EVIDENCE FROM RAT HEPATOCYTES OF AN UNRECOGNIZED PATHWAY OF 5-FLUOROURACIL METABOLISM WITH THE FORMATION OF A GLUCURONIDE DERIVATIVE

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  • 1 January 1985
    • journal article
    • research article
    • Vol. 45  (6) , 2450-2455
Abstract
Isolated rat hepatocytes in suspension were exposed to [3H]-5-fluorouracil for intervals over 2 h, following which the cells were removed from the media and sonicated, and the cytoplasm was sampled. High-performance liquid chromatography was used to separate 5-fluorouracil (FUra) from its known anabolites and catabolites, with subsequent quantitation of these metabolites by measurement of radioactivity. As the extracellular concentration of FUra was increased to > 30 .mu.M, the intracellular levels of FUra increased, with detection of a new peak of radioactivity distinct from any of the known anabolites or catabolites. This new metabolite, G, increased in concentration as the extracellular concentration of FUra was raised to > 1 mM. Inhibition of FUra catabolism by 2 mM thymine resulted in a further increase in intracellular FUra (approaching the extracellular FUra concentration) and was accompanied by a further increase in the intracellular concentration of G, demonstrating that G was not formed via the catabolic pathway. The increase in intracellular FUra and G was not accompanied by an increase in intracellular anabolites, suggesting that G was formed via a novel metabolic pathway. G was retained within the hepatocytes, although it was not bound to intracellular macromolecules. G was converted to FUra in the presence of .beta.-D-glucuronidase; this reaction was inhibited with the addition of saccharo-1,4-.beta.-lactone, a specific inhibitor of the .beta.-D-glucuronidase. This data, together with evidence from hepatocyte homogenates in which formation of G was shown to be dependent on the concentration of uridine-5''-diphosphoglucuronic acid, demonstrates that G is a glucuronide of FUra. The formation of G suggests that FUra is metabolized via a previously unrecognized metabolic pathway.