Stearoyl-CoA desaturase that utilizes O(2) and electrons from reduced cytochrome b5 to introduce the first double bond into saturated fatty acyl-CoA substrates. Catalyzes the insertion of a cis double bond at the delta-9 position into fatty acyl-CoA substrates including palmitoyl-CoA and stearoyl-CoA. Contributes to the biosynthesis of membrane phospholipids, cholesterol esters and triglycerides. 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 + octadecanoyl-CoA = (9Z)-octadecenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Expected to bind 2 Fe(2+) ions per subunit. Expression is highest in liver, followed by brain and intestine, and lowest in spleen. Also expressed in heart, gill and muscle. Expression is transcriptionally down-regulated by intraperitoneal injection of human leptin at 24 hours and 48 hours post-injection. The histidine box domains are involved in binding the catalytic metal ions. Belongs to the fatty acid desaturase type 1 family.