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Abstract— Animals maintained on rat chow and water ad libitum in quarters illuminated for 12 h/day show diurnal rhythms in serum methionine and brain S‐adenosylmethionine (SAM) concentrations. Brain methionine exhibits no such variation, nor does the ratio of serum methionine to the serum concentrations of six neutral amino acids which are believed to compete with methionine for uptake into brain. Administration of methionine to rats in doses that elevate serum methionine, but keep it within the daily physiological range, significantly increases brain concentrations of both methionine and SAM. The acute feeding of either a protein‐free or a 40% casein meal also increases brain methionine and SAM, but does not affect serum methionine; however, both diets also increase the ratio of serum methionine to tyrosine, an amino acid whose postprandial concentration is indicative of the concentrations of the other amino acids that compete with methionine for transport into brain. These findings suggest that brain methionine levels increase physiologically after eating as a result of changes in the serum amino acid pattern. Furthermore, such naturally occurring increases in brain methionine appear to be associated with elevations in brain SAM.

The disaggregation of brain polysomes which is produced by giving large doses of (L)-dopa to rats is not reproduced by administering its metabolite, 3-O-methyldopa, by giving D-dopa, which also depletes the brain of S-adenosylmethionine but is not converted to catecholamines, or by giving the L-dopa after a decarboxylase inhibitor. Polysome disaggregation is potentiated by the prior administration of a monoamine oxidase inhibitor, indicating that formation of a catecholamine is an obligatory requirement. These observations suggest that the mechanism by which L-dopa disaggregates brain polysomes involves its conversion to dopamine within the majority of brain cells.

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