22] Ceramidases

Abstract

Ceramidase activity was first characterized as an enzyme with an acidic pH optimum present in all rat tissues examined. This enzyme was later shown to account for a rare genetic disorder called Farber's disease where there is accumulation of ceramide in the lysosomes of spleen, cerebellum, fibroblasts, and kidney. An activity with alkaline pH optimum, however, was retained in Farber's patients, indicating that at least two ceramidases hydrolyze ceramide. Both enzymes are membrane associated; however, the acidic enzyme is localized primarily in the lysosomes and activated by Saposine D, which inhibits the alkaline enzyme (IC 50 1-5μM). The purified acid ceramidase shows little specificity with respect to the fatty acid chain length of ceramidases, but prefers sphingosine over sphinganine-containing ceramides. In addition to the acidic and alkaline activities, hydrolysis of ceramide is also found at neutral pH in rat liver, small intestine, and epithelial cells. Ceramidase (N-acylsphingosine deacylase) hydrolyzes ceramide to a free sphingoid base and fatty acid. Because ceramide can induce cell death, whereas the phosphorylated product of sphingosine, sphingosine-1-phosphate, is a potent inducer of cell proliferation and survival signals, ceramidase(s) play a key role in regulating the amounts of the bioactive sphingolipids. This chapter discusses methods for assaying ceramidases as well as factors known to participate in the regulation of these enzymes.