Incorporation of D-[3H]glucosamine into the sialoglycoprotein GP-350 of adult rat brain

Autoradiographic study of the production of secretory material by the subcommissural organ of frogs (Rana temporaria) after injection of several radioactive precursors, with special reference to the glycosilation and turnover rate of the secretory material

The suitability of several radioactive precursors for studying the secretory processes in the cells of the subcommissural organ (SCO) of frogs (Rana temporaria) was tested by means of autoradiography. Special attention was paid to: the contributions made by different cellular compartments to the glycosilation of the secretory product, and the intracellular turnover rate of the secretory material. From the results it is concluded that: 3H-glucosamine excellently labels Reissner's fibre (RF) in autoradiographs, much better than any other of the radioactive precursors applied. 3H-glucosamine molecules are attached to the protein moiety of the secretory product within the peri- and subnuclear granular endoplasmic reticulum, whereas 3H-fucose and additional 3H-glucosamine molecules are added to the oligosaccharide moiety in the supranuclear Golgi apparatus, previous to apical release; consequently, the subnuclear secretory material and the material that is released into the brain ventricle are chemically different so far as the oligosaccharide moiety is concerned. The oligosaccharide portion of the apical secretory product belongs (at least partially) to the class of the N-linked complex type oligosaccharides. The intracellular half-life of the subnuclear secretory material is at least 5.5 days. The subnuclear secretory material in the ependymal SCO-cells presumably has to pass through the Golgi apparatus before it can be released; this release probably occurs at the apical cell border.

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.

The effect of lithium on the incorporation of 3H-glucosamine into glycopeptides and the transformation of 3H-glucosamine into sialic acid in rat brain

Rats were fed a lithium-containing diet (40 mmol LiCl/kg of diet) for five weeks. The plasma-lithium concentration was 0.48 mmol Li+/l of plasma. 3H-glucosamine (400 nCi/kg body weight was injected intraperitoneally, and 24 hrs later the brain was divided anatomically into hemisphere, cerebellum, pons and thalamus. The brain parts were defatted and the sialic acid-containing glycopeptides were separated by column chromatography before determination of 3H-glucosamine, 3H-sialic acid and total sialic acid. Neither the total amount nor the specific activity of sialic acid were influenced by the lithium treatment. Also the 3H-content of the sialic acid-free glycopeptides remained unchanged.