Hydrolytic activity of acid ceramidase on <i>N</i>-acylethanolamines, anti-inflammatory and anorexic lipid mediators: an analysis using saposin D-knockout mice

N-Acylethanolamines (NAEs) constitute a class of lipid mediators and include palmitoylethanolamide, oleoylethanolamide, and anandamide, which exert anti-inflammatory/analgesic, anorexic, and cannabimimetic actions, respectively. NAE acid amidase (NAAA) is a lysosomal enzyme hydrolyzing NAEs. On the other hand, acid ceramidase (AC) is another lysosomal enzyme hydrolyzing ceramide (N-acylsphingosine), which shows 33% amino acid identity with NAAA. We previously showed that not only NAAA but also AC is capable to hydrolyze NAEs by using purified enzyme and cultured cells. Here, we examined NAE-hydrolyzing activity of AC in several tissues by using mice lacking saposin D (SAP-D), a presumed activator protein of AC. As analyzed by RT-qPCR, AC mRNA levels in brain, kidney, and liver were not significantly different between wild-type and SAP-D–/– mice. In contrast, C12-ceramide-hydrolyzing activities in the homogenates of these tissues in SAP-D–/– mice were decreased to 16–32% of those in wild-type mice, reflecting the capability of SAP-D to activate AC. Furthermore, the C12:0-NAE-hydrolyzing activities were also lowered to 15–29% in the tissue homogenates of SAP-D–/– mice. These results suggest a role of AC in the degradation of NAEs in tissues.