Involved in the biosynthesis of highly unsaturated fatty acids (HUFA) from the essential polyunsaturated fatty acids (PUFA) linoleic acid (LA) (18:2n-6) and alpha-linolenic acid (ALA) (18:3n-3) precursors, acting as a fatty acyl-coenzyme A (CoA) desaturase that introduces a cis double bond at carbon 6 of the fatty acyl chain. Catalyzes the first and rate limiting step in this pathway which is the desaturation of LA (18:2n-6) and ALA (18:3n-3) into gamma-linoleate (GLA) (18:3n-6) and stearidonate (18:4n-3), respectively (PubMed:9867867). Subsequently, in the biosynthetic pathway of HUFA n-3 series, it desaturates tetracosapentaenoate (24:5n-3) to tetracosahexaenoate (24:6n-3), which is then converted to docosahexaenoate (DHA)(22:6n-3), an important lipid for nervous system function (By similarity). It can also desaturate (11E)-octadecenoate (trans-vaccenoate) at carbon 6 generating (6Z,11E)-octadecadienoate (By similarity). In addition to Delta-6 activity, this enzyme exhibits Delta-8 activity with slight biases toward n-3 fatty acyl-CoA substrates (By similarity). (9Z,12Z)-octadecadienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (6Z,9Z,12Z)-octadecatrienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z,12Z,15Z)-octadecatrienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (6Z,9Z,12Z,15Z)-octadecatetraenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z,12Z,15Z,18Z,21Z)-tetracosapentaenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (6Z,9Z,12Z,15Z,18Z,21Z)-tetracosahexaenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (11E)-octadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (6Z,11E)-octadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (11Z,14Z)-eicosadienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (8Z,11Z,14Z)-eicosatrienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (11Z,14Z,17Z)-eicosatrienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (8Z,11Z,14Z,17Z)-eicosatetraenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. Highly expressed in the adrenal gland, liver, brain, and testis, tissues where lipogenesis and steroidogenesis are active. Also detected in lung, heart, and skeletal muscle. Found in 13-day-old embryo heart. Induced by dietary PUFA-deficient diet. Induced by a fat-free diet and by a diet containing triolein (18:1n-9) as the only fat source. Down-regulated in liver by dietary PUFA. The protein sequence includes a number of characteristic features of microsomal fatty acid desaturases including the three histidine boxes HXXXH, HXXHH, and QXXHH (these domains may contain the active site and/or be involved in metal ion binding), and the N-terminal cytochrome b5 domain containing the heme-binding motif, HPGG, similar to that of other fatty acid desaturases. Belongs to the fatty acid desaturase type 1 family.