Subcellular localization and tissue distribution of sialic acid precursor-forming enzymes

Abstract

The enzymes UDP-N-acetylglucosamine pyrophosphorylase, UDP-N-acetylglucosamine 2-epimerase, N-acetylmannosamine kinase, N-acetylglucosamine kinase and N-acetylglucosamine 2-epimerase, which are involved in the metabolism of N-acetylneuraminic acid, were studied in rat with regard to their subcellular localization and tissue distribution. The subcellular distribution studies in liver indicated that the enzymes are localized in the soluble cell fraction. In other tissues the comparison of enzyme activities in homogenates with that in high-speed supernatants led to a similar conclusion. UDP-N-acetylglucosamine pyrophosphorylase, N-acetylglucosamine kinase and N-acetylglucosamine 2-epimerase were detected in almost all tissues studied. UDP-N-acetylglucosamine 2-epimerase and N-acetylmannosamine kinase, two enzymes considered to be key enzymes in the N-acetylneuraminic acid biosynthesis, were detected only in sialoglycoprotein-secreting tissues, i.e. liver, salivary gland and intestinal mucosa. The low activity of the key enzymes in other tissues suggests that the biosynthetic pathway of N-acetylneuraminic acid is not the same in various tissues.

A Bifunctional Enzyme Catalyzes the First Two Steps inN-Acetylneuraminic Acid Biosynthesis of Rat Liver

“…But the activity of ManNAc kinase is directly linked to that of UDP-GlcNAc 2-epimerase by the bifunctionality of UDP-GlcNAc 2-epimerase/ManNAc kinase. In this case GlcNAc kinase, occurring in several rat tissues (39), may phosphorylate ManNAc (26). A very appropriate method for testing for an alternative pathway of Neu5Ac biosynthesis is to construct a gene knock-out of UDP-GlcNAc 2-epimerase/ManNAc kinase in mouse.…”

Pyrimidine nucleotide metabolism in rat hepatocytes: evidence for compartmentation of nucleotide pools

“…RNA synthesis takes place almost exclusively in the cell nucleus, whereas most activation processes that require pyrimidine ribonucleotides occur in the cytoplasm [9][10][11][12]. It is not known whether the distinct cellular locations at which these processes take place require a similar compartmentation of the nucleoside triphosphates, although many studies are in support of such a concept for pyrimidine nucleotides (for reviews, see [13][14][15][16]).…”

“…In contrast to the cytosolic localization of other nucleotide sugar synthesizing enzymes, CMP-neuraminate (CMP-NeuAc) synthase (EC 2.7.7.43) is located in the nucleus {9,10,20-22], although it is debated whether this is the actual site of CMPNeuAc synthesis [10,23]. Elucidation of metabolic channelling in cytidine metabolism is therefore of particular relevance.…”

Pyrimidine metabolism and sugar nucleotide synthesis in rat liver

“…It is known that the sialic acid-activating enzyme, CMPNeuAc (N-acetylneuraminate) synthase (EC 2.7.7.43) is located in the nucleus [2][3][4][5][6][7], whereas all other known sugar-activating enzymes are located in the cytosol [6,7]. Until now the reason for this peculiar localization is unknown, although it is questionable whether the enzyme is functionally active in the nucleus [8].…”