Microsomal metabolism of arylamides by the rat and guinea pig-I. Oxidation of N-3-fluorenylacetamide at carbon-atom-9-A major metabolic reaction

Eli Kaplan, Helmut R. Gutmann, Tim H Emory

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The N-hydroxylation of N-3-fluorenylacetamide (3-FAA), an isomer of the carcinogen, N-2-fluorenylacetamide (2-FAA), by hepatic microsomes of untreated and 3-methylcholanthrene (3-MC)-treated guinea pigs was found to be of a similar low order as that previously observed in the rat. Hepatic microsomes of the guinea pig and of the rat converted 3-FAA to N-(9-hydroxy)-3-FAA and to N-(9-oxo)-3-FAA. These new metabolites were separated and identified by high-pressure liquid chromatography (h.p.l.c.). N-(9-hydroxy)-3-FAA was the major product of the hydroxylation of 3-FAA by hepatic microsomes of the rat or guinea pig exceeding the formation of N-(7-hydroxy)-3-FAA, the principal phenolic metabolite of 3-FAA. The amounts of N-(9-oxo)-3-FAA formed were about one-third of the amounts of N-(9-hydroxy)-3-FAA produced. In contrast to the formation of phenolic metabolites, the hydroxylation of 3-FAA to N-(9-hydroxy)-3-FAA was not increased by pretreatment of guinea pigs or rats with 3-MC. Similarly, pretreatment of rats with PB did not enhance the yield of N-(9-hydroxy)-3-FAA. Co inhibited the formation of N-(9-hydroxy)-3-FAA by 80 per cent. These data lead us to conclude that the formation of N-(9-hydroxy)-3-FAA is catalyzed by a microsomal hemoprotein not identical with cytochrome P1-450 or P-450. In contrast to 3-FAA, 2-FAA appeared to be a poor substrate for hydroxylation to the N-(9-hydroxy)-2-FAA. The susceptibility of 3-FAA to hydroxylation at carbon-atom 9 of the fluorene moiety may be rationalized by resonance structures in which carbon-atom 9 is positively charged and acts as a electrophilic center. Similar resonance structures cannot be written for 2-FAA.

Original languageEnglish (US)
Pages (from-to)1581-1589
Number of pages9
JournalBiochemical Pharmacology
Issue number11
StatePublished - 1978

Bibliographical note

Funding Information:
Previous investigations have shown that the carcinogenic arylamide, 2-FAA,* is bound to cytochrome Pt-450 of rat liver microsornes, as indicated by a type I binding spectrum \[I \], and that the compound is metabolized by hepatic microsomes of 3-MC-treated rats to the proximate carcinogen, N-hydroxy-2-FAA\[l-3\]. In contrast, the noncarcinogenic isomer, 3-FAA, produces an identical type I binding spectrum with hepatic microsomes of 3-MC-treated rats or guinea pigs, but yields only trace amounts of N-hydroxy-3-FAA on incubation with hepatic microsomes of the 3-MC-stimulated rat \[1\]. In the current study, we have explored the N-hydroxylation and C-hydroxylation of 3-FAA by hepatic microsomes of the untreated and 3-MC-treated guinea pig. On the basis of structural considerations, we hypothesized that 3-FAA would be hydroxylated at C-9 of the fluorene moiety. Accordingly, we investigated the conversion of 3-FAA to N-(9-hydroxy)-3-FAA by hepatic microsomes of the * This work was supported in part by grant CA-02571 awarded by the National Cancer Institute, DHEW. A preliminary report was presented at the meetings of the Federation of American Societies of Experimental Biology. Chicago, IL, Apr. 1977. * The following abbreviations are used in the text: 2-FAA, N-2-Fluorenylacetamide; 3-MC, 3-methylcholanthrene; PB, phenobarbital; DMSO, dimethyl sulfoxide; 3-FAA, N-3-fluorenylacetamide; t.l.c., thin-layer chromatography; h.p.l.c., high-pressure liquid chromatography; and C-9, carbon-atom 9.


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