Converts linoleic acid to alpha-eleostearic acid (18:3(9Z,11E,13E)) and alpha-linolenic acid to alpha-parinaric acid (18:4(9Z,11E, 13E, 15Z)). Converts a single cis double bond at carbon 12 to two conjugated trans bonds at positions 11 and 13. a (9Z,12Z)-octadecadienoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (9Z,11E,13E)-octadecatrienoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z,12Z,15Z)-octadecatrienoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (9Z,11E,13E,15Z)-octadecatetraenoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. Expressed in developing seeds, but not in leaves. The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the fatty acid desaturase type 1 family.