RT Journal Article SR Electronic T1 Topological Alteration of the CYP3A4 Active Site by the Divalent Cation Mg2+ JF Drug Metabolism and Disposition JO Drug Metab Dispos FD American Society for Pharmacology and Experimental Therapeutics SP 1198 OP 1201 VO 28 IS 10 A1 Schrag, Michael L. A1 Wienkers, Larry C. YR 2000 UL http://dmd.aspetjournals.org/content/28/10/1198.abstract 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