Characterization of the function of mammalian folylpolyglutamate synthetase (FPGS)

The function and characteristics of mouse folylpolyglutamate synthetase have been examined. Folate polyglutamates were poor substrates for the efflux mechanism for monoglutamates in L1210 mouse leukemia cells, with negligible loss of preformed folate polyglutamates to the medium over four hours. Disruption of folate metabolism with methotrexate did not augment efflux of folate polyglutamates. Folylpolyglutamate synthetase, partially purified from mouse liver, was found to accept a variety of folate derivatives as substrates, including pteroic acid and methotrexate; however, the concentration of these substrates that saturated the reaction varied considerably. The enzyme that catalyzed the addition of glutamic acid to methotrexate and to the naturally-occurring folate monoglutamates appeared to be the same. The cytotoxicity of folylpolyglutamate synthetase inhibitors was predicted to require continued cell division since their effects would be based upon a decreased rate of synthesis of folate cofactors capable of retention by the cell membrane. Hence folylpolyglutamate synthetase inhibitors should have low toxicity to nonproliferative cell populations.