Glucuronidation of N-acetylbenzidine by human liver.
N-Glucuronidation is an important pathway in aromatic amine metabolism. This study assessed N-glucuronidation of N-acetylbenzidine by human liver slices and microsomes. With slices, considerable metabolism of [3H]N-acetylbenzidine (0.2 mM) was observed during a 2-hr incubation. N-Acetylbenzidine N'-glucuronide represented significant metabolism in four different human liver samples (6-33% of the total recovered radioactivity following HPLC). Benzidine (11-43%), benzidine N-glucuronide (8-11%), and N,N'-diacetylbenzidine (0-2%) were also formed. The kinetics of N-acetylbenzidine N'-glucuronide formation were investigated using Triton X-100-pretreated microsomes. Data were best described by a two-component Michaelis-Menten model composed of both high-affinity (low KM) and low-affinity (high KM) UDP-glucuronsyltranosferases. The high- and low-affinity KMs were 0.36 +/- 0.02 and 1.07 +/- 0.12 mM, respectively. To help identify the UDP-glucuronosyltransferases metabolizing N-acetylbenzidine, 23 transferase substrates were tested for their ability to inhibit glucuronidation. At 0.25 mM, bilirubin, estriol, and 17-epiestriol were good inhibitors (< 50% of control). Dose-response inhibition studies with estriol and 4-aminobiphenyl demonstrated that each agent reached a plateau as its concentration was increased. IC50 for estriol and 4-aminobiphenyl was 0.15 +/- 0.03 and 0.57 +/- 0.06 mM, respectively. Complimentary inhibition was observed when these agents were combined at maximal inhibitory concentrations. These results suggest that more than one UDP-glucuronosyltransferase metabolizes N-acetylbenzidine. N-Glucuronidation represents a major pathway for N-acetylbenzidine metabolism in humans.
Babu
S R
SR
Veterans Administration Medical Center, St. Louis, MO 63125-4199.
Lakshmi
V M
VM
Zenser
T V
TV
Davis
B B
BB
D000368:Aged; D001560:Benzidines / Q000378:metabolism*; D005260:Female; D005965:Glucuronates / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D008099:Liver / Q000378:metabolism*; D008297:Male; D008875:Middle Aged
D001560:Benzidines; D005965:Glucuronates; D014453:Glucuronosyltransferase; C034938:N-acetylbenzidine