PT  - JOURNAL ARTICLE
AU  - Schrag, Michael L.
AU  - Wienkers, Larry C.
TI  - Topological Alteration of the CYP3A4 Active Site by the Divalent  Cation Mg<sup>2+</sup>
DP  - 2000 Oct 01
TA  - Drug Metabolism and Disposition
PG  - 1198--1201
VI  - 28
IP  - 10
4099  - http://dmd.aspetjournals.org/content/28/10/1198.short
4100  - http://dmd.aspetjournals.org/content/28/10/1198.full
SO  - Drug Metab Dispos2000 Oct 01; 28
AB  - Phenyldiazene reacted with lymphoblast-expressed CYP3A4 to give a stable phenyl-iron complex that could be induced to rearrange in situ producing approximately equal amounts of fourN-phenyl-protoporphyrin IX isomers (NB:NA:NC:ND, 01:01:02:02). In the presence of 10 mM MgCl2, the formation profile of the protoporphyrin isomers was markedly altered compared with control, favoring the NA isomer (NB:NA:NC:ND, 01:34:01:02). In addition, an investigation of MgCl2effects on CYP3A4-mediated metabolism of triazolam revealed that 10 mM MgCl2 increased the apparent Kmof triazolam 4-hydroxylation from 83 to 173 μM and reduced theVmax for the reaction from 3.4 to 2.4 min−1. Moreover, when the reaction kinetics of the oxidation of pyrene by CYP3A4 was examined in the absence of MgCl2, it was found that the substrate-velocity curve was best approximated by a sigmoidal velocity curve (Hill coefficient 1.7 ± 0.1). However, when the reaction was conducted in the presence of 10 mM MgCl2, the resulting pyrene kinetics was not sigmoidal but rather biphasic (Hill coefficient 0.80 ± 0.07). Based on the current results, it appears that CYP3A4 is conformationally sensitive to its in vitro environment and parameters, such as the presence of a divalent magnesium, can have a measurable effect on active site topography and consequently catalytic activity. The American Society for Pharmacology and Experimental Therapeutics