BIOSYNTHESIS OF TRIACYLGLYCEROL IN THE FILAMENTOUS FUNGUS MUCOR-CIRCINELLOIDES

Lipid metabolism was studied in 2-d-old liquid cultures of Mucor circi nelloides grown at 25 degrees C. Under these conditions, oil accumulat ed to 0.5 g l(-1) with a gamma-linolenic acid content (gamma 18:3) of 60 mg l(-1). The major labelled lipids in cultures incubated with [C-1 4]acetate were triacylglycerol (TAG), phosphatidylcholine (PC) and pho sphatidylethanolamine (PE). The proportion of label declined in the ph ospholipids and increased in TAG with time. [C-14]18:1 and [C-14]18:2 rapidly appeared in PC and PE and later accumulated in [C-14]gamma 18: 3. TAG-synthesizing capacity was greatest in the microsomal membrane f raction, which accumulated high levels of phosphatidic acid in the pre sence of glycerol 3-phosphate and acyl-CoA substrates at ph 7.0. Furth er metabolism of phosphatidic acid to diacylglycerol and TAG was achie ved by increasing the ph to 8.0. Lysophosphatidic acid:acyl-CoA acyltr ansferase (LPAAT) activity was particularly high and may have accounte d for the rapid accumulation of phosphatidic acid in the membranes. Th e glycerol-3-phosphate :acyl-CoA acyltransferase (GPAAT) and LPAAT wer e non-specific for a range of saturated and unsaturated species of acy l-CoA although the GPAAT showed a marked selectivity for palmitoyl-CoA and the LPAAT for oleoyl- and linoleoyl-CoA. gamma-Linolenic acid was detected at all three positions of sn-TAG and was particularly enrich ed at the sn-3 position. The preparation of active in vitro systems (m icrosomal membranes) capable of the complete biosynthetic pathway for TAG assembly may be valuable in understanding the assembly of oils in future transgenic applications.