Inhibition dissociation constant

Note Ka and Kis are inhibition (dissociation) constants for the formation of the inhibitor complexes in which the subscripts denote the intercept effect and slope effect, respectively. 

Figure 5. Relationship between the bimolecular Inhibition reaction constant (kj) of substituted phenyl N-methylcarbamates and the inhibition dissociation constant (Kj) of the corresponding N-methyl N-methoxycarbamates in their reaction with house fly acetylcholinesterase, k Values were recalculated from (42) and Kj values were from (41).

Securinine nitrate was approved for medical use in the USSR as a substitute to imported strychnine.For its stimulant and antispasmodic effects, securinine nitrate was marketed as a drug in this country until the early 1990s. ° The intravenous toxicity of securinine nitrate in mice was quantified with a LD50 value of 3.5 0.9 mg/kg. Securinine was also found to be a weak inhibitor of acetylcholine esterase with an enzyme-inhibition dissociation constant of 1.6 0.1 10... 

Aim To measure the affinity of a ligand by observing the inhibition it produces of a receptor-bound radioligand (or ligand that is traceable by other means). The object is to obtain an estimate of the equilibrium dissociation constant of the nonradioactive ligand receptor complex (alternately denoted KB or Kj. The pattern of displacement curves can also be used to determine whether or not the antagonism is competitive. 

Uncompetitive antagonism, form of inhibition (originally defined for enzyme kinetics) in which both the maximal asymptotic value of the response and the equilibrium dissociation constant of the activator (i.e., agonist) are reduced by the antagonist. This differs from noncompetitive antagonism where the affinity of the receptor for the activating drug is not altered. Uncompetitive effects can occur due to allosteric modulation of receptor activity by an allosteric modulator (see Chapter 6.4). 

In such inhibition, the inhibitor and die substrate can simultaneously bind to the enzyme. The nature of the enzyme-inhibitor-substrate binding has resulted in a ternary complex defined as EIS. The Ks and Kt are identical to the corresponding dissociation constants. It is also assumed that the EIS does not react further and is unable to deliver any product P. The rate equation for non-competitive inhibition, unvAX, is influenced ... 

Substrate and product inhibitions analyses involved considerations of competitive, uncompetitive, non-competitive and mixed inhibition models. The kinetic studies of the enantiomeric hydrolysis reaction in the membrane reactor included inhibition effects by substrate (ibuprofen ester) and product (2-ethoxyethanol) while varying substrate concentration (5-50 mmol-I ). The initial reaction rate obtained from experimental data was used in the primary (Hanes-Woolf plot) and secondary plots (1/Vmax versus inhibitor concentration), which gave estimates of substrate inhibition (K[s) and product inhibition constants (A jp). The inhibitor constant (K[s or K[v) is a measure of enzyme-inhibitor affinity. It is the dissociation constant of the enzyme-inhibitor complex. 

Inhibition of Glycosidases by AIdono-l,S-lactones and Aldohexoses Expressed by the Dissociation Constant K of the Enzyme-Inhibitor Complex... 

A case similar to the slow, practically irreversible inhibition of jack bean a-D-mannosidase by swainsonine is represented by the interaction of castanospermine with isomaltase and rat-intestinal sucrase. Whereas the association constants for the formation of the enzyme-inhibitor complex were similar to those of other slow-binding glycosidase inhibitors (6.5 10 and 0.3 10 M s for sucrase and isomaltase, respectively), the dissociation constant of the enzyme-inhibitor complex was extremely low (3.6 10 s for sucrase) or could not be measured at all (isomaltase), resulting in a virtually irreversible inhibition. Danzin and Ehrhard discussed the strong binding of castanospermine in terms of the similarity of the protonated inhibitor to a D-glucosyl oxocarbenium ion transition-state, but were unable to give an explanation for the extremely slow dissociation of the enzyme-inhibitor complex. 

Inhibition of EGF binding by palytoxin could be due to a decrease in receptor affinity, as in the case of TPA-type tumor promoters, and/or a decrease in receptor number. In Swiss 3T3 cells there are two classes of EGF receptors. The dissociation constants for the two EGF receptor classes were determined to be approximately 2 X 10 M and 2 x 10" M, corresponding to approximately 1 x 10 and 1 X 10 receptor molecules per cell, respectively (33). Scatchard analysis revealed that treatment of Swiss 3T3 cells with palytoxin, like PDBu, caused an apparent loss in high-affinity binding (Figure 2). However, in contrast to PDBu, palytoxin also caused a significant (approximately 50%) loss of low affinity EGF binding. 

In this chapter we described the thermodynamics of enzyme-inhibitor interactions and defined three potential modes of reversible binding of inhibitors to enzyme molecules. Competitive inhibitors bind to the free enzyme form in direct competition with substrate molecules. Noncompetitive inhibitors bind to both the free enzyme and to the ES complex or subsequent enzyme forms that are populated during catalysis. Uncompetitive inhibitors bind exclusively to the ES complex or to subsequent enzyme forms. We saw that one can distinguish among these inhibition modes by their effects on the apparent values of the steady state kinetic parameters Umax, Km, and VmdX/KM. We further saw that for bisubstrate reactions, the inhibition modality depends on the reaction mechanism used by the enzyme. Finally, we described how one may use the dissociation constant for inhibition (Kh o.K or both) to best evaluate the relative affinity of different inhibitors for ones target enzyme, and thus drive compound optimization through medicinal chemistry efforts. 

Characterization of inhibition modality, and from this quantitative determination of enzyme-inhibitor dissociation constants, constitutes the only rational, quantitative means of assessing relative compound affinity for a target enzyme. 

DETERMINING REVERSIBLE INHIBITION MODALITY AND DISSOCIATION CONSTANT... 

Determining Reversible Inhibition Modality and Dissociation Constant 129... 

By experiments performed as discussed here, the reversible inhibition modality of each lead compound, representing a distinct pharmacophore or chemical structural class, can be defined and the dissociation constant (A) or aK<) can be determined from the data fitting to Equations (3.1) through (3.6). As lead analogues are produced within a structural series, one can generally assume that the inhibiton modality will be the same as that of the founder molecule (i.e., the lead) of that structural series. This assumption simplifies the determination of dissociation constants for other series molecules, as described below. However, this assumption must be... 

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