Regional Distribution of Methionine Adenosyltransferase in Rat Brain as Measured by a Rapid Radiochemical Method

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Abstract
The distribution of methionine adenosyltransferase (MAT) in the CNS of the rat was studied by use of a rapid, sensitive and specific radiochemical method. The S-adenosyl-[methyl-14C]L-methionine ([14C]SAM) generated by adenosyl transfer from ATP to [methyl-14C]L-methionine was quantitated by use of a SAM-consuming transmethylation reaction. Catechol-O-methyltransferase (COMT), prepared from rat liver, transferred the methyl-14C group of SAM to 3,4-dihydroxybenzoic acid. The 14C-labeled methylation products, vanillic acid and isovanillic acid, were separated from unreacted Met by solvent extraction and quantitated by liquid scintillation counting. Compared to other methods of MAT determination, which include separation of generated SAM from Met by ion-exchange chromatography, the assay described exhibited the same high degree of specificity and sensitivity but proved to be less time consuming. MAT activity was uniformly distributed between various brain regions and the pituitary gland of adult male rats. In the pineal gland the enzyme activity was .apprx. 10-fold higher.