Betaine Lipids in Lower Plants. Biosynthesis of DGTS and DGTA in <italic>Ochromonas danica</italic> (Chrysophyceae) and the Possible Role of DGTS in Lipid Metabolism

Membrane lipids and fatty acids of Ochromonas danica were analyzed. Of the two betaine lipids, the homoserine lipid DGTS mainly contains 14:0 and 18:2 fatty acids, while the alanine lipid DGTA is enriched in 18:0, 18:2 and 22:5 fatty acids. Of the polar moiety of DGTA, improved NMR data are presented. On incubation of cells with [3,4-14C]methionine, DGTS as well as DGTA were labelled. With [1-14C]methionine as a substrate, the label appeared in DGTS only. If double labelled [3H](glycerol)/[14C](polar part)DGTS was used as a precursor, radioactivity was incorporated specifically into DGTA in which the isotope ratio was unchanged compared to the precursor. Thus, the glyceryltrimethylhomoserine part of DGTS acts as the precursor of the polar group of DGTA. Labelling of cells with [1-14C]oleate in a pulse-chase manner and subsequent analysis of the label in the fatty acids and molecular species of different lipids including DGTS and DGTA, suggested a clearly different role of the two betaine lipids: DGTS acts as a i) primary acceptor for exogenous C18 monoene acid, ii) substrate for the desaturation of 18:1 to 18:2 acid, and iii) donor of mainly 18:2 fatty acid to be distributed among PE and other membrane lipids. Into DGTA, in contrast, fatty acids are introduced only after elongation and desaturation. As a result, the biosynthesis of DGTA from DGTS involves a decarboxylation and recarboxylation of the polar part and a simultaneous deacylation and reacylation of the glycerol moiety.