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 reac 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.