The separation, structure and metabolism of monogalactosyl diglyceride species in Chlorella vulgaris

Compared selectivities of the phosphatidylinositol-synthase from maize coleoptiles either in microsomal membranes or after solubilization

PI-synthase selectivity from etiolated maize coleptiles was studied either associated with the microsomal membranes or after solubilization by CHAPS and prepurification on a DEAE-trisacryl M column. When maize microsomes were incubated with [3H]inositol without any exogenous CPM-PA, the most heavily labelled molecular species were 16:0/18:2-PI (77%), 16:0/18:3-plus 18:2/18:2-PI (15%), 16:0/18:1-PI (4%) and 18:0/18:2-PI (4%). Addition to the incubation medium of up to 300 microM 16:0/16:0-CMP-PA unexpectedly resulted in the formation of very little labelled 16:0/16:0-PI. When the solubilized fraction from microsomes was incubated with [3H]inositol in absence of 16:0/16:0-CPM-PA, the same PI molecular species as above were synthesized. However, with increasing concentrations of 16:0/16:0-CMP-PA in the medium, increasing amounts of labelled 16:0/16:0-PI appeared as well. With prepurified PI-synthase eluted from a DEAE column, endogenous CMP-PA was poorly utilized for PI biosynthesis whereas the exogenous 16:0/16:0-CPM-PA was used actively. With time, the endogenous CMP-PA was utilized first and the exogenous substrate was utilized, albeit, much more slowly. The results demonstrate that the selectivity displayed by PI-synthase towards various molecular species of CMP-PA depends on the integration of the enzyme in the membrane structure. Solubilization of the enzyme, i.e., inclusion of the protein in micelles with detergents and lipids, results in an apparent loss of the selectivity for CMP-PA.

Comparison of the molecular species patterns of phosphatidic acid, CDP-diacylglycerols and phosphatidylinositol in potato tuber, pea leaf and soya-bean microsomes: consequences for the selectivity of the enzymes catalyzing phosphatidylinositol biosynthesis

Microsomes prepared from pea leaf, potato tuber or germinated soya-beans, were incubated for 30 min with [14C]glycerol 3-phosphate. In the three tissues, phosphatidic acid (PA), CDP-diacylglycerols (CMP-PA) and phosphatidylinositol (PI) were labelled and could be separated by TLC. After methylation of phosphatidic acid, or treatment of CMP-PA by a nucleotidase, the molecular species composition of the three lipid classes could be determined by radio-HPLC. The similarity observed between the distributions of radioactivity among CMP-PA and PA molecular species, in the three tissues, indicates that the enzyme CTP:PA cytidylyltransferase did not present any selectivity towards any molecular species of PA. In contrast, only two molecular species containing palmitic acid (16:0/18:2 and 16:0/18:3) were labelled in PI whereas labelled PA and CMP-PA contained molecular species possessing stearic acid (18:0/18:2, 18:0/18:3 and 18:0/18:1). This indicates that the enzyme PI-synthase utilizes preferentially those molecular species of CMP-PA containing palmitic acid as substrates. However, mass analyses of PI prepared from the microsomes of the three tissues used in this study, indicated the presence of molecular species containing stearic acid (18:0/18:2 and 18:2/18:2). Except in soya-bean microsomes (where 18:0/18:2-PI represented 16% of total PI), those last molecular species were always present in small amounts.

Release of diacylglycerol moieties from various glycosyl diacylglycerols

Periodic acid oxidation in methanol followed by incubation with 1, 1-dimethylhydrazine results in release of diacylglycerols from 1,2-diacyl-3-glycosyl-sn-glycerols. During hydrazinolysis of oxidized monogalactosyl diacylglycerol, an intermediate hydrazone derivative was observed which was isolated and identified. The diacylglycerols recovered are 1,2-diacyl isomers containing the same fatty acid mixtures as the intact glycolipids. The yields of diacylglycerols released from plant monogalactosyl-, digalactosyl-, and sulfoquinovosyl diacylglycerols were in the range of 30-50%. The method may be used for analysis of molecular species and for preparative purposes.

Temperature shift-induced responses in lipids in the blue-green alga, Anabaena variabilis: the central role of diacylmonogalactosylglycerol in thermo-adaptation

Changes in fatty acids and lipid molecular species after shift of growth temperature were studied in the blue-green alga, Anabaena variabilis. In the first 10 h after a temperature shift from 38 to 22 degrees C, lipid synthesis was markedly suppressed. During this period most of the palmitic acid of the diacylmonogalactosylglycerol was desaturated to palmitoleic acid. Thereafter lipid synthesis resumed, and, in the following hours, the relative contents of palmitic and palmitoleic acids were almost restored to original levels. On the other hand, the oleic and linoleic acids were almost restored to original levels. On the other hand, the oleic and inoleic acids were desaturated to alpha-linolenic acid in all the lipid classes. The desaturation reaction of the C18 acids were slower than those of the C16 acids. In the first 5 h after a growth-temperature shift from 22 to 38 degrees C, lipid synthesis was considerably stimulated. During this period, the relative content of palmitic acid increased and that of palmitoleic acid decreased in the diacylmonogalactosylglycerol, and in the following 20 h, they were restored. The oleic and linoleic acids increased with a concomitant decrease in alpha-linolenic acid in all the lipid classes. The decreases in unsaturation in the C16 and C18 acids were due to the stimulated synthesis of more saturated fatty acids. Among the major molecular species of the lipids a particular change was seen in 1-oleoyl-2-palmitoylmonogalactosyl-sn-glycerol. This species rapidly decreased after the downward temperature shift and rapidly increased after the upward temperature shift. From the viewpoint of thermo-adaptive regulation of membrane fluidity, the central role played by the diacylmonogalactosylglycerol in temperature acclimation is discussed.

Linoleate and alpha-linolenate synthesis by isolated spinach (Spinacia oleracea) chloroplasts

Diacylgalactosylglycerol synthesis was a prerequisite for the incorporation of [1-14C]-acetate into linoleate and alpha-linolenate of isolated spinach (Spinacia oleracea) chloroplasts. Oleate at position 1 of diacylgalactosylglycerol was desaturated to linoleate and alpha-linolenate both in the light and in the dark. Some desaturation of palmitate was also observed after prolonged incubations.

Fatty acid biosynthesis in the leaves of barley, wheat and pea

1. The incorporation of radioactivity from [1-14C]acetate into the leaf lipids of barley, pea and wheat has been studied in pulse-labelling experiments. 2. There was little increase in the total labelling of lipids after the leaves were transferred to non-radioactive medium. However, there was an increase in the relative labelling of unsaturated fatty acids. In addition, there was an increase in the relative labelling of diacylgalactosylglycerol. 3. The principal radioactively labelled acyl lipids were diacylgalactosylglycerol and phosphatidylcholine. Phosphatidylcholine showed a decreasing proportion of [14C]oleate and an increasing amount of [14C]linoleate with time. Diacylgalactosylglycerol also had decreasing amounts of [14C]oleate but, in addition, had an increasing proportion of [14C]linolenate with time. 4. The absence of significant amounts of [14C]linolenate in phosphatidylcholine appeared to exclude a role for this phospholipid in linoleate desaturation. 5. The specific radioactivities of oleate and linoleate in phosphatidylcholine, diacylgalactosylglycerol and diacylgalabiosylglycerol were very similar in any single experiment. It was concluded that these fatty acids can rapidly exchange between the three intact lipids.

Labelling studies in vivo on the metabolism of the acyl and glycerol moieties of the glycerolipids in the developing maize leaf

1. When [2-3H]glycerol was supplied to developing maize-leaf laminae, label entered 3-sn-phosphatidycholine at a linear rate essentially from zero time, whereas other lipids were labelled at accelerating rates. On transfer of laminae from [3H]glycerol to unlabelled glycerol, radioactivity was rapidly lost from 3-sn-phosphatidylcholine and accumulated in other lipids, principally monogalactosyl diacyglycerol. 2. Degradation of these lipids showed that 3H was present only in the glycerol moiety of the lipids. 3. In double-labelling pulse-chase experiments with [14C]acetate, which labelled essentially only fatty acids and [3H]glycerol similar amounts of 14C and 3H radioactivity were lost from 3-sn-phosphatidylcholine and accumulated by monogalactosyl diacylglycerol. 4. The different molecular species of both lipids isolated from laminae during a double-labelled pulse-chase study were separated by argentation t.l.c., and the changes in the amount of radioactivity and the 14C/3H ratio in different species were compared. The greatest loss of radioactivity during the period in unlabelled substrates occurred from the 3-sn-phosphatidylcholine species containing oleate and from the dilinoleate species, and radioactivity accumulated by monogalactosyl diacyglycerol was mainly in the dilinolenate species. However, despite the considerable change in the radioactivity in these species during the chase, the 14C/3H ratio in each of them remained relatively unchanged. 5. It is proposed that 3-sn-phosphatidylcholine in the developing leaf may serve as a donor or linoleate-containing diacyl-glycerols which are incorporated into other lipids, principally monogalactosyl diacylglycerol.

The breakdown of lipid reserves in the endosperm of germinating castor beans

1. Lipid extracts were obtained from castor-bean endosperm tissue at various times during germination and, after purification, the total lipid content was determined. Quantitative measurements of the triglyceride and phospholipid content together with the fatty acid composition were made. 2. The total lipid content of the endosperm rapidly decreased during germination; after 10 days less than 20% of the original weight of lipid remained. In contrast, the phospholipid content (initially less than 0.5% of the total lipid) increased slightly during this time. The fatty acid composition and the relative proportions of the triglyceride species of the total lipid extract remained constant during 10 days of germination. 3. Gibberellic acid (0.3 mM) markedly stimulated the rate of lipid breakdown but did not alter either the fatty acid composition or the relative proportion of triglyceride species. 4. The embryo had little effect on lipid metabolism in the endosperm tissue; only after 6 days of germination were differences observed in the rate of fat utilization in the presence and absence of the embryo.