10-Formyl-dihydrofolic acid is bioactive in human leukemia cells

The bioactivity of 10-formyl-7,8-dihydrofolic acid and 10-formyl-folic acid was determined in human leukemia (CCRF-CEM) cells grown in a folate-depleted medium containing methotrexate. Excess 10-formyl-7,8-dihydrofolic acid, (but not 10-formyl folic acid) supported the growth of these cells, but it was less potent than5-formyl-5,6,7,8-tetrahydrofolic acid (a control). 10-formyl-7, 8-dihydrofolic acid (not 10-formyl folic acid) was active as substrate for aminoimidazole carboxamide ribotide transformylase and dihydrofolate reductase. This is the first experimental evidence that 10-formyl-7,8-dihydrofolic acid is a bioactive folate in mammalian cells. These experiments and several other lines of evidence in the literature suggest that 10-formyl-folic acid must be metabolized to bioactive folate by enteric bacteria before it can be utilized by the vertebrate host.