Stimulation of acid ceramidase activity by saposin D

Ceramide is ubiquitously present in plasma membranes and plays a pivotal role in metabolism of sphingolipids. In addition, ceramide and its hydrolytic product, sphingosine, may have important roles as second messengers for cell function and survival. Ceramide is hydrolyzed by both acid and alkaline ceramidase. In this investigation, saposin D was found to stimulate the acid, but not the alkaline, ceramidase. With a crude membrane enzyme preparation, the stimulation was about fourfold. Saposin D is one of four saposins (sphingolipid activator proteins) that are derived from a single precursor protein, prosaposin. Saposins, A, B, and C had no significant effect on ceramidase activity. A kinetic study showed that the stimulation by saposin D was caused by decreasing its Km value for ceramide without a significant change in its Vmax value. A binding study demonstrated that saposin D, as well as saposins A, B, and C, binds to ceramide at neutral pH but not at acidic pH. Preliminary evidence suggests that partially purified acid ceramidase contains bound saposin D. These findings suggest that saposin D facilitates acid ceramidase activity primarily by interaction with the enzyme. The function of saposin D as a modulator of ceramide concentration in vivo has been suggested by the accumulation of ceramide in tissues of patients with total saposin deficiency.