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. Can also act as a 12(E) desaturase when acting on the monounsaturated fatty acids oleate and palmitoleate, stereoselectively introducing a trans double bond. 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 a (9Z)-octadecenoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (9Z,12E)-octadecadienoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O a (9Z)-hexadecenoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (9Z,12E)-hexadecadienoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. Expressed exclusively in developing seeds. 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.