This group represents acid ceramidases and some related proteins of currently unknown function. Ceramide is hydrolyzed by both acid and alkaline ceramidase []. Acid ceramidase requires saposin D, a sphingolipid activator protein, for the lysosomal breakdown of ceramide to a fatty acid and sphingosine []. Ceramide is found in plasma membranes and plays a pivotal role in the metabolism of sphingolipids. In addition, ceramide and sphingosine may have important roles as second messengers for cell function and survival []. Mutations of the human acid ceramidase gene, ASAH1, cause Farber lipogranulomatosis (FRBRL), an autosomal recessive lysosomal storage disorder characterised by subcutaneous lipid-loaded nodules, excruciating pain in the joints and extremities, and marked accumulation of ceramide in lysosomes []. Mutations of ASAH1 also cause spinal muscular atrophy with progressive myoclonic epilepsy (SMA-PME) [].
