ELSEVIER Biochimica et Biophysica Acta 1259 (1995) 49-55 BB Biochi~ic~a et Biophysica A~ta Alkaline sphingomyelinase activity in rat gastrointestinal tract: distribution and characteristics Rui-Dong Duan a,*, Lena Nyberg b, Ake Nilsson c ~ Department of Cell Biology 1, Experimental Research Center, University Hospital ofLund, Lund, Sweden b Research Park ldeon, Swedish Dairies Association, Land, Sweden c Gastroenterology Division, Department of Medicine, University Hospital ofLund, Lund, Sweden Received 6 March 1995; accepted 8 June 1995 Abstract Previous studies indicated that there was an alkaline sphingomyelinase (SMase) activity in small intestine, but its properties have not been studied in detail. In the present work, we studied the distribution of this enzyme activity in rat gastrointestinal tract and characterized it in intestinal mucosal homogenates. Little alkaline SMase activity was detected in the stomach and the duodenum. The activity in both mucosa and intestinal content increased in the small intestine and reached the maximum at the distal jejunum, then declined in the ileum and slightly increased again in the colon. The activity distribution pattern differed markedly from those of acid SMase and alkaline phosphatase. Little alkaline SMase activity could be found in bile, liver and pancreas before or after treatment with trypsin. The optimum pH of the alkaline SMase was 9. It specifically hydrolyzed sphingomyelin (SM), not phosphatidylcholine, to ceramide and phospho- choline. The alkaline SMase was bile salt dependent and was optionally activated by 3 mM bile salts. Triton X-100 could not mimic the effect of bile salt, rather dose-dependently inhibited the enzyme activity. Ca 2+, Mg 2÷ did not change the alkaline SMase activity in the presence of bile salts, and reduced the activity in the absence of bile salt. Trypsin inactivated acid SMase in pancreas, liver and duodenum but had no influence on intestinal alkaline SMase activity. In conclusion, the intestinal alkaline SMase has a specific distribution pattern and the characters of it differ in several respects from the known acid and neutral SMases. Keywords: Sphingomyelinase; Alkaline; Intestine; Bile salt 1. Introduction The interest in sphingomyelin (SM) metabolism has been greatly increased recently, mainly because the hydrol- ysis products of SM have been found to have important signalling effects on cell proliferation and differentiation [1-3]. The first step of hydrolysis of SM is catalysed by sphingomyelinase (SMase), which cleaves SM to ceramide and phosphocholine. Two types of SMase have been iden- tified, characterized, and purified [4]. The first one is the acid SMase which is predominantly localized in lyso- somes, with optimum pH being 4.5 to 5.5 [4-6]. It digests cellular SM and also SM associated with low-density lipoproteins that is endocytosed into the cells. The other known SMase is the neutral SMase with an optimum pH of 7.5. The neutral SMase is characterized as Mg2+-depen - * Corresponding author. Fax: +46 46 137277. 0005-2760/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0005-2760(95)00137-9 dent and its activity was abolished by EDTA [7-10]. The neutral SMase is located in plasma membrane and is believed to be important in generating lipid messengers to regulate cellular functions including proliferation, differen- tiation, and apoptosis [1-3]. About 25 years ago, Nilsson [11,12] found another type of SMase which was located in intestinal mucosa and was different from the known acid and neutral SMases with its alkaline optimal pH of 9.2. However, the distribution and the characters of the alkaline SMase has not been specifi- cally studied in detail. Such studies are of considerable interest for at least two reasons. First, SM is a component of normal diet [13-15] and the enzyme responsible for digestion and absorption of dietary SM has not been well documented. Second, the digestion of SM may potentially have clinical implications, as indicated by the findings that the amount of SM in the enterocytes may regulate the absorption of cholesterol [16], and that administration of dietary SM was able to inhibit the promotion of colon cancers in mice exposed to a chemical carcinogen [17].