Uridine catabolism by the isolated perfused rat liver

A new approach in the treatment of gastrointestinal tumors with 5-fluorouracil involves the infusion of high doses of uridine to improve the chemotherapeutic efficiency of the former. High amounts of uracil formed from uridine can interfere with the hepatic catabolism of 5-fluorouracil and thus increase its bioavailability and toxicity. In our study, we analysed the metabolite pattern of uridine in the effluent of isolated perfused rat livers in relation to portal uridine levels. The livers were perfused hemoglobin-free without recirculation at a constant flow. In the perfusate, uridine was changed from 0.5 to 100 mumol/l. The complete degradation of [2-14C]uridine and [2-14C]uracil was monitored via the release of labeled CO2. Radioactive catabolites of uridine including uracil and the sum of dihydrouracil and beta-ureidopropionate were separated by high-performance liquid chromatography and counted using a radioactivity flow monitor. Portal uridine concentrations were increased from 0.5 to 100 mumol/l and were accompanied by a rise in the relative amount of non-metabolized uridine in the effluent from 13 to 78%. At uridine concentrations above 50 mumol/l, there was a constant release of uracil into the effluent, indicating saturation of uridine phosphorolysis or transport. The amount of 14CO2 formed by the liver reflecting complete uridine breakdown was higher than any other uridine metabolite when uridine concentration varied from 0.5 to 15 mumol/l. Saturation of 14CO2 formation was achieved at a uridine concentration of 25 mumol/l. Higher peak values of 14CO2 release were observed after direct infusion of equivalent amounts of uracil into the portal vein.(ABSTRACT TRUNCATED AT 250 WORDS)