In Vitro Methotrexate Polyglutamate Synthesis by Rat Liver Folylpolyglutamate Synthetase and Inhibition by Bromosulfophthalein

Abstract

We have investigated the properties of the rat liver folylpoly-glutamate synthetase using methotrexate (MTX; 4-NH₂-10-CH₃-PteGlu) as a substrate. Many characteristics of the synthetase (e.g., the apparent K_m values for L-glutamate and ATP, and the optimal concentrations of KCl and 2-mercaptoethanol) are virtually identical whether MTX or tetrahydrofolate is the “folate” substrate. There are, however, several significant differences between the reactions catalyzed with these two substrates. The length of products synthesized from tetrahydrofolate are inversely related to the initial monoglutamate concentration. Low tetrahydrofolate concentrations allow synthesis of longer (n = 3) polyglutamates, up to pentaglut-amate length, while high concentrations lead to predominantly di-glutamate synthesis. However, 4-NH₂-10-CH₃-PteGlu₂ predominates regardless of the initial MTX concentration, under otherwise identical conditions. Also, tetrahydrofolate can be readily converted to pentaglutamate lengths, the same as predominates in rat liver in vivo. In contrast, MTX forms species containing only up to a total of three glutamates, i.e., 4-NH₂-10-CH₃-PteGlu₃. Finally, the ultimate product of synthesis from tetrahydrofolate, H₄teGlu₅, is a fairly good inhibitor of synthetase activity with either MTX or tetrahydrofolate as the substrate. The ultimate product of MTX synthesis, 4-NH₂-10-CH₃-PteGlu₃, however, is a poor inhibitor of activity with either substrate.

This investigation was supported by Grants CA 08010 and CA 28097 from the National Cancer Institute. Joseph R. Bertino is an American Cancer Society Professor.