a1-Antitrypsin is an a1 proteinase inhibitor that is expressed in the liver and the intestine. It acts as an acute phase protein by inhibiting serine proteases (e.g. elastase, trypsin, or chymotrypsin) and inflammatory processes by forming reversible complexes. In faeces, a1 antitrypsin is found both free or conjugated to either trypsin or elastase.
The higher concentration of proteinases in the faeces is caused by an increased permeability of the intestinal epithelial layer which in turn is set off by an abnormal array of the intestinal “tight junctions” that subsequently results in an enteral loss of plasma a1-antitrypsin.
Faecal loss of a1-antitrypsin has now become an accepted parameter for the evaluation of an enteral protein loss. The ELISA is now an established method, especially for the diagnosis of M. Crohn. It could be established, that this marker is well suited for indicating changes of intestinal permeability in cases other than colitis or M. Crohn. Immunological processes in the intestinal mucous layer, that may be caused by food intolerance, also result in an increase of faecal a1-antitrypsin concentrations.