The Role of Folylpolyglutamates in the Regulation of Folate Metabolism

Summary

Folylpolyglutamates have been shown to function as potent inhibitors, relative to the corresponding monoglutamates, of two enzymes involves in the metabolism of methylenetetrahydrofolate (CH₂-H₄folate) in mammalian tissues. Our initial studies on methylenetetrahydrofolate reductase from pig liver [Matthews, R. G., and Baugh, C. M., Biochemistry, 19, 2040–2045 (1980)] established that this enzyme shows a marked specificity for a hexaglutamyl side chain of both CH₂-H₄folate substrates and dihydrofolate inhibitors. The K_i values for dihydropteroylpolyglutamate (H₂PteGlu_n) inhibitors decreased from 6.5 μM for the monoglutamate to 0.013 μM for the hexaglutamate and then increased to 0.065 μM for the heptaglutamate. The I_so values for inhibition of the enzyme by H₂PteGlu_n in the presence of 1 μM CH₂-H₄PteGlu_n of the same chain length decreased from 7.4 μM for the monoglutamate pair to 0.13 μM for the hexaglutamate pair. We have now shown that methyltetrahydropteroylpolyglutamates (CH₃-H₄PteGlu_n) are potent inhibitors of pig liver serine hydroxymethyltransferase. In this case, the decrease in free energy associated with binding of the polyglutamate side chain is expressed not only as an increased affinity of the enzyme for CH₃-H₄PteGlu_n derivatives but also as an increased affinity of the enzyme-CH₃-H₄PteGlu_n binary complex for glycine. Glycine is bound 33-fold more tightly to the E-CH₃-H₄PteGlu₃ complex than to the E-CH₃-H₄PteGlu₁ complex. Additional glutamyl residues on CH₃-H₄PteGlu_n increase the affinity of the enzyme for CH₃-H₄PteGlu_n and the heptaglutamyl derivative is the most tightly bound of those studied. In both cases, inhibition by folylpolyglutamates may play a role in regulation of the flux of H₄folate-bound one carbon units in vivo. Inhibition of methylene-tetrahydrofolate reductase may occur when cellular levels of H₂PteGlu_n derivatives are elevated, e.g., during administration of methotrexate. Inhibition of serine hydroxymethyltransferase activity in the presence of both CH₃-H₄PteGlu_n derivarives and glycine may provide a means of regulating that enzyme by the conjoint concentrations of these two metabolites. Such inhibition may contribute to the rapid onset of megaloblastic anemia following administration of nitrous oxide.

This research has been supported in part by U.S. Public Health Service Grants GM 24908 (RGM), AM 16950 (LD) and by Institutional Research Grants IN40T and IN40U to the University of Michigan from the American Cancer Society.