title: Cytochrome P450 3A4-mediated oxidative conversion of a cyano to an amide group in the metabolism of pinacidil.
abstract: The conversion of nitriles to amides is generally considered to be a hydrolytic process that does not involve redox chemistry. We demonstrate here that cytochrome P450 (CYP) is responsible for the conversion of the cyano group of pinacidil to the corresponding amide. The reaction in human liver microsomes was NADPH-dependent and was nearly completely inhibited by an anti-CYP3A4 antibody. Incubations of pinacidil with recombinant CYP enzymes confirm that CYP3A4 is the principal catalyst of this reaction. The kinetics of pinacidil amide formation by CYP3A4 yielded an apparent K(m) of 452 +/- 33 microM and k(cat) of 0.108 min(-1) (k(cat)/K(m) = 0.238 mM(-1).min(-1)). Incubation of pinacidil with CYP3A4 in the presence of (18)O(2) or H(2)(18)O showed that the amide carbonyl oxygen derived exclusively from molecular oxygen. The CYP3A4-mediated reaction also was supported by hydrogen peroxide when incubations were carried out in the absence of cytochrome P450 reductase and NADPH. The reaction can be explained by a nucleophilic attack of a deprotonated ferric peroxide intermediate (Fe(3+)-O-O(-)) on the carbon atom of the -C triple bond N triple bond to form an Enz-Fe(III)-O-O-C(=NH)R intermediate, followed by cleavage of the O-O bond to give pinacidil amide. This nucleophilic addition of an Fe(3+)-O-O(-) intermediate to a -C=N pi-bond in a P450 system resembles the analogous reaction catalyzed by the nitric oxide synthases.
authors.affiliation: Department of Drug Metabolism, Merck Research Laboratories, Rahway, New Jersey 07065, USA. zhoupeng_zhang@merck.com
authors.forename: Zhoupeng
authors.initials: Z
authors.lastname: Zhang
authors.forename: Ying
authors.initials: Y
authors.lastname: Li
authors.forename: Ralph A
authors.initials: RA
authors.lastname: Stearns
authors.forename: Paul R
authors.initials: PR
authors.lastname: Ortiz De Montellano
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Baillie
authors.forename: Wei
authors.initials: W
authors.lastname: Tang
mesh_terms: D000577:Amides / Q000378:metabolism*; D002384:Catalysis; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009570:Nitriles / Q000378:metabolism*; D010084:Oxidation-Reduction; D020110:Pinacidil / Q000378:metabolism*
chemical_list: D000577:Amides; D009570:Nitriles; D020110:Pinacidil; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human