enterokinase, proteolytic enzyme (q.v.), secreted from the duodenal mucosa, that changes the inactive pancreatic secretion trypsinogen into trypsin, one of the enzymes that digest proteins. Enterokinase is believed to be produced by the glands of Brunner in the membrane lining of the duodenum. It resists destruction from the various enzymes in the small intestine but is destroyed by bacteria in the large intestine. Enterokinase can also change inactive procarboxypeptidase into the active enzyme carboxypeptidase.