Methenyltetrahydrofolate synthetase prevents the inhibition of phosphoribosyl 5-aminoimidazole 4-carboxamide ribonucleotide formyltransferase by 5-formyltetrahydrofolate polyglutamates.
Methenyltetrahydrofolate synthetase (EC 6.3.3.2) catalyzes the irreversible ATP and Mg2+-dependent transformation of 5-formyltetrahydrofolate (N5-HCO-H4-pteroylglutamic acid (PteGlu] to 5,10-methenyltetrahydrofolate. The physiological function of this reaction remains unknown even though it is potentially involved in the intracellular metabolism of the large doses of N5-HCO-H4-PteGlu (leucovorin) administered to cancer patients. We have tried to elucidate methenyltetrahydrofolate synthetase's physiological role by examining the consequences of its inhibition in MCF-7 human breast cancer cells by the folate analog 5-formyltetrahydrohomofolate (fTHHF), a potent competitive inhibitor with a Ki of 1.4 microM. fTHHF inhibited MCF-7 cell growth with an IC50 of 2.0 microM during 72-h exposures, and this effect was fully reversible by hypoxanthine but not thymidine, indicating specific inhibition of de novo purine synthesis. A correlation was observed between increases in intracellular N5-HCO-H4-PteGlu concentrations following fTHHF and cell growth inhibition. De novo purine synthesis was inhibited at the second folate-dependent enzyme, phosphoribosyl aminoimidazole-carboxamide formyltransferase (AICAR transferase; EC 2.1.2.3), as determined by aminoimidazole carboxamide rescue and azaserine inhibition studies. N5-HCO-H4-PteGlu pentaglutamate was a potent inhibitor of purified MCF-7 cell AICAR transferase with a Ki of 3.0 microM while the monoglutamate was not an inhibitor up to 10 microM and fTHHF was only weakly inhibitory with a Ki of 16 microM. These findings suggest that methenyltetrahydrofolate synthetase activity is needed to prevent de novo purine synthesis inhibition by N5-HCO-H4-PteGlu polyglutamates.
Bertrand
R
R
Institut du Cancer de Montréal, Québec, Canada.
Jolivet
J
J
D000217:Acyltransferases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D001943:Breast Neoplasms; D019731:Carbon-Nitrogen Ligases; D002460:Cell Line; D005260:Female; D005492:Folic Acid / Q000378:metabolism; D006801:Humans; D019877:Hydroxymethyl and Formyl Transferases; D007700:Kinetics; D002955:Leucovorin / Q000494:pharmacology*; D008025:Ligases / Q000378:metabolism*; D010455:Peptides / Q000031:analogs & derivatives*; D051027:Phosphoribosylaminoimidazolecarboxamide Formyltransferase; D011099:Polyglutamic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology
D010455:Peptides; D011099:Polyglutamic Acid; D005492:Folic Acid; D019877:Hydroxymethyl and Formyl Transferases; D051027:Phosphoribosylaminoimidazolecarboxamide Formyltransferase; D000217:Acyltransferases; D008025:Ligases; D019731:Carbon-Nitrogen Ligases; C021557:5,10-methenyltetrahydrofolate synthetase; D002955:Leucovorin