Biosynthesis of membrane glycoproteins in the rat small intestine

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.

Glycosylation in intestinal epithelium

This chapter reviews the glycosylation reactions in the intestinal epithelium. The intestinal epithelium represents a good model system in which the glycosylation process can be studied. The intestinal epithelium is composed of two basic epithelial cell types: the absorptive enterocyte and the mucus-producing goblet cell. Gastrointestinal epithelial renewal ensues through the processes of cell proliferation, migration, and differentiation. This renewal occurs in discrete proliferative zones along the gastrointestinal tract. In the small intestine, this proliferative zone is restricted to the base of the crypts, whereas in the large intestine it is less restrictive, occurring in the basal two thirds of the crypt. A longitudinal section along the crypt-to-surface axis, cells in various degrees of differentiation is observed, providing a unique in vivo system in which to investigate differentiation-related glycosylation events. The glycoconjugate repertoire displayed by a given cell reflects its endogenous expression of glycosyltransferases. The role played by terminal oligosaccharide structures in cell–cell recognition phenomena and the expression of glycosyltransferases occupy a key position in the post-translational processing of glycoconjugates and thus influence cellular function.

[Kinetic parameters of a soluble purified intestinal fucosyl-transferase]

The soluble fucosyl-transferase of the rat small intestinal mucosa was purified by passing through a Sephadex G-100 column, then resolved in two isoenzymes F1 (pHi = 4,47) and F2 (pHi = 4,96) by isoelectric focusing. The use in the first step of DEAE-cellulose instead of Sephadex G-100 leads to another component F3 (pHi = 8,70). Kinetic parameters (KM et Vm) for the three isoenzymes are given. The enzymatic mechanism seems to be a bi-bi random type for the three isoenzymes, and F3 appears as the most active species.

The differentiation and redifferentiation of the intestinal epithelium and its brush border membrane

This paper briefly reviews the progress of investigations during the past 30 years into the development of the intestinal epithelium, and surveys the directions in which research in this field is now advancing. Early studies showed that the differentiation of the epithelium is a discontinuous process, and also focused attention on the enzyme specialization of the brush border. The phases of differentiation were found to be under the control of the pituitary--adrenal system, with the outflow of glucocorticoids influencing both structural and biochemical aspects of enterocyte development in fetal and postnatal stages. More recent evidence has shown that thyroid hormones can also regulate epithelial differentiation, even in the absence of adrenocortical function. Organ culture is now becoming an important means of probing into the processes underlying overt differentiation, and methods for culturing pure populations of enterocytes are being perfected. Studies of developmental alterations in the constituents of the brush border membrane, and of synthesis and glycosylation of membrane constituents, are contributing to the development of an integrated account of the differentiation of this membrane, through which all nutrients must pass.

Glycoprotein synthesis in the adult rat pancreas. IV. Subcellular distribution of membrane glycoproteins

Zymogen granule membranes from the rat exocrine pancreas displays distinctive, simple protein and glycoprotein compositions when compared to other intracellular membranes. The carbohydrate content of zymogen granule membrane protein was 5-10-fold greater than that of membrane fractions isolated from smooth and rough microsomes, mitochondria and a preparation containing plasma membranes, and 50-100-fold greater than the zymogen granule content and the postmicrosomal supernate. The granule membrane glycoprotein contained primarily sialic acid, fucose, mannose, galactose and N-acetylglucosamine. The levels of galactose, fucose and sialic acid increased in membranes in the following order: rough microsomes less than smooth microsomes less than zymogen granules. Membrane polypeptides were analyzed by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The profile of zymogen granule membrane polypeptides was characterized by GP-2, a species with an apparent molecular weight of 74 000. Radioactivity profiles of membranes labeled with [3H]glucosamine or [3H]leucine, as well as periodic acid-Schiff stain profiles, indicated that GP-2 accounted for approx. 40% of the firmly bound granule membrane protein. Low levels of a species similar to GP-2 were detected in membranes of smooth microsomes and the preparation enriched in plasma membranes but not in other subcellular fractions. These results suggest that GP-2 is a biochemical marker for zymogen granules. Membrane glycoproteins of intact zymogen granules were resistant to neuraminidase treatment, while those in isolated granule membranes were readily degraded by neuraminidase. GP-2 of intact granules was not labeled by exposure to galactose oxidase followed by reduction with NaB3H4. In contrast, GP-2 in purified granule membranes was readily labeled by this procedure. Therefore GP-2 appears to be located on the zymogen granule interior.

Glycoprotein alterations in human colonic adenocarcinoma

A marked diminution in membrane glycoproteins containing blood group A activity was observed in colonic cancer tissues. This change was associated with a reduction of the enzyme responsible for its biosynthesis and a decreased concentration of N-acetylgalactosamine in the cancer tissues. Glycosidase activities were unchanged. In addition to the changes associated with blood group A, we also found a decrease in sugar content, alterations in other antigens, and changes in the levels of several glycosyltransferases in cancerous tissues.