Thoughts on Physiological Function of Sialic Acids

Eylar (1965) proposed that the function of protein-bound carbohydrate was as a signal for secretion of a particular protein species. This suggestion was based on the observation that most extracellular proteins contained carbohydrate, while intracellular proteins did not. The addition of carbohydrate would either be a signal or would be an intrinsic part of the secretion mechanism. This theory has been updated by Winterbum and Phelps (1972). The major problem with this proposal is that many proteins that are secreted do not contain carbohydrate (e.g., albumin). Also, many intracellular proteins do contain carbohydrate. Nevertheless, this proposal, in one form or another, is still the basis for current hypotheses on the role of bound carbohydrates. Schachter and Roden (1973) have modified this theory and suggest that the addition of carbohydrate is a signal for movement of molecules across membranes (internal as well as external). Those molecules which lack carbohydrate, but nevertheless have participated in such a mechanism, would have had it removed after the membrane transport step had occurred. 

Roth et al. (1971) found that some glycosyltransferases were located 

A great effort is underway, predicated on the notion that, in some way, carbohydrates of cell surfaces are important in cell-cell interactions. A great deal of work at the phenomenological level has been 

and Kent, P. W., 1968, Studies on the enzymatic N-acylation of amino sugars in the sheep colonic mucosa, Biochem. J. 107 589-598. 

Aminoff, D., Dodyk, F., and Roseman, S., 1963, Enzymatic synthesis of colominic acid, J. Biol. Chem. 238 pcll77-pcll78. 

---