Palmitate and stearate serve as precursors of the two most common monounsaturated fatty acids of animal tissues: palmitoleate, 16:1(Δ9), and oleate, 18:1(Δ9) (Fig. 20-13). Each of these fatty acids has a single cis double bond in the Δ9 position (between C-9 and C-10). The double bond is introduced into the fatty acid chain by an oxidative reaction catalyzed by fatty acyl-CoA desaturase (Fig. 20-14). This enzymeis an example of a mixed-function oxidase (Box 20-1). Two different substrates, the fatty acid and NADPH, simultaneously undergo twoelectron oxidations. The path of electron flow includes a cytochrome (cytochrome b5) and a flavoprotein (cytochrome b5 reductase), both of which, like fatty acyl-CoA desaturase itself, are present in the smooth endoplasmic reticulum.
Mammalian hepatocytes can readily introduce double bonds at the Δ9 position of fatty acids but cannot introduce additional double bonds in the fatty acid chain between C-10 and the methyl-terminal end.