Desaturation of Fatty Acids on Complex-lipid Substrates

Abstract

Almost 20 years ago Gurr et al. (1), working with Chlorella, suggested that complex lipids in plants may serve as substrates for desaturase enzymes, particularly those that catalysed the synthesis of C18-polyunsaturated fatty acids. This proposai was contrary to our understanding of the desaturase enzymes in animals where acyl-CoA substrates were directly involved. Hence it was considered that perhaps in plants the polyunsaturated fatty acid products were beinq rapidly transferred from CoA to oxygen esters. More recently, with the advent of reliable techniques, our knowledge of the substrates involved in plants has increased considerably and the evidence, for instance, that the oleate in microsomal phosphatidylcholine (PC) is the substrate for Δ12-desaturase is well documented (2–4). There are also reliable data to show that α-linolenic acid (α-18: 3) is synthesised in leaves from linoleate (18: 2) associated with the chloroplast lipid, MGDG (5–7). On the other hand, although the evidence is of a less satisfactory nature, the α-18: 3, that is synthesised in certain oil-seeds occurs in microsomal PC (8, 9). Plants therefore differ from animals in that the synthesis of polyunsaturated fatty acids is characterised by complex lipid substrates. This raises further important problems, particularly those concerned with the mode(s) of entry of the fatty acid substrate into the complex lipid and the mechanism(s) by which the polyunsaturated products are then made available for the assembly of other lipids.