| The metabolic interrelationships between monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), the two major lipid components of chloroplast membranes, of Dunaliella salina (UTEX 1644) were studied though the use of combined techniques of radioactive labeling and high performance liquid chromatography. Cells were pulse labeled with ({dollar}sp{lcub}14{rcub}{lcub}rm C{rcub}{dollar}) - or ({dollar}sp{lcub}3{rcub}{lcub}rm H{rcub}{dollar}) - fatty acids, and individual molecular species were analyzed for their specific radioactivity. MGDG was initially made in the form of 18:1/16:0, the most saturated molecular species. This was then either rapidly desaturated to 18:1/16:1 MGDG or galactosylated to 18:1/16:0 DGDG. 18:1/16:1 MGDG was subject to rapid further desaturation, ultimately to 18:3/16:4 MGDG, the most unsaturated molecular species present. Galactosylation favored MGDG molecular species having both fatty acyl chains unsaturated. 18:3/16:0 DGDG was made by a separate path involving the sequential desaturation of 18:1/16:0 DGDG. Thus, contrary to widely held opinion, desaturation occurs on fatty acids bound to DGDG as well as MGDG.; The comparison of chloroplast envelopes and thylakoids revealed that the former were enriched in 16:0-containing DGDG molecular species. The analysis of specific radioactivity of individual molecular species suggested that in D. salina, as in other plants, galactolipids are synthesized in the envelopes, but that there is a very rapid transfer of all galactolipid molecular species between the two membrane types.; Although galactolipids do not turn over significantly in vivo, an MGDG acyl hydrolase was found in the chloroplasts isolated from D. salina. When incubated in vitro, this enzyme showed a marked preference for removing fatty acids bound at the sn -1 position. The acyl hydrolase was unlike the microsomal phospholipid acyl hydrolase previously found in D. salina in the following respects: (1) the pH optimum was 6.5-7.0, (2) the enzyme required 10 mM Ca{dollar}sp{lcub}+2{rcub}{dollar} for full activity, but was not affected by calmodulin or calmodulin antagonists, and (3) the activity was not increased in cells stressed by low temperature. |
