Photoaffinity analogues of methotrexate as folate antagonist binding probes

Folate coenzyme and antifolate transport proteins in normal and neoplastic cells

The transport systems for folate coenzymes and antifolate compounds into various types of normal and neoplastic cells display considerable diversity in their pharmacokinetics and also in terms of the apparent molecular weights of the proteins involved. Further, several uptake routes may exist in a given cell type. A variety of neoplastic tissues have been reported to rely upon a single major transport system that has a relatively high affinity for the reduced form of folate compounds and for antifolates such as methotrexate. Using a photoaffinity analogue of methotrexate, we have identified the involvement of a 48 kDa membrane protein and a 38 kDa cytosolic or peripheral membrane protein in the transport of this compound into murine L1210 leukemia cells. Such an uptake is absent in mutant L1210 cells that are defective in methotrexate transport. We propose a model for the uptake of reduced folate coenzymes in L1210 cells in which the compound is initially transported across the cell membrane by the 48 kDa protein and delivered on the cytoplasmic surface to the 38 kDa protein; the 38 kDa protein then carries the folate compound to a specific enzyme of folate metabolism. Antibodies to the membrane folate binding protein from human placenta cross-react with the 48 kDa protein in L1210 cell membranes indicating an immunological relationship between these two proteins. Comparison of the amino acid sequences of peptides of the placental receptor obtained by digestion with S. aureus V8 protease indicate the presence of two homologous forms of the folate binding protein in placenta; one of these forms appears to have an identical sequence to the soluble and membrane associated folate binding proteins in human epidermoid carcinoma (KB) cells, which in turn share the primary structure of the soluble and membrane associated folate binders in human milk in the regions that have been sequenced. These results indicate that the folate coenzyme transport proteins in various tissues may be structurally related and in several instances may even be identical. In the latter cases the observed differences in apparent molecular weights may be due to differences in glycosylation and/or proteolysis. In support of this view is our observation that the deglycosylated and/or partially proteolyzed placental receptor retains the ability to bind folate.