Methotrexate transport in the L1210 mouse leukemia cell is a carrier-mediated process. Riboflavin competitively inhibits the unidirectional influx of methotrexate and possibly utilizes this carrier, at least in part, for its own transport. The apparent affinity of riboflavin for this transport mechanism is, however, much lower than that of methotrexate. In addition to their competition at the cell membrane for the transport mechanism, riboflavin and methotrexate undergo a time-dependent, photochemical reaction, which results in the formation of methotrexate derivatives that are separable by column chromatography. The compounds were not identified, but some of their properties were characterized. The labeled methotrexate derivatives do not significantly bind to the intracellular enzyme, dihydrofolate reductase. Their transport occurs by a route different from that for methotrexate, since uptake is more rapid, temperature sensitivity is markedly different, and inhibitors of methotrexate uptake are without effect on these derivatives. Other flavin-folate interactions were demonstrated by the reaction between riboflavin and the parent compound, folic acid, and by the reaction between methotrexate and ethanol lumiflavin, a congener of riboflavin.
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