Sequestration of biological reactive intermediates by trapping as covalent enzyme-intermediate complex

Identification of New Flavone-8-Acetic Acid Metabolites Using Mouse Microsomes and Comparison with Human Microsomes

Flavone-8-acetic acid (FAA) is a potent anticancer agent in mouse but has not shown activity in humans. Because FAA metabolism could play a role in this interspecies difference, our aim was to identify the metabolites formed in vitro using mouse microsomes compared with those in human microsomes. Mouse microsomes produced six metabolites as detected by reversed-phase high-performance liquid chromatography-mass spectrometry (MS). Three metabolites were identified as the 3'-, 4'-, or 6-hydroxy-FAA, by comparison with retention times and UV and MS spectra of standards. Two metabolites presented a molecular weight of 296 (FAA = 280) indicating the presence of one oxygen but did not correspond to any monohydroxylated FAA derivative. These two metabolites were identified as epoxides because they were sensitive to epoxide hydrolase. The position of the oxygen was determined by the formation of the corresponding phenols under soft acidic conditions: one epoxide yielded the 3'- and 4'-hydroxy-FAA, thus corresponding to the 3',4'-epoxy-FAA, whereas the other epoxide yielded 5- and 6-hydroxy-FAA, thus identifying the 5,6-epoxy-FAA. The last metabolite was assigned to the 3',4'-dihydrodiol-FAA because of its molecular weight (314) and sulfuric acid dehydration that indicated that the 3'- and 4'-positions were involved. Compared with mouse microsomes, human microsomes (2 pools and 15 individual microsomes) were unable to metabolize FAA to a significant extent. In conclusion, we have identified six new FAA metabolites formed by mouse microsomes, whereas human microsomes could not metabolize this flavonoid to a significant extent. The biological importance of the new metabolites identified herein remains to be evaluated.

Inter-species comparison of liver and small intestinal microsomal metabolism of fluoranthene

The magnitude of susceptibility to toxicant exposure may depend on the ability of animals to metabolize the chemicals. The present study has been undertaken to see whether any differences exist among mammals in the metabolism of fluoranthene (FLA), a polycyclic aromatic hydrocarbon (PAH) compound. Microsomes were isolated from the small intestine and liver of rat, mouse, hamster, goat, sheep, pig, dog, cow, monkey, and humans (commercially procured), and incubated with FLA. Post-incubation, samples were extracted with ethyl acetate and analyzed for FLA/metabolites by reverse-phase HPLC with fluorescence detection. The metabolism of FLA in both liver and small intestine was in the order: human > monkey > cow > goat > sheep > dog > pig > hamster > rat > mouse under conditions of the test system used. The rate of metabolism (pmol of metabolite/min/mg protein) was found to be more in liver than in intestine in all the species studied. The FLA metabolites identified were FLA 2,3-diol, trans-2,3-dihydroxy-1,10b-epoxy-1,2,3,10beta tetrahydro FLA (2,3D FLA), 3-hydroxy FLA, and 8-hydroxy FLA. The rodent microsomes produced considerably higher proportion of FLA 2,3-diol, and 2,3D FLA than did pig, dog, and humans. On the other hand, microsomes from higher mammals converted a greater proportion of FLA to 3-hydroxy FLA, the detoxification product of FLA. Overall, our results revealed a great variation among species to metabolize FLA.

Soybean epoxide hydrolase: identification of the catalytic residues and probing of the reaction mechanism with secondary kinetic isotope effects

Soybean epoxide hydrolase catalyzes the oxirane ring opening of 9,10-epoxystearate via a two-step mechanism involving the formation of an alkylenzyme intermediate, which, in contrast to most epoxide hydrolases studied so far, was found to be the rate-limiting step. We have probed residues potentially involved in catalysis by site-directed mutagenesis. Mutation of His(320), a residue predicted from sequence analysis to belong to the catalytic triad of the enzyme, considerably slowed down the second half-reaction. This kinetic manipulation provoked an accumulation of the reaction intermediate, which could be trapped and characterized by electrospray ionization mass spectrometry. As expected, mutation of Asp(126) totally abolished the activity of the enzyme from its crucial function as nucleophile involved in the formation of the alkylenzyme. In line with its role as the partner of His(320) in the "charge relay system," mutation of Asp(285) dramatically reduced the rate of catalysis. However, the mutant D285L still exhibited a very low residual activity, which, by structural analysis and mutagenesis, has been tentatively attributed to Glu(195), another acidic residue of the active site. Our studies have also confirmed the fundamental role of the conserved Tyr(175) and Tyr(255) residues, which are believed to activate the oxirane ring. Finally, we have determined the secondary tritium kinetic isotope effects on the epoxide opening step of 9,10-epoxystearate. The large observed values, i.e. (T)(V/K(m)) approximately 1.30, can be interpreted by the occurrence of a very late transition state in which the epoxide bond is broken before the nucleophilic attack by Asp(126) takes place.