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-C-14]]acetate or [1-C-14]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-C-14]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-C-14]oleate, approx. 60% of the radioactivity was associated with 18:1. Subsequent chase resulted in a gradual shift of label and after 48 h [C-14]18:1 declined to 10% and [C-14]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 [C-14]18:4 formation, [C-14]18:2 and [C-14]18:3 accumulated instead. (5) Isolated chloroplast readily incorporated [C-14]oleate into MGD and PA. Considerable amounts of [C-14]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 [C-14]oleate into MGD and PA and desaturate 18:1 to 18:2 and 18:3.