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Biomedical subjects

L W Nichol

Publications and source records attributed to L W Nichol.

At least 19 recordsLinked to original sources
PubMed1990

The reversible cross-linking of receptors by ligands: theory for the prediction of binding responses.

The general concept of receptor aggregation through the action of a cross-linking ligand is considered, three models being examined in detail. In two, ligand self-interaction leads to the formation of receptor cross-links whereas, in the third, receptor cross-links are formed through a single ligand bridge. Binding equations in closed form are formulated for each case and are shown to predict both concave and convex Scatchard plots in binding studies conducted with such systems. The significant point which emerges is that while each of the systems gives rise to very different types of binding responses, the form of the binding response is always strongly dependent on the total concentration of receptor in the system.

Algorithms↗
PubMed1988

The binding of an indefinitely associating ligand to acceptor: consideration of monovalent ligand species binding to a multivalent acceptor.

Currently available binding theory is extended to incorporate the concept of indefinite self-association of the ligand. Binding equations are formulated in closed form for the case of the binding to a multivalent acceptor of a ligand capable of isodesmically indefinitely self-associating in a "head-to-tail" mode such that each ligand state bears one site capable of interacting with the acceptor. It is shown both mathematically and by way of numerical example that this system will give rise exclusively to binding curves convex to the r-axis in Scatchard format. Thus, the system provides another example of a binding mechanism capable of generating an apparent negatively co-operative binding response.

Binding Sites↗
PubMed1987

The self-association of zinc-free bovine insulin. A single model based on interactions in the crystal that describes the association pattern in solution at pH 2, 7 and 10.

Sedimentation equilibrium studies are used to establish that a new pattern for the self-association of zinc-free insulin in solution is applicable over a wide range of conditions of pH, ionic strength and temperature. In this pattern, which is based on information from the existing literature on the X-ray crystal structure of insulin, the insulin monomer is viewed as having two distinct faces both capable of self-interaction. Sedimentation equilibrium experiments were analysed using expressions formulated for this association pattern that describe the dependence of weight average molecular weight and monomer concentration on total protein concentration. It has thereby been possible to obtain values for the two association constants which govern the system for each set of conditions studied, due allowance having been made for composition dependent non-ideality effects. Furthermore, by relating the pH, temperature and ionic strength dependence of the association constants with properties of various amino acid residues on the surface of the insulin monomer, it has also been possible to assign tentatively each constant to a particular reaction domain.

Animals↗
PubMed1979

Chromatographic evidence of the self-association of oxyhemoglobin in concentrated solutions: its biological implications.

Expressions that take into account the effects of thermodynamic non-ideality, described in terms of a high-order virial expansion, are derived for the concentration-dependence of the weight-average partition coefficient in exclusion chromatography of a single solute and of a solute undergoing reversible self-association. Comparison of the concentration-dependences predicted by those expressions with results obtained for bovine and human oxyhemoglobins on CPG-10-120 porous glass beads in 0.156 I phosphate-chloride buffer, pH 7.3, shows that neither oxyhemoglobin conforms with the concept of it being a single alpha 2 beta 2 entity with Stokes radius of 3.13 nm, the experimental value. Previously published osmotic pressure and sedimentation equilibrium results are also shown to be inconsistent with this concept. On the other hand, both sets of exclusion chromatography results are consistent with the joint operation of thermodynamic non-ideality and reversible association of the alpha 2 beta 2 species. From the magnitude of the equilibrium constant, derived for either of two possible modes of association, it is calculated that only half of the oxyhemoglobin would be in the alpha 2 beta 2 states under conditions of oxygen saturation and a concentration of 320 g/liter, that pertaining in the red blood cell. The consequences of this association phenomenon are discussed in relation to the oxygen binding curves obtained by others in the presence and absence of 2,3-diphosphoglycerate (DPG). An explanation is provided of the observed dependence on hemoglobin concentration of oxygen-binding in the presence of DPG, and of the absence of such an effect in DPG-free solutions. It is concluded that the control of oxygen binding to hemoglobin in the physiological situation involves the joint operation of self-association and allosteric effects.

Animals↗
PubMed1979

Studies of macromolecular heterogeneous associations involving cross-linking: a re-examination of the ovalbumin-lysozyme system.

A system is considered in which a multivalent acceptor interacts with a bivalent ligand in solution to form an array of complexes via multiple binding and cross-linking reactions. With the use of reacted site probability functions expressions are derived in terms of a site binding constant which are of potential use in the interpretation of sedimentation equilibrium and binding results obtained with such systems. Their potential use is explored in relation to results obtained on the interacting ovalbumin-lysozyme system at pH 6.80, ionic strength 0.02. A comparison is made of this interpretation with that based on an interaction pattern involving only multiple binding of ligand in the absence of cross-linking effects. While both interpretations quantitatively describe certain results, it is shown, by invoking further experimental observations on apparent weight-average molecular weight and precipitation behavior, that the more favored interpretation is that involving the operation of a spectrum of forces leading to a large array of ovalbumin-lysozyme complexes, including those of the cross-linked type. It is stressed that the particular ovalbumin-lysozyme system is but one example of interaction between oppositely charged macromolecules and therefore that the derived equations may find wider application to such systems and those known to involve more specific cross-linking interactions.

Chemical Phenomena↗
PubMed1978

The role of zinc in the stabilization of the dimeric form of bacterial alpha-amylase.

Bacterial alpha-amylase was shown by equilibrium and velocity-sedimentation studies to be a monomer-dimer equilibrium system in 0.10M-NaCl/0.015M-calcium acetate/0.010M-EDTA, pH7.0; an association constant of 2.4 X 10(3)M-1 is obtained. Studies of the binding of Zn2+ to alpha-amylase in 0.10M-NaCl/0.005M-calcium acetate, pH7.0, yielded binding curves that exhibit dependence on the concentration of alpha-amylase (Zn2+-free) used in the equilibrium-dialysis experiments. Results are described very satisfactorily by a reaction scheme in which Zn2+ binds exclusively to the dimer of the above monomer--dimer system with an association constant of 1.0 X 10(6)M-1. The present results refute the earlier scheme for dimer stabilization by Zn2+ in which the metal ion formed a cross-link between two non polymerizing monomer units.

Amylases↗
PubMed1978

Uptake of n-(9-anthroyloxy) fatty acid fluorescent probes into lipid bilayers.

A description in thermodynamic terms is given of ligand-membrane interaction which may occur by either or both a binding and a partition process. Results obtained by fluorescence enhancement and polarization techniques on the uptake of n-(9-anthroyloxy) fatty acids by phospholipid bilayers are analysed to show that binding rather than partition effects primarily determine the extent of probe uptake. Liposome concentration-dependence effects are also reported which required that binding results obtained with different probes be compared at fixed lipid concentrations. On this basis it is concluded that as the separation of the anthracene and carboxyl groups within the fatty acid molecule is increased, and hence as the anthracene group moves deeper into the bilayer, the fluorescent probe is bound to the bilayer with greater affinity but is accepted by fewer binding sites within the membrane. Studies on probe uptake at high ionic strength and into negatively charged bilayers indicate that hydrophobic rather than electrostatic interactions make the dominant contribution to the free energy of binding.

Fatty Acids↗
PubMed1977

The polymerization pattern of zinc(II)-insulin at pH 7.0.

Sedimentation equilibrium experiments were conducted at pH 7.0 using solutions of bovine insulin containing 2 mol of zinc(II) ions per six base-mol of insulin. A detailed analysis of these results revealed the existence of a stable zinc-insulin hexamer together with linked polymerization reactions. Specifically these are a background polymerization of zinc-free insulin as previously described by Jeffrey et al. ((1976) Biochemistry 15, 4660--4665) and a slight tendency for the zinc-insulin hexamer to undergo indefinite self-association. Equilibrium constants governing these reactions are reported together with equations which permit calculation of the composition of the solution at any given total concentration. Comment is made on the possible biological significance of this linked polymerization pattern, and on the likely identity of the structure of the stable zinc-insulin hexamer with that previously reported from X-ray crystallographic studies.

Animals↗
PubMed1977

The simulation of the urea cycle: correlation of effects due to inborn errors in the catalytic properties of the enzymes with clinical-biochemical observations.

Steady-state rate equations are written on the basis of information obtained from the literature describing the kinetics of the four enzyme-catalysed reactions comprising the urea cycle. These equations are formulated into a set which also accounts for fluxes of input and output compounds external to the cycle. Numerical integration of this set of equations is performed employing parameters selected to approximate those pertaining to the operation of the urea cycle in normal liver. The result is a pattern of intermediate metabolite concentrations, which forms a basis for the comparison of patterns reflecting the effects of inborn errors of metabolism. The latter are calculated by varying specified kinetic parameters in the numerical integration. Each of the observed clinical syndromes. Hyperammonemia Types I and II, Hyperarginemia, Citrullinemia and Argininosuccinicaciduria, is discussed.

Amino Acid Metabolism, Inborn Errors↗
PubMed1976

Polymerization pattern of insulin at pH 7.0.

Sedimentation equilibrium results, obtained with bovine zinc-free insulin (with and without a component of proinsulin) at pH 7.0, I o.2, 25 degrees C, and up to a total concentration of 0.8 g/l., are shown to be consistent with three different polymerization patterns, all involving an isodesmic indefinite self-association of specified oligomeric species. The analysis procedure, based on closed solutions formed by summing infinite series, yields for each pattern a set of equilibrium constants, It is shown that a distinction between the possible patterns can be made by analyzing sedimentation equilibrium results obtained in a higher total concentration range (up to 4 g/1.) with insulin freed of zinc and proinsulin, account being taken of the composition dependence of activity coefficients. The favored pattern, which differs from that previously reported in the literature, involves the dimerization of monomeric insulin (mol wt 5734), governed by a dimerization constant of 11 X 10(4) M-1 and the isodesmic indefinite self-association of the dimer, described by an association constant of 1.7 X 10(4) M-1. This polymerization pattern is also shown to be consistent with the reaction boundary observed in sedimentation velocity experiments.

Hydrogen-Ion Concentration↗
PubMed1976

Ligand-induced polymerization.

Three models of ligand-induced polymerization are considered, encompassing dimerization of acceptor-ligand complex and cross-linking of monomer units via a ligand bridge to form either dimer or linear chains. For each model a binding equation is developed and examined in terms of limits, form, and intersection within a family of curves, each member constructed with fixed but different total acceptor concentration. The characteristics of the binding curves that emerge are correlated with those found by examining the dependence on ligand concentration of either the weight-average molecular weight of the acceptor constituent or a function of it. In the latter plots, a maximum exists for the cross-linking models at the same ligand concentration that the corresponding binding curves intersect: the existence of the maximum implies that at saturating concentrations of ligand only monomeric acceptor constituent remains. The practical implication of these findings is discussed in terms of experimental systems reported previously in the literature. In addition, particular comment is made on the forms of binding curves for the three models, which assume shapes generally associated with positive and negative cooperativity. Finally, comparison is made with those systems in which a shift toward polymeric constituent is effected by ligand perturbation of a preexisting association equilibrium.

Albumins↗
PubMed1976

The sedimentation equilibrium of heterogeneously associating systems and mixtures of non-interacting solutes: analysis without determination of molecular weight averages.

Sedimentation equilibrium is first considered of a system in which a ligand of any size binds to an acceptor at p sites, the experimental result, obtained with either interference or absorption optics, being a distribution of total solute concentration as a function of radial distance. Theory illustrated by a numerical example, is presented which shows that this distribution may be analysed to give the activity of the unbound ligand as a function of total weight concentration. It is shown that this information may be used together with conservation of mass equations written in terms of the initial mixing composition to evaluate the equilibrium constant(s) relevant to the system. Correlation with composition evaluation by use of absorption optics (when possible) is also discussed. The procedure does not involve solution of simultaneous equations which are sums of exponentials nor differentiation of experimental results to obtain apparent weight-average molecular weights. It is general in that it leads to the evaluation of the activity of the species characterized by the smallest M(1-vp) product and, accordingly, is shown to be useful in the analysis of non-interacting as well as of interacting systems.

Binding Sites↗