Mutual Exclusivity Studies

If one of the compounds of interest has a unique spectroscopic feature, or can be synthesized with a fluorescent or radioactive label, then a variety of equilibrium 

Chapter 3 Reversible Modes of Inhibitor Interactions with Enzymes 

Mutual exclusivity can also be tested for by the effects of combinations of two inhibitors on the activity of a target enzyme. The advantage of this approach is that it does not require any special labeling of either compound, and only catalytic quantities of enzyme are required for the studies. There are a number of graphical methods that can be used to determine the effects of inhibitor combinations on enzyme velocity (see Copeland, 2000). The most popular of these was introduced by Yonetani and Theorell (1964) and is based on the following reciprocal equation  

Here vy is the enzyme velocity in the presence of both compounds at concentrations [/] and [J], The term y is an interaction term that defines the degree to which binding of one compound perturbs the affinity of the enzyme for the second compound. 

If two compounds bind in a mutually exclusive fashion, then their effects on enzyme velocity are additive and the value of y is infinite (i.e., the combination term 

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As the systems studied become more complex, it will be preferable to use different, mutually exclusive, separations for specific products, rather than to attempt to separate all possible products with a single, although possibly complex process. There the difficulty will lie in the unstable species which under different procedures can give different products, and perhaps appear to be different initial species. 

Clearly, many questions remain. If NO is an intermediate in GTN-mediated relaxation, then the enzyme(s) involved in GTN-NO conversion are not identified with certainty. Moreover, several hypotheses, not necessarily mutually exclusive, compete to explain the phenomenon of tolerance. Finally, it is plausibly argued that pharmacologically relevant concentrations of GTN activate the sGC/GK-1 system by some yet unidentified mechanism that does not involve NO. There is still much scope for study. 

Several studies have simultaneously tested the IDM and the CNBM, although these two models are not mutually exclusive, i.e., the predictions are usually not... 

From these studies, there are several factors that change sufficiently to provide a mechanism for fatigue and they are not mutually exclusive ... 

Nuclear receptors exert their different transcriptional functions through interactions with and the recruitment of co-factors to responsive promoters. Co-factors are either positive or negative regulatory proteins and are classified as co-activators, which promote, or co-repressors, which attenuate the activity of nuclear hormone receptors [46]. The molecular mechanisms that regulate the mutually exclusive interactions of the nuclear receptor with either class of co-factors have been analysed by crystallographic studies. Functional and structural studies have shown that co-activators interact with the transactivation function (AF) of nuclear hormone receptors via short, leucine-rich motifs (LXXLL) termed NR boxes , thereby transducing hormonal signals to the basal transcription machinery [47]. 

These two mechanisms, the one emphasizing the adsorption of specific often polynuclear hydrolysis products, the other emphasizing the role of polymeric species, are clearly not mutually exclusive an earlier study on thorium (IV) adsorption suggested a combined mechanism (J). 

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