Cytochrome substrates, interactions with

Schenkman, J. B., Sligar, S. G., and Cinti, D. L. (1981) Substrate interaction with cytochrome P450. Pharmacol. Ther. 12, 43-71. 

Recombinant human CYP2D6 was shown to be selective for the O-demethyla-tion of dextromethorphan at or near the K n for this cytochrome s interaction with this substrate (at a concentration of 3 pM), whereas at higher substrate concentrations CYPs of the cytochrome 2C family contributed significantly to the metabolism of the drug. Similarly, diazepam at a concentration of 100 pM was N-demethylated principally by CYPs 3A4 and 2C19. However, at the lower concentration of 20 pM, the metabolism mediated by CYP3A4 was reduced to 30% that of CYP2C19. 

Satsmadjis J, Voutsinou-Taliadouri F (1983) Mytilus galloprovincialis and Parapenaeus longirostris as bioindicators of heavy metal and organochlorine pollution. Mar Biol 76 115-124 Schenkman JB, SligarSG, Cinti DL(1982) Substrate interaction with cytochrome P-450. In Schenkman JB, Kupfer D (eds) Hepatic cytochrome P-450 monooxygenase system. Pergamon Press, Oxford, pp 587-615... 

Schenkman JB, Sligar SG, Cinti DL (1981) Substrate interactions with cytochrome P-450. Pharmacol Ther 12 43-71... 

Active caspases 8, 9 and 10 can convert caspase-3, the most abundant effector caspase from its pro-form to its active cleaved form. Cleavage of a number of different substrates by caspase-3 and also by caspase-6 and -7 which are two other executioner caspases besides caspase-3 then results in the typical morphology which is characteristic of apoptosis. Yet, the activation of caspase-3 and also of caspase-9 can be counteracted by IAPs, so called inhibitor of apoptosis proteins. However, concomitantly with cytochrome C also other proteins are released from mitochondria, including Smac/DIABLO. Smac/DIABLO and potentially other factors can interact with IAPs and thereby neutralize their caspase-inhibitory activity. This releases the breaks on the cell death program and allows apoptosis to ensue. 

The metabolism studies with cyanide present showed no dehydrogenation whatsoever of the substrate. It is thus considered likely that the resazurin and the resorufin interact with some metal bearing system, possibly the cytochromes participating in the hydrogen transfer. Although the resazurin (or resorufin) may interact in a system several steps removed from the dehydrogenation of the particular substrates, the relative rates of reduction of the indicator are still comparable with the relative oxidation rates of the substrates. 

The role of every component of this process is well established. The interaction with a substrate takes place at cytochrome P-450 in accord with Reaction (2) ... 

While cytochrome P-450 catalyzes the interaction with substrates, a final step of microsomal enzymatic system, flavoprotein NADPH-cytochrome P-450 reductase catalyzes the electron transfer from NADPH to cytochrome P-450. As is seen from Reaction (1), this enzyme contains one molecule of each of FMN and FAD. It has been suggested [4] that these flavins play different roles in catalysis FAD reacts with NADPH while FMN mediates electron... 

Krainev AG, Weiner LM, Kondrashin SK, Kanaeva IP, Bach-manova GI. 1991. Substrate access channel geometry of soluble and membrane-bound cytochromes P450 as studied by interactions with type II substrate analogues. Arch Biochem Biophys 288 17-21. 

Fig. 14.8 Experimental ligand interactions with cytochrome P450 2C family. (A) X-ray structure ofthe sulfaphenazole derivate DMZ in rabbit CYP2C5 at 2.3 A resolution (PDB 1 N5B) from Wester et al. [191]. Only one ofthe two-ligand orientations for DMZ in accord with electron density is shown placing the benzylic methyl group in a 4.4 A distance to the heme iron. (B) X-ray structure of S-warfarin in human CYP2C9 at 2.55 A resolution (PDB 10G5) from Williams et al. [192]. The substrate is situated in a predominantly hydrophobic pocket. This binding mode places the 6- and 7-hydroxylation sites 10 A from the iron (arrow).

---