The regulation of uterine relaxation

The regulation of uterine relaxation is poorly understood but research in myometrial tissue and other types of smooth muscle has defined a number of receptors, ion channels and regulatory proteins that are likely to be involved. Some of these proteins are substrates for protein kinases, especially cyclic nucleotide dependent kinases. More research is necessary to identify the key molecules involved in the maintenance of uterine quiescence in pregnancy. The use of tocolytics in preterm labour remains controversial; there is a need to identify better pharmacological targets to provoke safe and selective uterine relaxation and improve neonatal outcome.     

Leukocyte relaxation properties.

Study of the mechanical properties of leukocytes is useful to understand their passage through narrow capillaries and interaction with other cells. Leukocytes are known to be viscoelastic and their properties have been established by micropipette aspiration techniques. Here, the recovery of leukocytes to their normal spherical form is studied after prolonged deformation in a pipette which is large enough to permit complete entry of the leukocyte. The recovery history is characterized by the time history of the major diameter (d1) and minor diameter (d2). When the cell is removed from the pipette, it shows initially a small rapid recoil followed by a slower asymptotic recovery to the spherical shape. In the presence of cell activation and formation of pseudopods, the time history for recovery is prolonged compared with passive cell recovery. If a protopod pre-existed during the holding period, the recovery only begins when the protopod starts to retract.     

Induction of vascular relaxation by hydroperoxides

Hydrogen peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, and 3-chloroperoxybenzoic acid (CPB) and 15-HPETE relaxed, in a concentration dependent manner rat aortic rings contracted with PGF2 alpha (1 X 10(-5)). Relaxation is not inhibited by either indomethacin (2 X 10(-5) M), a cyclo-oxygenase inhibitor or eicosatetraynoic acid (1 X 10(-5) M), a dual cyclo-oxygenase and lipoxygenase inhibitor. Rings with intact endothelium relaxed to a greater degree on exposure to CPB and 15-HPETE. Methylene blue, a soluble guanylate cyclase inhibitor (1 X 10(-5) M) blocked the relaxation elicited by the five peroxides, whereas both superoxide dismutase (scavenger of superoxide anion) and mannitol (scavenger of hydroxyl radical) have no effect. We conclude that relaxation of vascular smooth muscle is a general property of peroxides and that the endothelium may in some instances facilitate this effect.     

Proton relaxation studies of diphosphopyridine coenzymes

The proton relaxation times of DPNH and 1-(2,6-dichlorobenzyl)-dihydronicotinamide (DCB-DHN) and their respective C4 deuterium-labeled dihydronicotinamide compounds were studied by Fourier transform NMR spectroscopy. The geminal C4 protons were found to exhibit unusually short relaxation times in comparison to the other dihydronicotinamide protons. Deuterium substitution at C4 increases the relaxation time of the remaining C4 proton. The deuterium relaxation time of the C4 deuterium labels has been measured. The small value of the deuterium T₁ precludes any significant contribution of the deuterium-proton scalar coupling. The advantages of C4 deuterium labeling in binding studies of DPNH with enzymes using NMR spectroscopy are also presented.     

Flexible relaxation of rigid-body docking solutions

Molecular Dynamics (MD) simulations have been performed on a set of rigid-body docking poses, carried out over 25 protein-protein complexes. The results show that fully flexible relaxation increases the fraction of native contacts (NC) by up to 70% for certain docking poses. The largest increase in the fraction of NC is observed for docking poses where anchor residues are able to sample their bound conformation. For each MD simulation, structural snap-shots were clustered and the centre of each cluster used as the MD-relaxed docking pose. A comparison between two energy-based scoring schemes, the first calculated for the MD-relaxed poses, the second for energy minimized poses, shows that the former are better in ranking complexes with large hydrophobic interfaces. Furthermore, complexes with large interfaces are generally ranked well, regardless of the type of relaxation method chosen, whereas complexes with small hydrophobic interfaces remain difficult to rank. In general, the results indicate that current force-fields are able to correctly describe direct intermolecular interactions between receptor and ligand molecules. However, these force-fields still fail in cases where protein-protein complexes are stabilized by subtle energy contributions.     

The dipolar origin of protein relaxation

1. A set of parameters is proposed to check the interpretation of the dielectric behaviour of protein solutions as a rigid-dipole relaxation of prolate ellipsoids of revolution in the frequency range between 20 kHz and 10 MHz. Besides the delta(b)-function of Scheraga, another analogous function (delta(a)) is presented to establish size and shape of globular proteins. A study of the influence of solvent viscosity on the dielectric dispersion also gives strong evidence in favour of rigid-dipole relaxation. 2. Measurements of the dielectric dispersion of monomer solutions of bovine serum albumin and transferrin are reported. Monomers of bovine serum albumin were obtained by fractionation on Sephadex G-150. Low-conductivity solutions of both proteins are obtained by passage through an ion-exchange resin. 3. Computer analysis of the experimental dispersion curves by use of a two-term Debye dispersion gives valuable information about transferrin and leads to an axial ratio 4.5 for a prolate ellipsoid of revolution. The dielectric increment of bovine serum albumin is very low and no conclusive results have yet been obtained.     

Direct analysis of continuous relaxation spectra

The dynamic response of a complex biological or chemical system to a perturbation must often be described by an integral over an effectively continuous relaxation spectrum. Because of its well known instability to experimental error, the direct estimation of the spectrum is generally considered unfeasible. However, we show that good estimates can be obtained by constraining the spectrum to be the smoothest one that is consistent with the data. Also constraining the spectrum to be non-negative, if there is a priori knowledge of this, can further increase its accuracy. The method is completely automatic in that no initial estimates or assumptions about the functional form of the spectrum are necessary. Therefore models can be tested more rigorously and objectively since the functional form that they predict for the spectrum need not be assumed at the outset of the analysis as with parameter-fitting procedures. The method is illustrated on simulated data on the photodissociation of CO from heme proteins at low temperatures. The nonuniqueness of the solutions is discussed.