Methyl P-tolyl Sulfide Research Articles

Catalysis of oxo transfer to prochiral sulfides by oxovanadium(v) compounds that model the active center of haloperoxidases.

The catalytic properties of a new class of chiral vanadium compounds--[(S,S,S)-VO(OMe)L1] (5), [(S,S)-VO(OMe)L2] (6), [(S,S)-VO(OMe)L3] (7), and [(R,R,R)-VO(OMe)L4] (8), as well as the system VO(OiPr)(3)/(R,R,R)-H(2)L4 [H(2)L1=(S,S)-bis(2-hydroxypropyl)-(S)-1-phenylethylamine, 1; H(2)L2=(S,S)-bis(2-hydroxypropyl)benzylamine, 2; H(2)L3=(S,S)-bis(2-hydroxypropyl)isopropylamine), 3; (H(2)L4)=(R,R)-bis(2-phenylethanol)-(R)-1-phenylethylamine, 4]--in the asymmetric oxidation of prochiral sulfides by organic hydroperoxides have been investigated. Particular attention has been paid to the factors that guide the discrimination between the two prochiral faces of the sulfides (methyl p-tolyl sulfide and benzyl phenyl sulfide), to steric implications stemming from the oxidant (cumyl hydroperoxide and tert-butyl hydroperoxide), and to the specific complex used. As an example, (S)-methyl p-tolyl sulfoxide was obtained in a 31 % enantiomeric excess by use of cumyl hydroperoxide as oxidant and complex 5 as the catalyst, after 150 min at 0 degrees C and with 100 % conversion of the sulfide. The crystal and molecular structures of 5 and 6 reveal the close relationship between these complexes and the active center of vanadate-dependent haloperoxidases: the vanadium is in a slightly distorted trigonal-bipyramidal environment with the nitrogen and the methoxy group in the axial positions, and the oxo and alkoxide functions of L2 and L3 are the plane. The presence and equilibrium situation of isomers of the catalysts in solution has been investigated by (51)V EXSY and variable-temperature multinuclear NMR spectroscopy. An intermediately formed peroxo (ROO(-)) vanadium complex was detected by (51)V NMR spectroscopy.

Identification of metabolic pathways involved in the biotransformation of tolperisone by human microsomal enzymes.

The in vitro metabolism of tolperisone, 1-(4-methyl-phenyl)-2-methyl-3-(1-piperidino)-1-propanone-hydrochloride, a centrally acting muscle relaxant, was examined in human liver microsomes (HLM) and recombinant enzymes. Liquid chromatography-mass spectrometry measurements revealed methyl-hydroxylation (metabolite at m/z 261; M1) as the main metabolic route in HLM, however, metabolites of two mass units greater than the parent compound and the hydroxy-metabolite were also detected (m/z 247 and m/z 263, respectively). The latter was identified as carbonyl-reduced M1, the former was assumed to be the carbonyl-reduced parent compound. Isoform-specific cytochrome P450 (P450) inhibitors, inhibitory antibodies, and experiments with recombinant P450s pointed to CYP2D6 as the prominent enzyme in tolperisone metabolism. CYP2C19, CYP2B6, and CYP1A2 are also involved to a smaller extent. Hydroxymethyl-tolperisone formation was mediated by CYP2D6, CYP2C19, CYP1A2, but not by CYP2B6. Tolperisone competitively inhibited dextromethorphan O-demethylation and bufuralol hydroxylation (K(i) = 17 and 30 microM, respectively). Tolperisone inhibited methyl p-tolyl sulfide oxidation (K(i) = 1200 microM) in recombinant flavin-containing monooxygenase 3 (FMO3) and resulted in a 3-fold (p < 0.01) higher turnover number using rFMO3 than that of control microsomes. Experiments using nonspecific P450 inhibitors-SKF-525A, 1-aminobenzotriazole, 1-benzylimidazole, and anti-NADPH-P450-reductase antibodies-resulted in 61, 47, 49, and 43% inhibition of intrinsic clearance in HLM, respectively, whereas hydroxymethyl-metabolite formation was inhibited completely by nonspecific chemical inhibitors and by 80% with antibodies. Therefore, it was concluded that tolperisone undergoes P450-dependent and P450-independent microsomal biotransformations to the same extent. On the basis of metabolites formed and indirect evidences of inhibition studies, a considerable involvement of a microsomal reductase is assumed.

Expression and characterization of functional dog flavin-containing monooxygenase 1.

A full-length dog (beagle) flavin-containing monooxygenase 1 (FMO1) cDNA (dFMO1) was obtained from liver by reverse transcription-polymerase chain reaction. The amino acid sequence of dFMO1 was 89% homologous to human FMO1. Using a baculovirus expression system in Sf-9 insect cells, dFMO1 was expressed to protein levels of 0.4 nmol/mg, as determined by immunoquantitation. The flavin content of the expressed enzyme was consistent with immunodetectable dFMO1 protein levels. Expressed dFMO1 catalyzed NADPH-dependent methyl p-tolyl sulfide oxidation, with K(m) and V(max) values of 98.6 microM and 63.8 nmol of S-oxide formed/min/mg of protein, respectively. By comparison, human FMO1 showed similar values of 87.1 microM (K(m)) and 51.0 nmol/min/mg (V(max)). Activity for dFMO1 showed characteristic pH dependence, with a 4.5-fold increase in S-oxidase activity as the incubation pH increased from 7.6 to 9.0. Human FMO1 also showed an increase in reaction rate with pH but a somewhat lower optimum of 8.0 to 8.4. dFMO1 also catalyzed imipramine N-oxidation, with a K(m) of 4.7 microM and a V(max) of 82.1 nmol/min/mg of protein. This enzyme displayed other characteristics of FMO enzymes, with rapid depletion of enzyme activity upon heating in the absence of NADPH. Protein levels of 74 pmol of dFMO1/mg of microsomal protein were determined for a pooled liver microsome sample, suggesting that this enzyme is a major canine hepatic monooxygenase. In conclusion, the expression and characterization of catalytically active dFMO1 will allow the role of this enzyme in the metabolism of xenobiotics to be determined.

Smart Aqueous Reaction Medium

Smart materials show a nonlinear response to a stimulus. In aqueous solution, the reactions between methyl p-tolyl sulfide and peroxomonosulfate or m-chloroperbenzoic acid show the expected linear dependence of the logarithm of the measured rate constant on the reciprocal temperature. This constitutes Arrhenius behavior. In the presence of 5 or 15 g L-1 of the thermoresponsive poloxamer, P104, H(OCH2CH2)27(OCH(CH3)CH2)61(OCH2CH2)27OH, which forms micelles as the temperature is increased, anti-Arrhenius behavior or hyper-Arrhenius behavior is observed. Anti-Arrhenius behavior occurs when the organic sulfide partitions into the thermally induced poloxamer micelles while the peroxomonosulfate anion remains in the bulk aqueous phase, causing a decrease in rate. Hyper-Arrhenius behavior occurs when both the organic sulfide and the m-chloroperbenzoic acid partition into the thermally induced micelles, causing a much greater increase in rate with temperature than in the absence of poloxomer. These two different types of smart behavior of aqueous P104 are discussed.

Aerobic oxidation of methyl p-tolyl sulfide catalyzed by a remarkably labile heteroscorpionate RuII-aqua complex, fac-[RuII(H2O)(dpp)(tppm)]2+.

fac-[RuII(Cl)(dpp)(L3)]+ (L3 = tris(pyrid-2-yl)methoxymethane (tpmm) = [1A]+ and tris(pyrid-2-yl)pentoxymethane (tppm) = [1B]+ and dpp = di(pyrazol-1-yl)propane) rapidly undergo ligand substitution with water to form fac-[RuII(H2O)(dpp)(L3)]2+ (L3 = tpmm = [2A]2+ and tppm = [2B]2+). In the structure of [2A]2+, the distorted octahedral arrangement of ligands around Ru is evident by a long Ru(1)-O(40) of 2.172(3) A and a large angle O(40)-Ru(1)-N(51) of 96.95(14) degrees . The remarkably short distance between O(40) of H2O and H(45a) of dpp confirms the heteroscorpionate ligand effect of dpp on H2O. [2B]2+ aerobically catalyzes methyl p-tolyl sulfide to methyl p-tolyl sulfoxide in 1,2-dichlorobenzene at 25.0 +/- 0.1 degrees C under 11.4 psi of O2. Experimental facts in support of this aerobic sulfide oxidation are the absence of H2O2 and the oxidative reactivity of the putative Ru(IV)-oxo intermediate toward methyl p-tolyl sulfide, 2-propanol, and allyl alcohol. This study provides the first documented example of aerobic-sulfide oxidation catalyzed by the remarkably labile heteroscorpionate Ru(II)-aqua complex without the formation of a highly reactive peroxide as an intermediate.

New procedures for the enantioselective oxidation of sulfides under stoichiometric and catalytic conditions

Acid-catalyzed oxidation of 2-furylcarbinols with hydrogen peroxide affords alternatively 2-(1-hydroperoxyalkyl)-furans 2 or 6-hydroperoxy-2 H-pyran-3(6 H)-ones 3. Compounds of the type 2 and 3 have been used as oxygen donors in efficient stoichiometric or catalytic procedures for the asymmetric sulfoxidation of prochiral sulfides in the presence of Ti(O- i-Pr) 4/ L-DET or Ti(O- i-Pr) 4/( R)-BINOL/H 2O systems. Positive non linear effects, (+)-NLE, were observed in the enantioselective oxidation of methyl p-tolyl sulfide, promoted by enantiomerically enriched Ti(IV)/BINOL/H 2O complexes.

Homopiperazine platinum(II) complexes containing substituted disulfide groups: crystal structure of [Pt II(homopiperazine)(diphenylsulfide)Cl]NO 3

A series of new cationic platinum(II) complexes of the type [Pt(L)(R′R″S)Cl]NO 3 (where L=homopiperazine or 1-methylhomopiperazine and R′R″S=dimethylsulfide, diethylsulfide, dipropylsulfide, diisopropylsulfide, dibutylsulfide, diphenylsulfide, dibenzylsulfide, methylphenylsulfide, or methyl p-tolylsulfide) were synthesized and characterized by elemental analysis and infrared, 1H and 195Pt nuclear magnetic resonance spectroscopy. Among the complexes synthesized, [Pt II(homopiperazine)(diphenylsulfide)Cl]NO 3 was examined by single-crystal X-ray diffraction. The slightly distorted square plane of the platinum complex included the amino groups of the homopiperazine molecule in a cis orientation, the sulfur atom of diphenyl sulfide, and a chloride ion. The homopiperazine molecule adopts a boat conformation and forms five- and six-membered chelating rings with platinum. Hydrogen bonding plays an important part in holding the crystal together.

Alkyne Dicobalt Carbonyl Complexes with Sulfide Ligands. Synthesis, Crystal Structure, and Dynamic Behavior

Hexacarbonyl dicobalt complex of bis(tert-butylsulfonylethyne) [Co2(μ-ButSO2C)2(CO)6, 3] experiences a thermally induced ligand exchange process with methyl p-tolyl sulfide, dibenzyl sulfide, and diethyl sulfide to give the corresponding stable sulfide complexes [Co2(μ-ButSO2C)2(CO)5SR2] 4, 5, and 6, respectively, in good yield (59−65%). The reaction with tetrahydrothiophene gives a disubstituted complex 7 in 74% yield. Oxathiane 9, derived from (+)-(2R)-10-mercaptoisoborneol, also reacts with 3 to generate a chiral sulfide complex 8 (58%). The solid-state structures of 5 and 8 have been established by X-ray crystallography and reveal the preference of the incoming sulfur ligand to occupy an equatorial coordination site. Further structural studies on 5 have been performed by low-temperature 1H NMR analysis and by theoretical procedures at the PM3(tm) level of theory. Analysis of the low-temperature 1H NMR spectrum of 5 shows a signal splitting consistent with the freezing of an equilibrium between two equ...

Determination of gas phase peroxyacetic acid using pre-column derivatization with organic sulfide reagents and liquid chromatography

The first selective HPLC methods for the determination of peroxyacetic acid (PAA) in gas phase samples have been developed. PAA reacts with 2-([3-{2-[4-amino-2-(methylsulfanyl)phenyl]-1-diazenyl}phenyl]sulfonyl)-1-ethanol (ADS) to form the corresponding sulfoxide. Sampling may be performed in impingers using aqueous solutions of the reagent or by test tubes with the reagent coated on a solid sorbent. Sulfide and sulfoxide are separated by means of HPLC and detected at a wavelength of 410 nm. The method is highly selective for PAA in the presence of hydrogen peroxide when sampling in impingers. A 10 000-fold excess of hydrogen peroxide leads to the same peak area compared to PAA. Limit of detection is 10 −8 mol PAA, thus corresponding to PAA concentration of 46 ppb when using a sampling time of 10 min with a flow-rate at 500 ml/min. Another sulfide reagent, methyl- p-tolyl sulfide (MTS) has been used in a similar way with impinger sampling. Major advantages of ADS towards MTS are improved UV–Vis spectroscopic properties and reduced volatility.

Roles of FMO and CYP450 in the Metabolism in Human Liver Microsomes of S‐Methyl‐N, N‐Diethyldithiocarbamate, a Disulfiram Metabolite

The conversion of S-methyl-N,N-diethyldithiocarbamate (MeDDC) to MeDDC sulfine is the first step after methylation in the metabolic pathway of disulfiram, an alcohol deterrent, to its ultimate active metabolite. Various isoforms of CYP450 have recently been shown to catalyze this reaction, but the involvement of flavin monooxygenase (FMO) in this metabolism in humans has not been evaluated. In this study we examined the ability of recombinant human FMO3 in insect microsomes to metabolize MeDDC, and investigated the relative roles of FMO and CYP450 in the metabolism of MeDDC in human liver microsomes. HPLC-mass spectrometry was used to identify the products of MeDDC formed by human liver microsomes and by recombinant human FMO3. MeDDC metabolism in human liver microsomes was studied by using either heat inactivation to inhibit FMO, or N-benzylimidazole (NBI) or antibodies to the CYP450 NADPH reductase to inhibit CYP450. We confirmed by HPLC-mass spectrometry that MeDDC sulfine was the major product of MeDDC formed by human liver microsomes and by FMO3. Recombinant FMO3 was an efficient catalyst for the formation of MeDDC sulfine (5.3+/-0.2 nmol/min/mg, mean+/-SEM, n = 6). Inhibition studies showed MeDDC was metabolized primarily by CYP450 in human liver microsomes at pH 7.4, with a 10% contribution from FMO (total microsomal activity 3.1+/-0.2, n = 17). In the course of this work, methyl p-tolyl sulfide (MTS), sulfoxidation of which is used by some investigators as a specific probe for FMO activity, was found to be a substrate for both FMO and CYP450 in human liver microsomes. Our results prove that MeDDC sulfine is the major product of MeDDC oxidation in human liver microsomes, MeDDC is a good substrate for human FMO3, and MeDDC is metabolized in human liver microsomes primarily by CYP450. We also showed that use of MTS sulfoxidation as an indicator of FMO activity in microsomes is valid only in the presence of a CYP450 inhibitor, such as NBI.

Enantioselective Sulfoxidation Catalyzed by Vanadium Haloperoxidases.

Vanadium haloperoxidases catalyze the oxidation of halides by hydrogen peroxide to produce hypohalous acid. We demonstrate that these enzymes also slowly mediate the enantioselective oxidation of organic sulfides (methyl phenyl sulfide, methyl p-tolyl sulfide, and 1-methoxy-4 (methylthio)benzene) to the corresponding sulfoxides (turnover frequency 1 min(-)(1)). The vanadium bromoperoxidase from the brown seaweed Ascophyllum nodosum converts methyl phenyl sulfide to the (R)-enantiomer of the sulfoxide (55% yield and 85% enantiomeric excess (ee)). At low peroxide concentrations a selectivity of 91% can be attained. The enzyme catalyzes the selective sulfoxidation reaction over a broad pH range with an optimum around pH 5-6 and remains completely functional during the reaction. When the vanadium bromoperoxidase from the red seaweed Corallina pilulifera is used the (S)-enantiomer (18% yield and 55% ee) is formed. In contrast, the vanadium chloroperoxidase from the fungus Curvularia inaequalis catalyzes the production of a racemic mixture (54% yield), which seems to be an intrinsic characteristic of this enzyme.

Enantiopure p,p′-disubstituted 1,2-diphenylethane-1,2-diols as chiral inducers in the Ti-mediated oxidation of sulfides: a case of reversal of asymmetric induction by fluorine substitution

In the asymmetric oxidation of methyl p-tolyl sulfide, ( 2a), and benzyl phenyl sulfide ( 2b) by TBHP, mediated by a titanium complex with enantiopure (R,R)-p,p′-disubstituted-1,2-diphenylethane-1,2-diols, both the unsubstituted diol (R,R)- 1a and the p-OMe substituted diol (R,R)- 1b lead to sulfoxides of S configuration, with ee up to 99%. On the contrary the p-CF 3 substituted ligand (R,R)- 1c leads to significantly lower ee and in the case of 2a a reversal of asymmetric induction is observed.

Peroxide analysis in laundry detergents using liquid chromatography

Liquid chromatographic (LC) studies have been conducted to investigate the formation and decomposition of peroxyacetic acid (PAA) and hydrogen peroxide in laundry detergents: Both, methyl- p-tolylsulfide (MTS) and methyl-phenylsulfoxide (MPSO) are suitable reagents for the selective LC determination of peroxycarboxylic acids in the presence of a large excess of hydrogen peroxide. MTS is oxidized in acidic media to the corresponding sulfoxide (MTSO) and MPSO is oxidized in alkaline media to methyl-phenylsulfone (MPSO 2). In both cases, triphenyl phosphine (TPP) can be used for the subsequent determination of hydrogen peroxide using LC. TPP is oxidized to the corresponding phosphine oxide (TPPO). The formation and the decomposition of PAA and hydrogen peroxide in laundry detergents with time have been observed using both methods. Acidification prior to analysis and subsequent application of MTS and TPP for the simultaneous determination of PAA and hydrogen peroxide in alkaline samples has been identified to be best suited as means for preservation of the original sample composition.

Can chloramine-T be a nitrene transfer agent?

Abstract Reaction of chloramine-T and methyl p-tolyl sulfide to give the corresponding sulfimide appears to proceed via a nitrene transfer mechanism in the presence of a copper(I) catalyst and a second nitrogen ligand .

Simultaneous HPLC Determination of Peroxyacetic Acid and Hydrogen Peroxide

The first instrumental method for simultaneous determination of peroxyacetic acid (PAA) and hydrogen peroxide has been developed. The successive quantitative reaction of PAA with methyl p-tolyl sulfide (MTS) and hydrogen peroxide with triphenylphosphine (TPP) yields the corresponding sulfoxide MTSO and phosphine oxide TPPO. The reagents and their oxides are separated by HPLC on reversed-phase columns with acetonitrile/water gradient elution within 5 min. External calibration with the solid standards of MTSO and TPPO leads to a very accurate and reliable method. Samples are stable and can be stored after derivatization for several days prior to analysis. Real samples from brewery disinfection were analyzed in comparison to titration with excellent correlation.

Convenient Synthesis and Characterization of the Chiral Complexes cis- and trans-[Ru((SS)-Pr(i)pybox)(py)(2)Cl]PF(6) and [Ru((SS)-Pr(i)pybox)(bpy)Cl]PF(6).

The chiral oxazoline complexes cis- and trans-[Ru((SS)-Pr(i)pybox)(py)(2)Cl]PF(6) were prepared from the common precursor trans-[Ru((SS)-Pr(i)pybox)(py)Cl(2)], and all three complexes were spectroscopically and structurally characterized. The complex cis-[Ru((SS)-Pr(i)pybox)(py)(2)Cl]PF(6) crystallized in P2(1)2(1)2(1) with Z = 4, a = 8.396(2) Å, b = 18.396(3) Å, c = 20.954(3) Å, and R = 0.039 for 4131 reflections. The complex trans-[Ru((SS)-Pr(i)pybox)(py)(2)Cl]PF(6) crystallized in P2(1) with Z = 2, a = 10.110(2) Å, b = 13.566(2) Å, c = 12.043(2) Å, beta = 106.10(2) degrees, and R = 0.039 for 3869 reflections. The precursor trans-[Ru((SS)-Pr(i)pybox)(py)Cl(2)] crystallized in P2(1)2(1)2(1) with Z = 8, a = 15.856(2) Å, b = 17.216(1) Å, c = 17.768(1) Å, and R = 0.030 for 5826 reflections. A related reagent, [Ru((SS)-Pr(i)pybox)(bpy)Cl]PF(6), was prepared by a more direct route and was also characterized. Aqua complexes were formed by hydrolysis of the chloride ligand in the bis(pyridine) complexes with retention of the stereochemistry. Electrochemical oxidation of the aqua complexes yielded oxo species which are formally ruthenium(IV). Reactions of the oxo complexes with the prochiral reductant, methyl p-tolyl sulfide led to stereoselective formation of the sulfoxide with enantiomeric excess of the R isomer ranging from 7% to 13%.

Rapid high-performance liquid chromatographic method for the determination of peroxyacetic acid

The selective oxidation of methyl p-tolyl sulfide (MTS) to the corresponding sulfoxide (MTSO) by peroxyacetic acid and the subsequent rapid separation of the sulfide and sulfoxide are the basis for a fast and reliable HPLC method for the determination of this oxidizing agent in the presence of hydrogen peroxide. The time required for chromatographic separation was reduced to less than 1 min. To improve the long-term stability of the sulfoxide solution, hydrogen peroxide was decomposed catalytically by manganese dioxide. Even in the presence of a tenfold molar excess of hydrogen peroxide, a storability of at least 20 h without a significant increase in MTSO concentration was achieved. External calibration can be performed using the stable and commercially available MTSO. Real samples from a brewery cleaning-in-place disinfection process were analysed and the results were compared with those of the classical two-step titration.

Synthesis and reaction of cyano-substitued 1,2,4-trioxolanes

Carbonyl oxides, derived from the ozonolyses of vinyl ethers, readily undergo [3 + 2] cycloadditions with acyl cyanides affording the corresponding cyano-substituted 1,2,4-trioxolanes in isolated yields of 34–88%. In competition experiments, a relative order of reactivity of the carbonyl oxide trapping agents was tentatively deduced; trifluoroacetophenone > α,α-diphenyl-N-methylnitrone (N-methyldiphenyl-methylideneamine N-oxide) > benzoyl cyanide > methyl benzoylformate > α,α,N-triphenylimine [N-(diphenylmethylidene)aniline] benzaldehyde. As expected from the electron-withdrawing ability of the cyano group, 3-cyano-3-phenyl-1,2,4-trioxolane oxidized not only triphenylphosphine but also methyl p-tolyl sulfide and 2,3-dimethylbut-2-ene very easily.

Preparation and reactions of myoglobin mutants bearing both proximal cysteine ligand and hydrophobic distal cavity: protein models for the active site of P-450.

We have reported that H93C human myoglobin (Mb), in which proximal histidine (His93, F8) was replaced by cysteine, gave nearly identical spectroscopic features of P-450 [Adachi, S., Nagano, S., Ishimori, K., Watanabe, Y., Morishima, I., Egawa, T., Kitagawa, T., & Makino R. (1993) Biochemistry 32, 241-252]. More importantly, the thiolate ligand enhanced its oxygenation activities when supported by H2O2 due to the exclusive encouragement of heterolytic O-O bond cleavage of peroxides. While we have attributed the enhanced heterolysis to the electron donation from the thiolate ligand, possible participation of the distal histidine (H64, E7) in H93C Mb cannot be eliminated. In addition, the racemic product formation catalyzed by H93C Mb implied that its distal cavity could prevent substrates from accessing to the heme and the reactions may proceed other than by the P-450 type mechanism (ferryl oxygen transfer). In order to clarify whether the distal histidine is involved in the O-O bond cleavage step and to improve accessibility of substrates, the distal histidine of H93C Mb is replaced by smaller and nonpolar residues, glycine (H64G/H93C Mb) and valine (H64V/H93C Mb), by site-directed mutagenesis. Various spectroscopic studies on these double-mutated Mbs revealed the ligation of cysteine to the ferric heme as a thiolate form. In the reaction with cumene hydroperoxide, the anionic nature of the proximal cysteine in H64G/H93C and H64V/H93C Mbs was found to encourage the heterolytic O-O bond cleavage as observed for H93C Mb. The results clearly demonstrate that the distal histidine of H93C Mb is hardly involved in the O-O bond cleavage step and are in good agreement with the role of thiolate ligation for the formation of the reactive intermediate, equivalent to compound I, in the catalytic cycle of P-450 reactions. In the oxygenation of methyl p-tolyl sulfide, the ratios of ferryl oxygen transfer increased in H64G/H93C Mb (58%) and H64V/H93C Mb (78%) as compared to H93C Mb (53%). The increased ratios of ferryl oxygen transfer imply the active site of H64G/H93C and H64V/H93C Mbs being more accessible for substrates; however, the sulfoxidation by the ferric mutant Mbs/H2O2 system was much slower than that by H93C Mb. The poor activities of these mutant Mbs are attributed to the significantly discouraged binding of H2O2.

Prochiral sulfides as in vitro probes for multiple forms of the flavin-containing monooxygenase

A homologous series of alkyl-substituted p-tolyl sulfides have been synthesized and evaluated as in vitro, isozyme-selective substrate probes for the microsomal flavin containing monooxygenases. Straight-chain and branched-chain alkyl homologs were metabolized to the corresponding ( R)- and ( S)-sulfoxides which were analyzed by chiral phase high-performance liquid chromatography. Initial studies demonstrated that the stereochemical composition of alkyl p-tolyl sulfoxides generated by FMO2, purified from rabbit lung, was a function of the degree of steric crowding about the prochiral center. In contrast, purified rabbit liver FMO1 formed the ( R)-sulfoxide from the n-alkyl series of substrates in a highly stereoselective manner (> 90%). Similar results were obtained with these two rabbit cDNAs expressed in E. coli. In contrast to rabbit FMO1 and FMO2, a characteristic feature of catalysis by cDNA-expressed rabbit FMO3 was the lack of stereoselectivity observed for formation of methyl p-tolyl sulfoxide. Collectively, these data demonstrate that the stereochemical composition of sulfoxides generated from the n-alkyl series of sulfides is isozyme-dependent. Metabolism of methyl p-tolyl sulfide by detergent-solubilized hepatic microsomes from a wide variety of experimental animals yielded predominantly ( R)- methyl p-tolyl sulfoxide, which, at least in rabbit liver, is indicative of catalysis dominated by FMO1. However, solubilized human and macaque liver preparations catalyzed this reaction in a relatively non-stereoselective manner. Macaque liver FMO was purified and the metabolite profile generated from the n-alkyl p-tolyl sulfides was found to be most similar to rabbit FMO3. Moreover, antibodies directed against macaque liver FMO selectively reacted with rabbit FMO3 and a microsomal protein expressed in adult human, but not fetal human liver, adult human kidney or adult human lung. Therefore, an FMO isoform expressed selectively in adult primate liver has catalytic and immunochemical properties consistent with its classification in the FMO3 family.