Lack of stereospecificity at carbon 6 of methyltetrahydrofolate transport in Ehrlich ascites tumor cells. Carrier-mediated transport of both stereoisomers.

Nonlabeled and tritiated stereoisomers of 5-methyltetrahydrofolate were prepared and were both shown to be substrates for the high affinity H4 folate cofactor membrane transport carrier in Ehrlich ascites tumor cells. Both the enzymically active form and the isomer having the opposite configuration at carbon 6 inhibited the influx of enzymically synthesized (+)-5-methyltetrahydrofolate, methotrexate, and aminopterin. When added to the media of cells preloaded with methotrexate, both isomers stimulated a net efflux of the antifolate from the cell. Influx of the natural and unnatural isomers followed Michaelis-Menten kinetics with comparable Km values. Each isomer competitively inhibited influx of the other.