DEMONSTRATION OF HYPOMETHYLATION OF PROTEINS IN THE BRAIN OF PIGS (BUT NOT IN RATS) ASSOCIATED WITH CHRONIC VITAMIN-B12 INACTIVATION

1. Pigs treated with nitrous oxide for periods of 1, 2 and 4 months de monstrated markedly reduced levels of methionine synthase and concomit ant reduction in the ratio of S-adenosylmethionine to S-adenosylhomocy steine, 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 correl ated with the development of clinical ataxia, but was absent when the pigs were clinically normal. It also only occurred when the S-adenosyl methionine level fell. 4. Rats maintained in nitrous oxide for 4 month s showed a marked reduction of methionine synthase but no reduction in the methylation ratio or in brain hypomethylation. None of the rats b ecame clinically ataxic. 5. Using an exogenous protein as a methyl gro up acceptor, it was demonstrated in an in vitro assay that the methylt ransferase enzymes responsible for brain 'O' and 'N' methylation were not affected per se by nitrous oxide treatment. 6. It is concluded tha t reduction of the methylation ratio in the brain of pigs as a consequ ence 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.