Bio-activation of 4-alkyl analogs of 1,4-dihydropyridine mediated by cytochrome P450 enzymes

Abstract

4-Alkyl-substituted 1,4-dihydropyridines (DHP) exhibit inhibitory activity toward certain cytochrome P450 enzymes (P450) during their biotransformation by these enzymes, which is called mechanism-based inactivation. Though much experimental evidence had proved the essentiality of alkyl radical for P450 inactivation, the underlying mechanism of such radical formation remains elusive. In the present study, density functional calculations were employed to investigate the dealkylation mechanism of 4-alkyl-substituted DHPs mediated by P450. Interestingly, our results indicate that the initial N-H activation proceeds via a proton-coupled electron transfer process, not via the long presumed hydrogen atom transfer mechanism or the stepwise electron transfer/proton transfer one, to form the amino radical and Cpd II complex. Subsequently, homolytic C-C bond cleavage at the 4-position occurs to afford the product complex involving an alkyl radical, an aromatic pyridine derivative. This C-C cleavage step is rate determining for the whole metabolic reaction, with an energy barrier of 7.9/7.9 kcal/mol on the quartet/doublet state, to which aromatization contributes as an essential intrinsic driving force. The 4-substituent groups induce differences in activation energy barriers and in the transition state structures of hydrogen abstraction process. The substrate reactivity correlates well with the stability of the generated alkyl radical as well as the C-C bond dissociation energy. Understanding the metabolic mechanism of DHP analogs is indeed essential for the related design of safer and more efficient drugs. Furthermore, our findings also enrich the mechanistic picture of amine oxidation catalyzed by P450.

Suicide Inhibition of Cytochrome P450 Enzymes by Cyclopropylamines via a Ring-Opening Mechanism: Proton-Coupled Electron Transfer Makes a Difference

“…On RC , two spin states are nearly degenerated with the LS lying under the HS by -0.3/-0.1 kcal/mol at the E1/E2 level. Intriguingly, the following essential H-abstraction from the N-H bond of the secondary amine moiety proceeds through an extremely low energy barrier (0.6/0.4 kcal/mol for the HS/LS at the E1 level, indicating an involvement of two-state reactivity mechanism (Shaik et al, 1998 ; Schröder et al, 2000 ; de Visser and Tan, 2008 ), which is similar to our previous finding about the oxidation of 4-alkylated DHPs (Li et al, 2015 ) as well as that reported by Hirao et al ( 2013a ) but different from that obtained by Rydberg and Olsen ( 2011 ). Hirao et al attributed the tiny energy barrier to weak bond disassociation energy (BDE) of N-H bond rather than the involvement of PCET mechanism, which has been proved in our previous work.…”

“…Moreover, such N -HAT mechanism was subsequently validated by Hirao et al in their investigation on P450 inactivation by 1,1-dimethylhydrazine (Hirao et al, 2013a ). However, previous investigations have revealed that there might be an alternative proton-coupled electron transfer (PCET) mechanism for the polar X-H (X = O, N) bond activation (Mayer et al, 2002 ; Usharani et al, 2013 ; Hirao and Chuanprasit, 2015 ; Li et al, 2015 ). Thus, an obvious question to be answered is: Ring-opening of BCA is initiated by a direct HAT mechanism or a PCET one?…”

ROS-dependent catalytic mechanism of melatonin metabolism and its application in the measurement of reactive oxygen

“…However, the spin density distribution on the “substrate Mel” moiety in 4 Int1 p450 / 2 Int1 p450 approaches ±1. Therefore, this H-abstraction follows the proton-coupled electron transfer (PCET) ( Mayer et al, 2002 ; Hirao and Chuanprasit, 2015 ; Li et al, 2015 ; Zhang et al, 2017 ). Starting from the intermediate 4 Int1 p450 / 2 Int1 p450 , the OH atom rebounds to the C1 atom.…”