Availability of specific sugars for glycoconjugate biosynthesis: a need for further investigations in man.

We review the metabolism of specific sugars used for protein glycosylation, focusing on the fate of exogenously provided sugars. Theoretically, all glycoprotein sugars can derive from glucose, but previous studies show that other exogenous sugars can be incorporated into glycoproteins. From data obtained in congenital galactosemia, exogenous galactose may be important for correct glycosylation. Contrary to galactose, the metabolism of other sugars seems to depend on insulin regulation: stimulation of their endogenous production in diabetic subjects might participate in some diabetic complications, precluding the need for an exogenous supply. The metabolic fate of these sugars is different according to the administration route and exogenous supply may be important either in enteral nutrition or in some clinical situations as has been suggested for sialic acid in the newborn. Data in man are too sparse to reach firm conclusions, implying a need for further investigations. Our preliminary results in animals and man demonstrate that stable isotope methodology allows one to trace glycoprotein sugar metabolism in nutritionally relevant conditions with accuracy and sensitivity, using doses of specific sugars well below toxic levels.