Octadecatetraenoate synthesis in the unicellular alga Isochrysis galbana: studies with intact and broken chloroplasts.
(1) Monogalactosyldiacylglycerol (MGD) is the major lipid component of Isochrysis galbana. In cells incubated for 3 h with [1-14C]]acetate or [1-14C]oleate. MGD contained 35.9% and 52.8%, respectively, of the label incorporated into cellular lipids. (2) 18:4 amounted to 50-60% of the total FA of MGD. Separation of MGD species of cells grown with [1-14C]oleate on AgNO3 impregnated plates revealed 20 distinct spots. The slowest spot was identified as dioctadecatetraenoyl MGD. Fast moving species were enriched with 18:1. (3) In cells incubated for 3 h with [1-14C]oleate, approx. 60% of the radioactivity was associated with 18:1. Subsequent chase resulted in a gradual shift of label and after 48 h [14C]18:1 declined to 10% and [14C]18:4 reached 52%. This shift was also reflected in the labeling pattern of the MGD-species. Dioctadecatetraenoyl-MGD became labelled only after 24 h. (4) Addition of the substituted pyridazinone herbicide (SAN 9785) during the chase period inhibited [14C]18:4 formation; [14C]18:2 and [14C]18:3 accumulated instead. (5) Isolated chloroplast readily incorporated [14C]oleate into MGD and PA. Considerable amounts of [14C]18:1 were desaturated to 18:2 and 18:3. Only very small amounts of 18:4 were formed. O2 was required for desaturation. Cofactor requirement could not be shown. (6) Membranes isolated from broken chloroplasts retained the ability to incorporate [14C]oleate into MGD and PA and desaturate 18:1 to 18:2 and 18:3.
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Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Israel.
Tietz
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D002736:Chloroplasts / Q000378:metabolism; D056890:Eukaryota / Q000378:metabolism*; D005231:Fatty Acids, Unsaturated / Q000096:biosynthesis*
D005231:Fatty Acids, Unsaturated