LOCALIZATION AND CAPACITY OF SPHINGOMYELIN DIGESTION IN THE RAT INTESTINAL-TRACT

Dietary sphingomyelin (SM) undergoes sequential cleavage to ceramide a nd sphingosine in the intestine. A distinctive intestinal sphingomyeli nase (SMase) with alkaline pH-optimum was earlier identified by us. Th e activity was higher in middle and lower small intestine, but its rol e in SM digestion has not been clarified. In this study we examined th e extension and capacity of SM digestion has not been clarified. In th is study we examined the extension and capacity of SM digestion in viv o. After feeding rats 0.2, 6.6, or 32 mu mol SM containing 2 mu Ci(3)H -sphingosine-labeled milk SM (H-3-SM), radioactivity was analyzed in i ntestinal contents and tissues 2, 4, and 8 hr later. The proportion of radioactivity in the contents of small intestine increased with the d ose of SM; 9% of given dose with 0.2 mu mol, 34% with 6.6 mu mol, and 71% with 32 mu mol, respectively after 2 hr. Lowest tissue radioactivi ty was found in duodenum and proximal jejunum and highest in distal je junum and proximal ileum. Three to twenty one percent of radioactivity in the intestinal tissue was in ceramide, the proportion varying with the dose given, region of the intestine, and time after administratio n. After administration of 6.6 or 32 mu mol SM, significant amounts of intact SM and ceramide was found in intestinal contents, colon, and e xcreted faeces. Colon was exposed to ceramide derived from exogenous S M in amounts that were rather proportional to the dose of SM fed. SM d igestion is thus a process extending over the whole intestine and occu rring mainly in the middle and lower parts of the small intestine. The site of digestion coincides with the distribution of the alkaline SMa se, indicating that this enzyme catalyzes the first step in the digest ion. The extension and limited capacity of the SM digestion leads to a n exposure of the lower small intestine and colon to SM and sphingolip id metabolites. (C) Elsevier Science Inc. 1997.