Chemistry and biosynthesis of mucin glycoproteins

Ultrastructure and histochemistry of acid mucus glycoproteins in the estrous mammal oviduct

The mucous surfaces of the oviducts of mammals, especially humans, are dependent on estradiol. The mucus glycoproteins and glycocalyceal glycoproteins have, however, barely been studied. Biochemical analyses have focussed on the relatively low molecular weight glycoproteins likely to be found in more serous-type granules of the ampulla and not on the very high MW glycoproteins typical of mucus and represented in the isthmus by morphological evidence of mucus secretion. Quantitatively, secretion from the ampulla is likely to predominate, because of its huge surface area compared with the isthmus. But functional closure of the isthmus under the influence of estradiol in the absence of progesterone means that it is the isthmus where luminal secretions accumulate--and where mucus glycoproteins will exert their most important effects on spermatozoa ascending the reproductive tract, and then on fertilized ova en route to the uterus. Further study of the extracellular, intraluminal, carbohydrate-rich environment of the oviductal isthmus, especially in humans, is likely to prove rewarding. Sampling of these secretions is now feasible using transvaginal, transuterine fallopian tube catheters that are in clinical use.

Neuraminidase in Bacteroides fragilis

A neuraminidase from Bacteroides fragilis was purified 542-fold by isoelectric focusing, adsorption chromatography on Affi-Gel 202, and gel filtration chromatography on Sephadex G-200. On isoelectric focusing the neuraminidase was resolved into three differently charged fractions with pI values of 6.8, 7.1, and 7.4. The major component of pI 7.1 was used for further purification. The purified enzyme had optimal activity at pH 6.4 with N-acetylneuraminlactose as the substrate. Its molecular weight, determined by Sephadex G-200 gel filtration chromatography, was 92,000. The neuraminidase hydrolyzed terminal neuraminic acid residues from N-acetylneuraminlactose, fetuin, bovine submaxillary mucin, and porcine stomach lining mucin. A new method for the detection of neuraminidase activity is described which is based on rocket affinoelectrophoresis. It utilizes the differences in the interaction of sialylated and desialylated mucin with Helix pomatia lectin, enzymatic activity being detected by formation of affinorockets after incubation of the neuraminidase with bovine submaxillary mucin.

Structure and function of mucus

Discussing the available evidence a fairly strong case can be made for the existence of a basic glycoprotein unit, characterized by what may be a common protein backbone (Fig. 1). This is far less likely for the carbohydrate portion. The considerably more variability in the amount and composition of the carbohydrate coat and species and organ differences may arise because of this fact. Very large aggregates are built up from the basic unit using cross-links of disulfide bonds either intermolecularly, i.e. directly, or intramolecularly, i.e. indirectly via a possible lectin-like structure which forms its bond with some of the carbohydrate side chains. Structures of the order of 10-100 million molecular weight are to be expected which, being heavily entangled, give rise to the special rheological character of the mucus. In most instances mucus behaves rheologically like a gel. The concentration of glycoprotein in the mucus may be the most important parameter which determines the special rheological features required in a special functional context. A unified point of view, when discussing mucus structure and function, was taken. On the evidence available, it seems well justified to continue to do so.