Has sphingolipid-delta-4-desaturase activity. Converts D-erythro-sphinganine to D-erythro-sphingosine (E-sphing-4-enine) (By similarity). Catalyzes the equilibrium isomerization of retinols (By similarity). an N-acylsphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = an N-acylsphing-4-enine + 2 Fe(III)-[cytochrome b5] + 2 H2O all-trans-retinol = 11-cis-retinol all-trans-retinol = 9-cis-retinol all-trans-retinol = 13-cis-retinol 11-cis-retinol = 13-cis-retinol 11-cis-retinol = 9-cis-retinol Interacts with RLBP1; the interaction increases synthesis of chromophore-precursors by DEGS1. Belongs to the fatty acid desaturase type 1 family. DEGS subfamily.