Inhibition heme modification

Chloramphenicol and secobarbital exhibit properties similar to those of tienilic acid, but they have not been studied in humans (11). Oxidative dechlorination of chloramphenicol with formation of reactive acyl chlorides appears to be an important metabolic pathway for irreversible inhibition of CYP. Chloramphenicol binds to CYP, and subsequent substrate hydroxylation and product release are not impaired. The inhibition of CYP oxidation and the inhibition of endogenous NADPH oxidase activity suggest that some modification of the CYP has taken place, which inhibits its ability to accept electrons from the CYP reductase (11). Secobarbital completely inactivates rat CYP2B1 functionally, with partial loss of the heme chromophore. Isolation of the N-alkylated secobarbital heme adduct and the modified CYP2B1 protein revealed that the metabolite partitioned between heme N-alkylation, CYP2B1 protein modification, and epoxidation. A small fraction of the prosthetic heme modifies the protein and contributes to the CYP2B1 inactivation (12). 

MVH is inhibited by CO, cyanide, arsenite, and sulfide. Carbon monoxide, cyanide, and arsenite react only with the reduced enzyme. Spectral modifications of the heme and other results have indicated that the heme is the site of action of these inhibitors as well as the site at which sulfite, nitrite, and hydroxylamine are reduced. The Michaelis constants of the enzyme for sulfite and NADPH are both about 4r-5 itM. 

Mechanism-based CYP inhibition or irreversible inhibition, involves permanent inactivation of CYP enzymes during catalysis, where reactive intermediate(s) are formed, leading to apoprotein or heme-ion center modification. Typical characteristics of mechanism-based enzyme inhibition include time-dependent loss of enzyme activity, a rate of inactivation generally following saturation kinetics, enzyme activity that cannot be recovered after... 

More recently, the reductive activation of halothane (CFaCHBrCl), which is a hepatotoxic anaesthetic molecule, by human hemoglobin results in the modification of the prosthetic heme [57]. The inhibition of the reaction by adding exogeneous CO or the spin trapping agent N-t-butyl-a-phenyl nitrone to the incubation mixture indicated that (i) a reduced and free heme iron is required by Hb to activate the halogenated substrate and (ii) the formation of free radical species is responsible for Hb inactivation. However, no carbene species were detected in these reactions. The mechanism is shown in Scheme 7. 

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