Separation of a protein factor necessary for the oxidative desaturation of fatty acids in the rat

Microsomes from rat liver were extracted by low ionic strength solutions. Extracted microsomes lost most of the linoleic acid desaturation activity. The addition of the extract back into the extracted microsomes was necessary to restore full desaturation activity. The soluble fraction had no desaturation activity. The existence of a soluble factor loosely bound to the microsomes, stable to sonication, and unstable to heat and trypsin digestion was recognized. This protein could not be replaced by albumin. The factor was also essential for the oxidative desaturation of palmitic, stearic, linoleic, and gamma-linolenic acid. The present experiment suggests that the protein factor is not NADH-cytochrome b5 reductase, cytochrome b5, or the cyanide-sensitive factor.