ALBERT

Notes: Summary Two new forms of the plasma membrane ATP-ase ofMicrococcus lysodeikticus NCTC 2665 were isolated from a sub-strain of the microorganism by polyacrylamide gel electrophoresis. One of them had a mol.wt of 368,000 and a very low specific activity (0.80 µ mol.min−1.mg protein−1) that could not be stimulated by trypsin. This form has been called BI (strain B, inactive). If the electrophoresis was carried out in the presence of reducing agents (i.e., dithiothreitol) and the pH of the effluent maintained at a value of 8.5 another form of the enzyme was obtained. This had a mol.wt of 385,000 and a specific activity of 2.5–5.0 µ mol.min−1.mg protein−1 that could be stimulated by trypsin to 5–10 µ mol.min−1.mg protein−1. This preparation of the ATPase has been called form BA (strain B, enzyme active). The subunit composition of both forms has been studied by sodium dodecyl sulphate and urea gel electrophoresis and compared to that of the enzyme previously purified from the original strain (form A). The three forms of the enzyme had similarβ and δ subunits, with mol.wt of about 50,000 and 30,000 dalton, respectively. They also had in common the component(s) of relative mobility 1.0, whose status as true subunit(s) of the enzyme remains yet to be established. However, subunitα, that had a mol.wt of about a 52,500 in form A (Andreu et al. Eur. J. Biochem. (1973) 37, 505–515), had a mol.wt similar toβ in form BI and about 60,000 in form BA. Furthermore BA usually showed two types of this subunit (α′ andα′′) and an additional peptide chain (ε) with a mol.wt of about 25,000 dalton. This latter subunit seemed to account for the stimulation by trypsin of form BA. Forms BA could be converted to BI by storage and freezing and thawing. Conventional protease activity could not be detected in any of the purified ATPase forms and addition of protease inhibitors to form BA failed to prevent its conversion to form BI. The low activity form (BI) was more stable than the active forms of the enzyme and also differed in its circular dichroism. These results show thatM. lysodeikticus ATPase can be isolated in several forms. Although these variations may be artifacts caused by the purification procedures, they provide model systems for understanding the structural and functional relationships of the enzyme and for drawing some speculations about its functionin vivo.

Notes: Summary Basal and trypsin-stimulated adenosine triphosphatase activities ofEscherichia coli K 12 have been characterized at pH 7.5 in the membrane-bound state and in a soluble form of the enzyme. The saturation curve for Mg2+/ATP = 1/2 was hyperbolic with the membrane-bound enzyme and sigmoidal with the soluble enzyme. Trypsin did not modify the shape of the curves. The kinetic parameters were for the membrane-bound ATPase: apparent Km = 2.5mm, Vmax (minus trypsin) = 1.6µmol·min−1·mg protein−1, Vmax (plus trypsin) = 2.44µmol·min−1·mg protein−1; for the soluble ATPase: [S0.5] = 1.2mm, Vmax (-trypsin) = 4µmol·min−1·mg protein−1; Vmax (+trypsin) = 6.6µmol·min−1·mg protein−1. Hill plot analysis showed a single slope for the membrane-bound ATPase (n = 0.92) but two slopes were obtained for the soluble enzyme (n = 0.98 and 1.87). It may suggest the existence of an initial positive cooperativity at low substrate concentrations followed by a lack of cooperativity at high ATP concentrations. Excess of free ATP and Mg2+ inhibited the ATPase but excess of Mg/ATP (1/2) did not. Saturation for ATP at constant Mg2+ concentration (4mm) showed two sites (groups) with different K ms: at low ATP the values were 0.38 and 1.4mm for the membrane-bound and soluble enzyme; at high ATP concentrations they were 17 and 20mm, respectively. Mg2+ saturation at constant ATP (8mm) revealed michaelian kinetics for the membrane-bound ATPase and sigmoid one for the protein in soluble state. When the ATPase was assayed in presence of trypsin we obtained higher Km values for Mg2+. These results might suggest that trypsin stimulatesE. coli ATPase by acting on some site(s) involved in Mg2+ binding. Adenosine diphosphate and inorganic phosphate (Pi) act as competitive inhibitors ofEscherichia coli ATPase. The Ki values for Pi were 1.6 ± 0.1mm for the membrane-bound ATPase and 1.3 ± 0.1mm for the enzyme in soluble form, the Ki values for ADP being 1.7mm and 0.75mm for the membrane-bound and soluble ATPase, respectively. Hill plots of the activity of the soluble enzyme in presence of ADP showed that ADP decreased the interaction coefficient at ATP concentrations below its Km value. Trypsin did not modify the mechanism of inhibition or the inhibition constants. Dicyclohexylcarbodiimide (0.4mm) inhibited the membrane-bound enzyme by 60–70% but concentrations 100 times higher did not affect the residual activity nor the soluble ATPase. This inhibition was independent of trypsin. Sodium azide (20µ m) inhibited both states ofE. coli ATPase by 50%. Concentrations 25-fold higher were required for complete inhibition. Ouabain, atebrin and oligomycin did not affect the bacterial ATPase.

Notes: Summary The Arrhenius plots for the active and low activity soluble forms of the ATPase purified from the membranes ofMicrococcus lysodeikticus grown at 30°C presented discontinuities at 30 and 33°C, respectively. Their activation parameters differed, being highest for the low activity form of the enzyme. Both forms underwent changes in their molecular properties as a consequence of being enzymically active, i.e., upon incubation with substrates at an adequate temperature. These changes consisted of a decrease in the relative mobilities of some of their subunits in dodecyl sulphate polyacrylamide gel electrophoresis, and the temperature at which they occurred depended on the energy of activation of the particular form of the ATPase used. The low activity form required an incubation temperature of 50°C, whereas for an active form 37°C was sufficient.

Notes: Abstract. We have studied the magnetic structure of the orthorhombic compound Ho2Ni2Pb by means of neutron diffraction in zero field and in magnetic fields up to 4.5 T. Both powder and single-crystalline samples were used. Previous bulk measurements suggest two distinct magnetic phase transitions: one at TN = 7.0 K and the other at 4.8 K. Our neutron diffraction measurements, which were made in the range 1.5-20 K, showed that Ho2Ni2Pb has a collinear magnetic structure with unequal number of up and down Ho moments that are aligned parallel and antiparallel to the c axis. At the lowest temperatures the Ho moments are equal in size, each 8.3 μB in agreement with magnetization data. The magnetic structure can be described as having a 5a ×b ×c magnetic unit cell. Below Ts = 3.0 K the structure is squared up. A smooth development of all the magnetic moment magnitudes indicates that the magnetic structure remains in principle the same over the whole temperature range, the “phase transition” around 4.8 K can be identified as an inflection point in the temperature dependence of one of the Ho moments. With increasing temperature there is a clear development towards a simple transverse sine-wave modulated magnetic structure that is established just below TN.

Notes: Abstract. A beautifully simple model introduced a couple of decades ago, Toom’s cellular automaton, revealed that non-equilibrium systems may exhibit generic bistability, i.e. two-phase coexistence over a finite area of the (two-dimensional) phase diagram, in violation of the equilibrium Gibbs phase rule. In this paper we analyse two interfacial models, describing more realistic situations, that share with Toom’s model a phase diagram with a broad region of phase coexistence. An analysis of the interfacial models yields conditions for generic bistability in terms of physically relevant parameters that may be controlled experimentally.

Notes: Abstract It is shown that, if (a) the gravitational field is represented by the metric tensor of a Riemann space, (b) the geodesic hypothesis is admitted, (c) it is assumed that, for a particle moving in a scalar gravitational field, this last postulate must give, by approximation, the Hamilton principle for this particle in special relativity, then a metric tensor is obtained with some interesting properties (no assumptions are made about field equations). The geodesic hypothesis, with the Lorentz transformation of this metric tensor, gives, by approximation, the Hamilton principle, with the Lagrangian corresponding in special relativity to a particle in a vector field. Moreover, the equations of motion in a Riemann space, as they follow from the geodesic postulate, in terms of associated coordinates, give, by approximation, an expression in complete analogy to that of the Lorentz force. Hence, the vector-field theory of gravitation of the Maxwell-Lorentz kind (as electrodynamics, because of the formal analogy between the two theories) is obtained as a weak field approximation of a description of gravitational (electromagnetic) interaction by a metric tensor in a Riemann space (except the field equations, an issue that we do not touch, so far).

Notes: Abstract Mortar mixes with different water-cement ratios and consistency were impregnated with methyl-methacrylate monomer and polymerized thermally under water using the free radical initiatorα, α′-azobis (isobutyronitrile). Results on drying, impregnation and polymerization are presented. It is shown that a considerable amount of polymer remained strongly adhered or chemically inserted in the inorganic matrix. The molecular weight of the inserted polymer is higher than that obtained in the solvent extracted polymer and this is also higher than the polymer molecular weight obtained by bulk polymerization under the same conditions. The compressive and flexural strength of the impregnated mortar were found to be a function of the amount of polymer in the composites. Fracture behaviour under load, and polymer distribution inside the composites were examined by scanning electron microscopic techniques (SEM). It was observed that the polymer acts in two ways, first as a filler of porous and microcrack voids, secondly forming an anisotropic irregular network improving the bond characteristic of the interface between aggregate and matrix. Furthermore, due to the the adhesion of the polymer to both phases, it acts as a reinforcement and improves the mechanical properties, in particular the flexural strength.