Whole-Cell Detection by 13C NMR of Metabolic Flux through the C1-Tetrahydrofolate Synthase/Serine Hydroxymethyltransferase Enzyme System and Effect of Antifolate Exposure in Saccharomyces cerevisiae

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Abstract
Folate-mediated one-carbon metabolism is critical for the synthesis of numerous cellular constituents required for cell growth. A potential source of one-carbon units is formate. This one-carbon unit is activated to 10-formyltetrahydrofolate via the synthetase activity of the trifunctional enzyme C1-tetrahydrofolate (THF) synthase for use in purine synthesis or can be further reduced to 5,10-methylene-THF by the dehydrogenase activity of the same enzyme. 5,10-Methylene-THF is used by serine hydroxymethyltransferase (SHMT) in the synthesis of serine. Recently, 13C NMR has been used to establish that the C1-THF synthase/SHMT enzyme system is the only route from formate to serine in vivo in the yeast Saccharomyces cerevisiae [Pasternack et al. (1992) Biochemistry 31, 8713-8719]. In vitro studies have considered the kinetics of the C1-THF synthase/SHMT enzyme system in the catalytic conversion of formate to serine [Strong et al. (1987) J. Biol. Chem. 262, 12519-12525]. In the present work, we begin to study the kinetics of this two-enzyme system in its natural environment. Provision of [13C]formate and direct detection of an intracellular accumulating pool of [3-13C]serine by 13C NMR of whole cells allow us to monitor the rate of flux through this enzyme system in vivo. The rate of accumulation of soluble [3-13C]serine under [13C]formate-saturating conditions is 13.0 +/- 1.2 microM/min relative to an external standard of serine in D2O. The extracellular formate concentration at half-maximal flux was determined to be 900 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

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