Sulphonation of N-hydroxy-2-acetylaminofluorene by human dehydroepiandrosterone sulphotransferase

Regioselectivity prediction of CYP1A2-mediated phase I metabolism

A kinetic, reactivity-binding model has been proposed to predict the regioselectivity of substrates meditated by the CYP1A2 enzyme, which is responsible for the metabolism of planar-conjugated compounds such as caffeine. This model consists of a docking simulation for binding energy and a semiempirical molecular orbital calculation for activation energy. Possible binding modes of CYP1A2 substrates were first examined using automated docking based on the crystal structure of CYP1A2, and binding energy was calculated. Then, activation energies for CYP1A2-mediated metabolism reactions were calculated using the semiempirical molecular orbital calculation, AM1. Finally, the metabolic probability obtained from two energy terms, binding and activation energies, was used for predicting the most probable metabolic site. This model predicted 8 out of 12 substrates accurately as the primary preferred site among all possible metabolic sites, and the other four substrates were predicted into the secondary preferred site. This method can be applied for qualitative prediction of drug metabolism mediated by CYP1A2 and other CYP450 family enzymes, helping to develop drugs efficiently.

CYP2E1 active site residues in substrate recognition sequence 5 identified by photoaffinity labeling and homology modeling

Despite its biological importance, our knowledge of active site structure and relevance of critical amino acids in CYP2E1 catalytic processes remain limited. In this study, we identified CYP2E1 active site residues using photoaffinity labeling with 7-azido-4-methylcoumarin (AzMC) coupled with a CYP2E1 homology model. In the absence of light, AzMC was an effective competitor against substrate p-nitrophenol oxidation by CYP2E1. Photoactivation of AzMC led to a concentration-dependent loss in CYP2E1 activity and structural integrity resulting from the modification of both heme and protein. The photo-labeling reaction degraded heme and produced a possible heme adduct. Probe incorporation into the protein occurred at multiple sites within substrate recognition sequence 5 (SRS-5). Based on a CYP2E1 homology model, we hypothesize AzMC labels SRS-5 residues, Leu363, Val364, and Leu368, in the active site. In addition, we propose a series of phenylalanines, especially Phe106, mediate contacts with the coumarin.

Computational approaches for predicting CYP-related metabolism properties in the screening of new drugs

The site of biotransformation, the extent and rate of metabolism and the number of active metabolic pathways are among the most important characteristics of the pharmacokinetics of a drug. The catalytic activity of drug metabolizing enzymes is likely the most influential determinant of the pharmacokinetic variability. Metabolic stability is the prerequisite for sustaining the therapeutically relevant concentrations. Metabolic inhibition and induction can give rise to clinically important drug-drug interactions. A variety of computational approaches are currently available for predicting different cytochrome P450 (CYP)-related metabolism endpoints. The present review will describe these approaches and their impact on drug development process. Indications on the available software for the implementation will also be given.

Dietary flavonoids: effects on xenobiotic and carcinogen metabolism

Flavonoids are present in fruits, vegetables and beverages derived from plants (tea, red wine), and in many dietary supplements or herbal remedies including Ginkgo Biloba, Soy Isoflavones, and Milk Thistle. Flavonoids have been described as health-promoting, disease-preventing dietary supplements, and have activity as cancer preventive agents. Additionally, they are extremely safe and associated with low toxicity, making them excellent candidates for chemopreventive agents. The cancer protective effects of flavonoids have been attributed to a wide variety of mechanisms, including modulating enzyme activities resulting in the decreased carcinogenicity of xenobiotics. This review focuses on the flavonoid effects on cytochrome P450 (CYP) enzymes involved in the activation of procarcinogens and phase II enzymes, largely responsible for the detoxification of carcinogens. A number of naturally occurring flavonoids have been shown to modulate the CYP450 system, including the induction of specific CYP isozymes, and the activation or inhibition of these enzymes. Some flavonoids alter CYPs through binding to the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, acting as either AhR agonists or antagonists. Inhibition of CYP enzymes, including CYP 1A1, 1A2, 2E1 and 3A4 by competitive or mechanism-based mechanisms also occurs. Flavones (chrysin, baicalein, and galangin), flavanones (naringenin) and isoflavones (genistein, biochanin A) inhibit the activity of aromatase (CYP19), thus decreasing estrogen biosynthesis and producing antiestrogenic effects, important in breast and prostate cancers. Activation of phase II detoxifying enzymes, such as UDP-glucuronyl transferase, glutathione S-transferase, and quinone reductase by flavonoids results in the detoxification of carcinogens and represents one mechanism of their anticarcinogenic effects. A number of flavonoids including fisetin, galangin, quercetin, kaempferol, and genistein represent potent non-competitive inhibitors of sulfotransferase 1A1 (or P-PST); this may represent an important mechanism for the chemoprevention of sulfation-induced carcinogenesis. Importantly, the effects of flavonoids on enzymes are generally dependent on the concentrations of flavonoids present, and the different flavonoids ingested. Due to the low oral bioavailability of many flavonoids, the concentrations achieved in vivo following dietary administration tend to be low, and may not reflect the concentrations tested under in vitro conditions; however, this may not be true following the ingestion of herbal preparations when much higher plasma concentrations may be obtained. Effects will also vary with the tissue distribution of enzymes, and with the species used in testing since differences between species in enzyme activities also can be substantial. Additionally, in humans, marked interindividual variability in drug-metabolizing enzymes occurs as a result of genetic and environmental factors. This variability in xenobiotic metabolizing enzymes and the effect of flavonoid ingestion on enzyme expression and activity can contribute to the varying susceptibility different individuals have to diseases such as cancer. As well, flavonoids may also interact with chemotherapeutic drugs used in cancer treatment through the induction or inhibition of their metabolism.

Simultaneous effects of tocopheryl polyethylene glycol succinate (TPGS) on local hair growth promotion and systemic absorption of topically applied minoxidil in a mouse model

In this study, topical minoxidil solutions supplemented with TPGS in cosolvent systems of various compositions consisting of water, alcohol, and polyethylene glycol 400 were designed to evaluate the efficacy of promoting hair growth after topical application and the safety in terms of the amount of minoxidil absorbed through the skin into the circulation using C57BL/6J mice as a model. The commercial product of 2% Regaine) was used as the positive control. The role, which sulfotransferase activity plays in hair growth with treatment using minoxidil, was determined as well. The results revealed that the addition of 0.5% TPGS was able to enhance the proliferation of hair, but an increase in the amount of TPGS to 2% led to deterioration in the enhancement of hair growth. At the higher added amount (2.0%) of TPGS, the promotion of hair growth was slightly reduced for both cosolvent formulations F1 (100% water) and F3 (100% PEG 400), whereas it was reduced to a greater extent for the cosolvent formulations F8-F10. In comparison, the influences of cosolvent compositions with TPGS amounts of 0.0 and 2.0% on the promotion of hair growth were similar. On the contrary, variability in the promotion of hair growth by different solvent formulations was minimal when the added amount of TPGS was 0.5%. In general, a relationship between hair growth and sulfotransferase activities after topical application of 2% Regaine and minoxidil formulations containing various amounts of TPGS was not demonstrated. Plasma concentrations of minoxidil with 2% Regaine were found to be greater than those of 2% minoxidil in those cosolvent formulations containing various amounts of TPGS, while showing insignificant differences among those 10 cosolvent formulations with a fixed amount of TPGS. A tendency for the plasma concentration of minoxidil to increase after the topical administration of minoxidil formulations containing the higher amount of TPGS (2%) was noted.

Herbal interactions involving cytochrome p450 enzymes: a mini review

The metabolism of a drug can be altered by another drug or foreign chemical, and such interactions can often be clinically significant. Cytochrome P450 (CYP) enzymes, a superfamily of enzymes found mainly in the liver, are involved in the metabolism of a plethora of xenobiotics and have been shown to be involved in numerous interactions between drugs and food, herbs and other drugs. The observed induction and inhibition of CYP enzymes by natural products in the presence of a prescribed drug has (among other reasons) led to the general acceptance that natural therapies can have adverse effects, contrary to the popular beliefs in countries where there is an active practice of ethnomedicine. Herbal medicines such as St. John's wort, garlic, piperine, ginseng, and gingko, which are freely available over the counter, have given rise to serious clinical interactions when co-administered with prescription medicines. Such adversities have spurred various pre-clinical and in vitro investigations on a series of other herbal remedies, with their clinical relevance remaining to be established. Although the presence of numerous active ingredients in herbal medicines, foods and dietary supplements complicate experimentation, the observable interactions with CYP enzymes warrant systematic studies, so that metabolism-based interactions can be predicted and avoided more readily. This article highlights the involvement of CYP enzymes in metabolism-related drug-herb interactions and the importance of gaining a mechanism-based understanding to avoid potential adverse drug reactions, in addition to outlining other contributory factors, such as pharmacogenetics and recreational habits that may compound this important health issue.

A quantitative structure-activity relationship analysis on a series of alkyl benzenes metabolized by human cytochrome p450 2E1

The results of quantitative structure-activity relationships for eight alkyl benzenes undergoing oxidative metabolism via human CYP2E1 are reported. Molecular orbital calculations via the AM1 method were employed for the generation of electronic structural descriptors against experimentally generated kinetic data for CYP2E1-mediated metabolism. The findings point to the importance of electronic structural properties of the molecules themselves, particularly the role of frontier orbitals, in determining rates of metabolism. Other factors appear to be responsible for the affinity of these substrates for the CYP2E1 enzyme however, such as its lipophilic character. The results are consistent with the interactive molecular modeling of these compounds within the putative active site of human CYP2E1 constructed from the CYP2C5 template, where it was found that pi-pi stacking interactions between aromatic rings are important for the binding of substrates to the CYP2E1 active site, together with contributions from desolvation entropy changes accompanying substrate binding.

Enantioselectivity of human hydroxysteroid sulfotransferase ST2A3 with naphthyl-1-ethanols

Hydroxysteroid (alcohol) sulfotransferases catalyze the sulfation of several endogenous steroids and many hydrophobic xenobiotic alcohols. The substrate stereoselectivities of sulfotransferases may be critically important in determining their overall roles in metabolism of drugs, carcinogens, and other xenobiotics. In the present work, stereoselectivity of the human hydroxysteroid sulfotransferase ST2A3 (also variously named as SULT2A1 or human DHEA-ST) was examined through analysis of its catalytic activities with the enantiomers of 1-naphthyl-1-ethanol and 2-naphthyl-1-ethanol. The kcat/Km value for sulfation of the R-(+)-enantiomer of 1-naphthyl-1-ethanol catalyzed by ST2A3 was 3.3 min-1mM-1, whereas the S-(-)-enantiomer was not a substrate for the enzyme. S-(-)-1-naphthyl-1-ethanol did however interact with ST2A3 as an inhibitor of the sulfation of dehydroepiandrosterone. This substrate stereospecificity was not present with the enantiomers of 2-naphthyl-1-ethanol, since both were substrates for the enzyme. Such differences between the sulfation of 1- and 2-naphthyl-1-ethanol are consistent with the importance of steric interactions between the ethanol group and a hydrogen atom at the peri-position (C8) on the naphthyl ring in 1-naphthyl-1-ethanol that combine with the topology of the enzyme's active site to determine stereospecificity.

Differential activation of promutagens by alloenzymes of human sulfotransferase 1A2 expressed in Salmonella typhimurium

Various enzymatically formed sulfuric acid esters are chemically reactive and mutagenic. This metabolic activation pathway is not detected in standard in-vitro mutagenicity test systems. We describe the construction of Salmonella typhimurium TA1538-derived strains expressing alloenzymes *1, *2, *3, *5, *6 of human sulfotransferase 1A2 (SULT1A2). The reference compounds, 1-hydroxymethylpyrene (1-HMP), N-hydroxy-2-acetylaminofluorene (OH-AAF) and 2-hydroxylamino-5-phenylpyridine (OH-APP), were activated to mutagens in these strains. Their activity differed 7- to 16-fold between strains expressing various alloenzymes. It was strongest and weakest in the strains expressing the common alloenzymes, *1 and *2, respectively. The SULT1A2 protein expression levels, and the V(max) and K(m) values with the reference substrate 4-nitrophenol, varied 2.5-, 4-, and 110-fold, respectively, in cytosolic preparations from strains TA1538-SULT1A2*1 and *2. Strains with varying protein levels were constructed via insertion of silent mutations in the 5'-part of the cDNA. TA1538-SULT1A2*1Z and TA1538-SULT1A2*2Y showed equal expression levels of alloenzymes *1 and *2, respectively, which were 3 times above those of TA1538-SULT1A2*1. The mutagenicity of OH-AAF and OH-APP was unchanged in strain TA1538-SULT1A2*1Z versus *1, and moderately increased in TA1538-SULT1A2*2Y versus *2. The influence of the protein level was stronger with 1-HMP. Nevertheless, mutagenic activity of 1-HMP was still 11 times higher in TA1538-SULT1A2*1Z than in TA1538-SULT1A2*2Y. Thus, differences in the properties between alloenzymes can lead to differences in the activation of promutagens. The model compounds were also tested in strains expressing the other ten human SULTs identified. Whereas OH-AAF and OH-APP showed the highest mutagenic activities in strains expressing SULT1A2, 1-HMP was more potent in strains expressing other SULT forms. With the limitation that little is known about the tissue distribution and regulation of SULT1A2, the findings suggest that its polymorphism may affect the individual susceptibility towards procarcinogens, in particular certain aromatic amines and amides.

Modelling human cytochromes P450 involved in drug metabolism from the CYP2C5 crystallographic template

A historical background to homology modelling of human P450s involved in drug metabolism is outlined, showing that the progress in crystallographic studies of bacterial forms of enzyme and, latterly, determination of a mammalian P450 crystal structure, has enabled the production of increasingly satisfactory models of human P450 enzymes. The methodology for the generation of P450 models by homology with crystallographic template structures is summarized, and recent results of CYP2C5-constructed models of P450s are described. These indicate that selective substrates are able to fit within the putative active sites of each enzyme, where key contacts with complementary amino acid residues are largely consistent with the results of site-directed mutagenesis experiments and metabolic studies. Consequently, the CYP2C5 crystal structure can be regarded at the current paradigm for homology modelling of the drug metabolizing P450s, especially those from the CYP2 family.

Molecular modeling of human cytochrome P450-substrate interactions

The results of homology modeling of 10 human cytochrome P450 (CYP) enzymes involved in the Phase 1 metabolism of drugs and other foreign compounds are reported. The models have been constructed from the CYP102 hemoprotein domain template for which the substrate-bound crystallographic coordinates are available. Selective substrates of individual human P450s: CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP4A11 are all shown to fit within the corresponding enzymes' active sites in such a manner that is consistent with reported experimental data for both known pathways of substrate metabolism and from the results of site-directed mutagenesis, either in those particular human P450 enzymes concerned or for ones within the same subfamily. The self-consistency of these homology models indicates that they may have potential utility for the pre-screening of novel drug structures.

Structure-activity relationship for human cytochrome P450 substrates and inhibitors

Criteria governing the avidity of substrate binding to human hepatic cytochromes P450 (CYP) associated with Phase 1 metabolism of drugs are described. The results of extensive quantitative structure-activity relationship (QSAR) analyses are reported for substrates of human P450s: CYPIA2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4, representing the enzymes exhibiting major involvement in the metabolism of drug substrates in Homo sapiens. In particular, it is shown that hydrogen bond properties in each class of enzyme-substrate complex are especially important factors in determining substrate binding affinity towards those human P450s which are involved in drug metabolism.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Human cytosolic sulphotransferases: genetics, characteristics, toxicological aspects

Cytosolic sulphotransferases transfer the sulpho moiety from the cofactor 5'-phosphoadenosine-3'-phosphosulphate (PAPS) to nucleophilic groups of xenobiotics and small endogenous compounds (such as hormones and neurotransmitters). This reaction often leads to products that can be excreted readily. However, other sulpho conjugates are strong electrophiles and may covalently bind with DNA and proteins. All known cytosolic sulphotransferases are members of an enzyme/gene superfamily termed SULT. In humans, 10 SULT genes are known. One of these genes encodes two different enzyme forms due to the use of alternative first exons. Different SULT forms substantially differ in their substrate specificity and tissue distribution. Genetic polymorphisms have been described for three human SULTs. Several allelic variants differ in functional properties, including the activation of promutagens. Only initial results are available from the analysis of SULT allele frequencies in different population groups, e.g. subjects suffering from specific diseases and corresponding controls.

Sulfation of flavonoids and other phenolic dietary compounds by the human cytosolic sulfotransferases

The protective effects of diet, especially soya products, tea, and many fruits, against a variety of human cancers, as suggested by epidemiological studies, has focused attention on flavonoids, isoflavonoids, and other phenolic dietary compounds as chemoprotectants. Among the mechanisms suggested for their chemoprotective action, their ability to inhibit the bioactivation of carcinogens by the human cytosolic sulfotransferases (STs) and the direct effects of their sulfoconjugates are being increasingly studied. We report here a systematic study on the sulfation of representative flavonoids, isoflavonoids, anti-oxidants, and other phenolic dietary compounds by all ten known human cytosolic STs. All ten recombinant human cytosolic STs were prepared in a pure form and tested for their sulfating activities with a variety of these compounds. P-form (SULT1A1) phenol ST (PST) showed high sulfating activity with most of these compounds. M-form (SULT1A3) PST showed high activity with the flavonoids but not with the isoflavonoids. SULT1C ST #2 showed high activity with the isoflavonoids and also sulfated most of the other compounds. Possible relevance of these results to the chemoprotective effects of these dietary compounds is discussed.

Modelling human cytochromes P450 for evaluating drug metabolism: an update

Cytochrome P450 (CYP) enzymes represent the major catalysts for the Phase 1 metabolism of drugs and other xenobiotics in Mammalia, including Homo sapiens. There is considerable current interest in evaluating and, consequently, predicting the metabolic fate of new chemical entities (NCEs) via modelling molecular interactions with P450 constructs, such that sites of metabolism, particular CYP involvement and binding affinities, can be estimated. This paper focuses on the principles for homology modelling of typical enzyme-substrate interactions within the putative active sites of major P450s associated with drug metabolism in man. It also represents an update on previously published work in this journal /1/.