Digestion and Absorption of Sphingomyelin from Milk.

Sphingomyelin (SM) is a major component of mammalian cell membranes. Due to its widespread occurrence it is a dietary component, occurring mainly in milk, eggs, meat and fish. In the cell membrane SM was earlier considered to be only a structural element, which interacts with cholesterol and forms a system for bilayer stabilization. In the last decade there has been an increasing interest in sphingolipid metabolism, since hydrolysis products originating from both endogenous and dietary sphingolipids may have important signalling effects and thereby influence cellular functions, such as cell growth, cell differentiation and apoptosis. After some early studies in Lund in the sixties, rather few studies have concerned the digestion and absorption of dietary SM. In this thesis SM isolated from bovine milk was characterized chemically and physically, and used in studies on its intestinal digestion and absorption. SM digestion is extended all over the small intestine and occurs mainly in the middle and lower parts. This coincides with the intestinal distribution of an alkaline sphingomyelinase (SMase), which may be important for digestion. The capacity of SM digestion is limited. Also after administration of small amounts, all of the small intestine and colon are exposed to SM and its metabolites. When rats were fed a mixture of SM and cholesterol, the uptake of both components in the rat intestine was reduced. A novel alkaline SMase has been identified in human bile, with several properties similar to those of the intestinal SMase. In human milk, enzyme activities for the first two steps in the degradation of SM were identified; an acid SMase and an alkaline ceramidase activity. The latter activity was assigned to the bile salt stimulated lipase (BSSL).