DL-threo-4-fluoroglutamic acid. A chain-terminating inhibitor of folylpolyglutamate synthesis.

Abstract

The effects of 4-fluoroglutamate on the reaction catalyzed by partially purified rat liver folylpolyglutamate synthetase have been investigated. DL-threo-4-Fluoroglutamate was an effective, concentration-dependent inhibitor of polyglutamylation of both tetrahydrofolate and methotrexate, while the erythro isomer was weakly inhibitory. 4-Fluoroglutamate acted as an alternate substrate; the DL-threo isomer was incorporated only slightly less effectively than L-glutamate, while the erythro isomer was poorly incorporated. The resulting product, a pteroylglutamyl-gamma-(4-fluoro)glutamate, was a very poor substrate for further glutamylation. Thus, when tetrahydrofolate and 4-fluoroglutamate were substrates, the sole Zn/HCl cleavage product co-chromatographed on high performance liquid chromatography with chemically synthesized p-aminobenzoylglutamyl-gamma-(4-fluoro)glutamate. When [3H]methotrexate (4-NH2-10-CH3PteGlu) and 4-fluoroglutamate were the substrates, one product was obtained which co-chromatographed on high performance liquid chromatography with chemically synthesized 4-NH2-CH3PteGlu-gamma-(4-fluoro)glutamate. Further evidence that the product from [3H]methotrexate was a dipeptide came from gamma-glutamyl hydrolase digestion experiments and quantitative amino acid analysis. The appearance of trace amounts of a product having properties consistent with the addition of a second 4-fluoroglutamate occurred only under forcing conditions. The chemically and enzymatically synthesized fluoroglutamate-containing products were at least 15 times poorer than the analogous diglutamyl compound as substrates for rat liver folylpolyglutamate synthetase. These results are consistent with inhibition of polyglutamate synthesis by 4-fluoroglutamate through a "leaky" chain termination mechanism.