title: Biochemical determinants of tumor sensitivity to 5-fluorouracil: ultrasensitive methods for the determination of 5-fluoro-2'-deoxyuridylate, 2'-deoxyuridylate, and thymidylate synthetase.
abstract: Techniques have been developed to measure FdUMP, the active metabolite of 5-FUra; thymidylate synthetase (TMP synthase; 5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1.45), the target enzyme for this antimetabolite; and dUMP, the substrate that competes with FdUMP for binding to TMP synthetase. As little as 0.02 pmol of FdUMP can be quantitated with a competitive ligand binding assay by using homogeneous Lactobacillus casei/MTX TMP synthetase as a binding protein. A new binding assay for TMP synthetase allows detection of 0.005 pmol of enzyme. The quantitative enzymatic conversion of dUMP to [methyl-(14)C]-TMP using 5,10-methylene[(14)C]tetrahydrofolate by pure L. casei TMP synthetase is used as an assay for dUMP with a sensitivity of 10 pmol. Cultured CCRF-CEM human lymphoblastic leukemia cells formed high levels of FdUMP (2.6 nmol per 10(9) cells) within 11 hr after exposure to 30 muM 5-FUra. Tumor cell TMP synthetase levels dropped, and then free FdUMP appeared. The intracellular dUMP pool was low (2-5 nmol per 10(9) cells) in logarithmically growing cultures of several tumor cell lines but expanded rapidly in CCRF-CEM cells on exposure to 5-FUra after enzyme levels decreased. The levels of dUMP found after exposure to 5-FUra are sufficient to severely retard inhibition of TMP synthetase by FdUMP.The methods described are sufficiently sensitive to allow these biochemical parameters of 5-FUra action to be measured in cell culture or in needle biopsy samples of human tumors.
mesh_terms: D002460:Cell Line; D003856:Deoxyuracil Nucleotides / Q000032:analysis*; D005468:Fluorodeoxyuridylate / Q000032:analysis*; D005472:Fluorouracil / Q000378:metabolism* / Q000627:therapeutic use; D006801:Humans; D007780:Lacticaseibacillus casei / Q000032:analysis / Q000201:enzymology; D008780:Methyltransferases / Q000032:analysis*; D009369:Neoplasms / Q000188:drug therapy; D011869:Radioligand Assay; D013940:Thymidylate Synthase / Q000032:analysis*
chemical_list: D003856:Deoxyuracil Nucleotides; D005468:Fluorodeoxyuridylate; D008780:Methyltransferases; D013940:Thymidylate Synthase; D005472:Fluorouracil