PHYSIOLOGICAL DEPENDENCE OF BRAIN METHIONINE AND S-ADENOSYLMETHIONINE CONCENTRATIONS ON SERUM AMINO ACID PATTERN

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.