Inhibition constant, definition

Competitive, 249, 123, 146, 190 [partial, 249, 124 progress curve equations for, 249, 176, 180 for three-substrate systems, 249, 133, 136] competitive-uncompetitive, 249, 138 concave-up hyperbolic, 249, 143 dead-end, 249, 124 [for bireactant kinetic mechanism determination, 249, 130-133 definition of kinetic constants, 249, 220-221 effects on enzyme progress curves, nonlinear regression analysis, 249, 71-72 inhibition constant evaluation, 249, 134-135 kinetic analysis with, 249, 123-143 one-substrate systems, 249, 124-126 unireactant systems, theory,... 

A competitive inhibition screen is often the first step in understanding the DDI potential of a NCE. The definitive assessment of inhibition is the inhibition constant (K ), which provides not only the inhibition potency but also information on the mechanism of inhibition (competitive, non-competitive). However in the hit to lead profiling environment this approach is over-complex for the question being asked, and generates far too many samples to enable rapid screening of compound series. DDI assays based upon the IC50 principle are therefore favored. The relationship between K and IC50 for a competitive inhibitor is ... 

There are two Haldanes for Ordered Uni Bi mechanism, with n = 1, and two for Ordered Bi Bi, but the second one depends on the definition of the inhibition constants. Ping Pong mechanisms with two stable enzyme forms have four Haldanes, one each with n = 0 and n = 2, and two with n = 1. The Theorell-Chance mechanism with only six rate constants has 16 Haldanes with n equal to minus one (1), zero (4), one (6), two (4), and three (1), corresponding to 4,3,2,1, and 0 inhibition constants, respectively. Ordered mechanisms that are of Ter reactancy in either direction have two Haldanes, with n = 0 and n = (Cleland, 1982). 

The reaction rates, 91, 91 and 913, are in terms of the mole concentration of glucose, maltose and maltotriose, 5, S2 and. The maximum reaction velocities, Michaelis constants and inhibition constants in the reaction rates follow the Arrhenius dependency. The definitions of the symbols and their corresponding data can be obtained from the literature (Gee and Ramirez, 1988 Wang and Jing, 2(XX)). 

Discussion. The turbidity of a dilute barium sulphate suspension is difficult to reproduce it is therefore essential to adhere rigidly to the experimental procedure detailed below. The velocity of the precipitation, as well as the concentration of the reactants, must be controlled by adding (after all the other components are present) pure solid barium chloride of definite grain size. The rate of solution of the barium chloride controls the velocity of the reaction. Sodium chloride and hydrochloric acid are added before the precipitation in order to inhibit the growth of microcrystals of barium sulphate the optimum pH is maintained and minimises the effect of variable amounts of other electrolytes present in the sample upon the size of the suspended barium sulphate particles. A glycerol-ethanol solution helps to stabilise the turbidity. The reaction vessel is shaken gently in order to obtain a uniform particle size each vessel should be shaken at the same rate and the same number of times. The unknown must be treated exactly like the standard solution. The interval between the time of precipitation and measurement must be kept constant. 

When inhibited oxidation is nonstationary with respect to hydroperoxide, there is a definite, mechanism-dependent correlation between the amounts of the inhibitor consumed and hydroperoxide produced (see Table 14.2). The values of parameter a can be expressed through the rate constants k2 and k7 (the symbol denotes that these constants are measured at 333 K) from Table 14.5. 

It is true that completely independent experiments may be confused by subtle effects of inhibition by H2S, which is a byproduct in desulfurization reactions, but as discussed later, the magnitude of the inhibition at the levels produced in the experiments is not large enough to result in such drastic changes in the hydrogenation rates of biphenyl. A few well-chosen experiments in which competitive test reactions are conducted simultaneously can provide definitive information that can be used in setting reasonable limits on the ratios of the important rate constants. This is illustrated later. 

In a definitive series of experimental investigations H. N. Wilson showed that the quinolinium salt, (C isNJ fPCV I2M0O3]3- was anhydrous, contained exactly 12 moles of molybdenum trioxide per mole of phosphate, that the precipitate had a negligible solubility and could be dried to constant weight in two hours at 105 °C. This precipitate also lent itself to a precise alkalimetric titration. In the presence of citric acid interference by silica was inhibited so that the method was admirably suitable for the analysis of basic slags or fertilizers.34... 

It should be noted that the mechanism depicted in Scheme 1 is the simplest that is consistent with mechanism-based inhibition. The mechanism for a given inhibitor and enzyme may be considerably more complex due to (a) multiple intermediates [e.g., MIC formation often involves four or more intermediates (29)], (b) detectable metabolite that may be produced from more than one intermediate, and (c) the fact that enzyme-inhibitor complex may produce a metabolite that is mechanistically unrelated to the inactivation pathway. Events such as these will necessitate alternate definitions for Z inact, Kh and r in terms of the microrate constants of the appropriate model. The hyperbolic relationship between rate of inactivation and inhibitor concentration will, however, remain, unless nonhyperbolic kinetics characterize this interaction. Silverman discussed this possibility from the perspective of an allosteric interaction between inhibitor and enzyme (5). Nonhyperbolic kinetics has been observed for the interaction of several drugs with members of the CYPs (30). 

One of the most interesting things about human COX enzymes is that there is more than one of them—definitely two, and probably at least three. This is important to our understanding of the therapeutic effects of ibuprofen, aspirin, and acetaminophen. It had long been suspected that there was more than one COX enzyme, but it was not until 1991 that evidence for the existence of two forms, COX-1 and COX-2, materialized. It was then recognized that COX-1 is present at near constant levels in the body under all conditions (that is, it is a constitutive enzyme), whereas the levels of COX-2 could increase in response to inflammatory conditions (i.e., it is an inducible enzyme). This led to the idea that the side effects of ibuprofen and aspirin (including stomach ulcers) probably arose from inhibition of the constitutive COX-1 enzyme, whereas the therapeutic benefits arose from inhibition of the inducible COX-2 enzyme. 

Biochemists observe other kinds of enzyme inhibition. Noncompetitive inhibition consists of cases in which an inhibitor combines with either the E or the ES form of the enzyme. This requires definition of two new inhibitor constants ... 

The inhibited esterase can also undergo aging, e.g. via net loss of the R-group to yield the negatively charged phos-phonyl adduct on the active site serine of the enzyme. Aging is characterized by a first-order rate constant, fcj, and the operational definition of this reaction is the time-dependent... 

The alkaline phosphatase of both human intestine and placenta are L-phenyl-alanine-sensitive and undergo uncompetitive inhibition to the same extent (nearly 80%) by 0.005 M L-phenylalanine. However, we have been able to find several distinguishing biochemical characteristics of the two enzymes (1) the anodic mobility of intestinal alkaline phosphatase remains unchanged after neuraminidase treatment, whereas the placental enzyme is sialidase-seusitive and hence the electrophoretic mobility on starch gel is considerably reduced by such treatment, (2) the Michaelis constant of placental alkaline phosphatase at a definite pH is appreciably higher than that of the intestinal enzyme (at pH 9.3 the Km values of placenta and intestine are 316 and 160 ixM, respectively), and (3) the pH optima (with 0.018 Af phenyl phosphate as substrate) of the two enzymes are different the values for intestinal and placental enzymes with 0.006 Af n-phenylalanine are 9.9 and 10.6, respectively, and the respective values in the presence of 0.005 Af L-phenylalanine are 10.2 and 11.1. Finally, contrary to the behavior of intestinal alkaline phosphatase, placental enzyme is completely heat stable (P19). 

Alternatively we could measure the 1C50 or the Ki (inhibitory constant) for the perpetrator. The A) of a perpetrator that is capable of inhibiting an enzyme (or transporter) is the dissociation constant for the enzyme-inhibitor complex. Accurate estimation of the A) requires, among other things, the appropriate definition or specification of the type of enzyme inhibition (e.g., competitive, noncompetitive, or uncompetitive). The appropriate in vitro experiments require that multiple concentrations of the inhibitor must be used as well as a range of substrate concentrations that embrace the substrate Km, and from these experiments both the type of inhibition elicited by the perpetrator can be deduced and the A) value for the perpetrator can be estimated. The Ki will have units of concentration. Alternatively, K values can be computed from /C50 values for an inhibitor. The /C50 is defined simply as the inhibitor concentration that decreases the biotransformation of a substrate at a single, specified concentration by 50%. This parameter obviously also has units of concentration (e.g., pM), and can be related to the Ki as follows. 

The relationship between Na-K ATPase activity and active trans-membrane transport of Na" " and K, discussed in detail in earlier reviews [6,127,128] rests on the following arguments. Both Na-K ATPase and Na - K transport are activated by the simultaneous presence of internal ATP, Mg and Na and external K and both are inhibited by externally present cardiac glycosides like ouabain. The half-maximal activating concentrations of Na and K, the values for ATP and the half-inhibitory concentrations of ouabain are nearly equal for the two activities in the systems where they have been determined. For a large variety of tissues there is a remarkably constant ratio of 3 Na transported per ATP hydrolyzed ([6] pp. 271-272 [128] p. 158). The 3Na /2K stoichiometry for the transport agrees with the ratio of Na released to K bound upon phosphorylation of the enzyme (Section 2). Definitive proof for the involvement of the enzyme in transmembrane transport of Na and K has come from reconstitution studies in which a purified... 

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