Alkaline sphingomyelinases and ceramidases of the gastrointestinal tract.

In addition to the acid and neutral sphingomyelinases (SMase) that occur in most tissues, distinct alkaline sphingomyelinases occur in the mucosa of the gastrointestinal tract and human bile. These enzymes exhibit characteristic properties with regard to bile-salt dependence, protease resistance, and longitudinal distribution in the gut. Alkaline SMase has now been partially purified from human bile and from rat small intestine. It is thought to have a role in sphingomyelin (SM) digestion but may also be important for the generation of antiproliferative sphingolipid messengers in the gut. It occurs throughout the whole length of the intestine and also in the colon. It is decreased in colon cancer tissue compared to surrounding mucosa and is extremely low in colon mucosa from patients with familial adenomatous polyposis (FAP). This chapter reviews the properties and potential physiological and pathophysiological significance of alkaline SMase. It also briefly summarizes the knowledge about sphingolipid digestion and about the ceramidases of the gut.