Uptake and Recycling of Sialic Acids in Cells

The significance of the poor uptake of Neu5Ac has still to be evaluated. Evidence for a Neu5Ac permease activity has only been presented in C. perfringens (NEES and Schauer 1974). The administration of Neu5Ac orally, intravenously or intraperitoneally leads to partial uptake and incorporation into glycoproteins and 

NeuSAc was cleaved by NeuSAc pyruvate-lyase and entered the cell as ManNAc, followed by resynthesis and incorporation, or whether the same sequence occurs within the cell after transport of NeuSAc across the cell membrane. A final possibility is that the NeuSAc is not cleaved at all by the lyase and is taken up as NeuSAc and activated. 

Further studies using NeuSAc have been related to the metabolism of extracellular CMP-NeuSAc and the detection of ectosialyltransferase activity. On the basis of a lag period observed for NeuSAc uptake and incorporation into glycoconjugates, and the absence of such a lag for CMP-NeuSAc incorporation, two routes have been proposed. The uptake of NeuSAc and metabolism as described above has been studied in hamster and mouse fibroblasts, and cell surface labelling of glycoprotein and glycolipid demonstrated (Datta 1974). The breakdown of CMP-NeuSAc was shown, and incorporation due to NeuSAc uptake rather than direct CMP-NeuSAc transfer proposed (Hirschberg et al. 1976). The uptake of CMP-NeuSAc into the cells (NIL, BHK and 3T3 fibroblasts) could be ruled out, and the K , for NeuSAc uptake was estimated to be 10 mM. Other experiments with CMP-NeuSAc and intact cell cultures (Painter and White 1976, Cerven 1977, see section III.9) pointed to surface sialyltransferase. Further studies by Fan and Datta (1980) provided evidence that both transfer and transport occur, by localization of acceptors within the cell and on the cell surface (plasma membrane), and direct demonstration of the presence of a plasma membrane sialyltransferase. The sialylation due to NeuSAc uptake occurs (at least initially) with different acceptors in comparison with CMP-NeuSAc plasma membrane sialylation. 

A further important observation has been that of a niicrosomal intraluminal pool of CMP-NeuSAc (Carey et al. 1980). Penetration of CMP-NeuSAc into microsomal vesicles occurs via a carrier-mediated transport system, and is in accordance with the site of sialylation in these membranes (Carey et al. 1980, Creek and Morr 1981). 

A study in newborn rats compared the uptake of [4,S,6,7,8,9-i C]NeuSAc and IP-p4C]NeuSAc-Lac administered orally to the animals (Witt et al. 1979). The uptake and distribution of NeuSAc was similar to that reported above, but differences were observed for IP-[i4C]NeuSAc-Lac. The absorption of radioactivity from the trisaccharide was delayed by 30 min relative to NeuSAc, and slower but higher incorporation in the same tissues found for NeuSAc. 

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