Mucins: gastrointestinal

Mucins are a class of O-linked glycoproteins that are distributed on the surfaces of epithelial cells of the respiratory, gastrointestinal, and reproductive tracts. The Golgi apparatus plays a major role in glycosyla-tion reactions involved in the biosynthesis of glycoproteins. 

For novel oligosaccharides for which H-n.m.r. data have not previously been documented in the literature, it is necessary to supplement the H-n.m.r. data with composition, sequence, and linkage information provided by chromatographic and m.s. analyses. An illustrative example is the characterization of the following oligosaccharide of the glycopeptides of fetal gastrointestinal mucins of meconium (74) ... 

Gastrointestinal, pancreas, biliary, lung, transitional cell, sweat glands, mucosal squamous cell, mucinous carcinomas of female GU tract, medullary carcinoma of thyroid Breast, squamous cell, endometrioid, Brenner tumor Renal cell, hepatocellular, prostate, follicular thyroid, adrenal cortical, serous carcinomas of female GU tract, embryonal, yolk sac, mesothelioma... 

Given the vast molar excess of mucin to histamine in the gastrointestinal tract, it is difficult to envision that such inhibition of binding would play more than a secondary role in the potentiation of histamine toxicity. 

Absorption of some highly ionized compounds (e.g., sulfonic acids and quaternary ammonium compounds) from the gastrointestinal tract cannot be explained in terms of the transport mechanisms discussed earUer. These compounds are known to penetrate the Upid membrane despite their low Upid-water partition coefficients. It is postulated that these highly lipophobic drugs combine reversibly with such endogenous compounds as mucin in the gastrointestinal lumen, forming neutral ion pair complexes it is this neutral complex that penetrates the Upid membrane by passive diffusion. 

Braybrooks, M.P., B.W. Barry, and E.T. Abbs. 1975. The effect of mucin on the bioavailability of tetracycline from the gastrointestinal tract in vivo, in vitro correlations. J Pharm Pharmacol 27 508. 

Most substances are not readily absorbed in the mouth or esophagus one of several exceptions is nitroglycerin, which is administered for certain heart disfunctions and absorbed if left in contact with oral tissue. The stomach is the first part of the gastrointestinal tract where substantial absorption and translocation to other parts of the body may take place. The stomach is unique because of its high content of HC1 and consequent low pH (about 1.0). Therefore, some substances that are ionic at pH values near 7 and above are neutral in the stomach and readily traverse the stomach walls. In some cases, absorption is affected by stomach contents other than HC1. These include food particles, gastric mucin, gastric lipase, and pepsin. 

The thickness of the mucus layer changes in function of age. In rats a subsequent increase of microclimate pH from 6 to 6.5 is observed with increasing age [91 The influence of gastrointestinal mucin on enzyme absorption is not well documented, but it is an intestinal diffusion barrier for many nutrients [10],... 

S. Satchitbanandain, D. Kluribld, R. Calvert and M. Cassidy. Effects of dietary fibers on gastrointestinal mucin in rats. Nutrition. Heston k. 16(7) 1163-1177 (1996). 

Fig. 2.1 Multi-domain structural organization of gastrointestinal gel-forming mucins. Schematic drawings (not at scale) of the different domains found in gastrointestinal mucins and their roles. Note that while the cysteine-rich domains (D-, CS-, C-, and CK-domains) have a similar length among mucin alleles, the O-glycosylated regions, except in the case of MUC5B, differ in size, a feature not shown in the drawings...

Tetracycline has been shown to bind to gastrointestinal mucus by hydrogen-bonding and via electrostatic and hydrophobic interactions. It has been suggested that cephaloridine and gentamicin bind intestinal mucin via ionic interactions. In addition, gentamicin has been shown to bind to sputum and tobramycin binds to glycoprotein- and DNA-rich fractions of sputum. 

Hoskins, L.C. 1992. Mucin degradation in the human gastrointestinal tract and its significance to enteric microbial ecology. Eur. J. Gastroenterol. Hepatol. 5, 205-213. 

Positive Gastrointestinal, pancreas, biliary, lung, transitional cell, sweat glands, mucosal squamous cell, mucinous carcinomas of female GU tract, medullary carcinoma of thyroid... 

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