Assessment of the absorption, metabolism and excretion of [¹4C]pasireotide in healthy volunteers using accelerator mass spectrometry.

PURPOSE: Pasireotide (SOM230) is a multireceptor-targeted somatostatin analog designed to have a broader somatostatin receptor binding profile than other currently available somatostatin analogs. The purpose of this study was to evaluate the absorption, metabolism and excretion of pasireotide in healthy male subjects (N = 4) following a single, subcutaneous (sc), 600 µg dose of [¹4C]pasireotide. METHODS: Blood, plasma, urine and feces were collected for 240 h post-dose and analyzed for total ¹4C and metabolite profile by accelerator mass spectrometry (AMS) or high-performance liquid chromatography-AMS. Parent drug levels were analyzed by radioimmunoassay. RESULTS: [¹4C]pasireotide was rapidly absorbed, with a mean peak plasma ¹4C concentration of 16.6 ± 5.28 ngEq/mL at 0.5 h in plasma. The parent drug to total ¹4C AUC(0-24h) ratio was 1.08, indicating that little metabolite was present in plasma up to 24 h post-dose. In pooled plasma samples (0-12 h), only unchanged [¹4C]pasireotide was detected. Unchanged [¹4C]pasireotide accounted for approximately 84 % of total excretion (feces and urine). Approximately 56 % of the administered radioactive dose was recovered within 240 h, eliminated primarily in feces (48.3 ± 8.16 %) and minimally in urine (7.63 ± 2.03 %). No serious adverse events were reported. CONCLUSIONS: A single dose of [¹4C]pasireotide 600 µg sc administered to healthy male subjects was rapidly absorbed and excreted in its unchanged form primarily via the hepatic route.

Disposition and metabolism of [¹4C]PTZ601 in healthy volunteers.

1. Six healthy male subjects were given a single dose of 500 mg of [14C]PTZ601 (mean radioactivity 79.2 µCi) by intravenous (IV) infusion over 1 h, and observed for 5 days post-dose during which pharmacokinetic (PK) samples were collected. Plasma PTZ601 concentrations and metabolite identification were determined using LC-MS/MS; PK parameters were estimated by non-compartmental analysis. Excretion and mass balance were determined with liquid scintillation analysis and metabolites profiling was characterized by HPLC online radiochemical detection. 2. The disposition of PTZ601 was best described by a fast absorption, followed by a biphasic elimination phase. Peak PTZ601 plasma concentrations were reached within 0.5-1 h. The mean elimination half-life was 1.6 h and clearance was 13 L/h. 3. Recovery of the radioactivity dose was complete (mean 92%). The main route of excretion (parent and metabolites) was the renal route, as urine accounted for 69-77%, while feces only 13-22%, of the total radioactivity. 4. The majority of the drug was excreted in urine as multiple open ring metabolites: M17.3 (oxidative ring-opened product) and M22.2 (di-cysteine conjugate of 17.3); unchanged PTZ601 in urine contributed to 15% of radioactivity. The major metabolites detected in plasma were M17.3, M12.8 (acetylated M17.3), M22.2, and M41.4 (methylated M17.3). 5. PTZ601 was well tolerated.

Pharmacokinetics and metabolism of nateglinide in humans.

The pharmacokinetics and metabolism of nateglinide were studied in six healthy male subjects receiving a single oral (120 mg) and intravenous (60 mg) dose of [14C]nateglinide in randomized order. Serial blood and complete urine and feces were collected for 120 h post dose. Nateglinide was rapidly (approximately 90%) absorbed, with peak blood and plasma concentrations at approximately 1 h post dose. The maximal plasma concentrations of radioactivity (6360 ngEq/ml) and nateglinide (5690 ng/ml) were comparable, and plasma radioactivity concentrations were about twice those of blood at all times. Oral bioavailability was 72%, indicating only a modest first-pass effect. After either dose, plasma nateglinide concentrations declined rapidly with elimination half-lives of 1.5 to 1.7 h and plasma clearance of 7.4 l/h. Plasma radioactivity was eliminated more slowly with half-lives of 52 and 35 h in plasma and blood, respectively, after the oral dose. The contribution of this more slowly eliminated component to the AUC(0-infinity) was minor. Nateglinide was extensively metabolized, with excretion predominantly (84-87%) in urine. Only approximately 16% of the dose was excreted unchanged in urine after either dosing route. The major metabolites were the result of oxidative modifications of the isopropyl group. Three of these were monohydroxylated, two of which appeared to be diastereoisomers. Additionally, one metabolite with an unsaturation in the isopropyl group and two diol-containing isomers were identified. Glucuronic acid conjugates resulting from direct glucuronidation of the carboxylic acid were also present. The major metabolite in plasma and urine was the result of hydroxylation of the methine carbon of the isopropyl group.

Metabolism of cyclosporin G in the mouse, rat, and dog.

Cyclosporin G (CsG; Sandoz compound OG 37-325) is a cyclic undecapeptide with potent, immunosuppressive activity and is currently in clinical testing for prevention of transplanted solid organ rejection. Although structurally similar to cyclosporin A (CsA), results in animals suggest that CsG has a reduced potential for nephrotoxicity when compared with CsA, while retaining equivalent therapeutic efficacy. In the present study, the major metabolic pathways of CsG in the mouse, rat, and dog were investigated using radiolabeled drug substance to determine if interspecies differences in metabolism exist. The results indicated that the major metabolic pathways in these animal species are similar to those previously reported for CsA, including oxidative modifications at amino acids 1, 4, and 9, and concomitant cyclization of amino acid 1 in two of these metabolites. Moreover, the seven major CsG metabolites (designated GM19, GM1c9, GM4N9, GM1, GM9, GM1c, and GM4N) observed in animal excreta and/or blood were identical to those identified in humans. The major circulating metabolite in blood was GM9 (9-hydroxylated CsG) in all species. In addition, numerous unidentified minor metabolites were observed. Renal excretion was a minor elimination pathway, with the majority of drug-related material excreted via the fecal route. In conclusion, CsG was found to proceed through the same metabolic pathways in three animal species and humans, and that species differences in metabolism were primarily because of differences in the relative importance of the pathways observed.

Pharmacokinetics and metabolism of cyclosporin G in humans.

The pharmacokinetics and metabolism of cyclosporin G (CsG; Sandoz compound OG 37-325) were studied in 12 healthy male volunteers receiving a single oral dose of 150 or 600 mg of [14C]CsG. Serial blood and plasma samples and complete urine and feces were collected for 120-hr postdose. CsG was rapidly absorbed, and the extent of absorption was dose-independent. Maximum blood concentrations of CsG at 2- to 3-hr postdose averaged 342 and 1170 ng/ml after doses of 150 and 600 mg, respectively, each accounting for approximately 50% of the blood radioactivity level. The plasma:blood concentration ratio for both CsG and total radioactivity averaged approximately 0.8. Overall disposition of absorbed CsG was independent of the dose. The drug was extensively metabolized with excretion predominantly via the fecal route. Total recovery in urine was only approximately 3% of the dose. In blood, the terminal half-life of CsG and total radioactivity averaged 9-11 hr following both the 150 and 600 mg doses. In plasma, the half-life of CsG was 2-4 hr and that of total radioactivity was 27-29 hr. The major metabolic pathways resulted from oxidative modifications at amino acids 1, 4, and 9, with concomitant cyclization of amino acid 1 in two metabolites. These pathways resulted in formation of seven major metabolites (designated GM19, GM1c9, GM4N9, GM1, GM9, GM1c, and GM4N) observed in human excreta and/or blood. Major metabolites of CsG in blood involved monohydroxylation (GM1 and GM9) or demethylation (GM4N). In blood, monohydroxylated CsG metabolites (GM1 and GM9) achieved roughly equal levels, with a trend toward higher GM9 concentrations at peak radioactivity.

Effects of deuterium labeling on azido amino acid mutagenicity in Salmonella typhimurium.

The mutagenic effects of azide (N3-) anion in bacterial test systems require the formation of the novel mutagenic metabolite, 3-azido-L-alanine (AZAL). Although the mechanism of AZAL-induced mutagenicity is unknown, subsequent bioactivation of this metabolite appears likely. Earlier studies have shown that other azide-containing amino acids are mutagenic as well. In fact, the mutagenic potency of the synthetic AZAL homologue, L-2-amino-4-azidobutanoic acid (HomoAZAL), was several times that of AZAL. To gain insight into the biochemical processing and mutagenicity of azido amino acids in Salmonella typhimurium, several specifically deuterium-labeled azido amino acids have been prepared and tested for mutagenic potency. In addition, the effect of (aminooxy)acetic acid (AOA) (a potent inhibitor of pyridoxal-dependent processes) on AZAL-induced mutagenesis was examined. The results showed that 2-deuterium substitution of AZAL resulted in a slight increase in mutagenic potency, while AOA treatment resulted in no change in AZAL potency. Taken together these findings did not support the involvement of pyridoxal-dependent processes in AZAL bioactivation. In contrast, deuterium substitution adjacent to the azide group in HomoAZAL and 5-azido-L-norvaline (N3-Norval) resulted in a large decrease in mutagenic potency when compared to the non-deuterium labeled compounds. These observations are consistent with a bioactivation mechanism involving rate-limiting C-H bond cleavage in the formation of the ultimate mutagen. Moreover, this effect of deuterium labeling points to processing of the azide-containing side chain as a key feature in the mutagenic activation mechanism.

Sulfation of di- and tricyclic phenols by rat liver aryl sulfotransferase isozymes.

Aryl sulfotransferases (ASTs) catalyze the sulfation of a variety of hydroxyl-containing substrates, including phenols, aryl oximes, benzylic alcohols, and arylhydroxamic acids. Sulfation of the latter class of substrates (e.g., N-hydroxy-2-acetamidofluorene) can yield highly unstable sulfuric acid esters capable of covalently binding to cellular nucleophiles. Accordingly, these enzymes have been implicated in the bioactivation of the arylhydroxamic acid (and precursor arylamine) class of hepatocarcinogens. Rat liver contains three well-characterized isoforms of AST. To understand better the factors which influence isozymic substrate specificity, the present study focused on steric and regiochemical factors with the sulfation of polyaromatic phenols as a model system. Seven di- and tricyclic phenols were tested as substrates for ASTs I, II, and IV. Based on a comparison of kinetic constants and assuming an absence of substrate-specific pH effects, the results suggest that regiochemical and steric factors play an important role in substrate specificity and provide insight into isozymic differences in active-site topology. For both AST I and AST II, the kinetic results were consistent with an active-site model in which the hydrophobic substrate binding pocket is wider, but less elongated, than that for AST IV. In addition, kinetic results for AST II with 4-phenylphenol were indicative of negative cooperativity which was unique to this isozyme. In contrast to those for ASTs I and II, the kinetic results for AST IV suggest an active-site model that is linearly extended. This elongated active-site model accommodates lengthy substrates and appears to derive little catalytic benefit from additional aromatic rings which increase substrate width.

Aryl sulfotransferase-IV-catalyzed sulfation of aryl oximes: steric and substituent effects.

The aryl sulfotransferases (EC 2.8.2.1) catalyze the sulfation of a wide variety of hydroxyl-containing molecules. The enzyme reaction requires 3'-phosphoadenosine-5'-phosphosulfate as the sulfate donor and several isozymes with broad, overlapping substrate specificities have been identified. One of the isozymes in rat hepatic cytosol, isozyme IV, is a major contributor to enzymatic sulfation. It exhibits the broadest substrate specificity of the three isozymes which have been characterized to date. Its substrates include a wide variety of phenols, certain aromatic hydroxylamines and benzylic alcohols. The latter two substrate types have implicated this isozyme in the bioactivation of several toxic compounds. Relatively little information is available, however, on substrate molecular features which account for the ability of isozyme IV to sulfate compounds not utilized by isozymes I and II. A recent investigation of isozymes I and II with a series of model aryl-oxime substrates suggested that catalysis is influenced primarily by steric factors and in particular substrate planarity and hydroxyl group orientation (Mangold et al. (1989) Biochim. Biophys. Acta 991, 453-458). In the present study, isozyme IV was investigated to characterize its substrate requirements with a more extensive series of aryl oxime substrates. The results indicated that isozyme IV has a much less stringent requirement for planarity and hydroxyl-group orientation than isozymes I or II. Isozyme IV accepted a greater variety of aryl-oxime substrates, including several classes which were not substrates for isozymes I and II. A comparison of kinetic constants and catalytic efficiencies suggested that substituent effects play a role in the sulfation of aryl oximes by isozyme IV.

Self-catalyzed irreversible inactivation of rat hepatic aryl sulfotransferase IV by N-hydroxy-2-acetylaminofluorene.

Rat hepatic aryl sulfotransferase IV catalyzes the sulfonation of the hepatocarcinogen, N-hydroxy-2-acetylaminofluorene. The resulting reactive N-O-sulfate ester is believed to be the ultimate carcinogenic species responsible for the induction of hepatic neoplasia. Previous studies have shown that dietary administration of either 2-acetylaminofluorene or N-hydroxy-2-acetylaminofluorene to rats is accompanied by a rapid decline in hepatic aryl sulfotransferase activity in vivo. In the present study, preincubation of purified rat hepatic aryl sulfotransferase IV with N-hydroxy-2-acetylaminofluorene resulted in rapid, time-dependent enzyme inactivation. This in vitro inactivation was not reversed by dialysis or gel filtration. Inclusion of excess nucleophile, methionine, resulted in considerable but not complete protection from inactivation. The inactivation was PAPS dependent and blocked by the sulfotransferase inhibitor, pentachlorophenol. The above observations and the apparent pseudo first-order kinetics observed suggest that the inactivation was in part mechanism based. Mechanism-based inactivation of the aryl sulfotransferases has not been previously reported. Furthermore, the results of the present study indicate that the previously reported in vivo decline in rat hepatic aryl sulfotransferase activity may be attributable in part to enzyme inactivation by its own reactive product.

Selective protein arylation by acetaminophen and 2,6-dimethylacetaminophen in cultured hepatocytes from phenobarbital-induced and uninduced mice. Relationship to cytotoxicity.

To evaluate the mechanistic importance of covalent binding in acetaminophen (APAP)-induced hepatotoxicity, we compared the effects of 2,6-dimethylacetaminophen (2,6-DMA) to those of APAP in primary cultures of mouse hepatocytes. Immunochemical analysis of electrophoretically separated proteins has shown that the majority of covalent binding after a cytotoxic dose of APAP occurs on two major bands of 44 and 58 kD (Bartolone et al., Biochem Pharmacol 36: 1193-1196, 1987). At equimolar concentrations, 2,6-DMA bound proteins only 15% as extensively as did APAP and was not cytotoxic in hepatocytes from uninduced mice. However, when the hepatocytes were obtained from phenobarbital-induced mice, APAP administration resulted in increased protein arylation and a more rapid onset of cytotoxicity. Furthermore, in the cells from phenobarbital-induced mice, 2,6-DMA not only resulted in increased binding but also in overt cytotoxicity. Since our affinity-purified anti-APAP antibody did not cross-react with 2,6-DMA, a new antibody specific for 2,6-DMA was prepared and, after affinity purification, was used to detect 2,6-DMA protein adducts by Western blotting. Results indicated that, in hepatocytes from both phenobarbital-induced and non-induced mice, the binding of 2,6-DMA was also highly selective with the most prominent target being the 58 kD cytosolic protein. However, by contrast to APAP, only minimal binding to the 44 kD protein was detected after 2,6-DMA treatment. Although several additional protein adducts were increased in treated cells from phenobarbital-induced mice, the 58 kD protein was clearly the most prominently arylated target associated with both APAP and 2,6-DMA cytotoxicity. These data suggest that both the specificity of covalent binding as well as the extent of binding to the major targets may play an important role in the ensuing toxicity.

Sulfation of mono- and diaryl oximes by aryl sulfotransferase isozymes.

Aryl sulfotransferases (3'-phosphoadenylsulfate:phenol sulfotransferase, EC 2.8.2.1) catalyze the sulfonation of a wide variety of hydroxyl-containing substrates, including numerous xenobiotics. The chemical diversity of aryl sulfotransferase substrates is in part attributable to the presence of multiple isozymes, each of which has broad substrate specificity. Of the aryl sulfotransferase isozymes in rat liver cytosol, two (designated isozymes I and II) have previously been shown to sulfonate phenolic compounds exclusively and, moreover, have very similar substrate specificity patterns. The recently reported unusually efficient, rapid isozyme I-catalyzed sulfonation of 9-fluorenone oxime (Mangold, J.B., Mangold, B.L.K. and Spina, A. (1986) Biochim. Biophys. Acta 874, 37-43) was therefore unexpected and suggested that aryl oximes may represent a useful class of model compounds to probe isozymic differences in substrate steric and electronic requirements. In the present study, several mono- and diaryl oximes have been prepared and tested as potential substrates for partially purified aryl sulfotransferases I and II from rat liver cytosol. The results indicate that steric factors, specifically planarity and hydroxyl group position, appear to be important requirements for enzyme-catalyzed sulfonation. In addition, although isozymes I and II had comparable activity with diaryl oximes, some striking differences in the ability of these two isozymes to sulfonate both substituted and unsubstituted monoaryl oximes were observed. This dissimilarity is consistent with distinct differences in the active sites of these isozymes.

Azidoalanine mutagenicity in Salmonella: effect of homologation and alpha-methyl substitution.

Azide mutagenicity in susceptible non-mammalian systems involves the requisite formation of L-azidoalanine, a novel mutagenic amino acid. The biochemical mechanism(s) of azidoalanine-induced mutagenesis, however, is not known. Previous studies of the structural requirements for azidoalanine mutagenicity suggested the importance of free L-amino acid character, and that bioactivation of azidoalanine to the ultimate mutagenic species is required. To gain more insight into possible enzymatic processing, the alpha-methyl analogue, alpha-methyl-azidoalanine, and the homologue, 2-amino-4-azidobutanoic acid, were synthesized and tested for mutagenic potency in Salmonella typhimurium strain TA1530. In addition, azidoacetic acid, a possible azidoalanine metabolite, was prepared and tested. The results show that alpha-methyl substitution effectively blocks the mutagenic effects of azidoalanine with alpha-methyl-azidoalanine being nearly devoid of mutagenic activity. In contrast, homologation of azidoalanine to yield 2-amino-4-azidobutanoic acid produces a marked increase in molar mutagenic potency. As with azidoalanine, the mutagenic activity of this homologue is associated with the L-isomer. Azidoacetic acid, however, was only very weakly mutagenic when tested as either the free acid or ethyl ester. This low mutagenic potency may indicate that bioactivation does not involve the entry of azide-containing azidoalanine catabolite into the Kreb's cycle. The high potency of 2-amino-4-azidobutanoic acid may be indicative of more efficient bioactivation and/or greater intrinsic activity. Importantly, the latter finding clearly shows that potent azido-amino acid mutagenicity is not limited to azidoalanine alone.

Dissociation of covalent binding from the oxidative effects of acetaminophen. Studies using dimethylated acetaminophen derivatives.

The cytotoxic effects of 10 mM acetaminophen (APAP) in primary cultures of non-induced mouse hepatocytes are accompanied by depletion of intracellular glutathione (GSH), arylation of protein, and loss of protein sulfhydryl (PSH) groups. Investigation of the stoichiometry of the covalent binding and PSH loss after APAP exposure demonstrated a greater loss in PSH than could be accounted for by covalent binding to proteins and suggests that APAP exhibits both oxidative and arylative actions in cell culture. Subcellular fractionation revealed that the PSH oxidation induced by APAP was greatest in the microsomal fraction. Exposure of the hepatocytes to 10 mM 3,5-dimethyl-acetaminophen (3,5-DMA) or 2,6-dimethyl-acetaminophen (2,6-DMA) permitted dissociation of the oxidative and arylative properties of APAP. Even though treatment of cultured hepatocytes with 3,5-DMA did not result in covalent binding, there was a more rapid depletion of intracellular GSH, oxidation of PSH, and cytotoxicity compared to APAP. This investigation also provides the first evidence that the cytotoxic effects of both APAP and 3,5-DMA are accompanied by the formation of protein aggregates of high molecular weight that are not disulfide linked. The aggregates probably reflect the oxidative properties of these drugs and may be a mediator of their toxic effects. By contrast, 2,6-DMA, which did bind to cellular proteins and deplete GSH, did not lead to PSH loss, protein aggregation, or cytotoxicity. Since PSH oxidation and protein aggregation correlated well with cytotoxicity, these data suggest that the oxidative component of APAP and 3,5-DMA can play a significant role in eliciting cellular damage in cultured hepatocytes.

Structure-activity relationships of the azide metabolite, azidoalanine, in S. typhimurium.

Azide is metabolized to the proximate mutagen, L-azidoalanine in bacterial systems. While this novel mutagenic metabolite plays a key role in azide mutagenesis, the biochemistry of this role is unknown. The chemical synthesis of authentic racemic azidoalanine and several derivatives thereof allowed the exploration of structure-activity relationships with this unique mutagen. We found that whereas azide, azidoalanine and azidoalanine tert.-butyl ester were of comparable mutagenic potency, derivatives which lack the free amino group, such as azidopropionic acid and amino-blocked azidoalanine, were orders of magnitude less active. These findings demonstrate that the free amino group is essential for significant activity, while the carboxyl group may be less important. This conclusion together with the finding that DL-azidoalanine is a less potent mutagen than azide itself, suggests that the metabolite, while necessary for azide mutagenicity, may not be the ultimate mutagenic species. Instead, the data are consistent with the hypothesis that azidoalanine requires further bioactivation.

Rat liver aryl sulfotransferase-catalyzed sulfation and rearrangement of 9-fluorenone oxime.

The role of hepatic cytosolic aryl sulfotransferase (3'-phosphoadenylylsulfate:phenol sulfotransferase, EC 2.8.2.1) in the enzymic rearrangement of 9-fluorenone oxime to phenanthridone was investigated. 9-Fluorenone oxime was found to be an excellent substrate for a partially purified rat liver aryl sulfotransferase preparation. This compound was in fact superior to 2-naphthol, the standard assay substrate. This is the first reported observation of aryl oxime sulfation by the aryl sulfotransferases. 9-Fluorenone oxime sulfation exhibited pronounced substrate inhibition at high substrate concentrations. However, despite virtually complete conversion of 9-fluorenone oxime to the corresponding N-O-sulfate conjugate in enzyme incubation mixtures, only small amounts of rearrangement product were detected after long-term incubations. In addition, 9-fluorenone oxime-O-sulfonic acid was chemically synthesized and tested for stability. The results showed that rearrangement was pH-dependent and occurred slowly over several hours. It is therefore concluded that aryl sulfotransferase-catalyzed sulfation likely plays an important role in the in vitro and in vivo disposition of 9-fluorenone oxime. Moreover, sulfation facilitates the Beckmann-like conversion of 9-fluorenone oxime to phenanthridone. Sulfation alone, however, appears insufficient to account for all of the previously reported in vitro and in vivo rearrangement.

Synthesis and enantioselective mutagenicity of azidoalanine in Salmonella typhimurium.

Azide mutagenicity involves the requisite formation of the putative novel aminoacid metabolite, beta-azidoalanine. The role of this metabolite, however, is unclear. In order to confirm the identity of this metabolite and provide additional information on possible stereochemical requirements for mutagenicity, authentic racemic and L-azidoalanine were synthesized by an unambiguous route and tested for mutagenicity in Salmonella typhimurium TA100, TA1535, hisG46 and Escherichia coli WP2-. A marked antipodal potency ratio was observed in strains TA100 and TA1535 when racemic and L-azidoalanine were compared. The mutagenic activity resided primarily in the L-isomer. The molar potency of L-azidoalanine in TA100 and TA1535 was nearly identical to that of azide. The lack of mutagenic response for racemic or L-azidoalanine in hisG46 and E. coli WP2- was like that reported for azide and is consistent with similar modes of action for these agents.

Mechanism-based inactivation of dopamine beta-monooxygenase by beta-chlorophenethylamine.

Functionalization of the beta-carbon of phenethylamines has been shown to produce a new class of substrate/inhibitor of dopamine beta-monooxygenase. Whereas both beta-hydroxy- and beta- chlorophenethylamine are converted to alpha-aminoacetophenone at comparable rates, only the latter conversion is accompanied by concomitant enzyme inactivation ( Klinman , J. P., and Krueger , M. (1982) Biochemistry 21, 67-75). In the present study, the nature of the reactive intermediates leading to dopamine beta-monooxygenase inactivation by beta- chlorophenethylamine has been investigated employing kinetic deuterium isotope effects and oxygen- 18 labeling as tools. Mechanistically significant findings presented herein include: 1) an analysis of primary deuterium isotope effects on turnover, indicating major differences in rate-determining steps for beta-chloro- and beta- hydroxyphenethylamine hydroxylation, Dkcat = 6.1 and 1.0, respectively; 2) evidence that dehydration of the gem-diol derived from oxygen- 18-labeled beta- hydroxyphenethylamine hydroxylation occurs in a random manner, attributed to dissociation of enzyme-bound gem-diol prior to alpha-aminoacetophenone formation; 3) the observation of a deuterium isotope effect for beta- chlorophenethylamine inactivation, Dkinact = 3.7, implicating C--H bond cleavage in the inactivation process; and 4) the demonstration that alpha-aminoacetophenone can replace ascorbic acid as exogenous reductant in the hydroxylation of tyramine. As discussed, these findings support the intermediacy of enzyme-bound alpha-aminoacetophenone in beta- chlorophenethylamine inactivation, and lead us to propose an intramolecular redox reaction to generate a ketone-derived radical cation as the dopamine beta-monooxygenase-inactivating species.

Stereochemical aspects of conjugation reactions catalyzed by rat liver glutathione S-transferase isozymes.

Substrate enantioselectivity in the conjugation of phenethyl halides catalyzed by the glutathione S-transferases was studied with partially purified isozymes from rat liver. All of the isozymes tested possessed measurable activity with phenethyl chloride. Tranferase A was the most active isozyme tested. Each of the isozymes demonstrated a high degree of substrate enantioselectivity, with transferase A being the most enantioselective isozyme. The enantioselectivity was determined by high-pressure liquid chromatographic analysis of the enzymatically formed diastereomeric products. The effect of limiting glutathione concentrations on the stereochemical outcome of the transferase A catalyzed conjugation of the chiral substrate, (S)-phenethyl chloride (4 mM), was investigated. The stereochemical course of the enzymatic reaction was not significantly altered at glutathione concentrations as low as 25 microM. The major product of conjugation had the opposite stereochemistry at the benzylic carbon, indicating that the reaction proceeded primarily with inversion of configuration. The glutathione conjugates, S-[(R)-1-phenylethyl]glutathione, S-[(S)-1-phenylethyl]glutathione, S-benzylglutathione, and S-methylglutathione were studied as inhibitors of the transferase A catalyzed conjugation of 1-chloro-2,4-dinitrobenzene. The order of the inhibitory potency was S-[(S)-1-phenylethyl]glutathione = S-benzylglutathione greater than S-[(R)-1-phenylethyl]glutathione greater than S-methylglutathione. This represented the first demonstration of the stereoselective product inhibition of the glutathione S-transferases.