Dietary specific sugars for serum protein enzymatic glycosylation in man

All glycoprotein sugars can theoretically derive from glucose. However, die tary specific sugars could represent preferential substrates or have regula tory roles in enzymatic glycosylation. This hypothesis was tested in man us ing stable isotopes. Healthy subjects ingested different amounts (150, 300, or 550 mg) of artificially C-13-enriched sugar (galactose. mannose, or glu cose) diluted in 200 mL water containing 50 g C-13-poor sucrose. C-13 enric hment of expired CO2 was monitored for 8 hours during indirect calorimetry. Serum glycoproteins were precipitated and delipidated at various intervals . Glycoprotein neutral sugars were obtained by acidic hydrolysis, purified by ion-exchange chromatography. derivatized to alditol acetates, and analyz ed by gas chromatography-isotope ratio mass spectrometry. The oxidation rat e for galactose and mannose was slower than the rate for glucose. Total oxi dation over the 8-hour period was less than 10% of the ingested amount of g alactose or mannose. Galactose and mannose were readily incorporated into g lycoprotein glycans. in the native form or after interconversion. despite i ngestion of a large excess of sucrose: glycoprotein sugar C-13 enrichment w as strongly higher after C-13-galactose or C-13-mannose than after C-13-glu cose. Thus. the metabolism of these three sugars appears to be different. S pecific dietary sugars could represent a new class of non essential nutrien ts displaying interesting metabolic roles. This could have practical conseq uences especially in parenteral nutrition, where glucose is currently the o nly sugar available for metabolism. Copyright (C) 1998 by W.B. Saunders Com pany.