Current Drug Metabolism, Vol. 2, No. 2, 2001
Contents
Uncommon P450-Catalyzed Reactions Pp. 93-115
F. Peter Guengerich
[Abstract]
Analysis of Mammalian Cytochrome P450 Structure and Function by Site-Directed Mutagenesis. Pp. 117-137
T. L. Domanski and J.R. Halpert
Induction of Cytochromes P450 Pp139-147
Erin G. Schuetz
Induction of CYP1A1. The AhR/DRE Paradigm: Transcription, Receptor Regulation, and Expanding Biological Roles Pp. 149-164
Qiang Ma
Hepatic Cytochrome P450 Regulation in Disease States Pp.165-183
Po-Yung Cheng and Edward T. Morgan
Heterotropic Cooperativity of Cytochrome P450 3A4 and Potential Drug-Drug Interactions Pp.185-198.
Wei Tang and Ralph A. Stearns
Cytochromes P450 in Brain: Function and Significance Pp199-214
H.W. Strobel C. M. Thompson and L.
Antonovic
Abstracts
[Back to top] Uncommon P450-Catalyzed Reactions
F. Peter Guengerich
Cytochrome
P450 (P450) enzymes play major roles in the metabolism of drugs, carcinogens,
steroids, eicosanoids, alkaloids, pesticides, and other important xenobiotics,
as well as chemicals normally endogenous to the body. P450s are gene-rally
considered in a classical catalytic reduction-oxidation cycle and an
odd-electron abstraction/rebound chemical mechanism that can be used to
rationalize carbon hydro-xylation, dealkylation of heteroatomic substrates,
heteroatom oxygenation, and the oxidation of unsaturated compounds to epoxides
and phenols. However, many other reactions are catalyzed by P450s but not
generally appreciated. The classical catalytic mechanism requires some
expansion to explain all of these reactions. Reactions discussed here include
mechanism-based heme inactivation, mechanism-based protein modification,
1,2-shifts, 1- and 2-electron reductions, 1-electron oxidation, oxidative
cleavage of carboxylic acid esters, desaturation, deformylation of aldehydes,
ring formation, ipso mechanisms for aryl dehalogenation and O- and
N-dearylation, cis-trans bond isomerization, several rearrangements of oxidized
eicosanoids, aldoxime dehydration, and hydrolysis of phosphatidylcholine
[Back to top] Analysis of Mammalian Cytochrome P450 Structure and Function by Site-Directed Mutagenesis Analysis of Mammalian Cytochrome P450 Structure and Function by Site-Directed Mutagenesis
T. L. Domanski and J.R. Halpert T. L. Domanski and J.R. Halpert
Over
the past decade, site-directed mutagenesis has become an essential tool in the
study of mammalian cytochrome P450 structure-function relationships. Residues
affecting substrate specificity, cooperativity, membrane localization, and
interactions with redox partners have been identified using a combination of
amino-acid sequence alignments, homology modeling, chimeragenesis, and
site-directed mutagenesis. As homology modeling and substrate docking
technology continue to improve, the ability to predict more precise functions
for specific residues will also advance, making it possible to utilize
site-directed mutagenesis to test these predictions. Future studies will employ
site-directed mutagenesis to learn more about cytochrome P450 substrate access
channels, to define the role of residues that do not lie within substrate
recognition sites, to engineer additional soluble forms of microsomal
cytochromes P450 for x-ray crystallography, and to engineer more efficient
enzymes for drug activation and/or bioremediation.
[Back to top] Induction of Cytochromes P450
Erin G. Schuetz
Humans and rodents are exposed to many foreign compounds in their diet (e.g., herbal supplements such as St. John's wart), in their environment (e.g., organ-ochlorine pesticides and polychorinated biphenyls), and as clinically prescribed drugs (e.g., rifampin and phenobarbital). In response to these exposures mammals have evolved mechanisms to induce proteins involved in xenobiotic detoxification. Metabo-lism by Phase I enzymes, particularly the heme containing monooxygenases – cytochromes P450 is frequently the first line of defense against such xenobiotics.
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[Back to top] Induction of CYP1A1. The AhR/DRE Paradigm: Transcription, Receptor Regulation, and Expanding Biological Roles
Qiang Ma
The
CYP1A1 gene encodes microsomal cytochrome P4501A1 that catalyzes the metabolism
of many xenobiotics, including the oxygenation of polycyclic aromatic
hydrocarbons (PAH). Induction of CYP1A1 enhances the metabolism of PAHs, and
therefore, represents an adaptive response to chemical exposure in mammalian
cells. Mechanistic studies reveal an AhR/DRE paradigm for the induction, which
involves activation of the aryl hydrocarbon receptor (AhR) by an agonist,
dimerization of AhR with the Ah receptor nuclear translocator (Arnt), followed
by binding of the AhR/Arnt heterodimer to the dioxin-responsive enhancer (DRE)
and transcription of the gene. The AhR mediated transcription is tightly
regulated through, at least, two mechanisms: (a) the cytoplasmic AhR interacts
with hsp90 and an immunophilin chaperone AIP for proper folding and
receptivity, and (b) the agonist-activated, nuclear AhR is degraded through the
ubiquitin-26S proteasome mediated protein turnover, such that the transcription
by AhR is controlled at a physiologically adequate level. In addition to CYP1A1
induction, AhR mediates a broad range of biological responses to CYP1A1
inducers, typified by the environmental contaminant dioxin, via modulating gene
expression. Thus, mechanistic studies of CYP1A1 induction have provided
insights into P450 induction, PAH carcinogenesis, dioxin action, AhR function,
and receptor-mediated mammalian gene expression.
[Back to top] Hepatic Cytochrome P450 Regulation in Disease States
Po-Yung Cheng and Edward T. Morgan
Hepatic cytochrome P450 (P450) enzyme activities and gene expression can be profoundly altered in disease states. In general the levels of affected hepatic P450 enzymes are depressed by diseases, causing potential and documented impairment of drug clearance and clinical drug toxicity. However, modulation of P450s is enzyme selective and this selectivity differs among different diseases. This review will concentrate on regulation of P450s in diabetes, obesity and infectious and inflammatory disease, conditions that affect millions of people worldwide every day.
[Back to
top] Heterotropic Cooperativity of
Cytochrome P450 3A4 and Potential Drug-Drug Interactions
Wei Tang and Ralph A. Stearns
Cytochromes
P450 (CYP) 3A4 is the most abundant human hepatic CYP isoform catalyzing the
metabolism of approximately 50% of therapeutic agents. In addition to
inhibition or induction, CYP3A4 is subject to stimulation, termed homotropic
(substrate stimulation) and heterotropic (stimulation by effectors)
cooperativity. The heterotropic cooperativity of CYP3A4 may result from an
increase in Vmax, a decrease in Km or a combination of the two and sometimes
exhibits regio-selectivity when the enzyme is involved in two or more metabolic
pathways for a single substrate. An effector of CYP3A4 can also be a substrate;
its metabolism may or may not be inhibited by another substrate. These
characteristics of heterotropic cooperativity of CYP3A4 have been interpreted
in the context of two binding domains in the active site of the enzyme, two
substrate binding plus a distinct allosteric binding site, multiple enzyme
conformations or multiple binding sites accompanied by conformational changes.
Examples of in vivo CYP cooperativity are rare; representative cases include flavone-dependent stimulation of zoxazolamine metabolism in rats and enhancement of CYP3A-mediated hepatic clearance of diclofenac by quinidine in monkeys. Effector-induced increases in CYP3A4 activity were observed during the 1¢-hydroxylation of midazolam and 4¢- and 10-hydroxylation of warfarin in human hepatocyte systems. These data imply that CYP cooperativity has the potential to cause in vivo drug-drug interactions. Because cooperative and inhibitory responses from CYP3A4 are known to be substrate-dependent, projection of the pharmacokinetics of an investigational drug and CYP-associated risks of drug-drug interactions in humans can be very complex. Further investigation of CYP cooperativity is warranted.
[Back to top] Cytochromes P450 in Brain: Function and Significance
H.W. Strobel C. M. Thompson and L. Antonovic
The presence and activity of cytochromes P450 in brain regions and various brain cells have been extended and advanced over the last five years covered by this review. Using in situ hybridization and immunohistochemical techniques, many cytochrome P450 enzymes have been demonstrated to be present in brain and to have a regional rather than universal distribution. Many of these various cytochromes P450 have been shown to catalyze the metabolism of neurosteroids as well as other biologically significant compounds in brain. In addition, many cytochrome P450 enzymes have been implicated in the metabolism of psychoactive drugs such as neuroleptics and antidepressants. The regulation of cytochrome P450 expression has been studied at greater detail, the regulation of aromatase being a prominent example during the last five years.