Sphingolipid hydrolysis enzymes in the gastrointestinal tract

In the intestinal tract, there are enzymes that hydrolyze both endogenous and exogenous sphingolipids. The alkaline sphingomyelinases (SMase) of the gut and human bile have been most studied, and a major part of this chapter discusses these enzymes. It also discusses studies of ceramidase, glycosylceramidase, and the digestion of dietary sphingolipids. In the intestinal tract, a distinct enzyme that hydrolyzes SM was discovered in 1969 by Nilsson and named “alkaline SMase.” The alkaline SMase activity is localized specifically in the intestinal tract and is not detectable in other organs, including brain, kidney, lung, spleen, testis, pancreas, and stomach, or in milk and urine. The intestinal alkaline SMase is not bacterial in origin as similar activity is found in ordinary mice and germ-free mice and in meconium of human fetus as early as 26 weeks of gestation. In the intestines of different experimental animals, the highest activities were in rat, mouse, pig, and baboon, lower activity in rabbit, and little activity in the guinea pig. Whether these species differences indicate genetic variation or an influence of diet composition on the expression of the enzyme in the intestine is unknown.