Role of folylpolyglutamate synthetase in the metabolism and cytotoxicity of 5-deazaacyclotetrahydrofolate, an anti-purine drug.

Chinese hamster ovary (CHO) cells expressing human and Escherichia coli folylpolyglutamate synthetase (FPGS) activities were used as models to study factors regulating the cytotoxicity and metabolism of 5-deazaacyclotetrahydrofolate (DATHF), an anti-purine agent that requires conversion to polyglutamate forms to be a potent inhibitor of its target enzymes. The sensitivity of cells continuously exposed to DATHF was not influenced by FPGS activity. After short term exposure to DATHF, cells expressing higher levels of human FPGS were more sensitive to the drug while cells expressing low levels were resistant. Cells expressing E. coli FPGS solely in the cytosol were resistant to DATHF and accumulated only low levels of the drug. Expression of E. coli FPGS in the mitochondria of these cells restored drug sensitivity and cytosolic drug accumulation. DATHF is extensively metabolized in the mitochondria of mammalian cells and, despite an inability to enter the mitochondria, folylpolyglutamates and DATHF polyglutamates formed in the mitochondria are released into the cytosol without prior hydrolysis. DATHF was not solely an inhibitor of de novo purine biosynthesis but also inhibited glycine synthesis in the cell.