title: Effect of polyglutamylation of 5,10-methylenetetrahydrofolate on the binding of 5-fluoro-2'-deoxyuridylate to thymidylate synthase purified from a human colon adenocarcinoma xenograft.
abstract: CH2-H4PteGlu and H4PteGlu exist in human colon adenocarcinoma xenografts predominantly in the form of polyglutamate species at concentrations of less than 3 microM. The interaction of polyglutamates of [6R]CH2-H4PteGlu in the formation and stability of [6-3H]FdUMP-thymidylate synthase-CH2-H4PteGlun ternary complexes has therefore been examined using enzyme purified from a human colon adenocarcinoma xenograft. Dissociation of these complexes was first-order and was dependent upon the concentration of folate. [6R]CH2-H4PteGlu3-6 (0.9 to 1.6 microM) were greater than 200-fold and [6R]CH2-H4PteGlu2 (18.2 microM) was 18-fold more effective than [6R]CH2-H4PteGlu1 (335 microM) at stabilizing ternary complexes for a T1/2 for dissociation of 100 min. Polyglutamylation of CH2-H4PteGlu also increased the affinity of binding of [6-3H]FdUMP to thymidylate synthase as determined by Scatchard analysis at folate concentrations of 10 microM, where the Kd in the presence of [6R]CH2-H4PteGlu1 was in the order of 4.0 x 10(-8) M, and for [6R]CH2-H4PteGlu3-5 was between 3.7 and 5.5 x 10(-9) M. To examine whether this effect was due to differences in the rates at which [6-3H]FdUMP was bound (kon) or dissociated (koff) from the enzyme, the apparent rate of [6-3H]FdUMP binding was determined in the presence of [6R]CH2H4PteGlu1, [6R]CH2-H4PteGlu3 and [6R]CH2-H4PteGlu4. The kon values were similar and were in the range of 1.7 to 2.3 x 10(6) M-1 min-1 for 10 or 20 microM folate concentrations. Differences in binding affinity determined for [6R]CH2-H4PteGlu1 and longer polyglutamate forms of [6R]CH2-H4PteGlu were thus due to differences in koff. The Vmax for the initial velocity of [6-3H]FdUMP binding was achieved at 10 microM folate. Consequently, at concentrations of CH2-H4PteGlu polyglutamates present in tumors, inhibition of thymidylate synthase by FdUMP in vivo would be expected to be transient, based upon the concentration of [6R]CH2-H4PteGlun required for maximal formation and stability of the covalent ternary complex. It would be advantageous for modulation of CH2-H4PteGlun pools to increase the concentrations of the longer polyglutamate species (n greater than or equal to 3) to maximize the interaction between FdUMP, thymidylate synthase and CH2-H4PteGlu.
authors.affiliation: Department of Biochemical and Clinical Pharmacology, St. Jude Children's Research Hospital, Memphis, TN 38101.
authors.forename: S
authors.initials: S
authors.lastname: Radparvar
authors.forename: P J
authors.initials: PJ
authors.lastname: Houghton
authors.forename: J A
authors.initials: JA
authors.lastname: Houghton
mesh_terms: D000230:Adenocarcinoma / Q000201:enzymology*; D003110:Colonic Neoplasms / Q000201:enzymology*; D003856:Deoxyuracil Nucleotides / Q000378:metabolism*; D005468:Fluorodeoxyuridylate / Q000378:metabolism*; D005492:Folic Acid / Q000031:analogs & derivatives*; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D011624:Pteroylpolyglutamic Acids / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013763:Tetrahydrofolates / Q000378:metabolism*; D013940:Thymidylate Synthase / Q000378:metabolism*
chemical_list: D003856:Deoxyuracil Nucleotides; D011624:Pteroylpolyglutamic Acids; D013763:Tetrahydrofolates; C013123:5,10-methylenetetrahydrofolic acid; D005468:Fluorodeoxyuridylate; D005492:Folic Acid; D013940:Thymidylate Synthase