Conversion of dihydroceramide to ceramide occurs at the cytosolic face of the endoplasmic reticulum

Abstract
Dihydroceramide desaturase is responsible for the introduction of the 4,5‐trans double bond into ceramide. Here, we describe the localization of this enzyme in the endoplasmic reticulum (ER) using ER‐ and Golgi‐enriched fractions from rat liver. Furthermore, enzyme topology was studied. Mild proteolysis of ER‐derived vesicles under conditions which assure membrane integrity (latency of mannose 6‐phosphatase was at least 91%) resulted in an up to 90% inactivation of dihydroceramide desaturase activity. This indicates a cytosolic orientation of dihydroceramide desaturase activity in the ER membrane.

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