Abstract

Methanol poisoning in primates and humans is due to formate accumulation as a result of low rates of formate oxidation. This toxicity is not seen in rats, where formate oxidation rates are high. Formate oxidation in vivo is dependent on hepatic tetrahydrofolate levels and on the activity of the enzyme 10-formyl-tetrahydrofolate (10-formyl-H4folate) dehydrogenase (EC 1.5.1.6). Because hepatic 10-formyl-H4folate dehydrogenase activity is lower in human liver than in rat liver, studies were performed investigating the properties of this enzyme in rat and human liver. 10-Formyl-H4folate dehydrogenase was purified to homogeneity from rat and human liver and was found to possess similar subunit molecular weights on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (96,000). N-Terminal amino acid analysis of the pure proteins showed an identical sequence for the first 16 amino acids. Antibodies raised in rabbits against the rat liver enzyme were inhibitory toward the activity of both rat and human liver enzymes and appeared to recognize only the 10-formyl-H4folate dehydrogenase in cytosolic preparations of rat and human liver. Immunoblots of pure rat and human liver 10-formyl-H4folate dehydrogenase showed similar staining intensity. It is concluded that rat and human liver 10-formyl-H4folate dehydrogenase possess very similar properties and that the activity of the enzyme in human liver is lower than that of rat liver, due to a reduced amount of enzyme protein in human liver. This may be an important factor in regulating formate oxidation in humans and may explain, in part, the accumulation of formate and the mechanism of toxicity of methanol in humans.