Stimulation of mixed‐function oxidation of 7‐ethoxycoumarin in periportal and pericentral regions of the perfused rat liver by xylitol

Abstract

Rates of 0-deethylation of 7-ethoxycoumarin by perfused livers from fasted, phenobarbital-treated rats were 3.7 pmol x g-x h-l. Approximately 50 % of the product was conjugated. When rates of 7-ethoxycoumarin 0-deethylation were varied by infusing different concentrations of substrate, a good correlation (r = 0.91) was found between rates of 0-deethylation of 7-ethoxycoumarin and fluorescence of 7-hydroxycoumarin detected from the liver surface. Micro-light guides (tip diameter 170 pm) placed on periportal and pericentral regions on the liver surface were used to monitor the conversion of nonfluorescent 7-ethoxycoumarin to fluorescent 7-hydroxycoumarin. The 0-deethylation of 7-ethoxycoumarin to 7-hydroxycoumarin increased fluorescence 64 % and 28 % in pericentral and periportal regions of the liver lobule, respectively. Rates of 7-ethoxycoumarin 0-deethylation estimated from these increases in fluorescence were 5.2 lmol x g-' x h-' in pericentral and 2.2 pmol x g-' x h-' in periportal regions of the liver. During mixed-function oxidation of 7-ethoxycoumarin, the oxidation :reduction state of NADP(H) was similar in both regions of the liver lobule. Xylitol(2 mM) decreased the NADP+/NADPH ratio and stimulated rates of drug metabolism in both regions of the liver lobule. This indicates that conditions exist where the supply of NADPH is an important rate-determining factor for 7-ethoxycoumarin metabolism in both periportal and pericentral regions of the liver lobule.Many chemicals (e. g. carbon tetrachloride, acetominophen, bromobenzene) which are activated by the microsomal mixed-function oxidase system produce selective damage to pericentral regions of the liver lobule ; however, the mechanisms associated with localized injury within the hepatic lobule remain poorly defined. Since several enzymes, including cytochrome P-450 [l], vary in content across the liver lobule, selective injury to specific zones of the liver may be explained by different rates of mixed-function oxidation across the liver lobule. Immunohistochemical and microspectrophotometric studies indicate that total cytochrome P-450 is greater in pericentral than in periportal regions of the liver lobule [1,2]. Inducing agents also have selective effects on different zones of the liver. For example, phenobarbital treatment increases cytochrome P-450 content by more than 100 % in pericentral regions while causing only a 50 % increase in periportal areas[2]. In addition to catalytic components of the monooxygenase system, variations in concentrations of cofactors and substrates may also be important determinants of rates of mixedfunction oxidation across the liver lobule.While the rate of reduction of components of the mixedfunction oxidase system is rate-limiting for xenobiotic metabolism in isolated microsomes [3] in the presence of excess cofactor, previous reports from this laboratory and others have shown that the supply of NADPH is an important determinant of rates of mixed-function oxidation in the whole liver [4-71. Thus, differences in the capac...