Turnover of Free Sialic Acid, CMP‐Sialic Acid, and Bound Sialic Acid In Rat Brain

Abstract

Adult male rats were injected intraventricularly with N-[3H]acetylmannosamine. After different time intervals the rats were killed and free sialic acid, CMP-sialic acid, lipid- and protein-bound sialic acid were isolated from brain and the specific radioactivities determined. Maximal specific radioactivity was reached after approximately 4 h for CMP-sialic acid, after 10-12 h for free sialic acid and after approximately 42 h for lipid- and protein-bound sialic acid. After some days the specific radioactivities of all four pools were the same and decreased equally, with a calculated turnover rate of approximately 3.5 weeks. The conclusion was that this phenomenon was the result of reutilisation of sialic acid and/or precursors. Therefore, the calculated turnover is not the turnover of bound sialic acid, but merely the rate of leakage of sialic acid and/or precursors out of the brain, so that no real turnover can be measured by this method. The first few hours after injection the specific radioactivity of CMP-sialic acid rose above that of free sialic acid. It is supposed that a compartmentalization exists of free sialic acid. The newly synthesised sialic acid molecules are not secreted into the cytoplasmic pool but are preferentially used for the synthesis of CMP-sialic acid. The results and conclusions are discussed in view of the general problems concerning turnover measurements of glycoconjugates.

Pyrimidine metabolism and sugar nucleotide synthesis in rat liver

“…From earlier experiments it was known that, if ethanol was used for the extraction of the nucleotide sugars, minimal hydrolysis of the labile CMP-NeuAc occurred [16]. However, it appeared that, after extraction ofliver tissue with ethanol, a low recovery of other nucleotides was obtained, especially the di-and tri-phosphates.…”

Salvage of catabolic products in ganglioside metabolism: a study on rat cerebellar granule cells in culture

“…Available data on ganglioside turnover are extremely heterogeneous. After administering radioactive low molecular weight precursors to cells or animals, half-lives varying from hours to weeks were reported [4,[11][12][13][14][15][16][17][18]. A possible explanation for this variability may reside in the failure to consider 'salvage processes', that is the re-use of some catabolic products for biosynthesis.…”

“…A possible explanation for this variability may reside in the failure to consider 'salvage processes', that is the re-use of some catabolic products for biosynthesis. In fact, different studies [5,13,[19][20][21][22][23] indicate that some products (sugars, sphingosine, fatty acid) are rapidly and actively recycled for biosynthetic purposes, Thus, the issue is raised whether and how much salvage processes contribute to the overall ganglioside turnover.…”

Efficient biochemical engineering of cellular sialic acids using an unphysiological sialic acid precursor in cells lacking UDP‐N‐acetylglucosamine 2‐epimerase

“…Since £ux of these compounds through the sialic acid biosynthetic pathway is thought only to take a few hours [26], these much longer intervals for cells to reach maximal incorporation most likely re£ect the relatively slow turnover of sialic acids on preexisting cellular glycoconjugates. The average half-life of sialic acid on liver cell surface glycoproteins is in the range of 25^30 h [27].…”