Methylenetetrahydrofolate dehydrogenases in normal and transformed mammalian cells

Transformed mammalian cells express both the usual NADP-dependent trifunctional methylenetetrahydrofolate dehydrogenase-cyclohydrolase-synthetase as well as the bifunctional NAD-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase. Antisera to these proteins do not crossreact, and Western blots of cell extracts indicate that there is no inactive form of the NAD-dependent enzyme in normal tissues. Immunofluorescence studies suggest a cytosolic location for the NAD-dependent enzyme, although sequence homology seen in the N-terminal 10 residues indicates a closer relationship with the mitochondrial form of the yeast NADP dependent trifunctional enzyme.