Fig. 6. Ceramides differentially inhibit in vitro enzyme activity of activated PLD2 (A) and activated PLD1 (B). Representative absolute activities for enzymes and control vesicles were: 50 pmole PC hydrolyzed/30 minutes at 37°C for PLD2 plus activators, and 250 pmole PC hydrolyzed/30 minutes at 37°C for PLD1 plus activators. Results are normalized with respect to control vesicles, and represent the average values for at least four experiments with error bars indicating s.e.m. (C) Percent inhibition of PLD1 activity by ceramides compared with percent inhibition of Ca²⁺ mobilization in RBL cells. Filled bars, C₂-cer; open bars, C₂-dhcer; dot-filled bars C₁₆-cer.