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Effects of Sulfanilamide and Methotrexate on 13C
Fluxes through the Glycine Decarboxylase/Serine
Hydroxymethyltransferase Enzyme System in Arabidopsis1
Vikram Prabhu*,
K. Brock Chatson,
Helen Lui,
Garth D. Abrams, and
John King
Department of Biology, University of Saskatchewan, Saskatoon,
Saskatchewan, Canada S7N 5E2 (V.P., H.L., G.D.A., J.K.); and Plant
Biotechnology Institute, National Research Council of Canada,
Saskatoon, Saskatchewan, Canada S7N 0W9 (K.B.C.)
In C3 plants large
amounts of photorespiratory glycine (Gly) are converted to serine by
the tetrahydrofolate (THF)-dependent activities of the Gly
decarboxylase complex (GDC) and serine
hydroxymethyltransferase (SHMT). Using 13C
nuclear magnetic resonance, we monitored the flux of carbon through the
GDC/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 other sulfonamide analogs were tested with similar results,
although all three analogs competitively inhibited the partially
purified enzyme. However, methotrexate or aminopterin, which are
confirmed inhibitors of Arabidopsis dihydrofolate reductase, decreased
the flux through the GDC/SHMT system by 60% after 48 h and by
100% in 96 h. The uptake of [ -13C]Gly was not
inhibited by either drug class. The specificity of methotrexate action
was shown by the ability of 5-formyl-THF to restore flux through the
GDC/SHMT pathway in methotrexate-inhibited plants. The experiments with
sulfonamides strongly suggest that the mitochondrial THF pool has a
long half-life. The studies with methotrexate support the additional,
critical role of dihydrofolate reductase in recycling THF oxidized in
thymidylate synthesis.
1
This study was supported in part by
grants-in-aid of research from the Natural Sciences and Engineering
Research Council of Canada to J.K.
*
Corresponding author; e-mail prabhuv{at}duke.usask.ca; fax
1-306-966-4461.
Plant Physiol. (1998) 116: 137-144
Copyright Clearance Center: 0032-0889/98/116/0137/08
© 1998 American Society of Plant Physiologists
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