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.