The endocrine secretion of mammalian digestive enzymes by exocrine glands

The exocrine pancreas and certain salivary glands of mammals secrete a vari ety of enzymes into the gastrointestinal tract, where they digest food. The same glands also release these enzymes into the bloodstream. This latter p rocess has commonly been assumed to occur solely as the result of a patholo gical condition or as an inadvertent by-product of exocrine secretion due t o the leakage of trace quantities of the enzymes into blood. However, a var iety of evidence suggests that the endocrine secretion of digestive enzymes is a normal occurrence that can be of substantial magnitude in healthy ind ividuals, is responsive to various physiological stimuli, and is distinct f rom exocrine secretion. Recent research has focused attention on this proce ss as a promising means for the delivery of engineered proteins into the sy stemic circulation for pharmaceutical purposes. In this review, we survey r esearch in this area and consider the evidence for the existence of an endo crine secretion of digestive enzymes, the cause of enzyme release into the bloodstream, its source within the tissue, and, finally, the physiological purposes that this secretion process might serve.