title: Sequential metabolism of salicylamide exclusively to gentisamide 5-glucuronide and not gentisamide sulfate conjugates in single-pass in situ perfused rat liver.
abstract: Gentisamide (GAM), the hydroxylated metabolite of salicylamide (SAM), underwent sequential metabolism, and GAM-5-glucuronide was the only metabolite detected in the single-pass in situ rat liver preparation (10 ml/min/liver) perfused at varying SAM steady-state input concentrations (CIn). The exclusive formation of GAM-5-glucuronide was unexpected, because GAM, when administered to the rat liver, formed predominantly monosulfate conjugates at the 5- and 2-positions (Morris et al., J. Pharmacol. Exp. Ther. 245: 614-624, 1988). Kinetic parameters obtained from single-pass studies for SAM sulfation (CIn increasing from 31 to 347 microM) and glucuronidation and hydroxylation (CIn decreasing from 1383 to 130 microM, with 0.85 mM SO4(2-), and CIn increasing from 32 to 800 microM in an absence of SO4(2-), with 2,6-dichloro-4-nitrophenol, a sulfation inhibitor) revealed sulfation as a high-affinity, high-capacity pathway, glucuronidation as a lower-affinity, high-capacity pathway and hydroxylation as a low-affinity, low-capacity pathway. However, these parameters would not explain the exclusive glucuronidation of GAM when generated from SAM. Rather the zonal localization of metabolizing activities [a periportal sulfation, evenly distributed glucuronidation, and perivenous hydroxylation system (Xu and Pang, J. Pharmacokinet. Biopharm. 17: 645-671, 1989; Morris et al., J. Pharmacokinet. Biopharm. 16: 633-656, 1988)] could explain the complete lack of sulfation in SAM sequential metabolism. The different metabolite patterns arising from the administrations of a metabolite precursor (SAM) versus a preformed metabolite (GAM), due to the proximity of enzymes for formation and sequential metabolism, may serve to explain the differential toxic or pharmacologic effects observed in other precursor-metabolite pairs.
authors.affiliation: Faculty of Pharmacy, University of Toronto, Ontario, Canada.
authors.forename: X
authors.initials: X
authors.lastname: Xu
authors.forename: B K
authors.initials: BK
authors.lastname: Tang
authors.forename: K S
authors.initials: KS
authors.lastname: Pang
mesh_terms: D000818:Animals; D001549:Benzamides / Q000097:blood*; D002851:Chromatography, High Pressure Liquid; D005965:Glucuronates / Q000378:metabolism; D006900:Hydroxylation; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D012457:Salicylamides / Q000097:blood / Q000378:metabolism*; D013431:Sulfates / Q000378:metabolism
chemical_list: D001549:Benzamides; D005965:Glucuronates; D012457:Salicylamides; D013431:Sulfates; C055881:gentisamide; C031060:salicylamide