Sphingolipid metabolism. De novo sphingolipid production starts with the reaction of serine and palmitoyl CoA, mediated by serine palmitoyl transferase. Sphingolipids are metabolized in many reversible and irreversible reactions. Ceramide is at the center of the sphingolipid metabolism network and can result from the hydrolysis of SM, which in turn results in the production of sphingosine and S1P downstream.