Copy number variations in three children with sudden infant death

Sudden death of an infant is a devastating event that needs an explanation. When an explanation cannot be found, the case is labeled as sudden infant death syndrome or unclassified sudden infant death. The influence of genetic factors has been recognized for sudden infant death, but copy number variations (CNVs) as potential risk factors have not been evaluated yet. Twenty-seven families were enrolled in this study. The tissue specimens from deceased children were obtained and array-based comparative genomic hybridization (array-CGH) experiments were performed on the genomic DNA isolated from these specimens using Agilent Technologies Custom 4 x 44K arrays. Quantitative polymerase chain reaction experiments were performed to confirm the overlapping duplication and deletion region in two different cases. A de novo CNV is detected in 3 of 27 cases (11%). In case 1, an approximately 3-Mb (chr 8: 143,211,215-qter) duplication on 8q24.3-qter and a 4.4-Mb deletion on the 22q13.3-qter (chr 22: 45,047,068-qter) were detected. Subtelomeric chromosome analysis of the father and the surviving sibling of case 1 showed a balanced reciprocal translocation, 46,XY,t(8;22)(q24.3;q13.3). A 240-kb (chr 6: 26,139,810-26,380,787) duplication and a 1.9-Mb deletion (chr 6: 26,085,971-27,966,150) at chromosome 6p22 were found in cases 2 and 3, respectively. Array-CGH and conventional cytogenetic studies did not reveal the observed CNVs in the parents and the siblings of cases 2 and 3. The detected CNVs in cases 2 and 3 encompassed several genes including the major histone cluster genes. Array-CGH analysis may be beneficial during the investigations after sudden infant death.

Enzyme kinetics of cytochrome P450-mediated reactions

The most common drug-drug interactions may be understood in terms of alterations of metabolism, associated primarily with changes in the activity of cytochrome P450 (CYP) enzymes. Kinetic parameters such as Km, Vmax, Ki and Ka, which describe metabolism-based drug interactions, are usually determined by appropriate kinetic models and may be used to predict the pharmacokinetic consequences of exposure to one or multiple drugs. According to classic Michaelis-Menten (M-M) kinetics, one binding site models can be employed to simply interpret inhibition (pure competitive, non-competitive and uncompetitive) or activation of the enzyme. However, some cytochromes P450, in particular CYP3A4, exhibit unusual kinetic characteristics. In this instance, the changes in apparent kinetic constants in the presence of inhibitor or activator or second substrate do not obey the rules of M-M kinetics, and the resulting kinetics are not straightforward and hamper mechanistic interpretation of the interaction in question. These unusual kinetics include substrate activation (autoactivation), substrate inhibition, partial inhibition, activation, differential kinetics and others. To address this problem, several kinetic models can be proposed, based upon the assumption that multiple substrate binding sites exist at the active site of a particular P450, and the resulting kinetic constants are, therefore, solved to adequately describe the observed interaction between multiple drugs. The following is an overview of some cytochrome P450-mediated classic and atypical enzyme kinetics, and the associated kinetic models. Applications of these kinetic models can provide some new insights into the mechanism of P450-mediated drug-drug interactions.

Studies on the metabolism of troglitazone to reactive intermediates in vitro and in vivo. Evidence for novel biotransformation pathways involving quinone methide formation and thiazolidinedione ring scission

Therapy with the oral antidiabetic agent troglitazone (Rezulin) has been associated with cases of severe hepatotoxicity and drug-induced liver failure, which led to the recent withdrawal of the product from the U.S. market. While the mechanism of this toxicity remains unknown, it is possible that chemically reactive metabolites of the drug play a causative role. In an effort to address this possibility, this study was undertaken to determine whether troglitazone undergoes metabolism in human liver microsomal preparations to electrophilic intermediates. Following incubation of troglitazone with human liver microsomes and with cDNA-expressed cytochrome P450 isoforms in the presence of glutathione (GSH), a total of five GSH conjugates (M1-M5) were detected and identified tentatively by LC-MS/MS analysis. In two cases (M1 and M5), the structures of the adducts were confirmed by NMR spectroscopy and/or by comparison with an authentic standard prepared by synthesis. The formation of GSH conjugates M1-M5 revealed the operation of two distinct metabolic activation pathways for troglitazone, one of which involves oxidation of the substituted chromane ring system to a reactive o-quinone methide derivative, while the second involves a novel oxidative cleavage of the thiazolidinedione (TZD) ring, potentially generating highly electrophilic alpha-ketoisocyanate and sulfenic acid intermediates. When troglitazone was administered orally to a rat, samples of bile were found to contain GSH conjugates which reflected the operation of these same metabolic pathways in vivo. The finding that metabolism of the TZD ring of troglitazone was catalyzed selectively by P450 3A enzymes is significant in light of the recent report that troglitazone is an inducer of this isoform in human hepatocytes. The implications of these results are discussed in the context of the potential for troglitazone to covalently modify hepatic proteins and to cause oxidative stress through redox cycling processes, either of which may play a role in drug-induced liver injury.

Metabolites of caspofungin acetate, a potent antifungal agent, in human plasma and urine

Caspofungin acetate (MK-0991) is a semisynthetic pneumocandin derivative being developed as a parenteral antifungal agent with broad-spectrum activity against systemic infections such as those caused by Candida and Aspergillus species. Following a 1-h i.v. infusion of 70 mg of [(3)H]MK-0991 to healthy subjects, excretion of drug-related material was very slow, such that 41 and 35% of the dosed radioactivity was recovered in urine and feces, respectively, over 27 days. Plasma and urine samples collected around 24 h postdose contained predominantly unchanged MK-0991, together with trace amounts of a peptide hydrolysis product, M0, a linear peptide. However, at later sampling times, M0 proved to be the major circulating component, whereas corresponding urine specimens contained mainly the hydrolytic metabolites M1 and M2, together with M0 and unchanged MK-0991, whose cumulative urinary excretion over the first 16 days postdose represented 13, 71, 1, and 9%, respectively, of the urinary radioactivity. The major metabolite, M2, was highly polar and extremely unstable under acidic conditions when it was converted to a less polar product identified as N-acetyl-4(S)-hydroxy-4-(4-hydroxyphenyl)-L-threonine gamma-lactone. Derivatization of M2 in aqueous media led to its identification as the corresponding gamma-hydroxy acid, N-acetyl-4(S)-hydroxy-4-(4-hydroxyphenyl)-L-threonine. Metabolite M1, which was extremely polar, eluting from HPLC column just after the void volume, was identified by chemical derivatization as des-acetyl-M2. Thus, the major urinary and plasma metabolites of MK-0991 resulted from peptide hydrolysis and/or N-acetylation.

Hormonal effects on tirilazad clearance in women: assessment of the role of CYP3A

This study assessed whether the previously reported difference in tirilazad clearance between pre- and postmenopausal women is reversed by hormone replacement and whether this observation can be explained by differences in CYP3A4 activity. Ten healthy women from each group were enrolled: premenopausal (ages 18-35), postmenopausal (ages 50-70), postmenopausal receiving estrogen, and postmenopausal women receiving estrogen and progestin. Volunteers received 0.0145 mg/kg midazolam and 3.0 mg/kg tirilazad mesylate intravenously on separate days. Plasma tirilazad and midazolam were measured by HPLC/dual mass spectrophotometry (MS/MS) assays. Tirilazad clearance was significantly higher in premenopausal women (0.51 +/- 0.09 L/hr/kg) than in postmenopausal groups (0.34 +/- 0.07, 0.32 +/- 0.06, and 0.36 +/- 0.08 L/hr/kg, respectively) (p = 0.0001). Midazolam clearance (0.64 +/- 0.12 L/hr/kg) was significantly higher in premenopausal women compared to postmenopausal groups (0.47 +/- 0.11, 0.49 +/- 0.11, and 0.53 +/- 0.19 L/hr/kg, respectively) (p = 0.037). Tirilazad clearance was weakly correlated with midazolam clearance (r2 = 0.129, p = 0.02). Tirilazad clearance is faster in premenopausal women than in postmenopausal women, but the effect of menopause on clearance is not reversed by hormone replacement. Tirilazad clearance in these women is weakly related to midazolam clearance, a marker of CYP3A activity.

Biotransformation of tirilazad in human: 3. tirilazad A-ring reduction by human liver microsomal 5alpha-reductase type 1 and type 2

Tirilazad mesylate (FREEDOX), a potent inhibitor of membrane lipid peroxidation in vitro, is under clinical development for the treatment of subarachnoid hemorrhage. In humans, tirilazad is cleared almost exclusively via hepatic elimination with a medium-to-high extraction ratio. In human liver microsomal preparations, tirilazad is biotransformed to multiple oxidative products and one reduced, pharmacologically active metabolite, U-89678. Characterization of the reduced metabolite by mass spectrometry and cochromatography with an authentic standard demonstrated that U-89678 was formed via stereoselective reduction of the Delta4 bond in the steroid A-ring. Kinetic analysis of tirilazad reduction in human liver microsomes revealed that kinetically distinct type 1 and type 2 5alpha-reductase enzymes were responsible for U-89678 formation; the apparent KM values for type 2 and type 1 were approximately 15 and approximately 0.5 microM, respectively. Based on pH dependence and finasteride inhibition studies, it was inferred that 5alpha-reductase type 1 was the high affinity/low capacity microsomal reductase that contributed to tirilazad clearance in vivo. In addition, a role for CYP3A4 in the metabolism of U-89678 was established using cDNA expressed CYP3A4 and correlation studies comparing U-89678 consumption with cytochrome P450 activities across a population of human liver microsomes. Collectively, these data suggest that formation of U-89678, a circulating pharmacologically active metabolite, contributes to the total metabolic elimination of tirilazad in humans and that clearance of U-89678 is mediated primarily via CYP3A4 metabolism. Therefore, concurrent administration of therapeutic agents that modulate 5alpha-reductase type 1 or CYP3A activity are anticipated to affect the pharmacokinetics of PNU-89678.

Biotransformation of tirilazad in human: 4. effect of finasteride on tirilazad clearance and reduced metabolite formation

The effect of oral finasteride, an inhibitor of 5alpha-reductase, on the clearance of tirilazad, a membrane lipid peroxidation inhibitor, was assessed in eight healthy men who received: 1) 10 mg/kg tirilazad mesylate solution orally on the 7th day of a 10-day regimen of 5 mg finasteride once daily, 2) 10 mg/kg tirilazad mesylate orally, 3) 2 mg/kg tirilazad mesylate i.v. on the 7th day of a 10-day regimen of 5 mg finasteride once daily and 4) 2 mg/kg tirilazad mesylate i.v., in a four-way cross-over design. Plasma concentrations of tirilazad and its active reduced metabolites (U-89678 and U-87999) were measured by liquid chromatography with tandem mass spectrometry (LC-MS-MS). Finasteride increased mean tirilazad areas under the curve by 21 and 29% for i.v. and p.o. tirilazad, respectively. Mean U-89678 areas under the curve were decreased 92 and 75% by finasteride administration with i.v. and p.o. tirilazad, respectively, and decreases of 94 and 85% in mean U-87999 area under the curve values were observed. These differences were statistically significant. These results indicate that finasteride inhibits the metabolism of tirilazad to U-89678. However, this inhibition has only a moderate effect on the overall clearance of tirilazad. These results thus confirm earlier in vitro work that showed that tirilazad is predominantly metabolized by CYP3A4. Although the major circulating metabolites of tirilazad are formed via reduction, this represents a minor route of tirilazad elimination in man.

In vitro metabolic transformations of 2,4-dipyrrolidinylpyrimidine: a chemical probe for P450-mediated oxidation of tirilazad mesylate

1. We have determined that 2,4-dipyrrolidinylpyrimidine (2,4-DPP), used as a model for studies of the metabolism of therapeutic agents containing this moiety, undergoes three characteristic hydroxylations when incubated with male rat liver microsomes. Analysis of microsomal incubates of stable isotope labelled analogues of 2,4-DPP by particle beam-liquid chromatography-mass spectrometry (LC-PB-MS) has shown that the three metabolites are 4-(3-hydroxypyrrolidinyl)-2-(pyrrolidinyl)-pyrimidine (M1), 4-(2-hydroxypyrrolidinyl)-2-(pyrrolidinyl)-pyrimidine (M2) and 2-(2-hydroxypyrrolidinyl)-4-(pyrrolidinyl)-pyrimidine (M3). 2. We determined that enzymes of the cytochrome P450 family are responsible for the in vitro hydroxylations of 2,4-DPP. 3. We observed that in microsomal incubations carried out in the presence of cyanide, a single cyanide adduct is formed implicating an iminium ion intermediate in the oxidation of the 2-pyrrolidine ring. 4. We also determined the intermolecular deuterium isotope effects for the formation of each of the three products. For M1, kH/kD = 14.55 +/- 0.54; for M2, kH/kD = 6.01 +/- 0.65; and for M3, kH/kD = 5.35 +/- 1.18. 5. We interpret these data as suggesting that M2 and M3 are formed by the same mechanism, probably including the formation of an iminium ion, and that M1 is formed by direct hydrogen abstraction.

Replacing 14C with stable isotopes in drug metabolism studies

After administration of a mixed dose of both radioisotope and stable-isotope-labeled tirilazad, we carried out a parallel set of HPLC analyses for drug metabolites in bile samples from monkeys and dogs using either radioactivity monitoring (RAM) for 14C or the chemical reaction interface mass spectrometry technique (CRIMS) to detect 13C or 15N. CRIMS is a novel method where analytes are decomposed in a microwave-induced plasma and the elements contained in the analytes are reformulated into small gaseous species that are detected by a mass spectrometer. The comprehensiveness of detection, chromatographic resolution, sensitivity, signal/noise, and quantitative abilities of CRIMS were compared with RAM and in no case was RAM superior. This implies that stable isotopes may be substituted for radioisotopes in studies of drug metabolism where the ability of the latter approach to detect a label independent of the structures in which the label appears has been the primary reason for continuing to use a hazardous and expensive tracer. With HPLC-CRIMS, stable isotopes such as 13C and 15N can be comprehensively detected and quantitative patterns of drug metabolism from biological fluids can be produced that mirror the results when 14C is used.

Biotransformation of tirilazad in human: 1. Cytochrome P450 3A-mediated hydroxylation of tirilazad mesylate in human liver microsomes

Tirilazad mesylate (Freedox), a potent inhibitor of membrane lipid peroxidation in vitro, is under clinical development for the treatment of subarachnoid hemorrhage. In humans, tirilazad is cleared almost exclusively via hepatic elimination. Characterization of three major microsomal metabolites of tirilazad by mass spectrometry indicated that hydroxylation had occurred in the pyrrolidine ring(s) and at the 6 beta-position of the steroid domain. A role for CYP3A4 in the formation of the three major metabolites (tirilazad hydroxylase activity) was established in human liver microsomal preparations: 1) Tirilazad hydroxylation was potently inhibited by troleandomycin and ketoconazole, specific inhibitors of CYP3A4. 2) The rates of tirilazad hydroxylation within a population of 14 human livers displayed a 9-fold interindividual variation and a significant correlation (r2 = .95) between tirilazad hydroxylation and testosterone 6 beta-hydroxylation. 3) Kinetic analysis of tirilazad hydroxylase activity in three human livers resulted in an apparent Km of 2.12, 1.68 and 1.66 microM, and Vmax = 0.85, 0.44 and 3.45 (nmol/mg protein/min) for HL14, HL17 and HL21, respectively. In addition, an apparent Km of 2.07 microM was established for tirilazad hydroxylation in a cDNA-expressed CYP3A4 microsomal system. Collectively, these data indicate that the metabolic clearance of tirilazad in humans is catalyzed primarily by CYP3A4 and provide an insight into factors (i.e., age, sex, drug-drug interactions) that modulate the metabolic clearance of tirilazad in vivo.

Biotransformation of tirilazad in human: 2. Effect of ketoconazole on tirilazad clearance and oral bioavailability

The effect of ketoconazole, a CYP3A inhibitor, on the oral bioavailability of tirilazad mesylate was assessed in 12 healthy subjects, who received the following treatments in a crossover design: a) 10 mg/kg tirilazad mesylate solution orally on the fourth day of a 7-day regimen of 200 mg ketoconazole once daily, b) 10 mg/kg tirilazad mesylate solution orally, c) 2 mg/kg i.v. tirilazad mesylate solution on the fourth day of a 7-day regimen of 200 mg ketoconazole once daily and d) 2mg/kg i.v. tirilazad mesylate solution. Plasma concentrations of tirilazad mesylate and its active reduced metabolites (U-89678 and U-87999) were measured by high-performance liquid chromatography. Urinary ratios of 6 beta-hydroxycortisol to cortisol (6 beta-OHC/C) were measured as an index of hepatic CYP3A activity. Ketoconazole increased mean tirilazad mesylate area under the curve (AUC) values by 67% and 309% for i.v. and oral administration, respectively. Mean AUC values for U-89678 were increased 472% and 720% by ketoconazole coadministration with i.v. and oral tirilazad, respectively, whereas increases of > 10-fold in mean U-87999 AUC values were observed. These differences were statistically significant. These results indicate that ketoconazole inhibits the metabolism of these three compounds, which suggests that all of the compounds are substrates for CYP3A. Urinary 6 beta-OHC/C ratios did not reflect this level of effect of ketoconazole on CYP3A; this probe may not be useful for assessing the effect of CYP3A inhibitors. The absolute bioavailability of oral tirilazad was 8.7 +/- 4.8%; ketoconazole increased the bioavailability to 20.9 +/- 6.5%. Ketoconazole increased tirilazad mesylate bioavailability by decreasing the first-pass liver and gut wall metabolism of tirilazad mesylate to similar degrees.

Deuterium isotope effect on the metabolism of the flame retardant tris(2,3-dibromopropyl) phosphate in the isolated perfused rat liver

The metabolism of tris(2,3-dibromopropyl) phosphate (Tris-BP) was compared with that of completely deuterated Tris-BP (D15-Tris-BP) in an isolated, recirculating rat liver perfusion system in order to determine the relative quantitative importance of two different biotransformation pathways of Tris-BP: (i) cytochrome P450-mediated metabolism and (ii) GSH S-transferase-mediated metabolism. To accomplish this we quantitated the biliary excretion of S-(3-hydroxypropyl)glutathione (GSOH) as a marker metabolite for cytochrome P450-mediated metabolism and that of S-(2,3-dihydroxypropyl) glutathione (GSOHOH) as a marker metabolite for GSH S-transferase-mediated metabolism. Complete deuterium substitution of Tris-BP significantly decreased the formation of GSOH, whereas there was no effect on the formation of GSOHOH. Because our previous studies showed a large decrease in genotoxicity of D15-Tris-BP compared to Tris-BP, the present results support our hypothesis that cytochrome P450-mediated metabolism is responsible for the genotoxic effects of Tris-BP in the rat liver.

Identification of N-acetylcysteine conjugates of 1,2-dibromo-3-chloropropane: evidence for cytochrome P450 and glutathione mediated bioactivation pathways

The haloalkane 1,2-dibromo-3-chloropropane (DBCP) is a carcinogen, mutagen, nephrotoxin, and testicular toxin. The identification of N-acetylcysteine conjugates of DBCP provides information on GSH mediated and cytochrome P450 mediated bioactivation pathways in the expression of DBCP-induced toxicities. N-Acetylcysteine conjugates excreted in the urine of male Sprague-Dawley rats administered DBCP, C1D2-DBCP, C2D1-DBCP, C3D2-DBCP, or D5-DBCP (80 mg/kg) were purified by reverse-phase HPLC as their methyl ester derivatives and characterized by fast atom bombardment tandem mass spectrometry. These metabolites were also converted to tert-butyldimethylsilyl ether derivatives and analyzed by gas chromatography-mass spectrometry (GC-MS) to facilitate the identification of N-acetyl-S-(2,3-dihydroxypropyl)cysteine (Ia), an apparent regioisomer of Ia, 2-(S-(N-acetylcysteinyl))-1,3-propanediol (Ib), N-acetyl-S-(3-hydroxypropyl)cysteine (IIa), and N-acetyl-S-(3-chloro-2-hydroxypropyl)-cysteine (III). Metabolites Ia, Ib, and III displayed quantitative retention of deuterium, an observation consistent with the formation of episulfonium ion intermediate(s) in their biogenesis. Mercapturate IIa retained three atoms of deuterium from D5-DBCP, and two atoms of deuterium from the dideuterio analogs (C1D2-DBCP and C3D2-DBCP), thus invoking P450 mediated formation of 2-bromoacrolein (2-BA) as an intermediate in the biogenesis of IIa. A mechanism is proposed in which conjugate addition of GSH to 2-BA, subsequent episulfonium ion formation, and addition of GSH afford 1,2-(diglutathion-S-yl)propanal. Glutathione mediated reduction is invoked to afford S-(3-hydroxypropyl)GSH which would be excreted in the urine as IIa. The quantitative retention of deuterium from C1D2-DBCP or C3D2-DBCP was indicative of isotopically sensitive branching of P450 metabolism at either C1 or C3 to afford 2-BA. C2D1-DBCP showed a 30% retention of 1 deuterium atom in IIa; the loss of the deuterium is consistent with 2-BA formation, whereas the retention of one deuterium atom is indicative of the formation of metabolite IIa through GSH conjugation of either 2,3-dibromopropanal or 2-bromo-3-chloropropanal. These data indicate that IIa is a marker metabolite for the potent direct-acting mutagen, 2-BA, or its metabolic precursors 2,3-dibromopropanal or 2-bromo-3-chloropropanal. Therefore, evidence has been presented for bioactivation of DBCP by glutathione and cytochrome P450 mediated mechanisms.

In vitro metabolism of tirilazad mesylate in male and female rats. Contribution of cytochrome P4502C11 and delta 4-5 alpha-reductase

Tirilazad mesylate, a potent inhibitor of membrane lipid peroxidation in vitro, is under clinical development for the treatment of subarachnoid hemorrhage and head injury. In rat, tirilazad seems to be highly extracted and is cleared almost exclusively via hepatic elimination. The biotransformation of tirilazad has been investigated in liver microsomal preparations from adult male and female Sprague-Dawley rats. Tirilazad metabolism in male rat liver microsomes resulted in the formation of two primary metabolites: M1 and M2. In incubations with female rat liver microsomes, M2 was the only primary metabolite detected. Structural characterization of M1 and M2 by mass spectrometry demonstrated that M2 was formed by reduction of the delta 4-double bond in the steroid A-ring, whereas M1 arose from oxidative desaturation of one pyrrolidine ring. Further structural analysis of M2 by proton NMR demonstrated that reduction at C-5 had occurred by addition of hydrogen in the alpha-configuration. Using metabolic probes and antibodies specific to individual hepatic microsomal enzymes, CYP2C11 and 3-oxo-5 alpha-steroid:NADP+ delta 4-oxidoreductase (5 alpha-reductase) were identified as responsible for the formation of M1 and M2, respectively. The formation of M1 was inhibited by testosterone, nicotine, cimetidine, and anti-CYP2C11 IgG. The formation of M2 was inhibited by finasteride, a potent inhibitor of 5 alpha-reductase. Kinetic analysis of CYP2C11-mediated M1 formation in male rat liver microsomal incubations revealed that M1 formation occurred through a low-affinity/low-capacity process (KM = 16.67 microM, Vmax = 0.978 nmol/mg microsomal protein/min); the formation of M2 was mediated by 5 alpha-reductase in a high-affinity/low-capacity process (KM = 3.07 microM, Vmax = 1.06 nmol/mg microsomal protein/min). In contrast, the formation of M2 in female rat liver microsomes was mediated by 5 alpha-reductase in a high-affinity/high-capacity process (KM = 2.72 microM, Vmax = 4.11 nmol/mg microsomal protein/min). Comparison of calculated intrinsic formation clearances (Vmax/KM) for M1 and M2 indicated that the female rat possessed a greater in vitro metabolic capacity for tirilazad biotransformation than the male rat. Therefore, the clearance of tirilazad mesylate in the rat is mediated primarily by rat liver 5 alpha-reductase, and the capacity in the female rat is 5-fold the capacity in the male. These observations correlate with documented differences in 5 alpha-reductase activity and predict a gender difference in tirilazad hepatic clearance in vivo.

Blocking of in vitro DNA replication by deoxycytidine adducts of the mutagen and clastogen 2-bromoacrolein

Calf thymus single-stranded DNA was modified with 2-bromoacrolein (2BA), a genotoxic metabolite of tris(2,3-dibromopropyl)phosphate. This DNA was used as a template for in vitro DNA replication by T7-polymerase and Klenow fragment of Escherichia coli DNA polymerase I. Increasing levels of 2BA modification led to decreased DNA synthesis as measured by [methyl-3H]dTTP incorporation. M13 mp19 single-stranded DNA template modified with 2BA was used to determine the sites of termination of DNA replication by T7 polymerase and Klenow fragment of Escherichia coli DNA polymerase I. It was found that DNA replication stopped frequently before and occasionally opposite deoxycytidine nucleotides. These results indicated that an as yet unidentified deoxycytidine:2BA adduct may have been formed in the reaction of 2BA with M13 DNA. To investigate if such adducts were formed, we reacted 2BA with deoxycytidine in vitro at pH 4.4, and putative deoxycytidine:2BA adducts were isolated by high-performance liquid chromatography. They were characterized by 1H and 13C nuclear magnetic resonance and with fast atom bombardment mass spectrometry as two diastereomeric 3-bromo-7-(beta-D-deoxyribofuranosyl)- 3,4-dihydro-2-hydroxy-(2H,7H)[1,6-a]pyrimidin-6-one adducts and a 3-bromo-7-(beta-deoxyribofuranosyl)-(4H,7H)-pyrimido[1,6-a]pyrimidin-6 -one adduct. Only the latter adduct, however, was formed in the reaction of 2BA with calf thymus single-stranded DNA in vitro. Tris(2,3-dibromopropyl)phosphate is clastogenic. Because clastogenicity may result from DNA adducts that block replication, the role of the presently identified deoxycytidine adducts of the reaction metabolite 2BA in the clastogenicity of tris(2,3-dibromopropyl)phosphate is discussed.

Inhibition of in vitro lipid peroxidation by 21-aminosteroids. Evidence for differential mechanisms

In a previous report (Ryan and Petry, Arch Biochem Biophys 300: 699-704, 1993), the effects of two 21-aminosteroids (U-74500A and U-74006F) on the oxidation and reduction of iron in a buffer/organic solvent system were investigated. In those studies, U-74500A was found to be an efficient iron reductant and potential iron chelator, whereas U-74006F had little effect on iron redox chemistry. As an extension of those studies, we now report the effects of U-74006F and U-74500A on lipid peroxidation in systems that are dependent upon iron oxidation/reduction. In liposomes, U-74500A inhibited ADP:Fe(II)-dependent lipid peroxidation in a concentration-dependent manner, whereas U-74006F was minimally effective in this system. The mechanism of U-74500A-dependent inhibition probably involved interactions with iron, as iron oxidation was inhibited in the presence of this compound. No effects on iron oxidation were observed in the presence of U-74006F. Addition of Ferrozine to liposomal incubation mixtures indicated that at least two iron pools were present in samples containing U-74500A, one immediately bound by Ferrozine, and another that was bound more slowly. Furthermore, ADP:Fe(III)/ascorbate-dependent lipid peroxidation was blocked completely by U-74500A, presumably by formation of a redox inert complex upon reduction of the iron. U-74500A partially protected ADP:Fe(II) from oxidation by H2O2 and lipid hydroperoxides, indicating that the U-74500A:iron complex was stable in the presence of biologically relevant oxidants. U-74006F did not markedly affect iron oxidation or reduction when incorporated into phospholipid liposomes. In microsomal lipid peroxidation systems containing ADP:Fe(III) and NADPH, both U-74500A and U-74006F inhibited lipid peroxidation. U-74006F-dependent inhibition of microsomal lipid peroxidation was dependent on both NADPH and Fe(III). Further, it was enhanced when U-74006F was allowed to preincubate in this system prior to iron addition. Preincubation of U-74006F with microsomes, NADPH, and ADP:Fe(III) produced several metabolites detectable by HPLC. These results suggest that U-74500A inhibits lipid peroxidation by directly affecting iron redox chemistry, whereas U-74006F-mediated inhibition is enhanced by preincubation with a metabolically competent microsomal system.

Studies on the metabolic fate of caracemide, an experimental antitumor agent, in the rat. Evidence for the release of methyl isocyanate in vivo

Following administration to rats of a single ip dose (6.6 mg kg-1) of the investigational antitumor agent caracemide (N-acetyl-N,O-bis[methylcarbamoyl]hydroxylamine), the mercapturic acid derivative N-acetyl-S-(N-methylcarbamoyl)cysteine (AMCC) was identified in urine by thermospray LC-MS. Quantification of this conjugate was carried out by stable isotope dilution thermospray LC-MS, which indicated that the fraction of the caracemide dose recovered as AMCC in 24-h urine collections was 54.0 +/- 5.5% (n = 4). Since AMCC is known to represent a major urinary metabolite of methyl isocyanate (MIC) in the rat, the results of this study support the contention that caracemide yields MIC as a toxic intermediate in vivo. Furthermore, with the aid of a specifically deuterium-labeled analog of caracemide ([carbamoyloxy-C2H3]caracemide), it was shown that the methylcarbamoyl group of AMCC derived from both the O-methylcarbamoyl (72%) and N-methylcarbamoyl (28%) side chains of the drug. In view of these findings, it is concluded that caracemide acts as a latent form of MIC in vivo and that this reactive isocyanate (or labile S-linked conjugates thereof) may contribute to the antitumor properties and/or adverse side-effects of caracemide.

Metabolic activation of tris(2,3-dibromopropyl)phosphate to reactive intermediates. I. Covalent binding and reactive metabolite formation in vitro

Analogs of tris(2,3-dibromopropyl)phosphate (Tris-BP) either labeled at specific positions with carbon-14, phosphorus-32, or oxygen-18 or dual-labeled with both deuterium and tritium were used as metabolic probes to study the chemical and metabolic events in the bioactivation of Tris-BP to chemically reactive metabolites in liver microsomal preparations. Oxidation at the terminal (C-3) carbon atom of the propyl groups of Tris-BP yielded the direct-acting mutagen 2-bromoacrolein as the major metabolite that binds to DNA. Although this reactive metabolite also appears to bind to microsomal protein, the rate of binding of radiolabeled Tris-BP to protein is 15-20x greater than binding to DNA, and some metabolites that retain the phosphate group are bound. Studies with deuterated analogs of Tris-BP implicate oxidation at C-2 of the propyl group as a major pathway that leads to protein binding which is enhanced by phenobarbital pretreatment of rats. Moreover, investigations with 18O-Tris-BP and H2(18)O show that Bis-BP that is formed from oxidation of Tris-BP incorporates one atom of oxygen from water. Deuterium isotope studies suggest that most of the Bis-BP arises from initial oxidation at C-2. Taken together these studies indicate that P-450 oxidation of Tris-BP at C-2 of the propyl group yields a reactive alpha-bromoketone metabolite of Tris-BP that can either alkylate proteins directly or be hydrolyzed to Bis-BP and an alpha-bromo-alpha'-hydroxyketone that can alkylate microsomal proteins.

Metabolic activation of tris(2,3-dibromopropyl)phosphate to reactive intermediates. II. Covalent binding, reactive metabolite formation, and differential metabolite-specific DNA damage in vivo

Analogs of tris(2,3-dibromopropyl)phosphate (Tris-BP) either labeled at specific positions with carbon-14 and phosphorus-32 or dual-labeled with both deuterium and tritium were administered to male Wistar rats at a nephrotoxic dose of 360 mumol/kg. The covalent binding of Tris-BP metabolites to hepatic, renal, and testicular proteins was determined after 9 and 24 hr, and plasma concentrations of bis(2,3-dibromopropyl)-phosphate (Bis-BP) formed metabolically from Tris-BP were measured at intervals throughout the initial 9-hr postdosing period. The covalent binding of 14C-Tris-BP metabolites in the kidney (2495 +/- 404 pmol/mg protein) was greater than that in the liver (476 +/- 123 pmol/mg protein) or testes (94 +/- 11 pmol/mg protein); the extent of renal covalent protein binding of Tris-BP metabolites was decreased by 82 and 84% when deuterium was substituted at carbon-2 and carbon-3, respectively. Substitution of Tris-BP with deuterium at carbon-2 or carbon-3 also decreased the mean area under the curve for Bis-BP plasma concentration by 48 and 57%, respectively. The mechanism of Tris-BP-induced renal and hepatic DNA damage was evaluated in Wistar rats by an automated alkaline elution procedure after the administration of analogs of Tris-BP or Bis-BP labeled at specific positions with deuterium. Renal DNA damage was decreased when Tris-BP was substituted with deuterium at either carbon-2 or carbon-3; the magnitude of the change correlated with both a decrease in the area under the Bis-BP plasma curve and a decrease in renal covalent binding of Tris-BP metabolites for each of the deuterated analogs. In marked contrast, analogs of Bis-BP labeled with deuterium at carbon-2 or carbon-3 did not show a decrease in the severity of renal DNA damage compared to unlabeled Bis-BP. On the basis of these observations a metabolic scheme for hepatic P-450-mediated oxidation at either carbon-2 or carbon-3 of Tris-BP affording Bis-BP by two alternate pathways that are susceptible to primary deuterium kinetic isotope effects is proposed. The Tris-BP metabolite, Bis-BP, is subsequently metabolized to reactive intermediates that cause DNA damage and bind to kidney proteins in a mechanism independent of cytochrome P-450.

2-Amino-3-(methylamino)propanoic acid (BMAA) bioavailability in the primate

2-Amino-3-(methylamino)-propanoic acid (BMAA) is a low potency excitatory amino acid present in the cycad plant that has been proposed as a factor in the high incidence of amyotrophic lateral sclerosis-parkinsonism dementia (ALS-PD) in the western Pacific region. We employed stable isotopic forms of BMAA to assess the oral bioavailability of this compound in cynomolgous monkeys (n = 3). The stable isotope labeled BMAA ([15N]-BMAA) was injected i.v. at the same time that the unlabeled compound was administered orally. Both forms of BMAA were then quantified in a 48h urine sample by gas chromatography-mass spectrometry (GC/MS). Following oral dosing, 80% of the administered BMAA was absorbed into the systemic circulation; thus, oral bioavailability was high and other routes of administration could not result in significantly higher circulating levels of BMAA for a given administered dose.

Partial characterization of biliary metabolites of pulegone by tandem mass spectrometry. Detection of glucuronide, glutathione, and glutathionyl glucuronide conjugates

The hepatotoxic monoterpene pulegone is a major constituent of the herbal abortifacient pennyroyal oil. An approximately equimolar mixture of 2H3- and 14C-labeled pulegone was administered to rats to study its phase II metabolism. Radioactive conjugates that were excreted into the bile were isolated by selective derivatization and HPLC separation, and subsequently characterized from the daughter ion mass spectra of protio- and deutero-analogs of each metabolite. The biliary metabolites characterized were glucuronide and glutathione (GSH) conjugates, accounting for approximately 3% of the radioactivity excreted in bile. The glucuronides, which were 2-fold more abundant than GSH conjugates, were mainly of hydroxylated pulegone and hydroxylated, reduced pulegone. The three GSH conjugates contained xenobiotic moieties that varied in their oxidation state; one of these was tentatively identified as the GSH conjugate of the proximate oxygenated metabolite, menthofuran. The two other GSH conjugates apparently underwent subsequent glucuronidation since novel glutathionyl glucuronide conjugates were identified that contained nonhydroxylated xenobiotic moieties. The results indicate that pulegone is bioactivated via at least three distinct pathways, each marked by a different GSH conjugate. Characterization of these conjugates represents a first step in the identification of the reactive metabolites from which they are derived.

Carbamoylation of peptides and proteins in vitro by S-(N-methylcarbamoyl)glutathione and S-(N-methylcarbamoyl)cysteine, two electrophilic S-linked conjugates of methyl isocyanate

The reactivity toward peptides and proteins of S-(N-methylcarbamoyl)glutathione (SMG), the glutathione conjugate of methyl isocyanate, and the corresponding cysteine adduct, S-(N-methylcarbamoyl)cysteine (SMC), was investigated with the aid of in vitro model systems. Incubation of SMC or a trideuteriomethyl analogue of SMC with either the reduced or oxidized forms of oxytocin afforded similar mixtures of mono-, bis- and tris-N-methylcarbamoylated peptides. Structure elucidation of the mono and bis adducts by fast atom bombardment tandem mass spectrometry indicated that carbamoylation of oxytocin occurred preferentially at Cys-6 and that Cys-1 and/or Tyr-2 were secondary sites of modification. Upon incubation of S-[N-([14C]methyl)carbamoyl]glutathione (14C-SMG) with native bovine serum albumin (BSA), radioactivity became bound covalently to the protein in a time- and concentration-dependent fashion. "Blocking" of the lone Cys-34 thiol group of BSA in the form of a disulfide prior to exposure of the protein to 14C-SMG failed to decrease significantly the extent or time course of this covalent binding. It is concluded that carbamate thioester conjugates of MIC are reactive, carbamoylating entities which can donate the elements of MIC to nucleophilic functionalities on peptides and proteins. Free thiols appear to be preferred sites for such carbamoylation processes, a phenomenon that may have important toxicological consequences in the pathology of tissue lesions induced by MIC and related isocyanates.

2-amino-3-(methylamino)-propanoic acid (BMAA) pharmacokinetics and blood-brain barrier permeability in the rat

2-Amino-3-(methylamino)-propanoic acid (BMAA) is a neurotoxic, excitatory amino acid which has been linked through cycad use and consumption with the onset of a variant of amyotrophic lateral sclerosis occurring with high incidence in the western Pacific region. We have studied BMAA pharmacokinetics, oral bioavailability and blood-brain barrier permeability in the rat in an attempt to better define the possible role for BMAA in this disease. To evaluate its kinetics and uptake, BMAA (25-400 mg/kg) was administered to rats, either acutely or chronically, and then plasma and brain concentrations were determined at various times thereafter by combined gas chromatography mass spectrometry. After single dose i.v. injection, BMAA was cleared from plasma in a rapid distribution phase (Vd approximately 16 liters/kg) followed by a slower elimination phase (t1/2 approximately 1 day). Brain uptake was limited by a low blood-brain barrier permeability-surface area product of 2 to 5 x 10(-5) ml/sed/g. Brain BMAA levels peaked within 8 hr after injection, and then declined with a t1/2 similar to that of plasma. After two weeks of continuous infusion (100 mg/kg/day), steady-state brain concentrations equalled 10 to 30 micrograms/g, and only moderately exceeded those in plasma. The results suggest that BMAA may reach potentially toxic levels in brain (i.e., greater than 250 microM) after large doses (greater than 100 mg/kg). However, such doses are orders of magnitude greater than those available from dietary or medicinal use of cycads.

Biotransformation of methyl isocyanate in the rat. Evidence for glutathione conjugation as a major pathway of metabolism and implications for isocyanate-mediated toxicities

S-(N-Methylcarbamoyl)-N-acetylcysteine (AMCC), a chemically labile mercapturic acid conjugate, was identified by liquid chromatography-mass spectrometry (LC-MS) in the urine of rats dosed intraperitoneally with methyl isocyanate (MIC; 45.2 mumol). The corresponding cysteine conjugate, however, was not detected in urine. Following methylation, urine extracts were analyzed by thermospray LC-MS and the AMCC methyl ester was quantified by means of a stable isotope dilution assay procedure which utilized S-(N-methylcarbamoyl)-N-[2H3]-acetylcysteine [( 2H3]AMCC) as internal standard. The results showed that the fraction of the injected dose of MIC which appeared in 24-h urine collections as AMCC was 24.8 +/- 1.9% (mean +/- SD, N = 4). Thus, conjugation of MIC with glutathione (GSH), followed by metabolism of the resulting adduct to AMCC, appears to represent a quantitatively important pathway of biotransformation of MIC in the rat. However, in view of the known carbamoylating properties and in vitro cytotoxicity of S-linked conjugates of MIC, it seems unlikely that the GSH pathway of metabolism fulfills a conventional detoxification role in the case of MIC. In contrast, it is proposed that carbamate thioester conjugates of MIC, which can revert spontaneously to free MIC under physiological conditions, may actually contribute to the multisystem adverse effects of this highly toxic isocyanate in vivo.

Metabolic activation of 1,2-dibromo-3-chloropropane to mutagenic metabolites: detection and mechanism of formation of (Z)- and (E)-2-chloro-3-(bromomethyl)oxirane

1,2-Dibromo-3-chloropropane (DBCP), a haloalkane nematocide and soil fumigant, is metabolically activated to chemically reactive species that are direct-acting mutagens in a Salmonella typhimurium TA 100 test system. Studies in vitro with rat liver microsomes indicated that oxidation at carbon 3 resulted in the formation of an unstable gem-chlorohydrin that rearranged with elimination of hydrogen bromide to form (Z)-2-chloro-3-(bromomethyl)oxirane [(Z)-CBPO] and (E)-2-chloro-3-(bromomethyl)oxirane [(E)-CBPO]. Gas chromatography-mass spectrometry (GC-MS) with positive ion chemical ionization (CI) was employed to identify (Z)-CBPO and (E)-CBPO by comparison of characteristic fragment ions in their CI mass spectra with those observed for authentic standards. Quantitative GC-MS methodology was exploited to quantitate the rate of formation of (Z)-CBPO and (E)-CBPO from DBCP and analogues of DBCP specifically deuterated at carbon 1 and carbon 3. The rate of formation of Z- and E-isomers of CBPO was 31 and 33 pmol/(min.mg of protein), respectively, from DBCP; substitution with deuterium at carbon 1 increased the rate of epoxide formation by 50%, whereas CBPO formation could not be detected from a substrate labeled with deuterium at carbon 3. Both epoxides were directly acting mutagens to S. typhimurium TA 100. (Z)-CBPO caused approximately twice as many his+ revertants/nmol compared to (E)-CBPO. Oxidation at carbon 2 of DBCP resulted in the formation of a bifunctional alkylating agent, 1-bromo-3-chloroacetone, presumably via the intermediacy of an unstable gem-bromohydrin.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic activation of 1,2-dibromo-3-chloropropane: evidence for the formation of reactive episulfonium ion intermediates

The nematocide and soil fumigant 1,2-dibromo-3-chloropropane (DBCP) is a carcinogen and a mutagen and displays target-organ toxicity to the testes and the kidney. It has been proposed that both cytochrome P-450 mediated activation and glutathione (GSH) conjugation pathways are operative in DNA damage and organotropy induced by DBCP. To determine the chemical mechanisms involved in the bioactivation of DBCP and to assess a role for an episulfonium ion intermediate, the mechanism of formation of GSH conjugate metabolites of DBCP was investigated. Five biliary GSH conjugates of DBCP were isolated from rats and identified by fast atom bombardment tandem mass spectrometry: S-(2,3-dihydroxy-propyl)glutathione (I), S-(2-hydroxypropyl)glutathione (IIA), S-(3-chloro-2-hydroxypropyl)glutathione (III), 1,3-di(S-glutathionyl)propan-2-ol (IV), and 1-(glycyl-S-cysteinyl)-3- (S-glutathionyl)propan-2-ol (V). The mechanisms of conjugate formation were addressed by assessing deuterium retention in conjugates derived from [1,1,2,3,3-2H5] DBCP (D5-DBCP). GSH conjugates I, III, IV, and V displayed quantitative retention of deuterium, an observation consistent with the formation of an episulfonium ion intermediate. GSH conjugate IIA, however, retained three atoms of deuterium, thus invoking a P-450 mechanism in its genesis. The involvement of glutathione transferase (GST) and sequential episulfonium ion intermediates in the formation of metabolites I, III, and IV was demonstrated in vitro. Upon incubation of DBCP with GST, metabolites I, III, and IV were identified by tandem mass spectrometry and were found to arise with quantitative retention of deuterium when D5-DBCP was employed as a substrate. An additional GSH conjugate, 1,2,3-tri(S-glutathionyl)propane (VI), was observed as the major metabolite in incubations of GST with DBCP. When the incubations of DBCP with GST were performed in H2(18)O, metabolite I incorporated two atoms of 18O, and metabolites III and IV incorporated one atom of 18O. The ability of GST to catalyze the formation of the four GSH conjugates observed in vivo, with quantitative retention of deuterium and incorporation of 18O from H2(18)O, may be rationalized by a mechanism invoking the initial formation of S-(2-bromo-3-chloropropyl)glutathione. Rearrangement of this unstable conjugate via several reactive episulfonium ions, with either hydrolysis by water or alkylation of GSH at various stages, would account for the pattern of metabolites and their status of isotopic enrichment observed under various incubation conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemical synthesis and cytotoxic properties of N-alkylcarbamic acid thioesters, metabolites of hepatotoxic formamides

The S-linked cysteine and glutathione conjugates of N-methylformamide and N-ethylformamide, together with a series of methyl ester derivatives thereof, have been synthesized and characterized by 1H NMR, 13C NMR, FAB-MS, and FAB tandem mass spectrometry. In vitro cytotoxicity assays showed that all of the title conjugates were toxic to isolated mouse hepatocytes when incubated at concentrations in excess of 1 mM and that they served as potent growth inhibitors of murine TLX5 lymphoma cells when present at levels of 10-100 microM. Both of these effects were reversed by the addition to incubation media of glutathione (10 mM). The possibility is raised that N-alkylcarbamic acid thioester conjugates, which are formed during the metabolism of N-alkylformamides in mammalian systems, may act as important mediators of the antineoplastic and/or hepatotoxic activity of the parent formamides, possibly through their ability to liberate methyl isocyanate at cell membranes.

S-(N-methylcarbamoyl)glutathione: a reactive S-linked metabolite of methyl isocyanate

S-(N-methylcarbamoyl)glutathione, a chemically-reactive glutathione conjugate, has been isolated from the bile of rats administered methyl isocyanate and characterized, as its N-benzyloxycarbonyl dimethylester derivative, by tandem mass spectrometry. The ability of this glutathione adduct to donate an N-methylcarbamoyl moiety to the free -SH group of cysteine was evaluated in vitro with the aid of a highly specific thermospray LC/MS assay procedure. The glutathione adduct reacted readily with cysteine in buffered aqueous media (pH 7.4, 37 degrees C) and after 2 hr, 42.5% of the substrate existed in the form of S-(N-methylcarbamoyl)cysteine. The reverse reaction, i.e. between the cysteine adduct and free glutathione, also took place readily under these conditions. It is concluded that conjugation of methyl isocyanate with glutathione in vivo affords a reactive S-linked product which displays the potential to carbamoylate nucleophilic amino acids. The various systemic toxicities associated with exposure of animals or humans to methyl isocyanate could therefore be due to release of the isocyanate from its glutathione conjugate, which thus may serve as a vehicle for the transport of methyl isocyanate in vivo.

Formation of cyclic 1,N2-propanodeoxyguanosine and thymidine adducts in the reaction of the mutagen 2-bromoacrolein with calf thymus DNA

The interaction of the mutagen 2-bromoacrolein (2BA) with DNA and thymidine was studied in vitro by reaction of [3-3H]2BA with thymidine, RNA, single-stranded DNA, and double-stranded DNA in phosphate buffer (pH 7.4). After purification of the nucleic acids, they were incubated at alkaline pH to convert any (hydroxybromo)propano(deoxy)-guanosine adducts to their dihydroxy analogues. After acid or enzymatic hydrolysis, the hydrolysates were analyzed by reversed-phase high-performance liquid chromatography. At a concentration of 1.6 mM, the fraction of 2BA that became covalently bound to DNA was 2.3% of the amount added. Only 3% of the radioactivity bound to DNA after extensive purification could be accounted for as cyclic 1,N2-(6,7-dihydroxy)-propanoguanine adducts. More 2BA became covalently bound to single-stranded DNA and RNA as compared with double-stranded DNA. However, high-performance liquid chromatographic analyses showed that formation of cyclic 1,N2-(6,7-dihydroxy)propanoguanine adducts was also a minor reaction with these macromolecules. Because these data showed that other type(s) of reaction(s) are more important in the reaction of 2BA with nucleic acids, we have investigated the reaction of 2BA with other nucleosides. It was found that 2BA reacted well with thymidine in vitro, and the major product was identified by 500 MHz 1H and 75.43 MHz 13C nuclear magnetic resonance and thermospray mass spectrometry as 3-(2"-bromo-3"-oxopropyl)thymidine. This adduct was unstable and decomposed upon storage. After enzymatic hydrolysis of [3H]2BA-modified double-stranded DNA and subsequent analysis of the hydrolysate by high-performance liquid chromatography, 22% of the covalently bound radioactivity to DNA coeluted with decomposition products of the 3-(bromooxypropyl)thymidine adduct. This indicates that reaction of 2BA with this nucleotide in DNA is a major reaction.

Effect of ethanol on hepatotoxicity of acetaminophen in mice and on reactive metabolite formation by mouse and human liver microsomes

The protective effect of a single dose of ethanol with regard to hepatotoxicity caused by acetaminophen (APAP) can be a consequence of either direct or indirect inhibition of APAP oxidation to its hepatotoxic intermediate (N-acetyl-p-benzoquinoneimine, NAPQI), or augmentation of repair mechanisms following the hepatotoxic insult. The mechanism of hepatoprotection appears to be species dependent. By varying the time of ethanol administration relative to APAP in mice (30 min before to 240 min after APAP), it was shown that ethanol must be administered early relative to APAP for hepatoprotection to be maximized. The role of direct inhibition of cytochrome P450 in the hepatoprotective effect of ethanol was evaluated by comparing the hepatoprotection afforded by ethanol and 4-methylpyrazole (4-MP) in vivo to the inhibition of APAP oxidation to NAPQI caused by each in mouse liver microsomes. At their respective peak in vivo concentrations attained following hepatoprotective doses, both ethanol and 4-MP inhibited the oxidation of APAP in microsomes by 25-30%. This result suggests that direct inhibition of cytochrome P450 by ethanol plays a role in the protection against acetaminophen-induced hepatotoxicity in mice. In human liver microsomes the inhibition of APAP oxidation to the hepatotoxic intermediate by 48 mM ethanol is less than half of the apparent inhibition of APAP oxidation reported in clinical studies in which the maximum ethanol concentration would have been 15-20 mM. Thus, in contrast to the mouse, inhibition of APAP oxidation to NAPQI in humans appears to be largely indirect, as has been reported previously in the rat.

The use of mass spectrometry in the study of chemically-reactive drug metabolites. Application of MS/MS and LC/MS to the analysis of glutathione- and related S-linked conjugates of N-methylformamide

The S-(N-methylcarbamoyl) derivatives of glutathione, cysteine and N-acetylcysteine, the S-linked conjugates derived from a reactive metabolite of N-methylformamide (NMF), were studied in mice dosed with an equimolar mixture of NMF and deuterium-labelled NMF. Following preparation of N-benzyloxycarbonyl derivatives in aqueous media, the title conjugates were isolated, purified as their methyl esters and subjected to analysis by fast atom bombardment mass spectrometry (FAB/MS), fast atom bombardment tandem mass spectrometry (FAB/MS/MS) or thermospray liquid chromatography/mass spectrometry (TSP LC/MS). Characteristic isotope clusters in the FAB or TSP mass spectra facilitated recognition of drug metabolites, while constant neutral loss (89 u) and daughter ion scanning tandem mass spectrometry (MS/MS) experiments provided unique structural information on the conjugates of interest. It is concluded that the combined use of stable isotopes, aqueous-phase derivatization and contemporary mass spectrometric techniques represents a powerful approach for the analysis of glutathione adducts and related S-linked conjugates of chemically-reactive drug metabolites.

Applications of tandem mass spectrometry to the characterization of derivatized glutathione conjugates. Studies with S-(N-methylcarbamoyl)glutathione, a metabolite of the antineoplastic agent N-methylformamide

Daughter ion spectra are reported for [M + H]+ ions generated by fast atom bombardment mass spectrometry of S-(N-methylcarbamoyl)glutathione (1) and a series of alkoxycarbonyl methyl ester derivatives thereof. Structurally informative, even-electron fragment ions, which serve to define the nature of both the xenobiotic and peptide components of the conjugate, are observed in the collisionally activated dissociation (CAD) spectra of 1 and its ethoxy- and benzyloxycarbonyl methyl esters. Studies with the t-butyloxycarbonyl (tBOC) methyl ester derivative, on the other hand, indicated that the tBOC group exerts a powerful directing influence on the CAD process, and that the major daughter ions in this case are associated with cleavage of the tBOC functionality itself and are of little diagnostic value. Of the derivatives examined, the benzyloxycarbonyl congener, which may be generated readily from 1 in aqueous media, is judged to be the most useful from the standpoints of ease of formation, desirable high-performance liquid chromatographic properties, and informative mass spectral fragmentation characteristics under CAD conditions.

Detection and mechanism of formation of the potent direct-acting mutagen 2-bromoacrolein from 1,2-dibromo-3-chloropropane

The nematocide 1,2-dibromo-3-chloropropane (DBCP) was converted to products which are mutagenic for Salmonella typhimurium TA 100 in the presence of rat liver microsomes, NADPH, and oxygen. Typical in vivo and in vitro inhibitors of cytochrome P-450 decreased DBCP mutagenicity in the presence of microsomes. Addition of glutathione to cytosolic preparations failed to bioactivate DBCP to mutagenic metabolites. Mutagenicity studies with selectively deuterated analogs showed that substitution of deuterium for hydrogen at C-1 or C-3 of DBCP modestly decreased mutagenicity, but that deuteration at both C-1 and C-3 markedly decreased mutagenicity. The formation rates of the potent direct-acting mutagen, 2-bromoacrolein (2-BA), in incubations of DBCP and its deuterated analogs with rat liver microsomes, correlated with the isotope effects on mutagenicity. Characterization of 2-BA was accomplished by gas chromatography-mass spectrometry using positive-ion chemical ionization. Mass spectral analysis of 2-BA formed from specifically deuterated analogs of DBCP indicated that initial oxidative dehalogenation at C-1 followed by a spontaneous beta-elimination reaction was the preferred pathway in the formation of 2-BA from DBCP. These results demonstrate that mutagenic metabolites of DBCP are formed by cytochrome P-450-mediated oxidative metabolism, and that 2-BA is a major mutagen formed.

Metabolism of N-methylformamide in mice: primary kinetic deuterium isotope effect and identification of S-(N-methylcarbamoyl)glutathione as a metabolite

S-(N-Methylcarbamoyl)glutathione has been identified by cesium ion liquid secondary ion mass spectrometry as a biliary metabolite in mice of the experimental antitumor agent and hepatotoxin N-methylformamide. Metabolism of N-methylformamide to urinary methylamine, urinary N-acetyl-S-(N-methylcarbamoyl)-cysteine and biliary S-(N-methylcarbamoyl)glutathione was found to be subject to large intermolecular primary kinetic isotope effects when hydrogen was replaced by deuterium in the formyl group (kH/kD = 5.5 +/- 0.2, 4.5 +/- 1.0 and 7 +/- 2, respectively), as shown by mass spectrometry of derivatives of these metabolites. These values indicate the existence of a common metabolic precursor for each of these metabolites. In particular, methylamine is shown not to arise from simple enzymatic hydrolysis of N-methylformamide but is associated with an oxidative process. Therefore, it is highly likely that N-methylformamide is oxidized and conjugated to form S-(N-methylcarbamoyl)glutathione which is metabolized further to N-acetyl-S-(N-methylcarbamoyl) cysteine. Either of these thiocarbamates could be hydrolyzed to give the parent thiol and the observed metabolic end products, methylamine and carbon dioxide. The presence of deuterium in the formyl moiety of N-methylformamide reduced markedly the hepatotoxicity of the compound, as shown by measurements of the activities of appropriate hepatic enzymes in plasma.

Hepatotoxicity of N-methylformamide in mice--II. Covalent binding of metabolites of [14C]-labelled N-methylformamide to hepatic proteins

Incubation of the hepatotoxin N-methylformamide (NMF) labelled either in the methyl group (OHCNH14CH3) or the formyl group (OH14CNHCH3) with mouse hepatic microsomes in the presence of NADPH, but not in its absence, led to covalent binding of metabolites to microsomal proteins. When [14C]NMF was injected into BALB/c mice radioactivity was found to be associated with liver and, to a much lesser extent, with kidney proteins. Association of radioactivity derived from OHCNH14CH3 with hepatic proteins was higher in BALB/c mice than in CBA/CA mice and in these it was higher than in BDF1 mice. Association of label derived from either isotopomer was significantly reduced but not abolished by pretreatment of mice with cycloheximide suggesting both covalent binding and metabolic incorporation of NMF metabolites. Depletion of hepatic glutathione by pretreatment of mice with buthionine sulfoximine or diethyl maleate prior to administration of OH14CNHCH3 enhanced the association of label with hepatic proteins measured 1 hr after drug injection. Covalent binding of [14C]NMF to hepatic microsomes in vitro was abolished in the presence of glutathione. It is argued that the generation of the toxic lesion and the association of NMF metabolites with hepatic proteins may be causally related even though certain mechanistic and enzymatic details of this link remain obscure.

Hepatotoxicity of N-methylformamide in mice--I. Relationship to glutathione status

In order to investigate the link between hepatotoxicity caused by N-methylformamide (NMF) and its ability to deplete hepatic glutathione experiments were conducted in three strains of mouse which differ in their susceptibility towards NMF-induced liver damage. NMF toxicity was measured by changes in plasma levels of sorbitol dehydrogenase and alanine and aspartate transaminases. In BALB/c mice, the most susceptible strain, a hepatotoxic dose of NMF (200 mg/kg) caused a depletion of hepatic glutathione to 21% of control levels 2 hr after drug administration. In CBA/CA and BDF1 mice the same dose of NMF depleted glutathione to 53% of control levels and did not cause hepatotoxicity. In BALB/c mice depletion of hepatic glutathione by pretreatment with buthionine sulfoximine decreased the hepatotoxic dose threshold of NMF from 150 mg/kg to 100 mg/kg. Conversely, pretreatment of mice with cysteine or N-acetylcysteine protected against both glutathione depletion and NMF-induced hepatotoxicity. The results are in accordance with the suggestion that the hepatotoxicity of NMF is associated with its metabolism to an intermediate which reacts with glutathione.