During the analysis of liver
protein expression in the offspring of male mice irradiated with fission-spectrum neutrons, one offspring displayed a heritable 50% decrease in the abundance of two
proteins. Homozygous mice lacking detectable quantities of these
proteins were obtained through breeding. Characterization of this
protein deficiency has identified these liver
proteins as forms of the
enzyme 10-formyltetrahydrofolate dehydrogenase (10-formyl-THF DH; 10-formyltetrahydrofolate:NADP+
oxidoreductase, EC 1.5.1.6). NH2-terminal sequence analysis demonstrated that both
proteins share identical sequences in the first 25 residues, and this sequence matches (96% identity) that of rat and human 10-formyl-THF DH. In addition, these
proteins showed cross-reactivity to polyclonal antiserum raised against purified rat 10-formyl-THF DH. Southern (
DNA) blot analysis revealed a restriction fragment length polymorphism consistent with a deletion mutation in the 10-formyl-THF DH structural gene in homozygous mice. Results of Northern (
RNA) blot analysis demonstrated the absence of 10-formyl-THF DH
mRNA in mice lacking 10-formyl-THF DH
protein. Furthermore, liver cytosolic 10-formyl-THF DH enzymatic activity was undetectable in homozygotes. Measurement of hepatic
folate pools showed that in homozygotes the total
folate pool is decreased and the level of
tetrahydrofolate is markedly depleted.