Rates of sulfation and glucuronidation of 7-hydroxycoumarin in periportal and pericentral regions of the liver lobule
Conway, J.G.; Kauffman, F.C.; Ji, S.; Thurman, R.G.
Molecular Pharmacology 22(2): 509-516
1982
ISSN/ISBN: 0026-895X PMID: 7144740 Document Number: 195081
Micro-light guides (tip diameter 170 .mu.m) were placed on periportal (light) and pericentral (dark) regions of perfused rat liver to monitor the fluorescence (3366 .fwdarw. 450 nm) of infused 7-hydroxycoumarin. During infusion of up to 30 .mu.M 7-hydroxycoumarin, fluorescence of free 7-hydroxycoumarin could be detected only in periportal regions during anterograde perfusion and in pericentral areas during retrograde perfusion. Over 95% of 7-hydroxycoumarin infused was converted into nonfluorescent sulfate and glucuronide conjugates. Under these conditions, rates of sulfation and glucuronidation can be studied in the periportal and pericentral regions of the lobule by measuring conjugates of 7-hydroxycoumarin in the effluent perfusate via anterograde or retrograde perfusions. Sulfation predominated over a glucuronidation when 2-10 .mu.m 7-hydroxycoumarin was infused in the anterograde direction; however, with 20-30 .mu.M 7-hydroxycoumarin, glucuronidation predominated. Rates of glucuronidation and sulfation were similar when 2-5 .mu.M 7-hydroxycoumarin was infused in the retrograde direction; however, glucuronidation predominated at higher substrate concentrations. At low concentrations of 7-hydroxycoumarin, sulfation predominated over glucuronidation in periportal hepatocytes but not in pericentral hepatocytes. When livers were perfused with sulfate-free media, the glucuronide was the perdominant conjugate formed in both periportal and pericentral hepatocytes. Sulfation evidently successfully competes with glucuronidation for 7-hydroxycoumarin at low substrate concentrations. When 7-hydroxycoumarin was generated via mixed-function oxidation following the infusion of 7-ethoxycoumarin (5-20 .mu.M), fluorescence from 7-hydroxycoumarin was detected predominantly in periportal areas during anterograde perfusions and in pericentral regions during retrograde perfusions. The predominant conjugate formed in both regions of the liver lobule during mixed-function oxidation was sulfate. Rates of sulfation were significantly greater in periportal regions than in pericentral regions with 7-hydroxycoumarin was infused directly or generated indirectly via mixed-function oxidation.