A L Hamill

Requirements of Δ9 and Δ12 fatty acid desaturation in Neurospora

Abstract Microsomes prepared from the wild-type strain and lipid auxotrophs of Neurospora were analyzed for Δ9-(stearoyl-CoA) and Δ12-(oleoyl-CoA) desaturase activities. The wild-type Δ9-desaturase was found to have a 20-fold higher specific activity and 2-fold lower activation energy than the Δ12-desaturase. In addition, Δ12-desaturase had higher Kmapp values for oleoyl-CoA and for NADH than the equivalent values for Δ9-desaturase. These properties were correlated with a rate-limiting role of Δ12-desaturase in the production of 18:2, the major fatty acid of Neurospora. The Δ12-desaturase also exhibited a higher tolerance to pH changes and to cyanide than did the Δ9-desaturase. Both activities could be measured in the same reaction mixture using stearoyl-CoA as the substrate, indicating a coupling of the two enzymes. Enrichment of cellular membranes of the wild-type Neurospora with 18:0 and 18:1, 18:2, 18:3 fatty acids led to the conclusion that the presence of excess substrate in the membrane induces activation of the appropriate desaturase. These experiments also suggested that the membrane fluidity, as determined by the degree of unsaturation of membrane fatty acids, may influence the activities of the desaturating enzymes. Perturbation of the polar head groups of the membrane phospholipids indicated that the correct composition of anionic phospholipids is an absolute requirement for the function of both desaturases. These studies show that the activities of the Δ9-desaturase and the Δ12-desaturase are regulated by a variety of factors and that the Δ12-desaturase is subjected to less stringent controls than the Δ9-desaturase.