A novel assay of glucuronidation of C- and N-hydroxylated metabolites of the carcinogen N-2-fluorenylacetamide

Sex hormone-mediated effects on the phase I and phase II metabolism of N-2-fluorenylacetamide. Modulation of 9-hydroxylation

Sex differences in the phase I (cytochrome P-450-catalyzed hydroxylations) and phase II (conjugations) metabolism of N-2-fluorenylacetamide (2-FAA) by the livers of 50-day-old Sprague-Dawley rats and effects of gonadectomy were determined. The higher level (1.4 times) of cytochrome P-450 in the microsomes of male rats correlated with their 8 and 1.3 times greater capacities to form 9-hydroxy(OH)-2-FAA and 7-OH-2-FAA respectively. One week after gonadectomy, the formation of 9-OH-2-FAA, the major metabolite in the male, was decreased by 70%, whereas in the female it was increased 1.3 times. Treatment of male rats with beta-naphthoflavone (beta-NF) increased the formation of phenolic metabolites and N-OH-2-FAA, but decreased that of 9-OH-2-FAA. The amounts of 9-OH-2-FAA were increased, however, in beta-NF-treated female and gonadectomized male rats. These sex hormone- and beta-NF-mediated differences in the extent of 9-hydroxylation of 2-FAA are discussed in relation to the fluctuations in the levels of specific cytochrome P-450 isozymes. In contrast to the phenolic metabolites and N-OH-2-FAA, 9-OH-2-FAA was a poor substrate for UDP-glucuronyltransferase; this conjugation was not induced by treatment of male rats with beta-NF. Hence, in the presence of male hormones, relatively large amounts of 9-OH-2-FAA were formed and possibly retained in the liver. A role of this alcohol as a potential promoter in hepatocarcinogenesis by 2-FAA is suggested.

Liquid chromatographic assay for the measurement of glucuronidation of arylcarboxylic acids using uridine diphospho-[U-14C] glucuronic acid

A general method for the assay of UDP-glucuronosyltransferase activity towards arylcarboxylic acids (clofibric acid, 1- and 2-naphthylacetic acid) using UDP-[U-14C] glucuronic acid in liver microsomes is described. The 14C-labelled glucuronide was separated by high-performance liquid chromatography, identified by hydrolysis by beta-glucuronidase, characterized by laser desorption mass spectrometry and quantified by scintillation counting. The coefficient of variation of the enzyme activity for the inter-assay repeatability was below 4.5%. As little as 2.5 nmol of the arylcarboxylic acid glucuronides could be detected and precisely quantified. The method was applied to the determination of the apparent kinetic constants for glucuronidation of the acids. Clofibric acid was the best substrate for UDP-glucuronosyltransferase (Vmax/KM, the ratio of the maximum initial velocity and the Michaelis-Menten constant, is 12.3). The two isomers, 1- and 2-naphthylacetic acids, were transformed at a similar rate. However, they exhibited different enzymatic affinities, as the KM values were 1.0 mM and 5.6 mM for 1- and 2-naphthylacetic acid, respectively. This indicates that the spatial organization of the substrates played a critical role in this acyl glucuronoconjugation.

3-Hydroxybenzo[a]pyrene glucuronidation by cells: a direct fluorometric assay in culture medium

A direct fluorometric assay for measuring the glucuronidation by cultured cells using 3-hydroxybenzo[a]pyrene (less than 1 microM) as substrate is described. The method is based on the different characteristics in the fluorescence spectra of 3-hydroxybenzo[a]pyrene and benzo[a]pyrene-3-glucuronide. The analytical procedure operates directly on the culture medium without extraction steps. The sensitivity of detection is 1 nmol glucuronide/1 mumol substrate.