Our knowledge of the biological effects of foreign compounds has been developed from many different kinds of studies. In vitro studies are usually designed to evaluate the plausibility of hypothetical events by which the foreign compound may cause a response. Because in vitro studies are designed to evaluate individual mechanisms and events, however, no single in vitro study can provide all the information needed to understand fully the mechanisms of all responses caused by a drug and the in vivo factors that may modulate them. For example, one ste of studies in vitro may provide an understanding of the enzymes that metabolize the biologically active foreign compound in various organs and ideally provide estimates of the intrinsic clearances of the enzymes in the organs. Another set of in vitro studies may provide information about the kinetics of the action of various biologically active substances, including perhaps metabolites of the parent foreign compound, on potential receptor sites in various organs. Neither set of studies by themselves, however, provides assurance that the concentrations required to evoke the response in vitro are actually attainable in vivo. Pharmacokinetic studies in vitro thus provide the necessary link between the two types of in vitro experiment. This paper discusses the two pharmacokinetic approaches currently being used, and illustrates the strength and weakness of each. It also discusses the ways in which in vitro studies clarify some of the uncertainties that may arise in the application of the pharmacokinetic approaches. Further, it suggests some possible mechanisms of action of drugs that may result in delayed responses.

Enzymes↗

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pH-dependent one- and two-electron oxidation of 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine catalyzed by horseradish peroxidase.

The porphyrinogenic agent 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP) is known to inactivate hepatic cytochrome P450 (P450) enzymes 2C11, 2C6, and 3A1 [Correia et al. (1987) Arch. Biochem. Biophys. 258, 436-451] by different mechanisms. The inactivation of P450 2C11 and 2C6 appears to be due to the ethylation of the heme in the active sites of the enzymes [Augusto et al. (1982) J. Biol. Chem. 257, 11288-11295], whereas the inactivation of P450 3A1 appears to involve the covalent binding of the heme to the apoprotein [Correia et al. (1987)]. Moreover, we have found that DDEP inactivates horseradish peroxidase (HRP) pretreated with hydrogen peroxide. In this system, DDEP was oxidized predominately to 3,5-dicarbethoxy-2,6-dimethyl-4-ethylpyridine (EDP) under weakly acidic conditions and predominately to 3,5-dicarbethoxy-2,6-dimethylpyridine (DP) under basic conditions. The loss of heme and the formation of altered heme products were also pH-dependent and were correlated with the formation of DP and the inactivation of HRP. Thus the inactivation of HRP appears to depend on the formation of an ethyl radical, which presumably reacts with the heme in the active site of the enzyme. Similar product ratios were obtained for the oxidation of DDEP by K3Fe(CN)6, indicating that product ratios of DP over EDP are mainly determined by the pH of buffer. These results, in addition to semiemperical calculations (AM1) for the oxidation of DDEP in the gas phase, are consistent with the idea that the inhibitor undergoes a single-electron oxidation to form the DDEP radical cation, the fate of which depends on the environment of the active site of the enzyme. The proposed formation of a radical cation by the abstraction of an electron from nitrogen is consistent with the finding of low intramolecular isotope effects of the metabolism of 3,5-dicarbethoxy-2,6-dimethyl-[4-2H,4-1H]-1,4-dihydropyridine by P450 2C11 and 3A4. Under basic or aprotic conditions, the radical dissociates to form DP and the ethyl radical, which reacts with the heme, thereby inactivating the enzyme. Under acidic or polar conditions, the radical undergoes an additional one-electron oxidation to form EDP.(ABSTRACT TRUNCATED AT 400 WORDS)

Catalysis↗

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Inactivation of horseradish peroxidase by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine.

In the presence of H2O2, horseradish peroxidase (HRP) catalyzes the one-electron oxidation of a porphyrinogenic agent, 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP), leading to formation of an ethyl radical, inactivation of the enzyme, and formation of an altered heme product. The loss of heme during the inactivation of HRP was dependent on the duration of exposure to DDEP as well as the concentration of H2O2 and DDEP. The pseudo first order rate constant for the oxidation of DDEP by compound I of HRP at pH 7.4 was 0.07 min-1, and the maximal extent of heme loss was 35%. The altered heme product, which was isolated by reverse phase HPLC, was characterized by the use of mass and 1H NMR spectrometry as a substitution product of a C2H4OH moiety for a meso proton of the prosthetic heme [meso-(hydroxyethyl)heme]. The source of the oxygen in the C2H4OH moiety appeared not to be H2O2 or H2O as 18O was not incorporated in the heme adduct when H2(18)O2 or H2(18)O was used. The DDEP-mediated reaction did not form the expected delta-meso-ethylheme adduct analogous to the ethyl radical-mediated inactivation of HRP by ethylhydrazine (EH) [Ator et al. (1987) J. Biol. Chem. 262, 14954-14960]. However, we have found that meso-(hydroxyethyl)heme was formed in the EH-mediated reaction, albeit in apparently lower amounts than delta-meso-ethylheme. Perhaps the proximity of the heme to the ethyl radical may play a role in determining the nature of the heme products formed.

Chromatography, High Pressure Liquid↗

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Determination of toxic pathways of metabolism by deuterium substitution.

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Animals↗

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12 alpha-hydroxytestosterone. A hitherto unidentified testosterone metabolite produced by cytochrome P-450 2A2.

Several cytochrome P-450 enzymes are able to hydroxylate testosterone to many different metabolites, some of which remain to be identified. One hitherto unidentified metabolite represents a major metabolite of P-450 2A2, present in the liver of adult male rats, and a minor metabolite of several other cytochrome P-450 enzymes. Using the techniques of HPLC and GC/MS, we have obtained unequivocal evidence that this metabolite is 12 alpha-hydroxytestosterone.

Animals↗

PubMedDate not supplied

Isotope effect studies on the mechanism of the cytochrome P-450IIA1-catalyzed formation of delta 6-testosterone from testosterone.

Testosterone metabolism by cytochrome P-450IIA1 results in four metabolites: 6 alpha-hydroxytestosterone; 7 alpha-hydroxytestosterone; 17 beta-hydroxy-4,6-androstadiene-3-one (delta 6-T); and 17 beta-hydroxy-4,6-androstadiene-3-one-6,7-oxide. The epoxide is formed upon further oxidation of delta 6-T, and its formation is in competition with the dissociation of delta 6-T from the active site. The analysis of the KM and Vmax values, as well as the product ratios for testosterone and three selectively deuterated analogs, strongly suggest that delta 6-testosterone formation occurs primarily by initial hydrogen atom abstraction at the 6 alpha-position followed by abstraction of the 7 alpha-hydrogen atom.

Animals↗

PubMedDate not supplied

Isozymes of cytochrome P-450 that metabolize naphthalene in liver and lung of untreated mice.

Four isozymes, designated P-450m50a, P-450m50b, P-450m51a, and P-450m51b, have been isolated from liver microsomes of untreated male CD-1 mice. Electrophoretograms revealed that the molecular weights of P-450m51a and P-450m51b were about 51,000, whereas those of P-450m50a and P-450m50b were about 49,700 and 49,500, respectively. The present study is concerned mainly with the elucidation of the characteristics of P-450m50b and P-450m51a, since they appear to be major forms of cytochrome P-450 in mouse liver. End group analyses have revealed that these forms differ not only from P1-450, isolated previously from mouse liver, but also from any other known isozymes of cytochrome P-450. Naphthalene was metabolized by both P-450m50b and P-450m51a, but the turnover number of P-450m50b was 2-4 times greater than that of P-450m51a. Moreover, P-450m50b formed predominantly (1R,2S)-naphthalene 1,2-oxide, as determined by the formation of trans-1(R)-hydroxy-2(R)-glutathionyl-1,2-dihydronaphthalene, whereas P-450m51a formed both (1R,2S)- and (1S,2R)-naphthalene 1,2-oxide in about equal amounts. Although cytochrome P-450c, isolated from rat liver, is also known to convert naphthalene predominantly to (1R,2S)-naphthalene 1,-2-oxide, an antibody prepared against cytochrome P-450c failed to inhibit the formation of the glutathione conjugate when the 1R,2S-oxide was formed by either pure P-450m50b or by microsomes from mouse lung. Thus, P-450m50b probably is not orthologous to cytochrome P-450c.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMedDate not supplied

Glutathione conjugates of 2-bromohydroquinone are nephrotoxic.

Incubation of either o-bromophenol or 2-bromohydroquinone with rat liver microsomes and 0.25 mM 35S-glutathione (GSH) gave rise to several isomeric 35S-GSH conjugates. A mixture of these isomeric GSH conjugates was prepared chemically and two were purified by HPLC; 1H-NMR spectroscopy revealed that one was 2-bromo-3-(glutathion-S-yl)hydroquinone and the other was a disubstituted GSH conjugate which could be either 2-bromo-3,5-(diglutathion-S-yl)hydroquinone or 2-bromo-3,6-(diglutathion-S-yl)hydroquinone. Injection of the disubstituted GSH conjugate intravenously to rats caused substantial elevations in blood urea nitrogen levels. Treatment of rats with AT-125 (Acivicin; NSC 163501; 10 mg/kg ip) caused a substantial inhibition of kidney gamma-glutamyl transpeptidase activity and decreased 2-bromohydroquinone-mediated elevations in blood urea nitrogen. These findings are consistent with the view that the kidney necrosis observed after administration of either bromobenzene (1), o-bromophenol (2), or 2-bromohydroquinone (3) might be due in part to 2-bromohydroquinone GSH conjugates formed in the liver and subsequently transported to the kidney and converted to ultimate nephrotoxic metabolite(s).

Animals↗

PubMedDate not supplied

An unusual metabolite of testosterone. 17 beta-Hydroxy-4,6-androstadiene-3-one.

A testosterone metabolite, 17 beta-hydroxy-4,6-androstadiene-3-one, possessing an absorbance maximum at 284 nm, was formed during incubation of testosterone with liver microsomes from dexamethasone-treated rats. The metabolite was identified by HPLC, UV spectroscopy, and thermospray liquid chromatography/mass spectrometry. The formation of this metabolite by rat liver microsomes required NADPH and oxygen and was inhibited markedly by SKF 525-A, 2,4-dichloro-6-phenylphenoxyethylamine, or CO/O2 (8:2, v/v), but not by cyanide, an inhibitor for stearyl-CoA desaturase. Pretreatment of rats with phenobarbital, pregnenolone 16 alpha-carbonitrile, and dexamethasone enhanced the formation of this metabolite in parallel with the increase in formation of 6 beta-hydroxytestosterone (r2 = 0.99). Although 16-methylprogesterone, a known 6 beta-hydroxylase inhibitor, competitively inhibited the formation of the metabolite and 6 beta-hydroxytestosterone by liver microsomes from dexamethasone-treated rats, the metabolite was not formed from either 6 beta-hydroxytestosterone or 7-hydroxytestosterone during incubation with liver microsomes. These findings are consistent with the view that cytochrome P-450 isozymes that catalyze 6 beta-hydroxylation of steroids in rat liver microsomes also catalyze the dehydrogenation of testosterone to form a double bond between the C-6 and C-7 positions.

Animals↗