Dissociation of methionine synthetase (EC 2.1.1.13) activity and impairment of DNA synthesis in fruit bats (Rousettus aegyptiacus) with nitrous oxide-induced vitamin B12 deficiency.

: 1. The effect of methylcobalamin inactivation by the anaesthetic gas nitrous oxide on the activity of the cobalamin-dependent methionine synthetase (5-methyltetrahydrofolate homocysteine methyltransferase; EC 2.1.1.13) reaction, and on DNA synthesis, in the fruit bat Rousettus aegyptiacus, was examined. 2. Methionine synthetase activity in the liver of bats exposed to N2O-oxygen (50:50, v/v) for 90 min/d averaged 32% of that of controls after 4 d of exposure and only 5% after 12-14 weeks of exposure. 3. DNA synthesis in the bone marrow, as reflected by the deoxyuridine suppression test, was unaffected by 4 d of exposure to N2O and only minimally affected after 5-10 weeks of exposure. 4. These results suggest that DNA synthesis in the fruit bat is unusually resistant to inhibition of methionine synthetase and imply the existence of a non-methylated circulating folate pool in this species.