Catalysis of the reaction: (9Z)-hexadecenoyl-CoA + 2 Fe(II)- + 2 H+ + O2 = (9Z,12Z)-hexadecadienoyl-CoA + 2 Fe(III)- + 2 H2O. Also converts (9Z)-octadecenoyl-CoA into (9Z,12Z)-octadecadienoyl-CoA. This microsomal enzyme introduces a cis double bond at position 12 of fatty-acyl-CoAs that contain a cis double bond at position 9.
delta12-fatty-acid desaturase activity