Isolation and lipid analyses of subcellular fractions from the arachidonic acid producing fungus Mortierella alpina 1S-4

The lipid profile, fatty acid composition and protein content in subcellular structures isolated from two growth stages of the fungus Mortierella alpina 1S-4 cultivated under lipid-accumulating and non-lipid-accumulating conditions were investigated. The method for fractionation of fungal subcellular organelles based on a combination of differential and density gradient centrifugation was developed for this purpose. An increase of the lipid/protein ratio in both lipid bodies and microsomes and a distinct rise in microsomal triacylglycerol level was observed at stationary phase only under lipid-accumulating conditions. High degree of fatty acid unsaturation in microsomes enriched with phosphatidylcholine and phosphatidylethanolamine may suggest that this fraction is the site of fatty acid desaturation assembly. The presence of typical mitochondrial phospholipids in other cellular membranes may indicate that mitochondria share the capacity to synthesize membrane lipids with the endoplasmic reticulum. Peroxisomes, lysosomes and cytosol contained increased levels of free fatty acids. Two additional fractions, PI-T and P2-T, with very low protein amounts were isolated. While the former displayed similar fatty acid profile as lipid bodies, the latter consisted of mainly saturated free fatty acids. Moreover, three unusual fatty acids that are not commonly found in such quantities in total lipid were detected in some fractions. The role of subcellular organelles in the fungal lipogenesis is discussed.