Structure and metabolism of glycoproteins: nutritional regulation

Use of compounds naturally labeled with stable isotopes for the study of the metabolism of glycoprotein neutral sugars by gas-liquid chromatography-isotope-ratio mass spectrometry. Technical validation in the rat

In order to develop an alternative method to radioactive labeling for the study of the glycoprotein sugar metabolism in man, the possible use of stable isotopes provided by naturally, 13C-enriched dietary compounds has been explored in rat intestine and serum. Rats were fed a semisynthetic diet containing 67% wheat starch (containing 1.08692 13C atom/100 carbon atoms) for a week, and then the same diet containing corn starch (1.10042% 13C) for a week. Neutral sugars were prepared from delipidated, trichloroacetic acid-precipitable material from 200-400 mg of intestinal mucosa or 1 mL of serum, separated, and analyzed as alditol acetates by gas-liquid chromatography coupled to isotope-ratio mass spectrometry. This technique allowed the determination, in a single experiment, of the amount and 13C abundance of six neutral sugars (including xylose used as internal standard). Despite the low abundance of 13C in natural products, the sensitivity of the technique was found to be sufficient to detect isotopic enrichment as low as 0.001% with good accuracy and reproducibility in 2 micrograms of each glycoprotein neutral sugar. As an example, the pattern of labeling by dietary D-glucose from corn starch appears to be very different for fucose, ribose, mannose, galactose, and glucose of intestine or serum macromolecules.

Nutritional and developmental regulation of glycosylation processes in digestive organs

We review the nutritional and developmental variations of the glycosylation processes in digestive organs, since glycoproteins play a prominent part as mucins or digestive enzymes in these tissues. The biosynthesis of the glycannic chains is demonstrated to be largely sensitive to various exogenous (such as nutritional) or endogenous (such as developmental) factors. Although the metabolic regulation by dietary variations appears as rather complex, according to the variety of experimental conditions and the diversity of the organs studied, available data demonstrate that this regulation does exist, depending on the quantity or sometimes the quality of the major or minor components of the diet, which induce significant variations in the glycosylation processes. The synthesis of the internal core of N-glycans is essentially regulated by diet-induced variations of the phosphoryl-dolichol level, whereas the modulation of the biosynthesis of the external part of N-glycans or the biosynthesis of O-glycans is controlled by diet-induced variations in the systems transferring fucose, galactose, sialic acid or hexosamines. Modifications in intestinal glycosylation during post-natal development in the rat control the quality of the glycannic chains of mucins and brush-border enzymes. The post-natal maturation of the intestinal rat tissue is characterized by a shift from sialylation to fucosylation, depending on coordinate changes in glycosyltransferase activities, in sugar-nucleotide breakdown or synthesis or in the activity of regulatory proteins. These activities are largely sensitive to dietary manipulations at weaning and to hormonal stimulations before weaning. However, glucocorticoid hormones do not appear as the triggering signal for the induction of these changes.

Participation of an endogenous inhibitor of fucosyltransferase activities in the developmental regulation of intestinal fucosylation processes

During the rat weaning period (about day 19 after birth) the intestinal maturation is accompanied by a drastic increase in the fucose content of mucosal glycoconjugates, concomitant with an increase in fucosyltransferase activities. The regulation of this fucosylation process appears to be a rather complex phenomenon, which involves several systems controlling fucosyltransferase activity or substrate availability. An endogenous protein inhibitor of the fucosyltransferase activities displays an opposite developmental pattern to that of fucosyltransferase activities, since its activity is high before weaning and is decreased 5-fold after weaning. Similarly, the GDP-fucose pyrophosphatase activity markedly decreases at weaning. The transformation of GDP-mannose into GDP-fucose increases early, at day 18, preceding the increase in fucosyltransferase activities. Before weaning, and especially at days 14 and 18, high levels of GDP-4-dehydro-6-deoxymannose, the product of the GDP-mannose 4,6-dehydratase activity, are produced during the transformation of GDP-mannose into GDP-fucose, even in excess of reduced coenzyme. This fact indicates that the second step of the transformation (epimerase-reductase reaction) could be a limiting factor for GDP-fucose availability before weaning, but not after weaning. The inverse relationship between the mucosal fucose content (or the fucosyltransferase activity) and the endogenous protein inhibitor during normal postnatal development supports the hypothesis of a physiological role for this inhibitor.

Evidence for the presence of an endogenous cytosolic protein inhibitor of intestinal fucosyltransferase activities

Soluble endogenous inhibitory activities for glycoprotein: alpha (1-2) and alpha (1-3) fucosyltransferases are demonstrated in rat small intestinal cytosol. These inhibitors are retained on DEAE-cellulose and are eluted as two fractions A and B. Fraction B is non dialyzable, heat stable and pronase-resistant and consists probably of poly-nucleotides. Fraction A is also non-dialyzable, but is thermolabile and pronase-sensitive, suggesting that it contains proteins. The inhibition of fucosyltransferase activity by fraction A is competitive for GDP-fucose and non-competitive for the glycoprotein substrate. Inhibition is not due to interfering enzymatic activities (glycosyl-nucleotide pyrophosphatases, glycosidases or proteases) and is reversible. This protein inhibitor, with a molecular weight of 60,000, is found only in the intestine and the pancreas and appears to be different from the previously reported inhibitors of brain glycolipid glycosyltransferases.