Stearoyl-CoA desaturase: rogue or innocent bystander?

Different lipid fractions in humans have characteristic fatty acid profiles and these are maintained partly through diet and to a lesser extent through endogenous synthesis. The enzyme stearoyl-CoA desaturase (SCD; EC 1.14.99.5) is the rate-limiting enzyme in the synthesis of monounsaturated fatty acids such as palmitoleic acid (16:1 n-7) and oleic acid (18:1 n-9). These are the two most abundant monounsaturated fatty acids in human plasma lipids, membranes and adipose tissue. Although in quantitative terms, the endogenous synthesis of fatty acids in humans is not great in most circumstances, it is becoming increasingly evident that SCD plays important structural and metabolic roles. In addition, 16:1 n-7 has been purported to act as a beneficial 'lipokine' in an animal model. Research in humans has relied on indirect measurements of SCD1 activity and therefore, much of our understanding has come from work on animal models. However, results have been somewhat counterintuitive and confusing, so the purpose of this review is to try to summarise our current understanding of this fascinating enzyme.