Methanol Toxicity and Formate Oxidation in NEUT2 Mice

NEUT2 mice are deficient in cytosolic 10-formyltetrahydrofolate dehydrogenase (FDH; EC 1.5.1.6) which catalyzes the oxidation of excess folate-linked one-carbon units in the form of 10-formyltetrahydrofolate to CO2 and tetrahydrofolate (Champion et al., Proc. Natl. Acad. Sci. USA 91, 11338–11342, 1994). The absence of FDH should impair the oxidation of formate via the folate-dependent pathway and as a consequence render homozygous NEUT2 mice more susceptible to methanol toxicity. Normal (CB6-F1) and NEUT2 heterozygous and homozygous mice had essentially identical LD50 values for methanol, 6.08, 6.00, and 6.03 g/kg, respectively. Normal mice oxidized low doses of [14C]sodium formate (ip 5 mg/kg) to 14CO2 at approximately twice the rate of homozygous NEUT2 mice, indicating the presence of another formate-oxidizing system in addition to FDH. Treatment of mice with the catalase inhibitor, 3-aminotriazole (1 g/kg ip) had no effect on the rate of formate oxidation, indicating that at low concentrations formate was not oxidized peroxidatively by catalase. High doses of [14C]sodium formate (ip 100 mg/kg) were oxidized to 14CO2 at identical rates in normal and NEUT2 homozygous mice. Pretreatment with 3-aminotriazole (1 g/kg ip) in this instance resulted in a 40 and 50% decrease in formate oxidation to CO2 in both normal and homozygous NEUT2 mice, respectively. These results indicate that mice are able to oxidize formate to CO2 by at least three different routes: (1) folate-dependent via FDH at low levels of formate; (2) peroxidation by catalase at high levels of formate; and (3) by an unknown route(s) which appears to function at both low and high levels of formate. The implications of these observations are discussed in terms of the current hypotheses concerning methanol and formate toxicity in rodents and primates.