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PubMed1988

Facilitated diffusion of p-nitrophenyl-alpha-D-maltohexaoside through the outer membrane of Escherichia coli. Characterization of LamB as a specific and saturable channel for maltooligosaccharides.

LamB, an outer membrane protein of Escherichia coli, is a component of the maltose-maltooligosaccharide transport system. We used p-nitrophenyl-alpha-D-maltohexaoside, a chromogenic analog of maltohexaose, and a periplasmic amylase that hydrolyzes this compound to study the LamB-mediated diffusion of p-nitrophenyl-alpha-D-maltohexaoside into the periplasm. Using this approach, we were able to characterize LamB in vivo as a saturable channel for maltooligosaccharides. Permeation through LamB follows Michaelis-Menten kinetics, with a Km of 0.13 mM and a Vmax of 3.3 nmol/min/10(9) cells. Previous studies suggested that maltose-binding protein increases the rate of maltooligosaccharide diffusion through LamB. We show here that, at least in strains that are unable to transport maltooligosaccharides into the cytoplasm, maltose-binding protein does not influence the rate of substrate diffusion. The periplasmic amylase had been previously described as being of the alpha-type. We have now purified this protein and analyzed its mode of action using chromogenic maltooligosaccharides of varying length. Analysis of the hydrolytic products revealed that the enzyme recognizes its substrate from the nonreducing end and preferentially liberates maltohexaose, in contrast to the behavior of classical alpha-amylases that are endohydrolases. Using p-nitrophenyl-alpha-D-maltohexaoside as a substrate, we determined a Km of 3 microM and a Vmax of 0.14 mumol/min/mg of protein.

Bacterial Outer Membrane Proteins↗
PubMed1993

A mathematical model for the elimination of carbon monoxide in humans.

Carboxyhaemoglobin (COHb) build-up in the blood as a result of exposures to carbon monoxide (CO) affects human beings. It is important to eliminate CO from the blood for treatment and health safety. A mathematical model is proposed to compute COHb level in the blood as a function of post-exposure time as CO is eliminated. The model takes into account molecular diffusion, facilitated diffusion, convection, non-equilibrium kinetics of CO with haemoglobin and the important physiological parameters, such as ventilation rate, blood flow rate and the total volume of blood in the body. Endogenous production of CO in the body is included in the formulation. The resulting coupled system of non-linear partial differential equations with physiologically relevant initial, entrance and boundary conditions is solved numerically. The COHb levels computed from our model agree with those measured experimentally (Pace et al., 1950; Peterson & Stewart, 1970). The half-life of COHb, i.e. the time required for the blood COHb to decrease from the initial level to its half-value is computed. The half-life values of COHb computed from our model are in good agreement with those based on experimental data collected under different physiological conditions (Pace et al., 1950). Also, the results predicted from our model give better approximation to the experimental values than the CFK equation (Coburn et al., 1965). It is found that the rate of elimination of CO increases with the increase of inspired PO2 and ventilation rate.

Carbon Monoxide↗
PubMed1986

[Mechanisms of intestinal absorption of nutrients].

The nutrient uptake from the intestinal lumen into the distributing blood circulation is mediated by the epithelial cell of the small intestine. The transfer process through this distinctly polar cell consists of three partial events: entrance of substances through the brush-border membrane, traversal of a metabolic active intracellular space and exit through the baso-lateral membrane. The fundamental transfer mechanisms--simple diffusion, facilitated diffusion, antiport and symport systems, electroneutral and electrogenic processes--are described. The significance of nutrient metabolization for transport processes is discussed: proton secretion by the epithelial cell coupled to the glucose and lactate metabolization is quoted as an illustration. The "acid microclimate" resulting from this proton secretion on the mucosal surface has a significant influence on weak-electrolyte absorption. This effect was clearly demonstrated for in vitro uptake of nicotinic acid into the intestinal tissue. It can be assumed that--similar to the role of a Na+-gradient--the proton gradient on the surface of absorptive epithelia is highly significant as a driving force of nutrient absorption.

Animals↗
PubMed1992

The effects of chemical kinetics on oxygen delivery to tissue.

A mathematical model has been formulated to analyze the effect of nonequilibrium kinetics on oxygen delivery to tissue. The model takes into account molecular diffusion, facilitated diffusion in the capillary blood, convection, chemical kinetics of O2 with hemoglobin, and the rate of metabolic consumption. A line iterative technique is described to solve numerically the resulting coupled system of nonlinear partial differential equations with physiologically relevant boundary and entrance conditions. With nonequilibrium kinetics the end-capillary PO2 is found to be lower than that in the venous blood. The effect is more pronounced during hypoxia and anemia. It is found that the tissue PO2 at the lethal corner decreases with the decrease in blood velocity, arterial PO2, hemoglobin concentration, P50, and increase in COHb concentration or metabolic rate, while the difference between end-capillary PO2 and venous PO2 increases, which reflects the effect of nonequilibrium kinetics on the delivery of O2 to tissue. Thus, the consideration of venous PO2 as an indicator of tissue PO2 in clinical and experimental studies may be questionable.

Biological Transport↗
PubMed1980

Dynamics of maternal-fetal nutrient transfer.

Several mechanisms account for transfer of substances across the placenta, including passive diffusion, facilitated diffusion, active transport, solute drag, and pinocytosis. The rates at which these processes occur and the rates of equilibration of various substances between maternal and fetal circulation vary widely. In general, gases and some ions equilibrate rapidly while large molecules such as proteins exchange slowly. Some nutrients such as amino acids, calcium, iron, and possibly some vitamins are transported against a concentration gradient. This report compares equilibration rates for various nutrients and discusses the use of mathematical modeling to quantitate certain aspects of the processes, and to gain a better understanding of factors determining placental exchange.

Animals↗
PubMed1984

Theoretical interpretation of extraction (in brain) of peptides including concentration variations.

The transport properties of several peptides across blood-brain barrier (BBB) have been investigated theoretically in terms of simple diffusion and facilitated diffusion processes. Comparison of the calculated results from the simple diffusion and the experimental data reveals the presence of the facilitated diffusion of these substances which we have conceived of as a carrier-mediated process. The values of the partition coefficients f for these peptides were in the range 7 X 10(-4) less than or equal to f less than or equal to 200 X 10(-4). The calculated f values gave permeabilities, Ps, in lipids between 10(-7) less than or equal to Ps less than or equal to 14 X 10(-7) cm/s. These values were then used to estimate the extraction for peptides from simple diffusion alone which vary from 0.3 to 3.5% compared with the experimental extraction (0.4-12%) indicating the inadequacy of the simple diffusion alone to explain the experimental data. As for the carrier-mediated facilitated diffusion process we have used the activated-complex theory. The extraction in this case depends on the maximal rate of transport (Tmax)f and the reciprocal of the affinity constant Kt for the transport of peptides through BBB. We have deduced that (Tmax)f approximately 0.46 X 10(-3) pmol/g X s and Kt approximately 0.35 nM for Met-enkephalin (Met-ENK), Leu-enkephalin (Leu-ENK), glutathione, carnosine, alpha-MSH and MIF and (Tmax)f approximately 10 X 10(-3) pmol/g X s and Kt approximately 7 nM for AVP, beta LT, beta E and alpha E to explain the observed results. We have also obtained the quantitative variation of extraction with concentration of peptides in the brain-capillary and have established that the extraction decreases with increasing concentration of peptides, tending to a small constant value at high concentrations. It has been inferred that carrier-mediated facilitated diffusion is important for the transport of peptides across BBB.

Animals↗
PubMed1976

Proton transport by phosphate diffusion--a mechanism of facilitated CO2 transfer.

We have measured CO2 fluxes across phosphate solutions at different carbonic anhydrase concentrations, bicarbonate concentration gradients, phosphate concentrations, and mobilities. Temperature was 22-25 degrees C, the pH of the phosphate solutions was 7.0-7.3. We found that under physiological conditions of pH and pCO2 a facilitated diffusion of CO2 occurs in addition to free diffusion when (a) sufficient carbonic anhydrase is present, and (b) a concentration gradient of HCO3- is established along with a pCO2 gradient, and (c) the phosphate buffer has a mobility comparable to that of bicarbonate. When the phosphate was immobilized by attaching 0.25-mm-long cellulose particles, no facilitation of CO2 diffusion was detectable. A mechanism of facilitated CO2 diffusion in phosphate solutions analogous to that in albumin solutions was proposed on the basis of these findings: bicarbonate diffusion together with a facilitated proton transport by phosphate diffusion. A mathematical model of this mechanism was formulated. The CO2 fluxed predicted by the model agree quantitatively with the experimentally determined fluxes. It is concluded that a highly effective proton transport mechanism acts in solutions of mobile phosphate buffers. By this mechanism; CO2 transfer may be increased up to fivefold and proton transfer may be increased to 10,000-fold.

Carbon Dioxide↗
PubMed1983

Biochemical modulation of blood-brain barrier permeability.

Hydrophilic substrates necessary for brain function cross the capillary by facilitated diffusion. The facilitation has many features in common with enzyme-catalyzed reactions and is probably subserved by protein entities in the endothelial wall. The proteins act as receptors, recognizing substrate molecules, and as translocators, giving the molecules access to an aqueous path through the endothelium. These receptor-translocators can be saturated, and the transport is subject to competitive inhibition by substrate analogs. Thus, amino acids inhibit the transport of each other, and galactose can inhibit glucose transport in suckling rats. The proteins can be induced, as in the case of ketone transport in starvation, and repressed, as in the case of glucose transport in hyperglycemia. In rats with hyperglycemia for three weeks, the maximum glucose transport capacity of the blood-brain barrier decreased from 400 to 290 mumol/hg/min. An important result of the description is the understanding that rigid distinctions between the function of receptors, translocators, and enzymes is impossible. Understanding of the biochemical properties of facilitated diffusion may help explain a variety of symptoms in many 'inborn errors of metabolism'. This understanding has followed greater, recent insights into the general properties of the blood-brain barrier (45,46,47).

Adaptation, Physiological↗
PubMed1977

The effect of carbon monoxide on haem-facilitated oxygen diffusion.

The aim of this paper is to quantify the effect of small quantities of carbon monoxide on the facilitated diffusion of oxygen by haemoglobin in the steady state. It is the first phase in the study of a mathematical model for carbon monoxide poisoning. Here we extend the Wyman model for facilitated diffusion to the case in which there are two ligands. The equations are solved using an asymptotic technique developed by Murray. We obtain accurate analytic approximations for the biologically important quantities of the problem for various percentages of carbon monoxide. These are the concentrations of free oxygen, haemoglobin, oxyhaemoglobin and carboxyhaemoglobin, and hence the saturation of the protein and the facilitated oxygen flux. The major effect of very small quantities of carbon monoxide on the oxygen flux is shown.

Carbon Monoxide↗