Pregnene 17 -hydroperoxides as possible precursors of the adrenosteroid hormones

Oxygen Surrogate Systems for Supporting Human Drug-Metabolizing Cytochrome P450 Enzymes

Oxygen surrogates (OSs) have been used to support cytochrome P450 (P450) enzymes for diverse purposes in drug metabolism research, including reaction phenotyping, mechanistic and inhibition studies, studies of redox partner interactions, and to avoid the need for NADPH or a redox partner. They also have been used in engineering P450s for more cost-effective, NADPH-independent biocatalysis. However, despite their broad application, little is known of the preference of individual P450s for different OSs or the substrate dependence of OS-supported activity. Furthermore, the biocatalytic potential of OSs other than cumene hydroperoxide (CuOOH) and hydrogen peroxide (H₂O₂) is yet to be explored. Here, we investigated the ability of the major human drug-metabolizing P450s, namely CYP3A4, CYP2C9, CYP2C19, CYP2D6, and CYP1A2, to use the following OSs: H₂O₂, tert-butyl hydroperoxide (tert-BuOOH), CuOOH, (diacetoxyiodo)benzene, and bis(trifluoroacetoxy)iodobenzene. Overall, CuOOH and tert-BuOOH were found to be the most effective at supporting these P450s. However, the ability of P450s to be supported by OSs effectively was also found to be highly dependent on the substrate used. This suggests that the choice of OS should be tailored to both the P450 and the substrate under investigation, underscoring the need to employ screening methods that reflect the activity toward the substrate of interest to the end application. SIGNIFICANCE STATEMENT: Cytochrome P450 (P450) enzymes can be supported by different oxygen surrogates (OSs), avoiding the need for a redox partner and costly NADPH. However, few data exist comparing relative activity with different OSs and substrates. This study shows that the choice of OS used to support the major drug-metabolizing P450s influences their relative activity and regioselectivity in a substrate-specific fashion and provides a model for the more efficient use of P450s for metabolite biosynthesis.

Monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 enzymes

This review examines the monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 (CYP) enzymes in bacterial, archaeal and mammalian systems. CYP enzymes catalyze monooxygenation reactions by inserting one oxygen atom from O2 into an enormous number and variety of substrates. The catalytic versatility of CYP stems from its ability to functionalize unactivated carbon-hydrogen (C-H) bonds of substrates through monooxygenation. The oxidative prowess of CYP in catalyzing monooxygenation reactions is attributed primarily to a porphyrin π radical ferryl intermediate known as Compound I (CpdI) (Por•+FeIV=O), or its ferryl radical resonance form (FeIV-O•). CYP-mediated hydroxylations occur via a consensus H atom abstraction/oxygen rebound mechanism involving an initial abstraction by CpdI of a H atom from the substrate, generating a highly-reactive protonated Compound II (CpdII) intermediate (FeIV-OH) and a carbon-centered alkyl radical that rebounds onto the ferryl hydroxyl moiety to yield the hydroxylated substrate. CYP enzymes utilize hydroperoxides, peracids, perborate, percarbonate, periodate, chlorite, iodosobenzene and N-oxides as surrogate oxygen atom donors to oxygenate substrates via the shunt pathway in the absence of NAD(P)H/O2 and reduction-oxidation (redox) auxiliary proteins. It has been difficult to isolate the historically elusive CpdI intermediate in the native NAD(P)H/O2-supported monooxygenase pathway and to determine its precise electronic structure and kinetic and physicochemical properties because of its high reactivity, unstable nature (t½~2 ms) and short life cycle, prompting suggestions for participation in monooxygenation reactions of alternative CYP iron-oxygen intermediates such as the ferric-peroxo anion species (FeIII-OO-), ferric-hydroperoxo species (FeIII-OOH) and FeIII-(H2O2) complex.

Hydroperoxides as inactivators of aromatase: 10 beta-hydroperoxy-4-estrene-3,17-dione, crystal structure and inactivation characteristics

The crystal structure of 10 beta-hydroperoxy-4-estrene-3,17-dione (10 beta-OOH) was determined, and its inhibition of human placental aromatase was investigated. In the absence of added NADPH, 10 beta-OOH caused a time-dependent loss of aromatase activity (e.g., 50% loss after 90 s with 2.16 microM 10 beta-OOH). Protection against this loss of activity was provided when a substrate, androstenedione, was included in the incubation. Centrifugation and resuspension of the 10 beta-OOH-treated microsomes in fresh buffer failed to restore the activity, but partial recovery could be effected by dithiothreitol. Experiments to detect destruction of aromatase protoheme were done but were inconclusive. In the presence of NADPH, 10 beta-OOH did not cause a time-dependent loss of activity but was instead a competitive inhibitor (Ki = 330 nM) of androstenedione (Km = 21 nM) aromatization. The added NADPH was not utilized for the aromatization of 10 beta-OOH to estrogens, and enhanced reduction of 10 beta-OOH to 10 beta-hydroxy-4-estrene-3,17-dione could not be detected. In addition, microsomes alone were incapable of using 10 beta-OOH to support the aromatization of androstenedione. Cumene hydroperoxide and H2O2 were also investigated as inactivators of aromatase. Losses of activity comparable to those found for 10 beta-OOH could only be observed at 500-1000-fold higher concentrations of these agents, and no protection was provided by either androstenedione or NADPH. Extensive destruction of microsomal protoheme was found with these nonsteroidal agents.

NADH and NADPH inhibit lipid peroxidation promoted by hydroperoxides in rat liver microsomes

Lipid peroxidation induced through cytochrome P-450 activation of cumene hydroperoxide, linolenic acid hydroperoxide and peroxidized phosphatidylcholine in rat liver microsomes is markedly inhibited by either NADH or NADPH. This inhibition is not due to an antioxidant effect. Conversely, cumene hydroperoxide decomposition is stimulated by the reduced pyridine nucleotides but not by some modifiers of cytochrome P-450 (SKF-525A, metyrapon and aniline). The mechanism by which NADH and NADPH prevent lipid peroxidation may involve a reduction of the hydroperoxides mediated by cytochrome P-450 and occurring without formation of free radical forms that are usual sparkers of lipid peroxidation.