Inhibitor incorporation mechanisms

The nonnucleoside reverse transcriptase inhibitors (NNRTIs), used in the treatment of AIDS, provide interesting examples of clinically relevant noncompetitive inhibitors. The causative agent of AIDS, HIV, belongs to a virus family that relies on an RNA-based genetic system. Replication of the vims requires reverse transcription of the viral genomic RNA into DNA, which is then incorporated into the genome of the infected host cell. Reverse transcription is catalyzed by a virally encoded nucleic acid polymerase, known as reverse transcriptase (RT). This enzyme is critical for viral replication inhibition of HIV RT is therefore an effective mechanism for abrogating infection in patients. 

Note that in some cases one may follow the time course of covalent E-A formation by equilibrium binding methods (e.g., LC/MS, HPLC, NMR, radioligand incorporation, or spectroscopic methods) rather than by activity measurements. In these cases substrate should also be able to protect the enzyme from inactivation according to Equation (8.7). Likewise a reversible competitive inhibitor should protect the enzyme from covalent modification by a mechanism-based inactivator. In this case the terms. S and Ku in Equation (8.7) would be replaced by [7r] and K respectively, where these terms refer to the concentration and dissociation constant for the reversible inhibitor. 

C-3 as determined by mass spectral analysis.(55) In a mechanism involving ketone hydration prior to bindTrTg, incorporation in recovered inhibitor should be at least 50%, a value corresponding to that expected for a single cycle of nonstereospecific addition/nonstereospecific elimination of water to the ketone carbonyl. The actual results then indicate that addition-elimination is a highly stereospecific process and thus enzyme-catalyzed. 

The thyroidal mechanism used for concentrating 1 may also concentrate other monovalent anions, including pertechnetate, perchlorate, and thiocyanate, within the follicular lumen. However, none of these anions become incorporated into Tg, although they may act as a competitive inhibitor of 1 transport. The ability of the thyroid gland to concentrate radioactive pertechnetate makes it a useful agent for thyroid imaging, since it is concentrated by the thyroid cells without further metabolism. The perchlorate and thiocyanate discharge tests make use of the ability of these anions to inhibit 1 transport to test for defects in the incorporation of 1 into Tg. 

Inhibition may be incorporated into the mechanism of micellar catalysis in the same way it is handled in enzyme kinetics. Representing the inhibitor by /, we can revise Reaction (G) as follows ... 

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