1. Pigs were maintained in air or in an atmosphere of nitrous oxide which dramatically changes the S-adenosyl-l-methionine to S-adenosyl-l-homocysteine ratio in neural tissues. Samples of cerebrospinal fluid, cortex, cerebellum and spinal cord were then extracted and analysed for S-adenosyl-l-methionine and S-adenosyl-l-homocysteine. Regression analyses were carried out on values obtained in cerebrospinal fluid and in neural tissues.

2. Highly significant correlations were obtained between levels of S-adenosyl-l-homocysteine (r2 = 0.42-0.69; P < 0.001) and S-adenosyl-l-methionine/S-adenosyl-l-homocysteine ratios (r2 = 0.56-0.65; P < 0.001) in cerebrospinal fluid and levels and ratios in cortex, cerebellum and spinal cord. The levels of S-adenosyl-l-methionine did not show a significant correlation.

3. We conclude that the ratio of these metabolites in the cerebrospinal fluid may reflect the ratio in the central nervous system and we suggest that this may also be true in human tissues. This finding will permit the determination of the probable methylation ratio in the central nervous system in human conditions, such as vitamin B12 deficiency and acquired immune deficiency syndrome, where a similar myelopathy occurs to that seen in the nitrous oxide-treated pig. All three myelopathies may arise from an inhibition of methyltransferases involved in the synthesis of myelin that would occur when the methylation ratio is reduced.

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