Degradation of rat hepatic cytochrome P-450 heme by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine to irreversibly bound protein adducts

Hepatic cytochromes P450: structural degrons and barcodes, posttranslational modifications and cellular adapters in the ERAD-endgame

The endoplasmic reticulum (ER)-anchored hepatic cytochromes P450 (P450s) are enzymes that metabolize endo- and xenobiotics i.e. drugs, carcinogens, toxins, natural and chemical products. These agents modulate liver P450 content through increased synthesis or reduction via inactivation and/or proteolytic degradation, resulting in clinically significant drug-drug interactions. P450 proteolytic degradation occurs via ER-associated degradation (ERAD) involving either of two distinct routes: Ubiquitin (Ub)-dependent 26S proteasomal degradation (ERAD/UPD) or autophagic lysosomal degradation (ERAD/ALD). CYP3A4, the major human liver/intestinal P450, and the fast-turnover CYP2E1 species are degraded via ERAD/UPD entailing multisite protein phosphorylation and subsequent ubiquitination by gp78 and CHIP E3 Ub-ligases. We are gaining insight into the nature of the structural determinants involved in CYP3A4 and CYP2E1 molecular recognition in ERAD/UPD [i.e. K48-linked polyUb chains and linear and/or "conformational" phosphodegrons consisting either of consecutive sequences on surface loops and/or disordered regions, or structurally-assembled surface clusters of negatively charged acidic (Asp/Glu) and phosphorylated (Ser/Thr) residues, within or vicinal to which, Lys-residues are targeted for ubiquitination]. Structural inspection of select human liver P450s reveals that such linear or conformational phosphodegrons may indeed be a common P450-ERAD/UPD feature. By contrast, although many P450s such as the slow-turnover CYP2E1 species and rat liver CYP2B1 and CYP2C11 are degraded via ERAD/ALD, little is known about the mechanism of their ALD-targeting. On the basis of our current knowledge of ALD-substrate targeting, we propose a tripartite conjunction of K63-linked Ub-chains, P450 structural "LIR" motifs and selective cellular "cargo receptors" as plausible P450-ALD determinants.

Acetaminophen induces accumulation of functional rat CYP3A via polyubiquitination dysfunction

Acetaminophen (APAP) is extensively used as an analgesic and antipyretic drug. APAP is partly metabolized to N-acetyl-p-benzoquinone imine, a reactive metabolite, by cytochrome P450 (CYP) 1A2, 2E1 and 3A4. Some reports have indicated that CYP3A protein production and its metabolic activity are induced by APAP in rats in vivo. The CYP3A subfamily is believed to be transcriptionally regulated by chemical compounds. However, the mechanism underlying these responses is not completely understood. To clarify these mechanisms, we assessed the effects of APAP on CYP3A1/23 protein levels according to mRNA synthesis and protein degradation in rat hepatocyte spheroids, a model of liver tissue, in vivo. APAP induced CYP3A1/23 protein levels and metabolic activity. However, no change in CYP3A1/23 mRNA levels was observed. Moreover, APAP prolonged the half-life of CYP3A1/23 protein. CYP3A is known to be degraded via the ubiquitin-proteasome system. APAP significantly was found to decrease levels of polyubiquitinated CYP3A1/23 and glycoprotein 78, an E3 ligase of CYP3A1/23. These findings demonstrate that APAP induces accumulation of functional CYP3A protein via inhibition of protein degradation. Our findings may lead to the determination of novel drug-drug interactions with APAP.

Hepatic cytochrome P450 ubiquitination: conformational phosphodegrons for E2/E3 recognition?

Hepatic endoplasmic reticulum (ER) integral cytochromes P450 (P450s) are monooxygenases engaged in the biotransformation and elimination of endo- as well as xenobiotics. Of the human liver P450s, CYP3A4 is the major and most dominant catalyst responsible for the biotransformation of over 50% of clinically prescribed drugs. CYP2E1 metabolizes smaller molecular weight compounds (EtOH), carcinogens, environmental toxins, and endobiotics, and is justly implicated in various toxigenic/pathogenic mechanisms of human disease. Both P450s are notorious for their potential to generate pathogenic reactive oxygen species (ROS) during futile oxidative cycling and/or oxidative uncoupling. Such ROS not only oxidatively damage the P450 catalytic cage, but on their escape into the cytosol, also the P450 outer surface and any surrounding cell organelles. Given their ER-monotopic topology coupled with this high potential to acquire oxidative lesions in their cytosolic (C) domain, not surprisingly these P450 proteins exhibit shorter lifespans and are excellent prototype substrates of ER-associated degradation ("ERAD-C") pathway. Indeed, we have shown that both CYP3A4 and CYP2E1 incur ERAD-C, during which they are first phosphorylated by protein kinases A and C, which greatly enhance/accelerate their ubiquitination by UBC7/gp78 and UbcH5a/CHIP/Hsp70/Hsp40 E2/E3 ubiquitin ligase complexes. Such P450 phosphorylation occurs on Ser/Thr residues within linear sequences as well as spatially clustered acidic (Asp/Glu) residues. We propose that such S/T phosphorylation within these clusters creates negatively charged patches or conformational phosphodegrons for interaction with positively charged E2/E3 domains. Such P450 S/T phosphorylation we posit serves as a molecular switch to turn on its ubiquitination and ERAD-C.

CHIP and gp78-mediated ubiquitination of CYP3A4: Implications for the pharmacology of anticancer agents

The autocrine motility factor receptor or glycoprotein-78 (gp78) and C-terminus of Hsp70-interacting protein (CHIP) are E3-ligases required for ubiquitination of cytochrome P450s of the 3A subfamily (CYP3A) in endoplasmic reticulum-associated degradation (ERAD). The CYP isozyme 3A4 (CYP3A4) is responsible for the metabolism of the majority of xenobiotics including anticancer agents. Much variability in clinical response to chemotherapy is observed and it has been suggested that variability in CYP3A4 expression could be a factor. The study reviewed in this journal club comments on the importance of further characterizing gp78 and CHIP as relevant proteins in ERAD of CYP3A4. This study demonstrated how both gp78 and CHIP play direct roles in reducing CYP3A4 protein content as well as CYP3A4 ubiquitination. Interestingly, when gp78 and CHIP were knocked down by siRNAs directed towards each protein, the stabilized CYP3A4 remained functional. This has implications for drug-drug interactions for agents metabolized by CYP3A4, which can influence drug exposure levels. This is relevant because most anticancer agents have very narrow therapeutic windows, thus even slight changes in CYP3A4 levels could alter the exposure of that drug and result in either insufficient efficacy or toxicity. Future studies must explore genetic variability in the ERAD pathway and identify new factors that influence CYP3A ERAD in order to better characterize how CYP3A variability affects anticancer drug pharmacology.

Ubiquitin-dependent proteasomal degradation of human liver cytochrome P450 2E1: identification of sites targeted for phosphorylation and ubiquitination

Human liver CYP2E1 is a monotopic, endoplasmic reticulum-anchored cytochrome P450 responsible for the biotransformation of clinically relevant drugs, low molecular weight xenobiotics, carcinogens, and endogenous ketones. CYP2E1 substrate complexation converts it into a stable slow-turnover species degraded largely via autophagic lysosomal degradation. Substrate decomplexation/withdrawal results in a fast turnover CYP2E1 species, putatively generated through its futile oxidative cycling, that incurs endoplasmic reticulum-associated ubiquitin-dependent proteasomal degradation (UPD). CYP2E1 thus exhibits biphasic turnover in the mammalian liver. We now show upon heterologous expression of human CYP2E1 in Saccharomyces cerevisiae that its autophagic lysosomal degradation and UPD pathways are evolutionarily conserved, even though its potential for futile catalytic cycling is low due to its sluggish catalytic activity in yeast. This suggested that other factors (i.e. post-translational modifications or "degrons") contribute to its UPD. Indeed, in cultured human hepatocytes, CYP2E1 is detectably ubiquitinated, and this is enhanced on its mechanism-based inactivation. Studies in Ubc7p and Ubc5p genetically deficient yeast strains versus corresponding isogenic wild types identified these ubiquitin-conjugating E2 enzymes as relevant to CYP2E1 UPD. Consistent with this, in vitro functional reconstitution analyses revealed that mammalian UBC7/gp78 and UbcH5a/CHIP E2-E3 ubiquitin ligases were capable of ubiquitinating CYP2E1, a process enhanced by protein kinase (PK) A and/or PKC inclusion. Inhibition of PKA or PKC blocked intracellular CYP2E1 ubiquitination and turnover. Here, through mass spectrometric analyses, we identify some CYP2E1 phosphorylation/ubiquitination sites in spatially associated clusters. We propose that these CYP2E1 phosphorylation clusters may serve to engage each E2-E3 ubiquitination complex in vitro and intracellularly.

Liver cytochrome P450 3A endoplasmic reticulum-associated degradation: a major role for the p97 AAA ATPase in cytochrome P450 3A extraction into the cytosol

The CYP3A subfamily of hepatic cytochromes P450, being engaged in the metabolism and clearance of >50% of clinically relevant drugs, can significantly influence therapeutics and drug-drug interactions. Our characterization of CYP3A degradation has indicated that CYPs 3A incur ubiquitin-dependent proteasomal degradation (UPD) in an endoplasmic reticulum (ER)-associated degradation (ERAD) process. Cytochromes P450 are monotopic hemoproteins N-terminally anchored to the ER membrane with their protein bulk readily accessible to the cytosolic proteasome. Given this topology, it was unclear whether they would require the AAA-ATPase p97 chaperone complex that retrotranslocates/dislocates ubiquitinated ER-integral and luminal proteins into the cytosol for proteasomal delivery. To assess the in vivo relevance of this p97-CYP3A association, we used lentiviral shRNAs to silence p97 (80% mRNA and 90% protein knockdown relative to controls) in sandwich-cultured rat hepatocytes. This extensive hepatic p97 knockdown remarkably had no effect on cellular morphology, ER stress, and/or apoptosis, despite the well recognized strategic p97 roles in multiple important cellular processes. However, such hepatic p97 knockdown almost completely abrogated CYP3A extraction into the cytosol, resulting in a significant accumulation of parent and ubiquitinated CYP3A species that were firmly ER-tethered. Little detectable CYP3A accumulated in the cytosol, even after concomitant inhibition of proteasomal degradation, thereby documenting a major role of p97 in CYP3A extraction and delivery to the 26 S proteasome during its UPD/ERAD. Intriguingly, the accumulated parent CYP3A was functionally active, indicating that p97 can regulate physiological CYP3A content and thus influence its clinically relevant function.

Cytochrome P450 regulation: the interplay between its heme and apoprotein moieties in synthesis, assembly, repair, and disposal

Heme is vital to our aerobic universe. Heme cellular content is finely tuned through an exquisite control of synthesis and degradation. Heme deficiency is deleterious to cells, whereas excess heme is toxic. Most of the cellular heme serves as the prosthetic moiety of functionally diverse hemoproteins, including cytochromes P450 (P450s). In the liver, P450s are its major consumers, with >50% of hepatic heme committed to their synthesis. Prosthetic heme is the sine qua non of P450 catalytic biotransformation of both endo- and xenobiotics. This well-recognized functional role notwithstanding, heme also regulates P450 protein synthesis, assembly, repair, and disposal. These less well-appreciated aspects are reviewed herein.

Liver cytochrome P450 3A ubiquitination in vivo by gp78/autocrine motility factor receptor and C terminus of Hsp70-interacting protein (CHIP) E3 ubiquitin ligases: physiological and pharmacological relevance

CYP3A4 is a dominant human liver cytochrome P450 enzyme engaged in the metabolism and disposition of >50% of clinically relevant drugs and held responsible for many adverse drug-drug interactions. CYP3A4 and its mammalian liver CYP3A orthologs are endoplasmic reticulum (ER)-anchored monotopic proteins that undergo ubiquitin (Ub)-dependent proteasomal degradation (UPD) in an ER-associated degradation (ERAD) process. These integral ER proteins are ubiquitinated in vivo, and in vitro studies have identified the ER-integral gp78 and the cytosolic co-chaperone, CHIP (C terminus of Hsp70-interacting protein), as the relevant E3 Ub-ligases, along with their cognate E2 Ub-conjugating enzymes UBC7 and UbcH5a, respectively. Using lentiviral shRNA templates targeted against each of these Ub-ligases, we now document that both E3s are indeed physiologically involved in CYP3A ERAD/UPD in cultured rat hepatocytes. Accordingly, specific RNAi resulted in ≈80% knockdown of each hepatic Ub-ligase, with a corresponding ≈2.5-fold CYP3A stabilization. Surprisingly, however, such stabilization resulted in increased levels of functionally active CYP3A, thereby challenging the previous notion that E3 recognition and subsequent ERAD of CYP3A proteins required ab initio their structural and/or functional inactivation. Furthermore, coexpression in HepG2 cells of both CYP3A4 and gp78, but not its functionally inactive RING-finger mutant, resulted in enhanced CYP3A4 loss greater than that in corresponding cells expressing only CYP3A4. Stabilization of a functionally active CYP3A after RNAi knockdown of either of the E3s, coupled with the increased CYP3A4 loss on gp78 or CHIP coexpression, suggests that ERAD-associated E3 Ub-ligases can influence clinically relevant drug metabolism by effectively regulating the physiological CYP3A content and consequently its function.

A role for protein phosphorylation in cytochrome P450 3A4 ubiquitin-dependent proteasomal degradation

Cytochromes P450 (P450s) incur phosphorylation. Although the precise role of this post-translational modification is unclear, marking P450s for degradation is plausible. Indeed, we have found that after structural inactivation, CYP3A4, the major human liver P450, and its rat orthologs are phosphorylated during their ubiquitin-dependent proteasomal degradation. Peptide mapping coupled with mass spectrometric analyses of CYP3A4 phosphorylated in vitro by protein kinase C (PKC) previously identified two target sites, Thr(264) and Ser(420). We now document that liver cytosolic kinases additionally target Ser(478) as a major site. To determine whether such phosphorylation is relevant to in vivo CYP3A4 degradation, wild type and CYP3A4 with single, double, or triple Ala mutations of these residues were heterologously expressed in Saccharomyces cerevisiae pep4Delta strains. We found that relative to CYP3A4wt, its S478A mutant was significantly stabilized in these yeast, and this was greatly to markedly enhanced for its S478A/T264A, S478A/S420A, and S478A/T264A/S420A double and triple mutants. Similar relative S478A/T264A/S420A mutant stabilization was also observed in HEK293T cells. To determine whether phosphorylation enhances CYP3A4 degradation by enhancing its ubiquitination, CYP3A4 ubiquitination was examined in an in vitro UBC7/gp78-reconstituted system with and without cAMP-dependent protein kinase A and PKC, two liver cytosolic kinases involved in CYP3A4 phosphorylation. cAMP-dependent protein kinase A/PKC-mediated phosphorylation of CYP3A4wt but not its S478A/T264A/S420A mutant enhanced its ubiquitination in this system. Together, these findings indicate that phosphorylation of CYP3A4 Ser(478), Thr(264), and Ser(420) residues by cytosolic kinases is important both for its ubiquitination and proteasomal degradation and suggest a direct link between P450 phosphorylation, ubiquitination, and degradation.

Toxicological Significance of Mechanism-Based Inactivation of Cytochrome P450 Enzymes by Drugs

Cytochrome P450 (P450) enzymes oxidize xenobiotics into chemically reactive metabolites or intermediates as well as into stable metabolites. If the reactivity of the product is very high, it binds to a catalytic site or sites of the enzyme itself and inactivates it. This phenomenon is referred to as mechanism-based inactivation. Many clinically important drugs are mechanism-based inactivators that include macrolide antibiotics, calcium channel blockers, and selective serotonin uptake inhibitors, but are not always structurally and pharmacologically related. The inactivation of P450s during drug therapy results in serious drug interactions, since irreversibility of the binding allows enzyme inhibition to be prolonged after elimination of the causal drug. The inhibition of the metabolism of drugs with narrow therapeutic indexes, such as terfenadine and astemizole, leads to toxicities. On the other hand, the fate of P450s after the inactivation and the toxicological consequences remains to be elucidated, while it has been suggested that P450s modified and degraded are involved in some forms of tissue toxicity. Porphyrinogenic drugs, such as griseofulvin, cause mechanism-based heme inactivation, leading to formation of ferrochelatase-inhibitory N-alkylated protoporphyrins and resulting in porphyria. Involvement of P450-derived free heme in halothane-induced hepatotoxicity and catalytic iron in cisplatin-induced nephrotoxicity has also been suggested. Autoantibodies against P450s have been found in hepatitis following administration of tienilic acid and dihydralazine. Tienilic acid is activated by and covalently bound to CYP2C9, and the neoantigens thus formed activate immune systems, resulting in the formation of an autoantibodydirected against CYP2C9, named anti-liver/kidney microsomal autoantibody type 2, whereas the pathological role of the autoantibodies in drug-induced hepatitis remains largely unknown.

Cellular proteolytic systems in P450 degradation: evolutionary conservation from Saccharomyces cerevisiae to mammalian liver

Mammalian hepatic cytochromes P450 (P450s) are endoplasmic reticulum (ER)-anchored haemoproteins with the bulk of their catalytic domains exposed to the cytosol and engaged in the metabolism of numerous xeno- and endobiotics. The native P450s exhibit widely ranging half-lifes and predominantly turn over via either autophagic-lysosomal degradation (ALD) or ubiquitin-dependent 26S proteasomal degradation (UPD). The basis for this heterogeneity and differential proteolytic targeting is unknown. On the other hand, structurally/functionally inactivated P450s are predominantly degraded via UPD in a process known as ER-associated degradation (ERAD). ALD/UPD/ERAD pathways are evolutionarily highly conserved. The availability of Saccharomyces cerevisiae mutants with specific genetic defects/deletions in various ALD/UPD/ERAD-associated proteins and corresponding isogenic wild-type strains has enabled the molecular dissection of the degradation pathways for heterologously expressed mammalian P450s, leading to the identification of specific protein participants. These findings collectively attest to a highly versatile cellular system for the physiological disposal of native, senescent and/or inactivated, structurally damaged mammalian liver P450s.

Characterization of the physiological turnover of native and inactivated cytochromes P450 3A in cultured rat hepatocytes: a role for the cytosolic AAA ATPase p97?

Mammalian hepatic cytochromes P450 (P450s) are endoplasmic reticulum (ER)-anchored hemoproteins engaged in the metabolism of numerous xeno- and endobiotics. P450s exhibit widely ranging half-lives, utilizing both autophagic-lysosomal (ALD) and ubiquitin-dependent 26S proteasomal (UPD) degradation pathways. Although suicidally inactivated hepatic CYPs 3A and "native" CYP3A4 in Saccharomyces cerevisiae are degraded via UPD, the turnover of native hepatic CYPs 3A in their physiological milieu has not been elucidated. Herein, we characterize the degradation of native, dexamethasone-inducible CYPs 3A in cultured primary rat hepatocytes, using proteasomal (MG-132 and MG-262) and ALD [NH4Cl and 3-methyladenine (3-MA)] inhibitors to examine their specific degradation route. Pulse-chase with immunoprecipitation analyses revealed a basal 52% 35S-CYP3A loss over 6 h, which was stabilized by both proteasomal inhibitors. By contrast, no corresponding CYP3A stabilization was detected with either ALD inhibitor NH4Cl or 3-MA. Furthermore, MG-262-induced CYP3A stabilization was associated with its polyubiquitylation, thereby verifying that native CYPs 3A were also degraded via UPD. To identify the specific participants in this process, cellular proteins were cross-linked in situ with paraformaldehyde (PFA) in cultured hepatocytes. Immunoblotting analyses of CYP3A immunoprecipitates after PFA-cross-linking revealed the presence of p97, a cytosolic AAA ATPase instrumental in the extraction and delivery of ubiquitylated ER proteins for proteasomal degradation. Such native CYP3A-p97 interactions were greatly magnified after CYP3A suicidal inactivation (which accelerates UPD), and/or proteasomal inhibition, and were confirmed by proteomic and confocal immunofluorescence microscopic analyses. These findings clearly reveal that native CYPs 3A undergo UPD and implicate a role for p97 in this process.

CYTOCHROME P450 UBIQUITINATION: Branding for the Proteolytic Slaughter?

The hepatic cytochromes P450 (P450s) are monotopic endoplasmic reticulum (ER)-anchored hemoproteins engaged in the enzymatic oxidation of a wide variety of endo- and xenobiotics. In the course of these reactions, the enzymes generate reactive O2species and/or reactive metabolic products that can attack the P450 heme and/or protein moiety and structurally and functionally damage the enzyme. The in vivo conformational unraveling of such a structurally damaged P450 signals its rapid removal via the cellular sanitation system responsible for the proteolytic disposal of structurally aberrant, abnormal, and/or otherwise malformed proteins. A key player in this process is the ubiquitin (Ub)-dependent 26S proteasome system. Accordingly, the structurally deformed P450 protein is first branded for recognition and proteolytic removal by the 26S proteasome with an enzymatically incorporated polyUb tag. P450s of the 3A subfamily such as the major human liver enzyme CYP3A4 are notorious targets for this process, and they represent excellent prototypes for the understanding of integral ER protein ubiquitination. Not all the participants in hepatic CYP3A ubiquitination and subsequent proteolytic degradation have been identified. The following discussion thus addresses the various known and plausible events and/or cellular participants involved in this multienzymatic P450 ubiquitination cascade, on the basis of our current knowledge of other eukaryotic models. In addition, because the detection of ubiquitinated P450s is technically challenging, the critical importance of appropriate methodology is also discussed.

Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs

Consistent with its highest abundance in humans, cytochrome P450 (CYP) 3A is responsible for the metabolism of about 60% of currently known drugs. However, this unusual low substrate specificity also makes CYP3A4 susceptible to reversible or irreversible inhibition by a variety of drugs. Mechanism-based inhibition of CYP3A4 is characterised by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-, time- and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYP isoenzymes to reactive metabolites capable of irreversibly binding covalently to CYP3A4. Approaches using in vitro, in silico and in vivo models can be used to study CYP3A4 inactivation by drugs. Human liver microsomes are always used to estimate inactivation kinetic parameters including the concentration required for half-maximal inactivation (K(I)) and the maximal rate of inactivation at saturation (k(inact)). Clinically important mechanism-based CYP3A4 inhibitors include antibacterials (e.g. clarithromycin, erythromycin and isoniazid), anticancer agents (e.g. tamoxifen and irinotecan), anti-HIV agents (e.g. ritonavir and delavirdine), antihypertensives (e.g. dihydralazine, verapamil and diltiazem), sex steroids and their receptor modulators (e.g. gestodene and raloxifene), and several herbal constituents (e.g. bergamottin and glabridin). Drugs inactivating CYP3A4 often possess several common moieties such as a tertiary amine function, furan ring, and acetylene function. It appears that the chemical properties of a drug critical to CYP3A4 inactivation include formation of reactive metabolites by CYP isoenzymes, preponderance of CYP inducers and P-glycoprotein (P-gp) substrate, and occurrence of clinically significant pharmacokinetic interactions with coadministered drugs. Compared with reversible inhibition of CYP3A4, mechanism-based inhibition of CYP3A4 more frequently cause pharmacokinetic-pharmacodynamic drug-drug interactions, as the inactivated CYP3A4 has to be replaced by newly synthesised CYP3A4 protein. The resultant drug interactions may lead to adverse drug effects, including some fatal events. For example, when aforementioned CYP3A4 inhibitors are coadministered with terfenadine, cisapride or astemizole (all CYP3A4 substrates), torsades de pointes (a life-threatening ventricular arrhythmia associated with QT prolongation) may occur.However, predicting drug-drug interactions involving CYP3A4 inactivation is difficult, since the clinical outcomes depend on a number of factors that are associated with drugs and patients. The apparent pharmacokinetic effect of a mechanism-based inhibitor of CYP3A4 would be a function of its K(I), k(inact) and partition ratio and the zero-order synthesis rate of new or replacement enzyme. The inactivators for CYP3A4 can be inducers and P-gp substrates/inhibitors, confounding in vitro-in vivo extrapolation. The clinical significance of CYP3A inhibition for drug safety and efficacy warrants closer understanding of the mechanisms for each inhibitor. Furthermore, such inactivation may be exploited for therapeutic gain in certain circumstances.

Metabolism-based inactivation of neuronal nitric-oxide synthase by components of cigarette and cigarette smoke

It has been shown that administration of cigarette smoke to rats leads to loss of neuronal nitric-oxide synthase (nNOS) activity and nNOS protein in penile tissue. The exact mechanism for this loss of activity and protein is not known. In the current study, we investigated whether extracts prepared from cigarette smoke or from the cigarette itself could directly inhibit nNOS activity. We discovered that the cigarette smoke extract and the cigarette extract cause a time-, concentration-, and calmodulin-dependent inactivation of nNOS in an in vitro system containing the purified enzyme. L-Arginine, but not D-arginine, protects nNOS from this time-dependent inactivation, suggesting an active site directed event. The kinetics of inactivation are consistent with the metabolism-based or suicide inactivation of nNOS. Based on studies with other metabolism-based inactivators, this cigarette-mediated inactivation may render nNOS more susceptible to proteasomal degradation and thereby may explain the loss of nNOS protein in vivo. The component(s) responsible for nNOS inactivation is not volatile, is not retained by a 3,000 molecular weight cut-off membrane, binds to activated charcoal, and is highly water-soluble under both acidic and basic conditions. The discovery of a direct inactivation of nNOS by an organic, cationic compound(s) present in tobacco and tobacco smoke provides a basis for further study of not only the mechanisms responsible for the biological effects of tobacco but also a search for a potentially novel inactivator of nNOS.

Hepatic cytochrome P450 degradation: mechanistic diversity of the cellular sanitation brigade

Hepatic cytochromes P450 (P450s) are monotopic endoplasmic reticulum (ER)-anchored hemoproteins that exhibit heterogenous physiological protein turnover. The molecular/cellular basis for such heterogeneity is not well understood. Although both autophagic-lysosomal and nonlysosomal pathways are available for their cellular degradation, native P450s such as CYP2B1 are preferentially degraded by the former route, whereas others such as CYPs 3A are degraded largely by the proteasomal pathway, and yet others such as CYP2E1 may be degraded by both. The molecular/structural determinants that dictate this differential proteolytic targeting of the native P450 proteins remain to be unraveled. In contrast, the bulk of the evidence indicates that inactivated and/or otherwise posttranslationally modified P450 proteins undergo adenosine triphosphate-dependent proteolytic degradation in the cytosol. Whether this process specifically involves the ubiquitin (Ub)-/26S proteasome-dependent, the Ub-independent 20S proteasome-dependent, or even a recently characterized Ub- and proteasome-independent pathway may depend on the particular P450 species targeted for degradation. Nevertheless, the collective evidence on P450 degradation attests to a remarkably versatile cellular sanitation brigade available for their disposal. Given that the P450s are integral ER proteins, this mechanistic diversity in their cellular disposal should further expand the repertoire of proteolytic processes available for ER proteins, thereby extending the currently held general notion of ER-associated degradation.

Comparison of the formation of N-alkylprotoporphyrin IX after interaction of porphyrinogenic xenobiotics with single cDNA-expressed human P450 enzymes in microsomes prepared from baculovirus-infected insect cells and human lymphoblastoid cell lines

In a previous study using microsomes from human lymphoblastoid cell lines (HLCL) containing single cDNA-expressed human cytochrome P450 (P450) enzymes, human P450 enzymes were identified that are susceptible to mechanism-based inactivation by the porphyrinogenic xenobiotics, 3-[(arylthio)ethyl]sydnone (TTMS), 3,5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine (4-ethylDDC) and allylisopropylacetamide (AIA). In this study, we tested the hypothesis that N-alkylprotoporphyrin IX (N-alkylPP) formation following interaction of porphyrinogenic xenobiotics with single cDNA-expressed human P450 enzymes in microsomes from HLCL would occur only with P450 enzymes that had undergone mechanism-based inactivation. In a previous study, when 4-ethylDDC and NADPH interacted with human liver microsomes possessing elevated levels of CYP1A2 and 2C9, N-ethylprotoporphyrin IX (N-ethylPP) was not formed despite the fact that it was formed in microsomes from baculovirus-infected insect cell lines (BIICL) containing either CYP1A2 or 2C9. In this study, we tested the hypothesis that 4-ethylDDC underwent biotransformation by CYP3A4 present in human liver microsomes, diverting the xenobiotic from CYP1A2 and 2C9. Fluorometry was used to measure N-alkylPP formation following interaction of porphyrinogenic xenobiotics and NADPH with cDNA-expressed human P450 enzymes in microsomes from HLCL or BIICL. With TTMS and 4-ethylDDC but not with AIA, N-alkylPP formation was observed only with human P450 enzymes CYP2D6, 1A2, 3A4, or 2C9 in microsomes from HLCL, which had undergone mechanism-based inactivation. Microsomes from BIICL containing CYP3A4 were added to a mixture of NADPH, 4-ethylDDC, and microsomes from BIICL containing CYP1A2 and 2C9. The addition of CYP3A4 to CYP1A2 and 2C9 did not decrease N-ethylPP formation, providing no support for the hypothesis.

Proteolytic degradation of nitric oxide synthase: effect of inhibitors and role of hsp90-based chaperones

Nitric oxide synthase (NOS) is a highly regulated enzyme that produces nitric oxide, a critical messenger in many physiological processes. In this perspective, we explore the role of proteolytic degradation of NOS, in particular the inducible and neuronal isoforms of NOS, as a mechanism of regulation of the enzyme. The ubiquitin-proteasome and calpain pathways are the major proteolytic systems identified to date that are responsible for this regulated degradation. The degradation of NOS is affected by diverse agents, including glucocorticoids, caveolin, neurotoxic compounds, and certain NOS inhibitors. Some irreversible inactivators of NOS enhance the proteolytic degradation of the enzyme, and this property may be of great importance in understanding the biological effects of these inhibitors, some of which are being developed for clinical use. Analogies with the regulated degradation of liver microsomal cytochromes P450, which are related to NOS, provide a framework for understanding these processes. Finally, a new perspective on the regulation of NOS by hsp90-based chaperones is presented that involves facilitated heme insertion into the enzyme.

Pathways of carbamazepine bioactivation in vitro I. Characterization of human cytochromes P450 responsible for the formation of 2- and 3-hydroxylated metabolites

In vitro studies were conducted to identify the cytochromes P450 (P450s) involved in the formation of 2- and 3-hydroxycarbamazepine, metabolites that may serve as precursors in the formation of protein-reactive metabolites. Human liver microsomes (HLMs) converted carbamazepine (30-300 microM) to 3-hydroxycarbamazepine at rates >25 times those of 2-hydroxycarbamazepine. Both the 2- and 3-hydroxylation of carbamazepine appeared to conform to monophasic Michaelis-Menten kinetics in HLMs (apparent K(m) values, approximately 1640 and approximately 217 microM; apparent V(max) values, approximately 5.71 and approximately 46.9 pmol/mg of protein/min, respectively). Rates of carbamazepine 2- and 3-hydroxylation correlated strongly with CYP2B6 activity (r >or= 0.757) in a panel of HLMs (n = 8). Carbamazepine 3-hydroxylation also correlated significantly with CYP2C8 activity at a carbamazepine concentration of 30 microM. Formation of 2- and 3-hydroxycarbamazepine did not correlate significantly with any other P450 activities. The chemical inhibitors ketoconazole (CYP3A) and 7-EFC (CYP2B6) inhibited both 2- and 3-hydroxycarbamazepine formation whereas 4-methylpyrazole (CYP2E1) markedly decreased 2-hydroxycarbamazepine formation. Several recombinant P450s catalyzed carbamazepine 2- and 3-hydroxylation, but after adjustment for relative hepatic abundance, CYP3A4 and CYP2B6 appeared to be the major catalysts of carbamazepine 3-hydroxylase activity, and at least five P450s were significant contributors to 2-hydroxycarbamazepine formation; CYP2E1 made the greatest contribution to the Cl(int) of carbamazepine 2-hydroxylation (approximately 30%), but P450s CYP1A2, 2A6, 2B6, and 3A4 also made significant contributions (approximately 13-18%). These results suggest that CYP2B6 and CYP3A4 are largely responsible for the formation of 3-hyrdoxycarbamazepine, whereas multiple P450s (CYP1A2, 2A6, 2B6, 2E1, and 3A4) contributed to 2-hydroxycarbamazepine formation.

Identification of human hepatic cytochrome P450 sources of N-alkylprotoporphyrin IX after interaction with porphyrinogenic xenobiotics, implications for detection of xenobiotic-induced porphyria in humans

Porphyrinogenicity of certain xenobiotics depends upon mechanism-based inactivation of specific cytochrome P450 (P450) enzymes, followed by formation of N-alkylprotoporphyrin IX (N-alkylPP). Examination of the porphyrinogenicity of xenobiotics in animals and extrapolation of the results to humans is associated with ambiguity due, in part, to differences between P450 enzymes. The goal of this study was to develop an in vitro test for the detection of N-alkylPPs, produced in human liver after administration of xenobiotics found to be porphyrinogenic in animals. This goal was achieved using fluorometry to detect N-alkylPP formation following mechanism-based inactivation by porphyrinogenic xenobiotics of single cDNA-expressed human P450 enzymes in microsomes prepared from baculovirus-infected insect cells (Supersomes) and in human liver microsomes. The following combinations of P450 enzymes were major sources of N-alkylPPs in Supersomes: CYP3A4 [3-[(arylthio)-ethyl]sydnone (TTMS)]; CYP1A2 and 2C9 [3,5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine (4-ethyl DDC)]; and CYP2C9, 2D6, and 3A4 [allylisopropylacetamide (AIA)]. Whereas similarities were found between results with human enzymes in Supersomes and their rat orthologs in rat liver microsomes, some differences were found. The results with TTMS and AIA, but not with 4-ethyl DDC, were the same in individual human enzymes expressed in Supersomes and human liver microsomes. We conclude that some differences exist between human liver P450 enzymes and their rat P450 orthologs in liver microsomes. It would therefore be prudent when dealing with xenobiotics in which porphyrinogenicity depends upon N-alkylPP formation to supplement animal data with studies using human P450 enzymes.

Mechanism-based inactivation of cytochrome P450 3A4 by 17 alpha-ethynylestradiol: evidence for heme destruction and covalent binding to protein

17 alpha-Ethynylestradiol (EE), a major constituent of many oral contraceptives, inactivated the testosterone 6 beta-hydroxylation activity of purified P450 3A4 reconstituted with phospholipid and NADPH-cytochrome P450 reductase in a mechanism-based manner. The inactivation of P450 3A4 followed pseudo first order kinetics and was dependent on NADPH. The values for the K(I) and k(inact) were 18 microM and 0.04 min(-1), respectively, and the t(1/2) was 16 min. Incubation of 50 microM EE with P450 3A4 at 37 degrees C for 30 min resulted in a 67% loss of testosterone 6 beta-hydroxylation activity accompanied by a 35% loss of the spectral absorbance of the native protein at 415 nm and a 70% loss of the spectrally detectable P450-CO complex. The inactivation of P450 3A4 by EE was irreversible. Testosterone, an alternate substrate, was able to protect P450 3A4 from EE-dependent inactivation. The partition ratio was approximately 50. The stoichiometry of binding was approximately 1.3 nmol of an EE metabolite bound per nmol of P450 3A4 inactivated. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [(3)H]EE was irreversibly bound to the P450 3A4 apoprotein. After extensive dialysis of the [(3)H]EE inactivated samples, high-pressure liquid chromatography (HPLC) analysis demonstrated that the inactivation resulting from EE metabolism led to the destruction of approximately half the heme with the concomitant generation of modified heme and EE-labeled heme fragments and produced covalently radiolabeled P450 3A4 apoprotein. Electrospray mass spectrometry demonstrated that the fraction corresponding to the major radiolabeled product of EE metabolism has a mass (M - H)(-) of 479 Da. HPLC and gas chromatography-mass spectometry analyses revealed that EE metabolism by P450 3A4 generated one major metabolite, 2-hydroxyethynylestradiol, and at least three additional metabolites. In conclusion, our results demonstrate that EE is an effective mechanism-based inactivator of P450 3A4 and that the mechanism of inactivation involves not only heme destruction, but also the irreversible modification of the apoprotein at the active site.

Ubiquitin-dependent 26S proteasomal pathway: a role in the degradation of native human liver CYP3A4 expressed in Saccharomyces cerevisiae?

Cytochrome P450, CYP3A4, is the dominant human liver endoplasmic reticulum (ER) hemoprotein enzyme, responsible for the metabolism of over 60% of clinically relevant drugs. We have previously shown that mechanism-based suicide inactivation of CYP3A4 and its rat liver ER orthologs, CYPs 3A, via heme-modification of their protein moieties, results in their ubiquitin (Ub)-dependent 26S proteasomal degradation (Korsmeyer et al. (1999) Arch. Biochem. Biophys. 365, 31; Wang et al. (1999) Arch. Biochem. Biophys. 365, 45). This is not surprising given that the heme-modified CYP3A proteins are structurally damaged. To determine whether the turnover of the native enzyme similarly recruited this pathway, we heterologously expressed this protein in wild-type Saccharomyces cerevisiae and mutant strains (hrd1Delta, hrd2-1, and hrd3Delta) previously shown to be deficient in the Ub-dependent 26S proteasomal degradation of the polytopic ER protein 3-hydroxy-3-methylglutaryl-CoA reductase (isoform Hmg2p), the rate-limiting enzyme in sterol biosynthesis, as well as in strains deficient in ER-associated Ub-conjugating enzymes, Ubc6p and/or Ubc7p (Hampton et al. (1996) Mol. Biol. Cell 7, 2029; Hampton and Bhakta (1997) Proc. Natl. Acad. Sci. USA 94, 12,944). Our findings reveal that in common with the degradation of Hmg2p, that of native CYP3A4 also requires Hrd2p (a subunit of the 19S cap complex of the 26S proteasome) and Ubc7p, and to a much lesser extent Hrd3p, a component of the ER-associated Ub-ligase complex. In contrast to Hmg2p-degradation, that of native CYP3A4 does not appear to absolutely require Hrd1p, another component of the ER-associated Ub-ligase complex. Furthermore, studies in a S. cerevisiae pep4Delta strain proven to be deficient in the vacuolar degradation of carboxypeptidase Y indicated that CYP3A4 degradation is also largely independent of vacuolar (lysosomal) proteolytic function. The degradation of two other native ER proteins, Sec61p and Sec63p, normal components of the ER translocon, were also examined in parallel and found to be stabilized to some extent in HRD2- and UBC7-deficient strains. Together these findings attest to the remarkable mechanistic diversity in the normal degradation of ER proteins.

Ubiquitination of neuronal nitric-oxide synthase in vitro and in vivo

It is established that suicide inactivation of neuronal nitric-oxide synthase (nNOS) with guanidine compounds, or inhibition of the hsp90-based chaperone system with geldanamycin, leads to the enhanced proteolytic degradation of nNOS. This regulated proteolysis is mediated, in part, by the proteasome. We show here with the use of human embryonic kidney 293 cells transfected with nNOS that inhibition of the proteasome with lactacystin leads to the accumulation of immunodetectable higher molecular mass forms of nNOS. Some of these higher molecular mass forms were immunoprecipitated by an anti-ubiquitin antibody, indicating that they are nNOS-polyubiquitin conjugates. Moreover, the predominant nNOS-ubiquitin conjugate detected in human embryonic kidney 293 cells, as well as in rat brain cytosol, migrates on SDS-polyacrylamide gels with a mobility near that for the native monomer of nNOS and likely represents a conjugate containing a few or perhaps one ubiquitin. Studies in vitro with the use of (125)I-ubiquitin and reticulocyte extracts could mimic this ubiquitination reaction, which was dependent on ATP. The heme-deficient monomeric form of nNOS is preferentially ubiquitinated over that of the heme-sufficient functionally active homodimer. Thus, we have shown for the first time that ubiquitination of nNOS occurs and is likely involved in the regulated proteolytic removal of non-functional enzyme.

Aminoguanidine-mediated inactivation and alteration of neuronal nitric-oxide synthase

It is established that aminoguanidine (AG) is a metabolism-based inactivator of the three major isoforms of nitric-oxide synthase. AG is thought to be of potential use in diseases, such as diabetes, where pathological overproduction of NO is implicated. We show here that during the inactivation of neuronal nitric-oxide synthase (nNOS) by AG that the prosthetic heme is altered, in part, to dissociable and protein-bound adducts. The protein-bound heme adduct is the result of cross-linking of the heme to residues in the oxygenase domain of nNOS. The dissociable heme product is unstable and reverts back to heme upon isolation. The alteration of the heme is concomitant with the loss in the ability to form the ferrous-CO complex of nNOS and accounts for at least two-thirds of the activity loss. Studies with [(14)C]AG indicate that alteration of the protein, in part on the reductase domain of nNOS, also occurs but at low levels. Thus, heme alteration appears to be the major cause of nNOS inactivation. The elucidation of the mechanism of inactivation of nNOS will likely lead to a better understanding of the in vivo effects of NOS inhibitors such as AG.

Heme: a regulator of rat hepatic tryptophan 2,3-dioxygenase?

The hepatic cytosolic hemoprotein tryptophan 2,3-dioxygenase (TDO) is the rate-limiting enzyme in tryptophan catabolism and thus plays a key role in regulating the physiological flux of tryptophan into relevant metabolic pathways. The TDO protein is induced by corticosteroids such as dexamethasone (DEX) and is stabilized by its prosthetic heme. In rats, acute chemically induced hepatic heme depletion reduces the functional hepatic TDO levels to 25-30% of basal levels within 1 h, and this decrease persists beyond 28 h of heme depletion at which time only 25-30% of the protein is available for heme incorporation. Since this could stem from impaired de novo synthesis and/or instability of the newly synthesized apoTDO protein in the absence of heme, we examined the specific role of heme in these events in a previously validated rat model of acute hepatic heme depletion triggered by the P450 suicide substrate 3, 5-dicarbethoxy 2,6-dimethyl-4-ethyl-1,4-dihydropyridine. We now show that exogenous heme can reverse the functional impairment of the enzyme observed during hepatic heme depletion and fully restore the impaired DEX-mediated induction of the enzyme to normal. Furthermore, through Northern/slot blot analyses coupled with nuclear run-on studies, we now document that this heme regulation of TDO is exerted primarily at the transcriptional level. Immunoblotting analyses also reveal corresponding changes in the TDO protein, thereby establishing that heme is necessary for DEX-inducible TDO mRNA transcription and subsequent translation. Thus, the TDO gene may contain heme-regulatory elements in addition to the reported glucocorticoid-responsive elements. Together, these findings suggest that clinically, hepatic heme deficiency may enhance the tryptophan flux into synthetic (serotonergic) pathways, not only by depriving prosthetic heme for a functionally competent TDO hemoprotein, its primary catabolic enzyme, but also by impairing the de novo synthesis of this enzyme.

Guanabenz-mediated inactivation and enhanced proteolytic degradation of neuronal nitric-oxide synthase

Guanabenz, a metabolism-based irreversible inactivator of neuronal nitric-oxide synthase (nNOS) in vitro, causes the loss of immunodetectable nNOS in vivo. This process is selective in that the slowly reversible inhibitor N(G)-nitro-L-arginine did not decrease the levels of nNOS in vivo. To better understand the mechanism for the loss of nNOS protein in vivo, we have investigated the effects of guanabenz and N(G)-nitro-L-arginine in HEK 293 cells stably transfected with the enzyme. We show here that guanabenz, but not N(G)-nitro-L-arginine, caused the inactivation and loss of nNOS protein in the HEK 293 cells. In studies with cycloheximide or in pulse-chase experiments with [(35)S]methionine, we demonstrate that the loss of nNOS was due in large part to enhanced proteolysis of the protein with the half-life decreasing by one-half from 20 to 10 h. Other metabolism-based irreversible inactivators to nNOS, N(G)-methyl-L-arginine, and N(5)-(1-iminoethyl)-L-ornithine, but not the reversible inhibitor 7-nitroindazole (7-NI), caused a similar decrease in the half-life of nNOS. Proteasomal inhibitors, lactacystin, Cbz-leucine-leucine-leucinal, and N-acetyl-leucine-leucine-norleucinal, but not the lysosomal protease inhibitor leupeptin, were found to effectively inhibit the proteolytic degradation of nNOS. Thus we have shown for the first time that the irreversible inactivators of nNOS, perhaps through covalent alteration of the enzyme, enhance the proteolytic turnover of the enzyme by a mechanism involving the proteasome.

Cytochrome P450 3A degradation in isolated rat hepatocytes: 26S proteasome inhibitors as probes

Mechanism-based inactivation of liver microsomal cytochromes P450 3A (CYP 3A, P450s 3A) in vivo and/or in vitro, via heme modification of the protein, results in accelerated proteolytic degradation of the enzyme that is preceded by the ubiquitination of the protein, thereby implicating the ubiquitin-ATP-dependent 26S proteasomal system. In this study, this involvement is confirmed with the use of the proteasomal inhibitors aclarubicin and MG-132 as probes, in isolated rat hepatocytes treated with the P450 3A mechanism-based inactivator, 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1, 4-dihydropyridine (DDEP). In addition, the findings reveal that during the course of this proteolysis, the endoplasmic reticulum (ER)-anchored DDEP-inactivated P450 3A is translocated from the ER to the cytosol in a brefeldin A-insensitive manner.

Proteolytic degradation of heme-modified hepatic cytochromes P450: A role for phosphorylation, ubiquitination, and the 26S proteasome?

The resident integral hepatic endoplasmic reticulum (ER) proteins, cytochromes P450 (P450s), turn over in vivo with widely varying half-lives. We and others (Correia et al., Arch. Biochem. Biophys. 297, 228, 1992; and Tierney et al., Arch. Biochem. Biophys. 293, 9, 1992) have previously shown that in intact animals, the hepatic P450s of the 3A and 2E1 subfamilies are first ubiquitinated and then proteolyzed after their drug-induced suicide inactivation. Our findings with intact rat hepatocytes and ER preparations containing native P450s and P450s inactivated via heme modification of the protein have revealed that the proteolytic degradation of heme-modified P450s requires a cytosolic ATP-dependent proteolytic system rather than lysosomal or ER proteases (Correia et al., Arch. Biochem. Biophys. 297, 228, 1992). Using purified cumene hydroperoxide-inactivated P450s (rat liver P450s 2B1 or 3A and/or a recombinant human liver P450 3A4) as models, we now document that these heme-modified enzymes are indeed ubiquitinated and then proteolyzed by the 26S proteasome, but not by its 20S proteolytic core. In addition, our studies indicate that the ubiquitination of these heme-modified P450s is preceded by their phosphorylation. It remains to be determined whether, in common with several other cellular proteins, such P450 phosphorylation is indeed required for their degradation. Nevertheless, these findings suggest that the membrane-anchored P450s are to be included in the growing class of ER proteins that undergo ubiquitin-dependent 26S proteasomal degradation.

Interaction of Angeli's salt with cytochrome P450 1A2 distal mutants: an optical absorption spectral study

Angeli's salt, Na2N2O3 or O-N=N+-(OH)(O-) in aqueous solution, is known to release NO- or NO., which relaxes vascular tissue and lowers blood pressure. In the liver, the most abundant heme enzyme is cytochrome P450. In the present study, we studied the effect of rat liver cytochrome P450 1A2 (P450 1A2) in regard to its catalysis of the N=N bond scission of Angeli's salt with optical absorption spectra. Also, we examined the contribution of putative distal amino acids of P450 1A2 to the reaction with the salt. We found that wild-type Fe3+ P450 1A2 markedly enhances the N=N scission of the salt up to 100 fold in terms of absorption spectroscopy. A Fe3+ P450 1A2-NO complex with an absorption peak at 435 nm was formed when the salt was added and the complex was then changed to a 6-coordinated Fe2+-NO complex having a 440-nm peak. Glu318Asp, Glu318Ala and Thr319Ala mutants at the putative distal site of P450 1A2 formed a 5-coordinated Fe2+-NO complex having a 400-nm absorption, that was not formed with the wild type. The Glu318Ala mutant, in particular, did not form the Fe3+-NO complex with the addition of Angeli's salt. The presence of L-Cys, reduced glutathione, catalase or superoxide dismutase markedly stabilized the Fe3+ wild type-NO complex. Thus, our data suggests that the N=N bond of Angeli's salt is cleaved with the P450 1A2 active site and NO- or NO. is released. We discuss mechanisms of redox and ligand changes of the P450 heme.

Hydroperoxide-mediated cytochrome P450-dependent 8-anilino-1-naphthalenesulfonic acid destruction, product formation and P450 modification

The interaction between hydroperoxides, cytochrome P450 and 8-anilino-1-naphthalenesulfonic acid (ANS) has been investigated. The addition of ANS to the cytochrome P450 solution did not effect the P450 Soret absorption peak or the reduced CO difference spectrum, suggesting that ANS may not bind to P450 home directly. H2O2 or CuOOH alone did not effect ANS fluorescence and absorption spectra indicating that no detectable reaction occurs between hydroperoxide and ANS in the absence of P450. The reconstituted system of cytochrome P450, P450 reductase, lipid and NADPH did not mediate ANS metabolism. In the presence of P450, the addition of either H2O2 or CuOOH, however, leads to a decrease in ANS absorption around 258 nm and 350 nm indicating possible destruction of ANS. ANS destruction was confirmed with the disappearance of the ANS elution peak in the reverse phase HPLC profiles and with the changes in P450-bound ANS fluorescence intensity and the shift of lambda max of ANS. Moreover, a very sensitive method to detect trace fluorescent products of ANS by thin layer chromatography has been developed based on the fact that ANS fluorescence is enhanced more than 1000-fold by the organic solvent butanol. A UV-sensitive fluorescent product was detected on thin layer chromatography profiles of the reaction mixtures. P450 was also observed to be modified by a fluorescent derivative of ANS, when the fluorescence was enhanced by butanol. These results also show that an organic compound which can not be metabolized by the reconstituted system of cytochrome P450 and NADPH-P450 reductase is metabolized by the reconstituted system of P450 and hydroperoxide, suggesting the activities of these two systems may not be completely comparable.

Inactivation of chick embryo hepatic cytochrome P450 1A, 2H and 3A following in ovo administration of 3,5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine and 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone

Rat hepatic cytochrome P450 (P450) isozymes 1A1, 2C6, 2C11, 3A1 and 3A2 are targets for mechanism-based inactivation by the porphyrinogenic compound 3,5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine (4-ethyl DDC). It is of interest to determine whether similar P450 isozymes are targets of porphyrinogenic drugs in the chick embryo liver. The chick embryo expresses P450 2H1/2 isozymes, which are similar to the rat P450 2B1/2 isozymes, a polycyclic aromatic hydrocarbon-inducible P450 1A isozyme, and a pregnenolone 16 alpha-carbonitrile-inducible P450 3A isozyme. We have found previously that chick embryo hepatic P450 1A and 3A isozymes are targeted for in vitro mechanism-based inactivation by 4-ethyl DDC and by the sydnone 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS). Marked differences have been observed between the in vitro and in vivo effects of porphyrinogenic drugs on P450 isozymes. Thus, the first objective of this study was to determine whether chick embryo hepatic P450 1A and 3A isozymes are subject to in ovo inactivation by these porphyrinogenic compounds. Our second objective was to determine whether the chick embryo hepatic P450 2H isozyme(s) was subject to in ovo and in vitro inactivation by 4-ethyl DDC and TTMS. Using hepatic microsomes prepared from beta-naphthoflavone-, dexamethasone-, phenobarbital-, and glutethimide-induced 19-day-old chick embryos, we found that total P450 content was decreased significantly in microsomes prepared from all treatment groups following in ovo administration of 4-ethyl DDC and TTMS. Moreover, in ovo administration of both 4-ethyl DDC and TTMS caused a significant decrease of 7-ethoxyresorufin O-deethylase, erythromycin N-demethylase, and benzphetamine N-demethylase activities, which are selective catalytic markers for chick embryo hepatic P450 1A, 3A and 2H isozymes, respectively. In addition, in vitro administration of 4-ethyl DDC and TTMS caused mechanism-based inactivation of benzphetamine N-demethylase activity in microsomes from phenobarbital- and glutethimide-treated chick embryos, showing that the chick embryo hepatic P450 2H isozyme is a target for mechanism-based inactivation. Therefore, it was concluded that the chick embryo hepatic P450 1A, 2H and 3A isozymes serve as targets for both in ovo and in vitro mechanism-based inactivation by 4-ethyl DDC and TTMS.

Evidence for mechanism-based inactivation of rat and chick embryo hepatic cytochrome P4501A and P4503A by dihydropyridines, sydnones, and dihydroquinolines

Rat hepatic P4501A1 and 3A1/2 have been shown previously to be targets for mechanism-based inactivation by the 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC), namely, 4-ethyl DDC and 4-isopropyl DDC. In this study we have shown that rat hepatic P4501A and P4503A are targets for mechanism-based inactivation by the sydnones, 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS) and 3-(2-phenylethyl)-4-methylsydnone (PEMS). The dihydroquinoline, 2,4-diethyl-2-methyl-1,2-dihydroquinoline (DMDQ), caused mechanism-based inactivation of rat hepatic P4501A but not of P4503A. The P4501A isozyme(s) of chick embryo liver was found to share the ability of rat liver P4501A to serve as a target for mechanism-based inactivation by the dihydropyridines, 4-ethyl DDC and 4-isopropyl DDC, the sydnones, TTMS and PEMS, and the dihydroquinoline, DMDQ. A P4503A-like isozyme of chick embryo liver shared the ability of the rat liver P4503A isozyme(s) to serve as a target for mechanism-based inactivation by the dihydropyridines, 4-ethyl DDC and 4-isopropyl DDC, and the sydnone, TTMS, but not of the sydnone PEMS. The dihydropyridine, DDC, was found to serve as a mechanism-based inactivator of the chick embryo P4501A isozyme(s), but not of the P4503A isozyme(s), in contrast to its previously reported inactivity with both the rat hepatic P4501A1 and 3A1/2 isozymes.

Degradation of rat liver cytochromes P450 3A after their inactivation by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine: characterization of the proteolytic system

The suicide substrate 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4- dihydropyridine (DDEP) inactivates rat liver cytochrome P450 (P450) 3A isozymes through prosthetic heme alkylation of the apoprotein in a mechanism-based fashion, which marks them for rapid proteolysis. In this article, through the use of epitope-specific monoclonal antibodies, we show that both 3A1 and 3A2 isozymes are targeted for proteolysis. Furthermore, using intact rats, isolated rat hepatocytes, and rat liver subcellular fractions supplemented with ATP and MgCl2, as well as various proteolytic inhibitors as probes, we now report that the hepatic cytosolic ubiquitin-dependent proteolytic system rather than hepatic lysosomes is involved in the rapid degradation of DDEP-induced heme alkylated P450s 3A.

Differential apoprotein loss of rat liver cytochromes P450 after their inactivation by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine: a case for distinct proteolytic mechanisms?

Suicide inactivation of hepatic cytochrome P450 (P450) enzymes 2C11, 2C6, and 3A1/A2 by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP) in intact rats results in prosthetic heme destruction, albeit by apparently distinct mechanisms. Such heme destruction is now shown to be associated with the loss of immunochemically detectable apoprotein of P450s 2C11 and 3A but with little of that of P450 2C6, in spite of their comparable DDEP-mediated functional inactivation. The loss of a approximately 50-kDa hepatic microsomal protein band along with the immunoreactive P450 3A loss strengthens the concept that such an in vivo loss indeed reflects proteolysis of the DDEP-inactivated P450. Furthermore, this propensity of DDEP-inactivated P450s 3A for proteolysis appears to correlate with the relative degree of prosthetic heme alkylation of their apoprotein rather than their functional inactivation per se. Thus, rapid degradation of apoP450s 3A was seen after DDEP treatment, which promoted extensive irreversible heme binding to apoP450s 3A, but not after exposure to allylisopropylacetamide (AIA), which inactivates these isozymes comparably, but induced minimal apoP450 3A heme alkylation. In addition, differences were observed in the relative sensitivities of proteolysis of DDEP-inactivated P450s 2C11 and 3A to hemin, which largely prevented the DDEP-induced proteolytic loss of P450 2C11 but apparently failed to prevent the loss of DDEP-inactivated P450s 3A, when coadministered with DDEP. This differential hemin sensitivity of the proteolysis of DDEP-inactivated P450 2C11, coupled with the observation that immunochemically detectable P450 2C11 loss occurs after its inactivation by both AIA and DDEP, provides compelling support for the existence of distinct proteolytic pathways for individual suicidally inactivated P450s.

Degradation of cytochrome P450 2E1: selective loss after labilization of the enzyme

Mechanism-based inactivation of cytochrome P450 can result in the chemical modification of the heme, the protein, or both as a result of covalent binding of modified heme to the protein. In the present study we took advantage of different modes of inactivation of P450 2E1 by CCl4, 1-aminobenzotriazole, or 3-amino-1,2,4-triazole to investigate parameters which target P450 2E1 for proteolysis from the microsomal membrane. Treatment of mice with CCl4 at the point of maximal induction of P450 2E1 after a single oral dose of acetone resulted in the complete loss of P450 2E1-dependent p-nitrophenol hydroxylation and a 75% loss of immunochemically detectable protein within 1 h of administration. Treatment with 1-aminobenzotriazole at the point of maximal induction caused a complete loss of P450 2E1-dependent p-nitrophenol hydroxylation but only a 12% loss of immunochemically detectable protein 1 h after administration. Treatment of mice with 3-amino-1,2,4-triazole caused a rapid loss of both catalytic activity and microsomal p-nitrophenol hydroxylase activity. However, unlike CCl4 treatment, the activity and enzyme level rebounded 5 and 9 h after treatment. The P450 2E1 ligand, 4-methylpyrazole, administered at the point of maximal induction maintained the acetone-induced catalytic and immunochemical level of P450 2E1. These results suggest that differentially modified forms of P450 2E1 show a characteristic susceptibility to degradation. While there are many potential pathways for protein degradation, the loss of P450 2E1 was associated with increased formation of high molecular weight microsomal ubiquitin conjugates. The formation of ubiquitin-conjugated microsomal protein which correlates with P450 2E1 loss suggests that ubiquitination may represent a proteolytic signal for the rapid and selective proteolysis of certain labilized conformations of P450 2E1 from the endoplasmic reticulum.

High-performance liquid chromatography of N-alkylprotoporphyrins: an investigation of their formation and loss under chemical and enzymatic conditions in vitro

A new technique for resolving N-alkylprotoporphyrins into each structural isomer is described. The technique has been used to investigate the rate of formation and loss of N-alkylporphyrins during reaction of the parent porphyrin with alkyl iodides and to establish the conditions required for optimal yields of the various isomers. Preferential loss of the isomers bearing the N-alkyl group on one of the vinyl-substituted pyrrole rings is observed on prolonged incubation and HI generated during the reaction has been shown to be responsible. A method for detection and partial resolution by HPLC of N-alkylprotoporphyrins produced by liver microsomes in vitro is also described. Microsomes from rats induced with 3-methylcholanthrene produce significantly more N-ethylprotoporphyrin from either 4-ethyl-3,5-diethoxycarbonyl-1,4-dihydro- 2,6-dimethyl-pyridine or ethylhydrazine than do microsomes from control animals, but the isomeric composition of the isolated N-alkylporphyrin differs from that reported in vivo. Evidence that authentic N-alkylporphyrins are lost during incubation with microsomes has been obtained, and here again, the isomers bearing the N-alkyl group on vinyl-substituted pyrrole rings are preferentially lost. Experiments with 14C-labeled N-methylprotoporphyrin show that approximately 40% of the lost porphyrin could be recovered bound covalently to the microsomal pellet.

Irreversible binding of heme to microsomal protein during inactivation of cytochrome P450 by 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine

The porphyrinogenicity of 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC) is related to the process of mechanism-based destruction of cytochrome P450 (P450) heme, accompanied by conversion of heme to N-alkylprotoporphyrins (N-alkylPPs). Certain DDC analogues (4-isopropyl, 4-isobutyl, 4-hexyl) are weakly porphyrinogenic in comparison to the potent porphyrinogen, 4-ethyl DDC. We have examined the abilities of these DDC analogues to promote irreversible binding of radiolabeled heme to protein in rat liver microsomal preparations. The goals of this study were to determine whether DDC analogues with different porphyrinogenicities differ in the extents to which they cause heme adduct formation, and whether P450 isozymes differ in their capacities to catalyze heme covalent binding. Incubation of microsomes with NADPH alone promoted heme covalent binding, while loss of spectral P450 heme was minimal or absent. In microsomal incubations containing NADPH, the 4-ethyl, 4-isopropyl, and 4-isobutyl analogues caused heme covalent binding to extents which paralleled their P450 destructive activities. In contrast, 4-hexyl DDC caused less heme covalent binding as a function of P450 loss than the other analogues in microsomes from untreated and beta-naphthoflavone (beta NF)-treated rats. Thus, the weakly porphyrinogenic DDC analogues do not cause greater heme covalent binding than 4-ethyl DDC. Weak porphyrinogenicity, therefore, cannot be explained by diversion of the heme moiety of P450 from conversion to N-alkylPPs towards utilization for formation of heme-derived protein adducts. Treatment of rats with P450 inducing agents altered the degree to which DDC analogues caused heme covalent binding. The greatest heme adduct formation occurred in microsomes from untreated and dexamethasone (DEX)-treated rats, whereas treatment with phenobarbital and especially beta NF reduced heme covalent binding as a function of P450 loss. Thus, these microsomal studies suggest that constitutive P450 isozymes and members of the DEX-inducible P450IIIA subfamily appear to catalyze heme covalent binding, while beta NF-inducible forms such as P450IA1 (P450c) seem to be relatively inactive in this regard.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

Free radical modification of prosthetic heme groups

Hemoproteins catalyze reductive and oxidative one-electron transformations. Not infrequently, the radicals produced by these one-electron reactions add to the prosthetic heme group of the enzyme and modify or terminate its catalytic function. Reactions of the radicals with the heme group include additions to the iron atom, pyrrole nitrogens, pyrrole carbons, vinyl groups, and meso carbons. The radicals involved in these reactions derive from the oxidizing agent, the substrate, or the amino acid residues of the catalytic site. The mechanism by which the radicals are generated, their steric and electronic properties, and the extent to which they have access to the heme group determine the nature and regiospecificity of the reaction. The reaction of heme prosthetic groups with radicals is relevant to the inhibition of hemoprotein enzymes, the normal and pathological degradation of heme, and our understanding of hemoprotein function.

Chemical synthesis, absolute configuration, and stereochemistry of formation of 10-hydroxywarfarin: a major oxidative metabolite of (+)-(R)-warfarin from hepatic microsomal preparations

The synthesis of a diastereomerically pure 10-hydroxywarfarin [4-hydroxy-3-(2-hydroxy-3-oxo-1-phenylbutyl)-2H-1 benzopyran-2-one] was accomplished in three steps from racemic warfarin. The relative configuration of the synthetic product was established by conversion to a cyclic derivative followed by NMR and X-ray diffraction analysis. Absolute stereochemistry was determined by enzymatic conversion of either of the pure enantiomers of warfarin to a 10-hydroxy metabolite of known relative configuration. Metabolic formation of 10-hydroxywarfarin was studied using hepatic microsomal preparations from female rats and man. The formation of 10-hydroxywarfarin catalyzed by hepatic microsomes from both dexamethasone-treated rats and man was highly stereoselective [(R)/(S): 3.4-9.0] for (R)-warfarin. In contrast, little stereoselectivity was observed in reactions catalyzed by untreated rat liver microsomes. The resultant stereochemistry at the site of oxidation was also found to be highly dependent on substrate stereochemistry. (R)-Warfarin gave (9R;10S)-10-hydroxywarfarin with only a trace of the (9R;10R) isomer irrespective of which enzyme preparation was used for catalysis, while (S)-warfarin gave (9S;10R)-10-hydroxywarfarin with only a trace of the (9S;10S) isomer, again irrespective of which enzyme preparation was used for catalysis.