Turnover of free sialic acid, CMP-sialic acid, and bound sialic acid in rat brain.

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.