ISOLATION AND CHARACTERIZATION OF A NOVEL EUKARYOTIC MONOFUNCTIONAL NAD+-DEPENDENT 5,10-METHYLENETETRAHYDROFOLATE DEHYDROGENASE

An NAD+-dependent 5,10-methylenetetrahydrofolate (THF) dehydrogenase has been purified to homogeneity from the yeast Saccharomyces cerevisiae. The purified enzyme exhibits a final specific activity of 5.4 units mg−1 and is represented by a single protein of apparent Mr = 33 000-38 000 as determined by sodium dodecyl sulfate gel electrophoresis. A native Mr = 64 000 was determined by gel filtration, suggesting a homodimer subunit structure. Cross-linking experiments with dimethyl suberimidate confirmed the dimeric structure. The enzyme is specific for NAD+ and is not dependent on Mg2+ for activity. The forward reaction initial velocity kinetics are consistent with a sequential reaction mechanism. With this model, Km values for NAD+ and (6 R,S)-5,10-methylene-THF are 1.6 and 0.06 mM, respectively. In contrast to all other previously described eukaryotic 5, 10-methylene-THF dehydrogenases, the purified enzyme is apparently monofunctional, with undetectable 5, 10-methenyl-THF cyclohydrolase and 10-formyl-THF synthetase activities. Subcellular fractionation of yeast indicates the enzyme is cytoplasmic, with no NAD+-dependent 5, 10-methylene-THF dehydrogenase detectable in mitochondria. The activity was found in all yeast strains examined, at all stages of growth from the lag phase through the stationary phase. © 1990, American Chemical Society. All rights reserved.