A Possible Mechanism for a Light-Driven Regulation of the Fatty Acid Composition in Galactolipids of Chloroplasts

Acylation Mechanism, Galactolipid Synthesis The occurrence of acylgalactosylglycerol (AGG) in spinach chloroplasts is proposed. In lipid se­ parations of whole spinach leaves this fraction is hidden by variable amounts of extraplastidary steryl glucoside (SG) which depended on the physiological state of the leaf material. Beside the phospholipid fraction, this lipid mixture with unique chromatographic behavior, recently termed GL, exhibited the highest specific activity in the lipid extract of infiltrated leaf sections of spinach after short periods of illumination (2 min) under 14C-fixing conditions. The main source of label was found in the fatty acid residues of the lipids described. The decrease of specific activity in GL after a cold chase was accompanied by an increasing 14C-incorporation into the fatty acid moieties of the MGDGas well as the phospholipid fraction. This effect was significantly reduced if the 14C-pulse was followed by a dark period. This incorporation behavior suggests, that AGG func­ tions as an intermediary acyl acceptor in the light driven fatty acid transfer between the lipids de­ scribed. SG-synthesis, on the contrary, is independent of additional illumination and seems to be localized outside the chloroplast. The last notion was derived from experiments involving the in­ corporation of [UDP-14C] glucose into SG by spinach leaf homogenates. With regard to the in­ creasing level of trienoic fatty acids in AGG and MGDG during the regeneration of dark pretreat­ ed spinach leaves in the light, the data are interpreted in terms of a light regulated acylation of AGG with specific unsaturated fatty acids.