THE SUBCELLULAR LOCALIZATION OF CYTIDINE 5''-MONOPHOSPHO-N-ACETYLNEURAMINIC ACID SYNTHETASE IN CALF BRAIN

Abstract— The subcellular distribution of cytidine 5'-monophospho- N -acetylneuraminic acid synthetase, which is a key enzyme involved in the biosynthesis of sialo-glycoproteins and gangliosides, was studied in the frontal grey cortex and the corpus callosum of the calf brain. It appeared that the enzyme was highly concentrated in nuclear fractions, which were shown to be relatively pure as evaluated from morphological, enzymic and chemical data. The possibility that the nuclear localization of the enzyme was due to an artifact, as the result of adsorption onto the nuclear membrane or absorption into the nucleus during the homogenization of the tissue, was ruled out completely and so it appeared that cytidine 5'-monophospho- N -acetylneuraminic acid synthetase, at least for the greater part, is a true nuclear enzyme. The subnuclear localization of the enzyme was shown to be in the nuclear sap. Both neuronal and glial cell nuclei contained the enzyme, which makes it very likely that biosynthesis of sialo-glycoproteins and/or gangliosides occurs in neurons as well as in glial cells. The significance of the nuclear localization of the enzyme for the biosynthesis of sialic acid containing macromolecules and the possible regulatory role played by the nucleus in this process are discussed.