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Browsable database of medical products, manufacturer, therapeutic categories, indication, contra indication, side effects, drug and food interaction.

Medical Product | Medical Manufacturer | Therapheutic | Indication | Contra Indication | Side Effect | Drug Interaction | Food Interaction

**constant** (kon´stant)

A quantity that, under stated conditions, does not vary with changes in the environment.Ambard'sSee Ambard'sc.laws, underlaw.associationc.1.in experimental immunology, a mathematical expression of hapten-antibody interaction: average associationc.,K= [hapten-bound antibody]/[free antibody][free hapten];2. (the equilibriumK_{a})c.involved in the association of two or more compounds or ions into a new compound; the reciprocal of the dissociationc.bindingc;Avogadro'sc.Avogadro'snumberbindingc.associationcdecaythe fractional change in the number of atoms of a radionuclide which occurs in unit time; the constant l in the equation for the fraction (DN/N) of the number of atoms (N) of a radionuclide disintegrating in timec.Dt, DN/N = - lDt.disintegrationc., radioactivec., transformationc;diffusionc.diffusioncoefficientdisintegrationc.decaycdissociationthe equilibriumc.(K_{d},K)c.involved in the dissociation of a compound into two or more compounds or ions. The reciprocal of the associationc.(2).dissociationexpressed by general equation [Hc.of an acid (K_{d},K_{a})^{+}][A^{-}]/[HA] =K_{a}, where HA is the undissociated acid.dissociationexpressed by the general equation [Bc.of a base (K_{b})^{+}][OH^{-}]/[BOH] =K_{b}, where BOH is the undissociated base.dissociationexpressed by the equation [Hc.of water (K_{w})^{+}][OH^{-}] =K_{w}= 10^{-14}at 25°C.equilibriumin the reactionc.(K_{eq})A+B<->C+Dat equilibrium (i.e., no net change in concentrations ofA,B,C, orD), the concentrations of the four components are related by the equationK_{eq}= [C][D]/[A][B];K_{eq}is the equilibriumc.If any component in the reaction has a multiplier (e.g., H_{2}<-> 2H), that multiplier appears as an exponent in the calculation ofK(e.g.,K_{eq}= [H]^{2}/[H_{2}]). When this equation is applied to the ionization of a substance in solution,K_{eq}is called the dissociationc.(K_{d}) and its negative logarithm (base 10) is the pK_{d}. See also Henderson-Hasselbalchequation, mass-actionratio.Faraday'sSee faraday.c.(F)flotationcharacteristic sedimentation behavior of a lipoprotein fraction of plasma in a centrifugal field in a medium of appropriate density, achieved by adding a salt or Dc.(S_{f})_{2}O to the plasma.negative S, Svedberg of flotation;gasc.(R)R(symbol for the constant) = 8.314 x 10^{7}ergs per degree Celsius per mole = 8.314 J K^{-1}mol^{-1}(joules per kelvin mole).Hillc.HillcoefficientMichaelisc.1.the true dissociation constant for the enzyme-substrate binary complex in a single-substrate rapid equilibrium enzyme-catalyzed reaction (usually symbolized byK_{s});2.the concentration of the substrate at which half the true maximum velocity of an enzyme-catalyzed reaction is achieved (when velocities are measured under initial rate and steady state conditions); the ratio of rate constants (k_{2}+k_{3})/k_{1}in the single-substrate enzyme-catalyzed reaction: E + S <-> ES <-> E + products where E represents the free enzyme, S is the substrate, and ES is the central binary complex. The expression for the Michaelisc.will be more complex for multisubstrate reactions. An apparent Michaelisc.is ac.determined either under conditions that are not strictly steady state and initial rate or one that varies with the concentration of one or more cosubstrates. See Michaelis-Mentenequation.Michaelis-Mentenc;Michaelis-Mentenc.(K_{m})MichaeliscNewtoniana universalc.of gravitation (G)c.relating the gravitational force,f., attracting two masses,m_{1}andm_{2}, toward each other when they are separated by a distance,r, in the equation:f= G(m_{1}m_{2}/r^{2}); it has the value of 6.67259 x 10^{-8}dyne cm^{2}g^{-2}= 6.67259 x 10^{-11}m^{3}kg^{-1}s^{-2}in SI units.permeabilitya measure of the ease with which an ion can cross a unit area of membrane driven by a 1.0c.(P with a subscript for the ion, P)mdifference in concentration; usually expressed in centimeters per second.Cf. permeabilitycoefficient.Planck'sac.(h)c., 6.6260755 x 10^{-34}J. s (joule-seconds) or 6.6260755 x 10^{-27}erg-seconds = 6.6260755 x 10^{-34}J Hz^{-1}(joule per hertz).radioactivec.(LAMBDA)decaycrateproportionalityc.'s(k)c.'sequal to the initial rate of a reaction divided by the concentration of the reactant(s); e.g., in the reaction A -> B + C, the rate of the reaction equals -d[A]/dt=k_{1}[A]. The ratec.k_{1}is a unimolecular ratec.since there is only one molecular species reacting and has units of reciprocal time (e.g., sec^{-1}). For the reverse reaction, B + C -> A, the rate equals -d[B]/dt= d[A]/dt=k_{2}[B][C]. The ratec.k_{2}is a bimolecular ratec.and has units of reciprocal concentration-time (e.g.,m^{-1}sec^{-1}).velocityc.'s;sedimentationthec.c.sin Svedberg's equation for estimating the molecular weight of a protein from the rate of movement in a centrifugal field: whereMis the molecular weight,Rthe gas constant,Tthe absolute temperature,Dthe diffusion constant (in square centimeters per second),Vthe partial specific volume of the protein, rho the density of the solvent. The constants, with dimensions of time per unit of field force (s = d_{rb}/dt / omega^{2}r_{o}wherer_{b}is the position at timet,r_{0}is the position at time 0, and omega is the angular velocity) is usually between 1 x 10^{-13}and 200 x 10^{-13}second. The Svedberg unit (S) is arbitrarily set at 1 x 10^{-13}second and is very often used to describe the sedimentation rate of macromolecules; e.g., 4 S RNA.sedimentationcoefficient;specificityratio of the maximum velocity (Vc._{max}) ork_{cat}to the trueK_{m}value for a specific substrate in an enzyme-catalyzed reaction.timethat part of a circuit that determines the time interval over which the rate of electrical events will be averaged; in pulmonary physiology, the factors determining rate of flow in the airways.c.transformationc.decaycvelocityc.'s(k)ratec.'s

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