1. The component fatty acids of the endogenous phospholipids of microsomal preparations of Mucor, when shaken at 30°C increased in both chain length and in degree of unsaturation. The net effect was the production of y-linolenic acid which, over 2 h, increased from 17% to 32% of total fatty acids present. No further significant changes occurred after t h s time.2. The major site for desaturation/elongation reactions was at the sn-2 position of PtdIns. PtdCho and PtdEtn were not implicated.3. Of numerous metabolites and cofactors added to the microsomes, only malate could prolong the elongation/desaturation reactions for up to 6 h. This effect was shown to be due to a membraneassociated malic enzyme [malate dehydrogenase (decarboxylating) NADP'] with the NADPH produced being used in fatty-acid desaturation.4. Kinetic analysis of cytosolic and microsomal enzymes [both in 0.1 % (mass/vol.) Chaps] could not distinguish between them. However, when the microsomal malic enzyme was dialysed to remove Chaps, it lost 90% of activity, although the cytosolic malic enzyme lost only 20% activity.5. The structural analogue of malate, tartronic acid, which is an inhibitor of malic enzyme, also inhibited the malate-induced stimulation of fatty-acyl group desaturation and elongation in the microsomal membranes.6. It is concluded that two distinct malic enzymes exist, one soluble and one membrane bound, with similar active sites. Both have different roles in the production of NADPH, for lipid metabolism. The former will produce NADPH for fatty-acid biosynthesis whilst the latter produces NADPH for fatty-acid desaturation.The desaturation of fatty acids beyond oleate [18: 1 (9)] is considered to occur in plants and microorganisms with fattyacyl groups attached to phospholipids and thus is a membrane(microsoma1)-associated activity in eukaryotic cells [l-61. The desaturation of oleate to linolenate [I 8 : 2 (9, 12)], and thence to either a-o r y-linoleate [18:3 (9, 12, 15) or 18:3 (6, 9, 12) respectively] and other similar desaturations. occurs by separate desaturases (in the above examples they would be referred to, respectively, as the 412, A15 or A6 desaturases, respectively). These desaturases require both molecular oxygen and NADPH: -CH2-CH2-

The lipid classes and component fatty acids of seven fungi were examined. Three marine fungi, Thraustochytrium aureum, Thraustochytrium roseum and Schizochytrium aggregatum (grown at 30, 25 and 25 degrees C, respectively), produced less than 10% lipid but contained docosahexaenoic acid (DHA) up to 30% and eicosapentaenoic acid (EPA) up to 11% of the total fatty acids. Mortierella alpinapeyron produced 38% oil containing solely n-6 polyunsaturated fatty acids (PUFA) with arachidonic acid (AA) at 11% of the total fatty acids. Conidiobolus nanodes and Entomorphthora exitalis produced 25% oil and contained both n-3 and n-6 PUFA, with AA at 16% and 18%, respectively. Saprolegnia parasitica produced 10% oil and contained AA and EPA, respectively, at 19% and 18%. The triacylglycerol fraction always represented the major component at between 44% and 68% of the total lipid. Each fungus, except T. aureum, had the greatest degree of fatty acid unsaturation in the phospholipid fraction. The triacylglycerol fraction of T. aureum was the most unsaturated with DHA representing 29% (w/w) of all fatty acids present. The presence of the enzyme ATP:citrate lyase correlated with the ability of molds to accumulate more than 10% (w/w) lipid when the fungi were grown in nitrogen-limiting media. In those molds that failed to accumulate more than 10% lipid, the enzyme was absent.

Sesamol, a nonoil component of sesame seed oil, inhibited growth, fatty acid synthesis, and desaturation by Mucor circinelloides in vivo. Although sesamol also inhibited the growth of other fungi and yeasts, its effect on the lipid metabolism of M. circinelloides was exceptional. An enzymological study demonstrated that sesamol affected lipid synthesis primarily by the inhibition of malic enzyme activity, thereby limiting the NADPH supply for fatty acid synthesis and desaturation. Sesamol itself had no inhibitory effect on malic enzyme activity in vitro. A metabolite of sesamol is therefore probably responsible for the in vivo effects of sesamol on lipid metabolism.

The oleaginous fungus Entomophthora exitalis was grown in continuous culture at a constant dilution rate (0.04 h -1) and over a range of temperatures (20-30 ° C). As the growth temperature was decreased from 30 to 20°C the percentage of polyunsaturated fatty acids (PUFA) increased proportionally from 18 to 27% (w/w) of the total fatty acids. The increase in unsaturation was as a result of an increased proportion of n-6 PUFA (particularly arachidonic acid) in the phospholipid and sphingo-plus glycolipid fractions. The triacylglycerol fraction of lipids displayed a negligible change. The proportion of phospholipids within the extracted lipid increased between 26 and 20°C without any change in the lipid content of the fungus. Although the changes in lipid unsaturation correlated, at first inspection, to the culture dissolved 02 tension (DOT), growth of the fungus at a constant dilution rate and temperature (22 ° C) over a range of DOT values failed to influence lipid unsaturation. Thus temperature is the principal regulation factor of the degree of unsaturation in the lipids of this organism.