| PAC | papular acrodermatitis of childhood; parent-adult-child; pericarditis-arthropathy-camptodactyly [syn... |
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| PAL | pathology laboratory; peptidyl-alpha-hydroxyglycine alpha-amidating lysine phase alteration plane; p... |
| PDP | pattern disruption point; piperidinopyrimidine; platelet-derived plasma; primer-dependent deoxynucle... |
| PI | first meiotic prophase; isoelectric point; pacing impulse; package insert; pancreatic insufficiency;... |
| PIN | product identification number |
| constant region | The stem and forking part of the Y-shaped antibody protein, consisting of amino acid chains, that is exactly the same in all antibody molecules within the same individual. (The ends of the Y-shaped molecule will vary widely between different antibodies). (09 Oct 1997) |
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| haemoglobin Constant Spring | An abnormal haemoglobin having an extended polypeptide chain (31 additional amino acid residues) on the a chain (thus, the a chain is 172 amino acids long); approximately 20% of the individuals with Hb H disease also have this defect. (05 Mar 2000) |
| Hill constant | A measure of cooperativity in a binding process. A Hill coefficient of 1 indicates independent binding, a value of greater than 1 shows positive cooperativity binding of one ligand facilitates binding of subsequent ligands at other sites on the multimeric receptor complex. Worked out originally for the binding of oxygen to haemoglobin (Hill coefficient of 2.8). (18 Nov 1997) |
| sedimentation constant | The constant s in Svedberg's equation for estimating the molecular weight of a protein from the rate of movement in a centrifugal field:where M is the molecular weight, R the gas constant, T the absolute temperature, D the diffusion constant (in square centimeters per second), V the partial specific volume of the protein, ρ the density of the solvent. The constant s, with dimensions of time per unit of field force (s = drb/dt /ω2ro where rb is the position at time t, r0 is the position at time 0, and ω is the angular velocity) is usually between 1 × 10-13 and 200 × 10-13 second. The Svedberg unit (S) is arbitrarily set at 1 × 10-13 second and is very often used to describe the sedimentation rate of macromolecules; e.g., 4 S RNA. Synonym: sedimentation coefficient. (05 Mar 2000) |
| specificity constant | Ratio of the maximum velocity (Vmax) or kcat to the true Km value for a specific substrate in an enzyme-catalyzed reaction. (05 Mar 2000) |
| Newtonian constant of gravitation | A universal constant relating the gravitational force, f., attracting two masses, m1 and m2, toward each other when they are separated by a distance, r, in the equation: f = G(m1m2/r2); it has the value of 6.67259 × 10-8 dyne cm2 g-2 = 6.67259 × 10-11 m3 kg-1 s-2 in SI units. (05 Mar 2000) |
| decay constant | <physics, radiobiology> The fraction of the amount of a radionuclide that undergoes transition per unit time. Formally: Lamda=dP/dt Where dP is the probability of a given nucleus undergoing spontaneous nuclear transition in the time interval dt. (16 Dec 1997) |
| diffusion constant | For the translational diffusion of solutes, diffusion is described by Fick's First Law, that states that the amount of a substance crossing a given area is proportional to the spatial gradient of concentration and the diffusion constant (D), that is related to molecular size and shape. A useful derived relationship is that the mean square distance moved by molecules in time t is 6Dt. (18 Nov 1997) |
| disintegration constant | <physics, radiobiology> The fraction of the amount of a radionuclide that undergoes transition per unit time. Formally: Lamda=dP/dt Where dP is the probability of a given nucleus undergoing spontaneous nuclear transition in the time interval dt. (16 Dec 1997) |
| dissociation constant | <chemistry> In a chemical equilibrium of form A + B = AB, the equilibrium concentrations (strictly, activities) of the reactants are related such that A x B/AB = a constant, Kd, the dissociation constant, that in this simplest case has the dimensions of concentration. When A is H, this is the acid dissociation constant often designated Ka and expressed as pKa (log10Ka). (18 Nov 1997) |
| dissociation constant of a base | Expressed by the general equation [B+][OH-]/[BOH] = Kb, where BOH is the undissociated base. (05 Mar 2000) |
| dissociation constant of an acid | Expressed by general equation [H+][A-]/[HA] = Ka, where HA is the undissociated acid. (05 Mar 2000) |
| dissociation constant of water | Expressed by the equation [H+][OH-] = Kw = 10-14 at 25캜. (05 Mar 2000) |
| immunoglobulin constant region | That region of the immunoglobulin (antibody) molecule that is invariable in its amino acid sequence within any class of immunoglobulins. It confers the biological specificity to the ig and is grossly species specific. It comprises the c-terminus half of the light chains and three quarters or more of the heavy chains, all of the fc fragment, and the c-terminus half of the fab fragment. (12 Dec 1998) |
| time constant | That 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. (05 Mar 2000) |