1. Pigs treated with nitrous oxide for periods of 1, 2 and 4 months demonstrated markedly reduced levels of methionine synthase and concomitant reduction in the ratio of S-adenosylmethionine to S-adenosylhomocysteine, the methylation ratio, at all time intervals.
2. Both ‘O’ and ‘N’ methylations were significantly reduced in pigs after 4 months in nitrous oxide but not after shorter periods.
3. Hypomethylation correlated with the development of clinical ataxia, but was absent when the pigs were clinically normal. It also only occurred when the S-adenosylmethionine level fell.
4. Rats maintained in nitrous oxide for 4 months showed a marked reduction of methionine synthase but no reduction in the methylation ratio or in brain hypomethylation. None of the rats became clinically ataxic.
5. Using an exogenous protein as a methyl group acceptor, it was demonstrated in an in vitro assay that the methyltransferase enzymes responsible for brain ‘O’ and ‘N’ methylation were not affected per se by nitrous oxide treatment.
6. It is concluded that reduction of the methylation ratio in the brain of pigs as a consequence of methionine synthase inhibition leads to brain hypomethylation. This hypomethylation could affect critical components of nerve tissue, inducing the vacuolar myelopathic changes seen in the spinal cord of these animals, which mimic those of subacute combined degeneration in man.