Chain elongation in the formation of polyunsaturated fatty acids by brain: Some properties of the microsomal system

Abstract

Chain elongation of polyunsaturated acids has been investigated using microsomes from developing rat brain. With 18:3(n − 6) in 0.05% detergent as an acceptor and [2- 14C]malonyl-coenzyme A (CoA) as a two-carbon donor, incorporation of radioactivity into 20:3 was optimal (and incorporation into other acyl chains was minimal) in the presence of 100 μ m substrate, 200 μ m p-bromophenacylbromide and 10 m m KCN. Up to 30% of the labeled products were incorporated into phospholipids and triacylglycerol. Maximal microsomal elongation activity was observed at 3–4 weeks of age. Several other fatty acid or acyl-CoA acceptors tested in this system were elongated at slower rates compared to 18:3(n − 6) [e.g., 16:0-CoA, 75%; 20:4(n − 6), 57%; 18:3(n − 3), 13%; 18:2(n −6), 10%; 20:3(n − 6), 6%]. The rate of elongation of chemically synthesized 18:3-CoA was only 50% of the detergent-suspended acid and was optimal at 6 μ m substrate; inhibition above 6 μ m 18:3-CoA was reduced by bovine serum albumin, but incorporation of label into palmitate was greatly stimulated. CoA markedly inhibited elongation of 18:3(n − 6) or 18:3-CoA; N-ethylmaleimide at equimolar amounts reversed this CoA inhibition but did not alter the inhibition caused by concentrations of 18:3-CoA above 6 μ m. ATP was absolutely required for elongation of either the free acid or the acyl-CoA derivative, whereas exogenous MgCl 2 had little effect.