Inhibitors inhibition constants

Fig. 3 Elaboration of the DCL of inhibitors inhibition constants K and the amplification of the constitutional dynamic library (CDL) against catalytically active human hCA I and hCA II cytosolic isozymes (adapted from [66])...

Inhibitors are characterized by inhibition constants which are defined as the ratio of the rate constant for transfer to inhibitor to the propagation constant for the monomer in analogy with Eq. (6.87) for chain transfer constants. For styrene at 50°C the inhibition constant of p-benzoquinone is 518, and that for O2 is 1.5 X 10. The Polymer Handbook (Ref. 3) is an excellent source for these and most other rate constants discussed in this chapter. 

An adjacent tnfluoromethyl group sharply increases the electrophilic character of the carbonyl carbon Compounds that readily form hydrates and hemiacetals show a time-dependent reversible mhibition of the en yme acetylcholinesterase (equation 2), in which the tight complex makes inhibition only partially reversible [75] In comparison with a nonfluormated analogue, several aliphatic ketones flanked by CFj and CF2 groups, are exceptionally potent reversible inhibitors of acetylcholinesterase, as documented by companson of inhibition constants shown in equation 3 [16 ... 

Cheng, Y. C., andPrasoff, W. H. (1973). Relationship between the inhibition constant (Ki) and the concentration of inhibitor which causes 50 percent inhibition (150) of an enzymatic reaction. Biochem. Pharmacol. 22 3099—3108. 

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. 

In the case of dmg interactions involving metabolic inhibition, little increase in the substrate concentration is expected when the inhibition constant (K ) determined in in vitro studies using human liver samples is larger than the inhibitor concentration in vivo. Various approaches have been adopted using mathematical models in attempts to quantitatively predict in vivo dmg interactions from in vitro data [5]. 

Common inhibitors include stable radicals (Section 5.3.1), oxygen (5.3.2), certain monomers (5.3.3), phenols (5.3.4), quinones (5.3.5), phenothiazine (5.3.6), nitro and nitroso-compounds (5.3.7) and certain transition metal salts (5.3.8). Some inhibition constants (kjkp) are provided in Table 5.6. Absolute rate constants (kj) for the reactions of these species with simple carbon-centered radicals arc summarized in Tabic 5.7. 

Tabic 5.6 Inhibition constants (kr/kv,60 °C, bulk) for Various Inhibitors with Some Common Monomers"... 

Double reciprocal plots distinguish between competitive and noncompetitive inhibitors and simpbfy evaluation of inhibition constants Aj. v, is determined at several substrate concentrations both in the presence and in the absence of inhibitor. For classic competitive inhibition, the lines that connect the experimental data points meet at they axis (Figure 8-9). Since they intercept is equal to IIV, this pattern indicates that wben 1/[S] approaches 0, Vj is independent of the presence of inhibitor. Note, however, that the intercept on the X axis does vary with inhibitor concentration—and that since is smaller than HK, (the apparent... 

In the first group of studies, involving kinetic inhibition studies, comparisons of the uilibrium (K ), phosphorylation (IC), and inhibition constant (K.) for the inhibition of electric eel and human erythrocyte AChE by ANTX-A(S) and DFP were done (Table II). From Table II it is seen that ANTX- A(S) has a higher affinity for human erythrocyte AChE (K =0.253 fiM) than electric eel AChE (K j=3.67 aM). AN DC-A(S) also shows greater affinity for AChE than DFP (K =300 fiM). And finally the bimolecular rate constant, Kj, which indicates the overall rate of reaction, shows AChE is more sensitive toward inhibition by ANTX-A(S) (Kj=1.36 pM- min- ) than DFP (K, = 0.033 /iM- min ). These studies add information to the comparative activity of ANTX-A(S) and other irreversible AChE inhibitors but do not show the site of inhibition. 

The most potent thrombin inhibitor is hirudin, originally isolated from the salivary glands of the medicinal leech Hirudo medicinalis. Its inhibition constant is in the femtomolar (10-15 M) range (57). It is a 65-amino-acid tyrosine-sulfated single-chain polypeptide. Recombinant hirudin differs from native hirudin by the absence of the sulfate group on tyrosine 63 (Tyr-63) and is referred to as desulfato hirudin. The loss of this sulfate group reduces the thrombin inhibitory potency by 10-fold. 

Inhibitor Approximate molecular mass (Da) Inhibition constant (K.)... 

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. 

Our inhibitor design strategy was based on the premise that structural modifications in the base of purine riboside that enhance purine base hydration without impairing the binding of the hydrated species to the ADA binding site would result in purine riboside (PR) analogues with high ADA inhibitory potency. Since the apparent inhibition constant (Kj (app)) is related to the hydration equilibrium constant (Keq) and the inhibitory constant for the hydrated molecule (Kj ) by... 

In this case, [I] represents the inhibitor concentration at the enzyme-active site and K is the inhibition constant for a single enzyme ... 

The inhibition constant Kt for the complex between the enzyme (E) and the inhibitor (7) is the dissociation constant for the enzyme inhibitor complex (El) ... 

A large number of potential reversible protease inhibitors exist (Laskowski Kato, 1980). Protein protease inhibitors like Strepromyces Subtilisin Inhibitor (SSI) (Hiromi et al, 1985) and Chymotrypsin Inhibitor (CI-2) (Jonassen, 1980 and McPhalen James, 1988) are known to be very strong inhibitors with inhibition constants at or below 10"10 M. 

The kinetic parameters kcat and Km were determined without inhibitor and at several inhibitor concentrations. The inhibition constants K,e and Kies were calculated from plots of KJkcat and /kcat as a function of the concentration of inhibitor. 

The inhibition constants found for the more promising inhibitors were typically below Kj = 10"4 M, that is several orders of magnitude lower than the inhibition constant for boric acid as expected. 

Much earlier, Wolfenden (Westerick and Wolfenden, 1972) and Thompson (1973), established a criterion for enzyme inhibitors working as TSAs. Iliey proposed that such activity should be reflected by a linear relationship between the inhibition constant for the enzyme K and its inverse second-... 

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