Distribution and properties of neutral ceramidase activity in rat intestinal tract

Ceramide plays an important role in regulating cell proliferation and apopt osis. Recent studies indicate that generation of ceramide in the intestine from sphingomyelin hydrolysis may be implicated in colon cancer development . The enzymes that catalyze the further hydrolysis of ceramide in the intes tine have, however, not been well investigated. Our data reveal the existen ce of a ceramidase (EC 3.5.1.23) in rat intestinal mucosa with an optimal p H of 7.0. One milligram of mucosal protein is able to hydrolyze 44.0 +/- 9. 6 nmol of ceramide in I hr. The activity is low in the proximal duodenum an d increases to a plateau in the proximal jejunum. The activity is then simi lar throughout the small intestine, until it declines in the distal part of ileum. Some activity is also detectable in the colon. The activity increas es slightly in the presence of monomeric bile salt concentrations and sharp ly at the critical micellar concentration. Similar patterns were observed f or both primary (taurocholate) and secondary (taurodeoxycholate) bile salts . The addition of Triton X-100 enhances the ceramidase activity at optimal bile salt concentration. The reaction is linear with time for the first 20 min and the hydrolytic rate declines slowly thereafter. Finally, the activi ty shows a considerable resistance against tryptic degradation, as 71% of t he ceramidase activity remained when the homogenates were preincubated with high concentrations of trypsin. Intestinal mucosa also has a ceramide synt hesis activity, with a distribution pattern generally paralleling ceramide hydrolysis activity. In conclusion, intestinal neutral ceramidase has a dis tinct distribution pattern and bile salt dependence, which enables it to co llaborate with intestinal sphingomyelinase in hydrolysis of sphingomyelin.