Digestion
Digestion begins in the mouth with the secretion of saliva and its digestive enzymes. Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis. Gastric juice contains hydrochloric acid and pepsin which would damage the walls of the stomach and mucus is secreted for protection. In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach. The partially digested food enters the duodenum as a thick semi-liquid chyme. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile, pancreatic juice and intestinal juice. The intestinal walls are lined with villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine.
ENPP7
ENPP7 is the key enzyme in the gut that digests sphingomyelin. Sphingomyelin is a lipid constituent of cell membrane, and a dietary component being particularly abundant in milk, cheese, egg, and meat. Digestion of sphingomyelin mainly occurs in the middle part of the small intestine, where ENPP7 is abundant, indicating a role of the enzyme in sphingomyelin digestion. Recent studies on ENPP7 knockout mice clearly showed that digestion of sphingomyelin and generation of ceramide is severely affected in ENPP7 deficiency mice. ENPP7 is fully developed in the intestine before birth, which gives the infant ability to digest sphingomyelin in the milk.