Evidence that the rate-limiting step for the biosynthesis of arachidonic acid in Mortierella alpina is at the level of the 18:3 to 20:3 elongase.

Mortierella alpina, a fungus used commercially as a source of arachidonic acid, 20:4(n-6), has been examined to see if growth on lipid-based carbon sources leads to repression of either fatty acid biosynthesis and/or fatty acid desaturation and elongation. Changes in the activities of ATP:citrate lyase, isocitrate lyase, carnitine acetyltransferase, malic enzyme, glucose-6-phosphate dehydrogenase and pyruvate kinase when the fungus was grown on fatty-acid-based (Tween) carbon sources were consistent with (i) the cells using the fatty acyl portion of the substrate as the sole carbon source, (ii) pyruvate kinase being the source of pyruvate for biosynthesis under these conditions and (iii) malic enzyme's major function being as a provider of NADPH for lipid biosynthesis. The abolition of fatty acid synthase activity when cells were grown on Tweens indicated the cessation of de novo fatty acid biosynthesis under these conditions. The fatty acyl composition of the lipid accumulated by the fungus grown on Tweens 20, 40 and 80 showed that desaturation and elongation of the substrate lipid still occurred. The absolute amount of arachidonic acid synthesized by Tween-grown cells was the same as for cells grown on glucose. The transformation of incorporated fatty acids into 20:4(n-6) was, it appeared, limited at the elongation of 18:3(n-6) to 20:3(n-6) as, in every case, 18:1, 18:2 and 18:3(n-6) increased in amount in the Tween-grown cells. These data show for the first time that fatty acid synthesis is regulated separately from fatty acid desaturation/elongation and that the latter reactions are not repressed by growth of the fungus on simple fatty acids. Furthermore, the data strongly implicate the elongation of 18:3(n-6) to 20:3(n-6) as the limiting step in arachidonic acid biosynthesis by Mort. alpina.