Alphatic 3,4-epoxyalcohols. Metabolism by epoxide hydrase and mutagenic activity.

Rabbit hepatic microsomal epoxide hydrase catalyzes the rapid hydrolysis of 1,2-epoxy-4-heptanol to 1,2,4-heptanetriol. Both diastereomers of the substrate are hydrolyzed, and both product diastereomers are formed. Similarly both cis- and trans-3,4-epoxy-1-hexanol are hydrolyzed, albeit more slowly, to give 1,3,4-hexanetriol. The trans isomer gives exclusively one diastereomer (erythro) of the triol, while the cis isomer gives the other diastereomer (threo). The product expected if a primary cationic intermediate were to be formed and trapped intramolecularly during the hydrolysis of 1,2-epoxy-4-heptanol, 2-propyl-4-tetrahydrofuranol, was not observed. A comparison of the mutagenic activity in the Ames test of 1-heptane, 1-hepten-4-ol, 1,2-epoxyheptane, and 1,2-epoxy-4-heptanol revealed that only the latter is a detectable mutagen. A vicinal hydroxyl therefore does not interfere significantly with enzymatic epoxide hydrolysis, but it does enhance the bioalkylating potential of even an aliphatic epoxide.

Ligation of the iron in the heme-heme oxygenase complex: X-ray absorption, electronic absorption and magnetic circular dichroism studies.

Heme oxygenase (HO) catalyzes the first steps in the breakdown of heme to biliverdin and carbon monoxide. It is a membrane-bound protein that has been shown to exist in two isoforms, HO-1 and HO-2. Recently, a soluble, truncated form of rat HO-1 (rHO) lacking the 23 amino-acid membrane anchor has been expressed in E. coli. Extended X-ray absorption fine structure (EXAFS) data on ferric rHO and its fluoride derivative support assignment of the axial iron ligands as oxygen and/or nitrogen donors having distances similar to ferric myoglobin. The electronic absorption and magnetic circular dichroism (MCD) spectra of the ferric and ferrous protoheme complexes of rHO as well as various ligand adducts are very similar to the corresponding spectra of myoglobin. The present study is the first investigation of the heme-heme oxygenase complex with EXAFS and MCD spectroscopy and establishes that the proximal ligand to the heme in rHO is histidine. Furthermore, the close similarity between the electronic absorption and MCD spectra of ferric rHO and myoglobin over the pH range 6 to 10 is consistent with distal heme ligation of ferric rHO as a water molecule or hydroxide ion, depending on pH. Taken together and in conjunction with the results of earlier studies, EXAFS, electronic absorption, and MCD spectroscopy solidly establish that the ligands to the heme in rHO are identical to those in myoglobin, namely, histidine/H2O at low pH and histidine/OH at high pH.

Cytochrome P-450 catalysis: radical intermediates and dehydrogenation reactions.

Cytochrome P-450 appears to catalyse most monooxygenation reactions by sequential one-electron steps rather than by a single, concerted transfer of the ferryl oxygen to the substrate. The radical intermediates, although very short-lived, can rearrange or undergo alternative reactions characteristic of radical species. As Paul Ortiz de Montellano explains, these alternative reactions yield novel metabolites, sometimes result in inactivation of the enzyme, and can provide information on the reaction mechanism.

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.

Porphyrinogenic activity and ferrochelatase-inhibitory activity of sydnones in chick embryo liver cells.

3-[2-(2,4,6-Trimethylphenyl)thioethyl]-4-methylsydnone was shown to be a potent porphyrinogenic agent in chick embryo liver cells. The accumulation of protoporphyrin IX was consistent with the finding that ferrochelatase activity was inhibited. 3-Benzyl-4-phenylsydnone did not inhibit ferrochelatase activity and protoporphyrin IX was found to constitute only a minor fraction of the prophyrins. These results support the idea that the porphyrinogenicity of 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone is due to its catalytic activation by cytochrome P-450 leading to heme alkylation and formation of N-vinylprotoprophyrin IX which inhibits ferrochelatase.

Identification of novel cellular proteins that bind to the LC8 dynein light chain using a pepscan technique.

Dynein is a minus end-directed microtubule motor that serves multiple cellular functions. We have performed a fine mapping of the 8 kDa dynein light chain (LC8) binding sites throughout the development of a library of consecutive synthetic dodecapeptides covering the amino acid sequences of the various proteins known to interact with this dynein member according to the yeast two hybrid system. Two different consensus sequences were identified: GIQVD present in nNOS, in DNA cytosine methyl transferase and also in GKAP, where it is present twice in the protein sequence. The other LC8 binding motif is KSTQT, present in Bim, dynein heavy chain, Kid-1, protein 4 and also in swallow. Interestingly, this KSTQT motif is also present in several viruses known to associate with microtubules during retrograde transport from the plasma membrane to the nucleus during viral infection.

AMP-activated protein kinase phosphorylation of endothelial NO synthase.

The AMP-activated protein kinase (AMPK) in rat skeletal and cardiac muscle is activated by vigorous exercise and ischaemic stress. Under these conditions AMPK phosphorylates and inhibits acetyl-coenzyme A carboxylase causing increased oxidation of fatty acids. Here we show that AMPK co-immunoprecipitates with cardiac endothelial NO synthase (eNOS) and phosphorylates Ser-1177 in the presence of Ca2+-calmodulin (CaM) to activate eNOS both in vitro and during ischaemia in rat hearts. In the absence of Ca2+-calmodulin, AMPK also phosphorylates eNOS at Thr-495 in the CaM-binding sequence, resulting in inhibition of eNOS activity but Thr-495 phosphorylation is unchanged during ischaemia. Phosphorylation of eNOS by the AMPK in endothelial cells and myocytes provides a further regulatory link between metabolic stress and cardiovascular function.

Arylhydrazines as probes of hemoprotein structure and function.

The reactions of arylhydrazines (ArNHNH2) or aryldiazenes (ArN = NH) with simple iron porphyrins or with hemoproteins that have relatively open active sites, including hemoglobin, myoglobin, cytochrome P450, chloroperoxidase, catalase, prostaglandin synthase, and indoleamine-2,3-dioxygenase yield sigma-bonded aryl-iron complexes. Denaturation of the protein complexes under aerobic, acidic conditions shifts the aryl group to the porphyrin nitrogens and produces mixtures of the four possible N-arylprotoporphyrin IX regioisomers. The regioisomers are obtained in approximately equal amounts if the iron-to-nitrogen shift occurs outside of the protein but the ratio of isomers differs if the rearrangement is controlled by the protein. Only in the case of cytochrome P450 enzymes can the shift be induced to occur without denaturation of the protein. The isomer ratios obtained when the shift occurs in the intact active site provide direct experimental information on the active site topology and dynamics. Topological information has thus been obtained for cytochromes P450 1A1, 1A2, 2B1, 2B2, 2B4, 2B10, 2B11, 2E1, 11A1, 51, 101, 102, and 108. In contrast to hemoproteins with open active sites, conventional peroxidases react with arylhydrazines to give delta-meso-aryl adducts and covalent protein adducts. Reaction with the delta-meso edge but not the heme iron provides key evidence that restricting access of substrates to the ferryl oxygen helps direct the reaction towards peroxidase rather than peroxygenase catalysis.

Alkylation of the prosthetic heme in cytochrome P-450 during oxidative metabolism of the sedative-hypnotic ethchlorvynol.

The clinically used sedative-hypnotic ethchlorvynol destroys hepatic microsomal cytochrome P-450 enzymes in a process catalyzed by the same hemoproteins. Abnormal porphyrins accumulate in the livers of phenobarbital-pretreated rats after administration of ethchlorvynol. The abnormal porphyrin fraction has been isolated and shown to consist of the four possible regioisomers of N-(5-chloro-3-ethyl-3-hydroxy-2-oxo-4-pentenyl)protoporphyrin IX. Cytochrome P-450 inactivation thus appears to result from alkylation of the prosthetic heme by the oxidatively activated acetylenic function in ethchlorvynol. The autocatalytic destruction of the hemoprotein is likely to alter the metabolism and elimination of ethchlorvynol and coadministered drugs and may be the cause of the porphyrinogenic properties of ethchlorvynol.

Mechanism-based inhibitors of prostaglandin omega-hydroxylase: (R)- and (S)-12-hydroxy-16-heptadecynoic acid and 2,2-dimethyl-12-hydroxy-16-heptadecynoic acid.

12-Hydroxy-16-heptadecynoic acid has been shown to selectively inactivate cytochrome P450 4A4, a pulmonary cytochrome P450 enzyme that catalyzes the omega-hydroxylation of prostaglandins [Muerhoff, A. S.; Williams, D. E.; Reich, N. O.; CaJacob, C. A.; Ortiz de Montellano, P. R.; Masters, B. S. S. J. Biol. Chem. 1989, 264, 749-756]. Potent, specific inhibitors of this enzyme are required to explore its physiological role. In a continuing effort to develop such agents, the two enantiomers of 12-hydroxy-16-heptadecynoic acid have been stereospecifically synthesized, their absolute stereochemistry confirmed, and the dependence of enzyme inactivation on absolute stereochemistry determined using cytochrome P450 4A4 purified from the lungs of pregnant rabbits. The 12S enantiomer is roughly twice as active (KI = 1.8 microM, t1/2 = 0.7 min) as the 12R enantiomer (KI = 3.6 microM, t1/2 = 0.8 min), but the chirality of the hydroxyl group is not a major determinant of the specificity for the prostaglandin omega-hydroxylase. The flexibility of the acyclic skeleton of the inhibitor may account for the relatively low enantiomeric discrimination. 2,2-Dimethyl-12-hydroxy-16-heptadecynoic acid, an analogue that cannot undergo beta-oxidation, has also been synthesized as a potential in vivo inhibitor of the enzyme and has been shown to inactivate the purified enzyme with KI = 4.9 microM and t1/2 = 1.0 min. These acetylenic agents, particularly the dimethyl analog, are promising in vivo inhibitors of cytochrome P450 4A4.

Haloperidol-based irreversible inhibitors of the HIV-1 and HIV-2 proteases.

The proteases expressed by the HIV-1 and HIV-2 viruses process the polyproteins encoded by the viral genomes into the mature proteins required for virion replication and assembly. Eight analogs of haloperidol have been synthesized that cause time-dependent inactivation of the HIV-1 protease and, in six cases, HIV-2 protease. The IC50 values for the analogues are comparable to that of haloperidol itself. Enzyme inactivation is due to the presence of an epoxide in two of the analogues and carbonyl-conjugated double or triple bonds in the others. Irreversible inactivation is confirmed by the failure to recover activity when one of the inhibitors is removed from the medium. At pH 8.0, the agents inactivate the HIV-1 protease 4-80 times more rapidly than the HIV-2 protease. Faster inactivation of the HIV-1 protease is consistent with alkylation of cysteine residues because the HIV-1 protease has four such residues whereas the HIV-2 protease has none. Inactivation of the HIV-2 protease requires modification of non-cysteine residues. The similarities in the rates of inactivation of the HIV-2 protease by six agents that have intrinsically different reactivities toward nucleophiles suggest that the rate-limiting step in the inactivation process is not the alkylation reaction itself. At least five of the agents inhibit polyprotein processing in an ex vivo cell assay system, but they are also toxic to the cells.

Dissociation of increased lauric acid omega-hydroxylase activity from the antilipidemic action of clofibrate.

Clofibrate, an antilipidemic drug that acts by a still obscure mechanism, is known to specifically increase up to 30-fold the activity of the hepatic cytochrome P-450 isozyme that omega-hydroxlates lauric acid. The thesis that accelerated catabolism of medium-length fatty acids initiated by omega-hydroxylation contributes significantly to the antilipidemic action of clofibrate has been examined by measuring the impact on serum triglyceride levels of coadministering clofibrate and a suicide substrate that inactivates the hydroxylase. The results suggest that the antilipidemic action of clofibrate does not depend critically on the enhanced omega-hydroxylation of fatty acids.

Effects of 3-(2-phenylethyl)-4-methylsydnone and related sydnones on heme biosynthesis.

3-[2-(2,4,6-Trimethylphenyl)thioethyl]-4-methylsydnone (TTMS) and 3-(2-phenylethyl)-4-methylsydnone (PEMS) cause mechanism-based inactivation of rat hepatic microsomal cytochrome P-450 and the formation of N-alkylprotoporphyrins in rat liver. In the present study, we have shown that both TTMS and PEMS cause mechanism-based inactivation of chick embryo hepatic microsomal cytochrome P-450. TTMS also caused the inhibition of ferrochelatase activity, the accumulation of protoporphyrin IX, and an increase in the activity of delta-aminolevulinic acid synthase in chick embryo liver cell culture. PEMS was devoid of effect on ferrochelatase activity, porphyrin accumulation, and delta-aminolevulinic acid synthase activity. There are two possible explanations for the lack of effect of PEMS on heme biosynthesis: (1) the ring-A- and/or ring-B-substituted regiosomers of the N-phenylethyl- and N-phenylethenylprotoporphyrins which are produced during the mechanism-based inactivation of cytochrome P-450 by PEMS are too bulky to fit into the active site of ferrochelatase to inhibit its activity, in contrast to the N-vinylprotoporphyrin formed from TTMS; and (2) the N-alkylprotoporphyrins produced consist of the ring-C- and/or ring-D-substituted regioisomers, which are not inhibitors of ferrochelatase activity.

Characterization of rat neuronal nitric oxide synthase expressed in Saccharomyces cerevisiae.

A cDNA encoding rat neuronal nitric oxide synthase (nNOS) was cloned into the yeast expression vector pMA56 to generate pA379. Transformation of Saccharomyces cerevisiae strain BJ2168 with this plasmid resulted in the synthesis of nNOS at levels of 0.5-1.0 mg/liter. The protein is localized in the cytosol and is catalytically active as determined by the conversion of [3H]-L-arginine to [3H]-L-citrulline and NO. The enzyme was purified by calmodulin-Sepharose affinity chromatography and its catalytic activity was found to be both calcium and calmodulin dependent. Overexpression of nNOS in S. cerevisiae and purification of the recombinant protein will facilitate detailed characterization of this important enzyme.

Drug metabolism biosensors: electrochemical reactivities of cytochrome P450cam immobilised in synthetic vesicular systems.

Biosensors containing cytochrome P450cam in a didodecyldimethylammonium bromide vesicular system were prepared by cross-linking onto a glassy carbon electrode (GCE) with glutaraldehyde in the presence of bovine serum albumin. Cyclic voltammetric responses of the sensor in air-free buffer solution showed that the sensor exhibited reversible electrochemistry due to direct electron exchange between the haem Fe(3+/2+) redox system and the GCE surface. In air-saturated solution containing camphor, the biosensor gave an irreversible electrocatalytic current which is compatible with the monooxygenation of the substrate. Steady state amperometric experiments with camphor, adamantanone and fenchone were performed with a biosensor prepared by cross-linking P450cam with glutaraldehyde onto a Pt disc electrode. The sensor was characterised by fast amperometric responses, attaining steady-state in about 20 s in a cobalt sepulchrate mediated electrochemical system. The kinetic parameters of the biosensor were analysed using the electrochemical Michaelis Menten equation. The estimated apparent Michaelis-Menten constant, Km, values for the biosensors were in the range of 1.41-3.9 mM.

Protein dynamics and imidazole binding in cytochrome P450 enzymes.

P450 (cytochrome P450) enzymes have major roles in the biosynthesis of endogenous factors such as steroids and eicosanoids, in the termination of the action of endogenous factors such as retinoic acid, in the metabolism of most drugs and xenobiotics and in the generation of toxic and carcinogenic products. Understanding the determinants of the substrate and inhibitor specificities of these enzymes is important for drug design. The crystallographic analysis of the deformability of two bacterial P450 active sites associated with the binding of azole (a class of inhibitors with an imidazole or triazole ring that co-ordinates to the haem iron) inhibitors described in the present study illustrates the importance of protein conformational malleability in the binding of imidazole derivatives.

Catalytic sites of hemoprotein peroxidases.

The structures of the active sites of horseradish and cytochrome c peroxidase, prototypical peroxidases with an imidazole heme ligand, suggest that small substrates are generally oxidized by peroxidases at the delta-meso edge of the heme group. This inference is supported by experimental results on the Coprinus macrorhizus peroxidase (52), manganese peroxidase (51), lignin peroxidase (50) and, less definitively, lactoperoxidase (90). Macromolecular substrates, exemplified by the cytochrome c peroxidase-cytochrome c interaction, are likely to be oxidized at peroxidase surface sites bearing no specific relationship to the delta-meso heme edge. The second oxidation equivalent in the two-electron Compound I states of the peroxidases is stored either as a porphyrin radical or as a protein radical, although some peroxidases have both types of compound I. The factors that control the location of the second oxidation equivalent remain unclear. Classical peroxidases do not generally catalyze olefin epoxidation and other monooxygenations but do catalyze sulfoxidation reactions. This is best rationalized by physical separation of the substrate from the ferryl oxygen, possibly by a protein barrier, because results with cytochrome c peroxidase show that there is no inherent mechanistic reason for the inability of peroxidases to epoxidize olefins. It is not yet clear why the barrier to oxygen transfer reactions is circumvented during sulfur oxidation reactions, although one possibility is that the relatively stable sulfur cation radical that is initially formed disrupts the barrier. Chloroperoxidase, the principal nonclassical hemoprotein peroxidase so far examined, has an open active site that readily catalyzes P450-like monooxygenation reactions. The active site of chloroperoxidase is a potentially useful model for that of myeloperoxidase, but caution must be used in extrapolating from one to the other because myeloperoxidase has a histidine rather than thiolate fifth heme ligand and therefore is a classical rather than nonclassical peroxidase.