Acyl exchange between oleoyl‐CoA and phosphatidylcholine in microsomes of developing soya bean cotyledons and its role in fatty acid desaturation

Microsomes of developing soya bean cotyledons transfer oleate from oleoyl‐CoA to phosphatidylcholine (PC) by two different mechanisms: one in which oleate transfer is accompanied by the release of free CoA and another which results in the exchange of oleate from oleoyl‐CoA for unsaturated 18‐carbon fatty acids of PC. The acyl exchange can be demonstrated only when bovine serum albumin is present in the incubation medium. ATP‐dependent acyl‐CoA synthetase is not involved in the exchange process, which apparently does not require any cofactors. In light of this exchange process, the oleate desaturase system was reinvestigated in order to determine what the actual substrate for this system is. Upon incubation of microsomes with high concentrations of [14C] oleoyl‐CoA, bovine serum albumin and NADH, it could be conclusively demonstrated that most oleic acid is desaturated while part of the PC molecule. The amounts of [14C] linoleoyl‐CoA formed could be explained entirely by the acyl exchange. The physiological significance of the acyl exchange system is discussed. A new method for separation of acyl‐CoA from other lipids and free CoA using reversed phase column chromatography also is described.