationship between population increment and limiting resources. It can be reduced to either the logistic or exponential equation under two extreme conditions. The new equation has three parameters, X_m, X′_m and μ_c, each of which has ecological significance. $\text{X}{}_{\mathrm{m}}\text{X′}{}_{\mathrm{m}}$ concerns the efficiency of nutrient utilization by an organism. Its value is between zero and one. With ratios approaching unity, the efficiency is high; lower ratios indicate that population increment is quickly restricted by limiting resources. μ_c, is a velocity parameter lying between μ_e, (exponential growth) and μ_L (logistic growth), and is dependent on the value of $\text{solX}{}_{\mathrm{m}}\text{X′}{}_{\mathrm{m}}$. From μ_c we can predict the time course of population incremental velocity ($\text{d}\text{X}\text{d}\text{t}$), and can observe that it is not symmetrical, unlike that derived from the logistic equation. At $\text{X}{}_{\mathrm{m}}\text{X′}{}_{\mathrm{m}}\text{= 1}$ the maximum velocity of the population increment predicted from the new equation is twice that of the logistic equation.Population growth in nature seems to support the new equation rather than the logistic equation, and it can be successfully fitted by means of a least square method.     

A comparison of the effects of ouabain and 2-deoxy-d-glucose on the thermodynamic variables of the frog skin

Previous studies support the validity of a linear thermodynamic formalism relating the rates of active Na⁺ transport and oxygen consumption J_r to the electrical potential difference ΔΨ an the affinity α (negative free energy) of the metabolic driving reaction. The formulation was further tested in paired control and experimental hemiskins by the use of two inhibitors of Na⁺ transport. Ouabain, a specific inhibitor of the Na⁺ pump, might be expected to diminish the dependence of J_r on ΔΨ without affecting α, whereas 2-deoxy-d-glucose, a competitive inhibitor of glucose metabolism, should be expected to diminish α. Both inhibitors were used at concentrations adequate to depress Na⁺ transport (i.e. short-circuit current J_o) to some 50°_o of control level. Measurements were made of I_o and $\text{d}\text{J}{}_{\mathrm{<mtext>r</mtext>}}\text{d}\text{(ΔΨ)}$, and the apparent value of the affinity α_app was calculated according to the thermodynamic formulation. Ouabain depressed $\text{d}\text{J}{}_{\mathrm{<mtext>r</mtext>}}\text{d}\text{(ΔΨ)}$ without affecting α_app whereas 2-deoxy-d-glucose depressed α_app without affecting $\text{d}\text{J}{}_{\mathrm{<mtext>r</mtext>}}\text{d}\text{(αΨ)}$. The demonstration of these effects indicates the utility of the formalism.     

On the active transport of organic acids (fluorescein) in the choroid plexus of the rabbit

The kinetics of active transport of an organic acid (fluorescein) through the membranes of the choroid plexus from the lateral ventricules of the brain of rabbit was studied both morphologically and functionally. It was shown that fluorescein is actively translocated through the apical and basal membrane of the epithelium and is accumulated in blood capillaries at a concentration exceeding one order of magnitude that in the incubation medium. The kinetic curves displaying saturation and the demonstration of inhibition by other acids shows that a specific carrier is involved in the transfer across the membrane. The active transport of fluorescein at 20°C was found to be sodium independent. Total exclusion of sodium from the incubation medium does not change the Michaelis constant ($\text{K}{}_{\mathrm{<mtext>m</mtext>}}$) and maximal velocity ($\text{V}$). The active transport depends on the operation of (Na⁺ + K⁺)-ATPase as energy source but obviously no specific complexes with the participation of sodium are involved.     

Studies on glutathione transport utilizing inside-out vesicle prepared from human erythrocytes

Adenosine triphosphate-dependent glutathione transport was characterized using inside-out vesicles made from human erythrocytes. Kinetic analysis of the glutathione disulfide (GSSG) transport showed a biphasic Line-weaver-Burk plot as a function of GSSG concentration suggesting the operation of two different processes. One phase had a high affinity for GSSG and a low transport velocity. Most active at acidic pH and at 25°C, this transport activity was easily lost during the storage of vesicles at 4°C. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for Mg-ATP was 0.63 mM; guanosine triphosphate (GTP) substituted for ATP gave a 340% stimulation of transport activity. Neither dithiothreitol nor thiol reagents affected this transport process. The other phase had a low affinity for GSSG and a high transport velocity. Most active at pH 7.2 and 37°C, this transport activity was stable during storage of vesicles at 4°C for several days. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for Mg-ATP was 1.25 mM; GTP substituted with no change in activity. Dithiothreitol increased the V but did not alter the $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$, and thiol reagents inhibited the transport. These findings suggest that there are two independent transfer processes for GSSG in human erythrocytes.     

Soluble and membrane-bound thiamine-binding proteins from Saccharomyces cerevisiae

Previous communications from this laboratory have indicated that there exists a thiamine-binding protein in the soluble fraction of Saccharomyces cerevisiae which may be implicated to participate in the transport system of thiamine in vivo.In the present paper it is demonstrated that both activities of the soluble thiamine-binding protein and thiamine transport in S. cerevisiae are greatest in the early-log phase of the growth and decline sharply with cell growth. The soluble thiamine-binding protein isolated from yeast cells by conventional methods containing osmotic shock treatment appeared to be a glycoprotein with a molecular weight of 140 000 by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The apparent $\text{K}{}_{\mathrm{<mtext>d</mtext>}}$ of the binding for thiamine was 29 nM which is about six fold lower than the apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ (0.18 μM) of thiamine transport. The optimal pH for the binding was 5.5, and the binding was inhibited reversibly by 8 M urea but irreversibly by 8 M urea containing 1% 2-mercaptoethanol. Several thiamine derivatives and the analogs such as pyrithiamine and oxythiamine inhibited to similar extent both the binding of thiamine and transport in S. cerevisiae, whereas thiamine phosphates, 2-methyl-4-amino-5-hydroxymethylpyrimidine and $\text{O-}\text{benzoylthiamine}$ disulfide did not show similarities in the effect on the binding and transport in vivo. Furthermore, it was demonstrated by gel filtration of sonic extract from the cells that a thiamine transport mutant of S. cerevisiae (PT-R₂) contains the soluble binding protein in a comparable amounts to that in the parent strain, suggesting that another protein component is required for the actual translocation of thiamine in the yeast cell membrane. On the other hand, the membrane fraction prepared from S. cerevisiae showed a thiamine-binding activity with apparent $\text{K}{}_{\mathrm{<mtext>d</mtext>}}$ of 0.17μM at optimal pH 5.0 which is almost the same with the apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for the thiamine transport system. The membrane-bound thiamine-binding activity was not only repressible by exogenous thiamine in the growth medium, but as well as thiamine transport it was markedly inhibited by both pyrithiamine and $\text{O-}\text{benzoylthiamine}$ disulfide. In addition, it was found that membrane fraction prepared frtom PT-R₂ has the thiamine-binding activity of only 3% of that from the parent strain of S. cerevisiae.These results strongly suggest that membrane-bound thiamine-binding protein may be directly involved in the transport of thiamine in S. cerevisiae.     

Kinetics of active and passive components of water exchange between the air and a mite, Dermatophagoides farinae

Female North American house dust mites were found to exchange water with the ambient air from two compartments. At humidities above the critical equilibrium activity (CEA), transpiration out of a single large compartment was observed using HTO as a tracer for water. Total sorption into this compartment was also observed by following changes in the specific radioactivity. The sorption data required that an active process or pump be present. The water in this pump is the second compartment above the CEA. Below the CEA the large compartment could be identified as a compartment characterized by a small transpiration rate constant. The pump below the CEA becomes a rapidly transpiring fast compartment. By separating the water pool into two compartments, it was possible to relate a_v to k and $\text{m}\text{?}{}_{\mathrm{<mtext>S</mtext>}}$. The major effect of a_v on k was related to its effect on the permeability of the cuticle. The influence of a_v on $\text{m}\text{?}{}_{\mathrm{<mtext>S</mtext>}}$ was different for active and passive sorption. Above the CEA the pump operated at full capacity and active $\text{m}\text{?}{}_{\mathrm{<mtext>S</mtext>}}$ was directly proportional to a_v. Passive sorption was influenced by a_v in two ways. The driving force for $\text{m}\text{?}{}_{\mathrm{<mtext>S</mtext>}}$ was further reduced below saturation by the effect of a_v on the permeability of the exchange surface.     

Preferential solvation of methylmercurated calf thymus deoxyribonucleic acid.

The changes in polymer-solvent interactions that occur when native calf thymus DNA is dialyzed against Na₂SO₄ solutions of a given ionic strength and buffer concentration but of varying concentrations in methylmercuric hydroxide have been investigated with the help of solution density measurements at 25 °C and pH 6.8–7.0. From measurements executed under equilibrium dialysis conditions at the three salt levels 5 mm, 0.05 m, and 0.5 m Na₂SO₄ (m refers to molality) and in the presence of 5 mm cacodylic acid buffer, the density increments $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ for native calf thymus DNA were determined as a function of CH₃HgOH concentration. $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ was found not to vary with organomercurial concentration, irrespective of the concentration of supporting electrolyte, until a certain CH₃HgOH concentration level has been reached, viz., , beyond which $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ increases strongly with increasing concentration of CH₃HgOH. As is shown by optical melting, $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ becomes a function of organomercurial concentration the moment DNA undergoes denaturation brought about by the complexing of CH₃HgOH with the various N-binding sites of the base residues in the DNA double helix.Polymer-solvent interactions, expressed in terms of preferential water interactions (“net hydration”) and preferential salt interactions (“salt solvation”), were derived from the $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ data in combination with data obtained on the preferential interaction of CH₃HgOH with denatured DNA and data on the partial specific volumes of all major solution components, gathered from density measurements on solutions with fixed concentrations of diffusible components. Evidence is presented which shows that denaturation in general decreases the net hydration while salt becomes preferentially associated with the polyelectrolyte. This process is further amplified by the interaction of CH₃HgOH with denatured DNA: Methylmercurated DNA alters the redistribution of diffusible components at dialysis equilibrium to such an extent that in a formal sense large amounts of water are rejected from the immediate vicinity of the polymer. The molecular implications of these findings are explored. The results are further discussed in the light of previous findings where the methylmercury-induced denaturation of DNA had been studied with the help of buoyant density measurements in a Cs₂SO₄ density gradient and by velocity-sedimentation in a variety of sulfate media.     

Flow analysis in a biological system adopting the buffer capacitor concept.

The flow measurement of each component in each compartment is important in works on transport phenomena in a biological system. The method of flow measurement was studied adopting the capacitor concept derived from network thermodynamics.A biologically active component i in a compartment is defined as follows,

$\text{n}{}_{\mathrm{i}}\text{=n}{}_1\text{+n}{}_2\text{=c}{}_1\text{V+c}{}_2\text{V}$

where the total quantity ni consists of a measurable form n_i (free form, conc. c₁) and concealed form n₂ (conc, c₂). Capacitor for the species i of a compartment is defined as follows,

$\text{C=}\text{d}\text{n}{}_{\mathrm{i}}\text{d}\text{μ}{}_{\mathrm{i}}\text{=}\text{1+}\text{c}{}_2\text{c}{}_1\text{c}{}_1\text{dV}\text{dμ}{}_{\mathrm{i}}\text{+}\text{1+}\text{dc}{}_2\text{dc}{}_1\text{v}\text{dc}{}_1\text{dμ}{}_{\mathrm{i}}$

,

$\text{=Ac}{}_1\text{dV}\text{dμ}{}_{\mathrm{i}}\text{+BV}\text{d}\text{c}{}_1\text{d}\text{t}$

Thus flow of each component is expressed as,

$\text{J}{}_{\mathrm{i}}\text{=}\text{d}\text{n}{}_{\mathrm{i}}\text{d}{}_{\mathrm{i}}\text{=}\text{d}\text{n}{}_{\mathrm{i}}\text{dμ}{}_{\mathrm{i}}\text{dμ}\text{n}{}_{\mathrm{i}}\text{dt}\text{=C}\text{d}\text{μ}{}_{\mathrm{i}}\text{dt}$

,

$\text{=Ac}{}_1\text{d}\text{V}\text{dt}\text{+BV}\text{d}\text{c}{}_1\text{dt}$

Method of determination of capacitor coefficients A and B by titration experiment was also considered. For an experimental case, the capacitance for H⁺ of blood compartment was determined. The relationship between the capacitor concept and the buffer value of Van Slyke was discussed.     

Hymenolepis diminuta: The non-saturable component of methionine uptake

Lussier P. E., Podesta R. B. and Mettrick D. F. 1982. Hymenolepis diminuta: the non-saturable component of methionine uptake. International Journal for Parasitoiogy12: 265–270. The concentration dependence of in vitro unidirectional methionine influx by Hymenolepis diminuta was analysed by the relation: $\text{J =}\text{(J}{}_{\mathrm{<mtext>m</mtext>}}\text{C}{}_{\mathrm{<mtext>b</mtext>}}\text{)}\text{(K}{}_{\mathrm{t}}\text{+ C}{}_{\mathrm{<mtext>b</mtext>}}\text{)}\text{+ K}{}_{\mathrm{d}}\text{(C}{}_{\mathrm{<mtext>b</mtext>}}\text{)}$, where J_m is the maximum uptake rate, K_t is the the apparent affinity constant and C_b is the medium substrate concentration. The linear component was separated using an asymptotic least squares curve fitting procedure and the resulting constant, K_d, is thought to be an apparent permeability coefficient. K_d may be a reflection of a simple diffusive component, a second mediated component or a combination of a passive and mediated influx. The low Q₁₀ value of the K_d's for methionine uptake (Q₁₀ = 1.31) indicated that this component is probably a reflection of diffusion within the membrane. However, the decrease in the K_d component in the presence of leucine and glycine, implies that there is also a small, second, mediated component in addition to the diffusive component. K_d derived from the asymptotic portion of the concentration-flux relation was compared with the residual flux of methionine after near complete inhibition of the mediated component with leucine and glycine. The K_d component was found to be pH-sensitive, increasing as the pH decreased and was not affected by external sodium. Results indicate that the mediated component of methionine influx was accelerated by increasing external Na⁺ and H⁺ concentrations.     

Sucrose transport by Streptococcus mutans. Evidence for multiple transport systems

The transport of sucrose by selected mutant and wild-type cells of Streptococcus mutans was studied using washed cocci harvested at appropriate phases of growth, incubated in the presence of fluoride and appropriately labelled substrates. The rapid sucrose uptake observed cannot be ascribed to possible extracellular formation of hexoses from sucrose and their subsequent transport, formation of intracellular glycogen-like polysaccharide, or binding of sucrose or extracellular glucans to the cocci. Rather, there are at least three discrete transport systems for sucrose, two of which are $\text{phospho}\text{enol}\text{pyruvate-dependent}$ phosphotransferases with relatively low apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values and the other a non-phosphotransferase (non-PTS) third transport system (termed TTS) with a relatively high apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$. For strain 6715-13 mutant 33, the $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values are 6.25·10^?5 M, 2.4·10^?4 M, and 3.0·10^?3 M, respectively; for strain NCTC-10449, the $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values are 7.1·10^?5 M, 2.5·10^?4 M and 3.3·10^?3 M, respectively. The two lower $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ systems could not be demonstrated in mid-log phase glucose-adapted cocci, a condition known to repress sucrose-specific phosphotransferase activity, but under these conditions the highest $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ system persists. Also, a mutant devoid of sucrose-specific phosphotransferase activity fails to evidence the two high affinity (low apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$) systems, but still has the lowest affinity (highest $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$) system. There was essentially no uptake at 4°C indicating these processes are energy dependent. The third transport system, whose nature is unknown, appears to function under conditions of sucrose abundance and rapid growth which are known to repress $\text{phospho}\text{enol}\text{pyruvate-dependent}$ sucrose-specific phosphotransferase activity in S. mutans. These multiple transport systems seem well-adapted to S. mutans which is faced with fluctuating supplies of sucrose in its natural habitat on the surfaces of teeth.     

The composition-dependence of the extent of reaction between a multivalent acceptor and a bivalent ligand: Implications of self-interaction of the ligand in precipitin effects

Theory is presented relating to the reversible interaction of an f-valent acceptor, A, with a bivalent ligand, B, which leads to the formation of a series of complexes comprising networks of alternating A and B molecules. An explicit expression is derived for the overall extent of reaction in terms of the total molar concentrations of reactants $\text{(}\text{m}{}_{\mathrm{A}}$ and $\text{m}{}_{\mathrm{B}}\text{)}$, the valency of the acceptor and the site-binding constant, k, governing the equilibria. It is shown by differentiation of this expression holding $\text{m}{}_{\mathrm{A}}$ (or $\text{m}{}_{\mathrm{B}}$) fixed that relations are available for the independent evaluation of f and k from a combination of precipitin and radioimmunoassay experiments. Moreover, it is established that dilution with solvent ($\text{m}{}_{\mathrm{A}}\text{/}\text{m}{}_{\mathrm{B}}$ fixed) cannot lead to the appearance of a precipitate with this type of crosslinking system. The latter observation forms the background for the development of theory pertaining to the joint operation of ligand dimerization, 2B?B₂, and crosslinking of the multivalent acceptor with bivalent B₂. The theoretical examination of this system is developed in terms of site-probability functions and involves the delineation of unique solutions for the extent of crosslinking reaction aided by the definition of the extent of binding in defined limits. It is shown with the use of numerical examples that the system involving self-associating ligand may result in the appearance of a precipitate on dilution with solvent and the conditions for the operation of this phenomenon are elucidated. It is noted that other types of ligand self-interaction may lead to similar effects in crosslinking systems, and the general principles emerging from this study are discussed in terms of systems in which antibody ligands are known to be involved in association reactions or are suspected to be so involved on the basis of precipitation effects observed on dilution with solvent.     

Galactose uptake by human platelets in vitro

Galactose transport by human platelets has been studied by measuring the cellular accumulation of the radiolabeled sugar during brief periods of suspension in varying concentrations of galactose. Weighted least-squares regression curves fitted to the measurements (initial velocity versus galactose concentration) indicate that a kinetic model with two saturable components is statistically more consistent with the data than a model based upon a single process ($\text{P < 0.001}$). For the two-component model $\text{K}{}_{\mathrm{<mtext>m</mtext><mtext>1</mtext>}}\text{= 0.29}$ mM, $\text{V}{}_1\text{= 1.2}\text{mmol/min per 10}{}^{15}\text{platelets}$, $\text{K}{}_{\mathrm{<mtext>m</mtext><mtext>2</mtext>}}\text{= 46}\text{mM}$, $\text{V}{}_2\text{= 117}\text{mmol/min per 10}{}^{15}\text{platelets}$. The fact that galactose metabolites did not accumulate during the initial phase of uptake indicates that the uptake process is not mediated by enzymatic catalysis. Surface binding also appears inadequate to explain the uptake. The most likely basis for the kinetic data, therefore, is membrane transport. The kinetics are consistent with transport by coexistent membrane structures as well as with transport by a single structure manifesting negative cooperativity.     

An allosteric pore model for sugar transport in human erythrocytes

Glucose transport in human erythrocytes is characterized by a marked asymmetry in the $\text{V}$ and $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values for entry and for exit. In addition, they show a high $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ and a high $\text{V}$ for equilibrium exchange but low $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values for infinite cis and for infinite trans exit and entry. An allosteric pore model has been proposed to account for these characteristics. In this model, substrate-induced conformational changes destabilize the interfaces between protein subunits (the pore gates).Pores doubly occupied from inside destabilize the transport gates and result in high $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ and high $\text{V}$ transport parameters. This effect is less marked when pores are doubly occupied from outside and therefore transport asymmetry results.     

Osmotic permeability of Novikoff hepatoma cells

The osmotic permeability coefficient (P_f) for water movement across Novikoff hepatoma cells was found to be 82 ± 3 (S.E.) · 10^?5 cm · s^?1 at 20°C. The corresponding diffusional permeability coefficient for ³HHO (P_d) was 97 ± 10 (S.E.) · 10^?5 cm · s^?1, therefore the ratio $\text{P}{}_{\mathrm{<mtext>f</mtext>}}\text{P}{}_{\mathrm{<mtext>d</mtext>}}$ is close to unity. The apparent activation energy for water filtration was 10.4 ± 0.4 (S.E.) kcal · mol^?1. This value is significantly greater than the activation energy for the self diffusion of water. The product of the hydraulic permeability coefficient and the viscosity coefficient for water was temperature-dependent. However, the product of the hydraulic permeability coefficient and the viscosity coefficient for membrane lipid did not vary with temperature. These data are interpreted as evidence for water movement across a lipid membrane barrier rather than through aqueous channels.     

Cockroaches on a treadmill: Aerobic running

Five species of cockroach were tested on a miniature treadmill at three velocities as O₂ consumption () was measured: Gromphadorhina chopardi, Blaberus discoidalis, Eublaberus posticus, Byrsotria fumagata and Periplaneta americana. All cockroaches showed a classical aerobic response to running: increased rapidly from a resting rate to a steady-state on-response varied from under 30 s to 3 min. Recovery after exercise was rapid as well; $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ off-response varied from under 30 s to 6 min. These times are faster or similar to mammalian values. varied directly with velocity as in running mammals, birds and reptiles. during steady-state running was only 4–12 times higher than at rest. Running is energetically much less costly per unit time than flying, but the cost of transport per unit distance is much more expensive for pedestrians. The minimal cost of transport (M_run), the lowest necessary to transport a given mass a specific distance, is high in cockroaches due to their small size. The new data suggest that insects may be less economical than comparable sized vertebrates.     

Quantitative structure-activity relationships of chymotrypsin-ligand interactions: An analysis of interactions in p3 space.

Fourteen derivatives of l-alanine of the type CH₃CH(NHCO-3-C₅H₄N)COOR₃ have been synthesized and their hydrolysis by chymotrypsin was studied with the object of characterizing enzymic space (?₃) to which R₃ binds. The binding of R₃ ($\text{log}\text{1}\text{K}{}_{\mathrm{m}}$) was shown via correlation analysis to correlate with molar refractivity (MR) of R₃ rather than hydrophobicity (π). The results confirmed our earlier predictions. A correlation equation for the hydrolysis of 77 acyl-amino acid esters of the general formula R₂CH(NHCOR₁)COOR₃ relating $\text{log}\text{(k}{}_{\mathrm{<mtext>cat</mtext>}}\text{k}{}_{\mathrm{m}}\text{)}$ to molar refractivity of R₁, R₂, and R₃ and to $\text{σ}{}^1$ (Taft's polar parameter) of R₃ was formulated. The general picture of ligand interactions with chymotrypsin as seen with correlation analysis is discussed.     

Phytoplankton photosynthesis in the subsurface chlorophyll-maximum layer of the tropical North Pacific Ocean

Maximum quantum yield (φ_m^B) and maximum photosynthetic rate (P_m^B) of light-saturation curves of phytoplankton photosynthesis were determined for nannoplankton (< 20 μm) and netplankton (>20 μm) from the subsurface chlorophyll-maximum layer at 14 stations in the tropical North Pacific Ocean in the spring of 1976. The maximum quantum yield $\text{(φ}{}_{\mathrm{m}}{}^{\mathrm{B}}\text{±}\text{s.e.}\text{)}$ was significantly higher for nannoplankton (0.056 ± 0.006 moles CO₂·Einstein^?1 absorbed) than netplankton (0.039 ± 0.002 moles CO₂·Einstein^?1 absorbed). The importance of nannoplankton in the maximum photosynthetic rate (P_m^B) appears to be less consistent. At least 60% of the theoretical maximum quantum yield (0.12 moles CO₂·Einstein^?1 absorbed) was probably incorporated into the particulate fraction at the subsurface chlorophyll-maximum layer.