SUBCELLULAR DISTRIBUTION OF ENZYMES OF TRANSMETHYLATION AND TRANSSULPHURATION IN RAT BRAIN

—Certain of the sulphur containing amino acids have been associated with synaptic transmission in the central nervous system. The enzymes involved in the synthesis of these putative neurotransmitter or modulator compounds have a different subcellular distribution in rat brain from those enzymes that catalyse the synthesis of other compounds in this pathway. Methionine adenosyltransferase and 5‐methyltetrahydrofolate‐homocysteine methyltransferase catalyse reactions that maintain the methylation functions of the pathway and are found in soluble fractions. Cystathionine β‐synthase, cystathionase, cysteine dioxygenase and cysteine sulphinic acid decarboxylase catalyse the synthesis of those sulphur‐containing amino acids implicated in neurotransmitter functions and these enzymes have both paniculate and soluble components. Serine hydroxymethyltransferase, which also has a particulate fraction in brain, is responsible for the synthesis of the neurotransmitter glycine, in addition to its role in the methionine‐related metabolism of folate.