Identification and quantification of serum proteins secreted into the normal human jejunum

Characterization and quantification of plasma proteins excreted in faeces from healthy humans

Faecal plasma protein loss was studied in 38 healthy adults. Using crossed immunoelectrophoresis and single radial immunodiffusion the most frequently found proteins were alpha 1-antitrypsin, IgA, alpha 1-antichymotrypsin (found in 97, 92, and 84% of subjects), prealbumin and IgM (both found in 55%). The major plasma proteins, albumin and IgG, were found in 37 and 13% of subjects, respectively, and in trace amounts only. alpha 2-macroglobulin could not be detected. There was no relation between the presence of proteins in faeces and their plasma concentration. When added to faeces, alpha 1-antitrypsin, alpha 1-antichymotrypsin, and prealbumin were resistant to incubation (37 degrees C, 48 h), whereas albumin, IgG, IgM, and IgA were rapidly degraded (within 8-24 h). Some IgA was bound to secretory component, indicating enteric secretion. alpha 2-macroglobulin was semi-resistant to degradation, but its passage to the intestinal lumen may have been prevented by its molecular size. In conclusion, resistance to degradation, enteric secretion, and low molecular weight are the primary factors which favour the excretion of plasma proteins in faeces. The technique used in this study allows further studies in patients with inflammatory changes and protein-losing enteropathy.

Bidirectional small-intestinal permeability in the rat to some common marker molecules in vitro

BACKGROUND

The barrier properties of the small intestine were investigated by studying the bidirectional permeability to five commonly used marker molecules.

METHODS

Proximal and distal small-intestinal segments from rats were mounted in diffusion chambers, and the permeation of the markers 3H-mannitol (Mw 182), 51Cr-ethylenediamineteraacetic acid (Mw 341), [mercaptopropionic acid], D-arginine8]-vasopressin (Mw 1069), fluorescein isothiocyanate (FITC)-dextran (mean Mw 3000), and inulin (Mw 5200) was measured across the mucosa in both directions.

RESULTS

A generally increased inward (mucosa to serosa) and a decreased outward (serosa to mucosa) permeation of the markers was found in the proximal to distal direction. The inward permeability showed increasing regional differences with decreasing size of the markers. In the absence of the villous epithelium, removed by scraping the intestinal wall, 86% to 62% of the proximal and distal barrier was lost in the inward direction but only 14% to 26% in the outward direction.

CONCLUSIONS

The intestinal epithelial barrier is more permeable in the outward than in the inward direction, and regional permeability differences exist in a size-dependent fashion. The results suggest two passage routes, one for the smallest molecule, mannitol, and a second for the larger markers in the present size range, both apparently different from the route for macromolecules such as intact proteins.

On the transfer of serum proteins to the rat intestinal juice

The in vivo pattern of serum proteins in the rat small-intestinal juice was characterized by crossed immunoelectrophoresis. Immunoglobulins and albumin, alpha-1-antitrypsin, transferrin, and orosomucoid were present. Larger serum proteins were absent (ceruloplasmin, haptoglobin, alpha-1-macroglobulin, alpha and beta lipoproteins). Thus, apart from immunoglobulins, only serum proteins with a molecular mass less than approximately 100 kDa were demonstrated. The origin and epithelial transfer were further characterized, using albumin as a model. No sign of local synthesis of albumin by the enterocytes was found by Northern blotting, and no albumin was found in the Golgi complex by immunogold electron microscopy. By immunogold electron microscopy a heavy labelling of albumin was observed in the interstitial spaces between the villus enterocytes. Where the enterocytes disintegrated, albumin was seen to leak out into the intestinal lumen from the opened interstitial spaces. A weak labelling was also found in the lysosomal/endosomal-like structures, especially in the crypt enterocytes, indicating pinocytosis of albumin. We conclude that the main reason for the occurrence of certain serum proteins in the intestinal juice is a selective passage through the capillary wall followed by passive intercellular transport via delivery of the serum in the interstitial space during disintegration of the enterocytes.