EFFECTS OF SULFANILAMIDE AND METHOTREXATE ON C-13 FLUXES THROUGH THE GLYCINE DECARBOXYLASE SERINE HYDROXYMETHYLTRANSFERASE ENZYME-SYSTEM INARABIDOPSIS/

In C-3 plants large amounts of photorespiratory glycine (Gly) are conv erted to serine by the tetrahydrofolate (THF)-dependent activities of the Gly decarboxylase complex (GDC) and serine hydroxymethyltransferas e (SHMT). Using C-13 nuclear magnetic resonance, we monitored the flux of carbon through the CDC/SHMT enzyme system in Arabidopsis thaliana (L.) Heynh. Columbia exposed to inhibitors of THF-synthesizing enzymes . Plants exposed for 96 h to sulfanilamide, a dihydropteroate synthase inhibitor, showed little reduction in flux through GDC/SHMT. Two othe r sulfonamide analogs were tested with similar results, although all t hree analogs competitively inhibited the partially purified enzyme. Ho wever, methotrexate or aminopterin, which are confirmed inhibitors of Arabidopsis dihydrofolate reductase, decreased the flux through the GD C/SHMT system by 60% after 48 h and by 100% in 96 h. The uptake of [al pha-C-13]Gly was not inhibited by either drug class. The specificity o f methotrexate action was shown by the ability of 5-formyl-THF to rest ore flux through the GDC/SHMT pathway in methotrexate-inhibited plants . The experiments with sulfonamides strongly suggest that the mitochon drial THF pool has a long half-life. The studies with methotrexate sup port the additional, critical role of dihydrofolate reductase in recyc ling THF oxidized in thymidylate synthesis.