HEK293 and SMS-KCNR cells were labeled with C12-dhCCPS for 1 hour and then treated with the indicated concentration of hydrogen peroxide for 1h (A,B) or with 200μM hydrogen peroxide for the indicated time (C,D). Percent of conversion of C12-dhCCPS into C12-CCPS (A,C) as well as levels of endogenous dihydroceramide (B,D) were measured by LC/MS. * p<0.05, **p<0.01, ***p<0.001.