Proteases, mechanism-based inactivation

Ernest, C.S., II, Hall, S.D. and Jones, D. R. (2005) Mechanism-based inactivation of CYP3A by HIV protease inhibitors. Journal of Pharmacology and Experimental Therapeutics, 312 (2), 583-591. 

Koudriakova T, latsimirskaia E, Utkin I, et al. Metabolism of the human immunodeficiency virus protease inhibitors indinavir and ritonavir by human intestinal microsomes and expressed cytochrome P4503A4/3A5 mechanism-based inactivation of cytochrome P4503A by ritonavir. Drug Metab Dispos 1998 26(6) 552-561. 

In contrast, 5-benzyl-6-chloro-2-pyrone is not a substrate for chymotrypsin, but inactivates the enzyme with the formation of a shoulder in the absorbance spectrum at 320 nm (Westkaemper and Abeles, 1983). The chromophore, which is similar to that of the pyrone ring, appears with kinetics corresponding to the rate of inactivation and is lost on reactivation of the enzyme. The crystal structure and C NMR studies of the modified enzyme demonstrate that the benzyl group is bound in the specificity pocket of the active site and that serine-195 is covalently attached to C-6 of the intact pyrone (Ringe et ai, 1985) (Fig. 49). Inactivation by the 5-benzyI analog is therefore an example of affinity labeling rather than mechanism-based inactivation and emphasizes the importance of the side chain of protease inactivators in proper orientation of the compound in the active site. 

Kraft GA, Katzenellenbogen JA. Synthesis of halo enol lactones. Mechanism-based inactivators of serine proteases. J Am Chem Soc 1981 103 5459-5466. 

Ritonavir has both a reversible and an irreversible component to its CYP inhibition. See Ernest II, C.S., et al. Mechanism-based inactivation of CYP3A by FOV protease inhibitors. J. Pharmacol. Exp. Ther. 2005,312, 583-591. 

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