"Leaky" cells of glandular epithelia

Conservation of digestive enzymes

The traditional understanding is that an entirely new complement of digestive enzymes is secreted by the pancreas into the small intestines with each meal. This is thought to be necessary because, like food itself, these enzymes are degraded during digestion. In this review we discuss experiments that bring this point of view into question. They suggest that digestive enzymes can be absorbed into blood, reaccumulated by the pancreas, and reutilized, instead of being reduced to their constituent amino acids in the intestines. This is called an enteropancreatic circulation of digestive enzymes.

The endocrine secretion of mammalian digestive enzymes by exocrine glands

The exocrine pancreas and certain salivary glands of mammals secrete a variety of enzymes into the gastrointestinal tract, where they digest food. The same glands also release these enzymes into the bloodstream. This latter process has commonly been assumed to occur solely as the result of a pathological condition or as an inadvertent by-product of exocrine secretion due to the leakage of trace quantities of the enzymes into blood. However, a variety of evidence suggests that the endocrine secretion of digestive enzymes is a normal occurrence that can be of substantial magnitude in healthy individuals, is responsive to various physiological stimuli, and is distinct from exocrine secretion. Recent research has focused attention on this process as a promising means for the delivery of engineered proteins into the systemic circulation for pharmaceutical purposes. In this review, we survey research in this area and consider the evidence for the existence of an endocrine 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.