19F Nuclear Magnetic Resonance Analysis of 5-Fluorouracil Metabolism in Four Differently Pigmented Strains of Nectria haematococca

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19F nuclear magnetic resonance spectroscopy was used to study the metabolism of 5-fluorouracil in four strains of Nectria haematococca which displayed similar sensitivities to growth inhibition by this compound but differed in their pigmentation. The major metabolites, 5-fluorouridine and α-fluoro-β-alanine, were excreted into the medium by all four strains. The classical ribofluoronucleotides (5-fluorouridine-5′-monophosphate, -diphosphate, and -triphosphate) and α-fluoro-β-alanine were identified in the acid-soluble fraction of perchloric acid extracts of mycelia. Two hydrolysis products of 5-fluorouracil incorporated into RNA were found in the acid-insoluble pool. They were unambiguously assigned to 5-fluorouridine-2′-monophosphate and 3′-monophosphate with specific hydrolysis reactions on isolated RNA. The lack of fluorodeoxyribonucleotides and the fact that the four strains incorporated similar amounts of fluororibonucleotides into their RNAs strongly suggest an RNA-directed mechanism of cytotoxicity for 5-fluorouracil. The heavily pigmented wild type differed from the three low-pigmented strains in its low uptake of 5-fluorouracil and, consequently, in its reduced biosynthesis of 5-fluorouridine and α-fluoro-β-alanine. At present, it is not clear whether this change in 5-fluorouracil metabolism is a side effect of pigment production or results from another event.

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