Further studies stereospecificity at carbon 6 for membrane transport of tetrahydrofolates. Diastereoisomers of 5-methyltetrahydrofolates as competitive inhibitors of transport of methotrexate in L1210 cells.

The unnatural d diastereoisomer at carbon 6 of 5-methyltetrahydrofolate was only slightly less effective than the natural 1 diastereoisomer as a competitive inhibitor of the carrier-mediated membrane transport of [3H]methotrexate into L1210 murine leukemia cells. The apparent Ki for a mixture containing equal amounts of both natural and unnatural diastereoisomers was not significantly different from that found for the unnatural form. These results show that the reduced folate carrier system in these cells has a strong affinity for the unnatural stereoisomer, a finding in contrast to that obtained with the corresponding diastereoisomer of 5-formyltetrahydrofolate.