Oxygen, cytochrome biosynthesis response

Squalene epoxidase, like most enzymes responsible for the later steps of sterol biosynthesis [43, 51], is membrane-bound which makes its purification in native form challenging. The purification is additionally complicated by the presence of a large number of cytochrome P450 and other enzymes that have similar hydro-phobicity and size as squalene epoxidase and are hence difficult to remove [52]. Most studies have been carried out with rat liver microsome squalene epoxidase either partially purified or as a homogenate of the cell membrane fraction. In vitro reconstitution of squalene epoxidase activity is absolutely dependent on molecular oxygen, NADPH, FAD, and NADPH-cytochrome c reductase [52, 53]. In this respect, squalene epoxidase resembles the cytochrome P450 enzymes described... 

Like the acyl-CoA desaturases (Chapter 7), the 1 -alkyl desaturase exhibits the typical requirements of a microsomal mixed-function oxidase. Molecular oxygen, a reduced pyridine nucleotide, cytochrome b, cytochrome reductase, and a terminal desaturase protein that is sensitive to cyanide are all required. The precise reaction mechanism responsible for the biosynthesis of ethanolamine plasmalogens is unknown, but it is clear from an investigation with a tritiated fatty alcohol that only the 15 and 25 (erythro)-labeled hydrogens are lost during the formation of the alk-l -enyl moiety of ethanolamine plasmalogens. 

---