K562 human leukaemia cells lack a significant delta 6-desaturase activity. However, they synthesize long-chain polyunsaturated fatty acids (PUFA) from linoleic (C18:2(9,12)) and linolenic (C18:3(9,12,15)) acids, by reactions involving a C2 chain elongation followed by a delta 5-desaturation step and, to some extent, a further elongation. The main products formed were separated by argentation t.l.c. and identified by g.l.c. as the uncommon fatty acids C20:3(5,11,14) and C20:4(5,11,14,17) respectively. These acids were also produced when cells were supplemented with C20:2(11,14) or C20:3(11,14,17) respectively. The presence of a delta 5-desaturase was further confirmed by using its corresponding normal substrates, C20:3(8,11,14) and C20:4(8,11,14,17), which led to C20:4(5,8,11,14) and C20:5(5,8,11,14,17) respectively. On the other hand, a high delta 9-desaturase activity, but no significant delta 4-desaturase activity, were detected in K562 cells. These results indicate the existence of an alternative pathway, involving delta 5-desaturase, which is the only route for PUFA biosynthesis in K562 cells. This pathway may be relevant for the biosynthesis of PUFA in cells lacking delta 6-desaturase activity.
Biosynthesis of Polyunsaturated Fatty Acids: Metabolic Engineering in Plants