Synthesis of mono- and digalactosyldiacylglycerol in isolated spinach chloroplasts

Purified, intact chloroplasts of Spinacia oleracea L. synthesize galactose-labeled mono- and digalactosyldiacylglycerol (MGDG and DGDG) from UDP-[U-(14)C]galactose. In the presence of high concentrations of unchelated divalent cations they also synthesize tri- and tetra-galactosyldiacylglycerol. The acyl chains of galactose-labeled MGDG are strongly desaturated and such MGDG is a good precursor for DGDG and higher oligogalactolipids. The synthesis of MGDG is catalyzed by UDP-Gal:sn-1,2-diacylglycerol galactosyltransferase, and synthesis of DGDG and the oligogalactolipids is exclusively catalyzed by galactolipid:galactolipid galactosyltransferase. The content of diacylglycerol in chloroplasts remains low during UDP-Gal incorporation. This indicates that formation of diacylglycerol by galactolipid:galactolipid galactosyltransferase is balanced with diacylglycerol consumption by UDP-Gal:diacylglycerol galactosyltransferase for MGDG synthesis. Incubation of intact spinach chloroplasts with [2-(14)C]acetate or sn-[U-(14)C]glycerol-3-P in the presence of Mg(2+) and unlabeled UDP-Gal resulted in high (14)C incorporation into MGDG, while DGDG labeling was low. This de novo made MGDG is mainly oligoene. Its conversion into DGDG is also catalyzed, at least in part, by galactolipid:galactolipid galactosyltransferase.

Separation of chloroplast polar lipids and measurement of galactolipid metabolism by high-performance liquid chromatography

Procedures are described for the separation of polar lipids from plant chloroplasts by high-performance liquid chromatography, using a polar-modified silica column. Glycolipids and phospholipids were eluted with a gradient of 2-propanol/n-hexane (80:55, v/v) and 2-propanol/n-hexane/water/methanol (80:55:15:10, v/v). The lipids were detected by uv absorbance at 202 nm. Diacylglycerol and mono-, di-, and trigalactosyldiacylglycerol and phosphatidylcholine were separated on a LiChrosorb NH2 column (7-microns particles, Merck, FRG), but acidic lipids were retained. These lipids could be quantified from their 202-nm absorbance recording. The absorption coefficients obtained depended on the mean number of double bonds in the different lipid classes. The separation was applied for a rapid monitoring of the lipid composition in thylakoids and in fractionated inner and outer envelopes. The activities of galactosyltransferases involved in galactolipid metabolism, UDPGal:diacylglycerol galactosyltransferase and galactolipid:galactolipid galactosyltransferase, could be measured quantitatively in specific assays for both enzymes.

Galactolipid formation in chloroplast envelopes. II. Isolation-induced changes in galactolipid composition

Spinach envelopes were isolated at pH 8.5, which is inhibitory to interlipid galactosyl transfer, and at pH 7.2, which is close to values used in most recent work on chloroplast envelopes. Peptide patterns were identical, but galactolipid compositions differed markedly. These changes are such as could be explained by interlipid galactosyl transfer during isolation at pH 7.2, i.e. a strong decrease in monogalactolipid and a corresponding increase in tri- and tetragalactolipids and diacylglycerol. Digalactolipid was little affected. It was further observed that the interlipid galactosyl transfer was restricted to lipids rich in hexadecatrienoic acid. Arguments are presented that the lipid composition of envelopes in vivo is better represented by the results obtained at high pH. The results will be discussed in relation to previous data on galactolipid composition and biosynthesis in envelopes.

Galactolipid formation in chloroplast envelopes. III. Some observations on galactose incorporation by envelopes with high and low content of diacylglycerol

Chloroplast envelopes from spinach, isolated at two different pH values, were incubated with UDP[14C]galactose at pH values of 6.0, 7.2 and 8.5 and rates and patterns of galactolipid synthesis were measured. Envelopes isolated at pH 8.5 and considered to be poor in initial diacylglycerol content were generally inhibited, especially at high pH. At this pH, where interlipid galactosyl transfer is very slow, incorporation rates reflect diacylglycerol content. At lower ph values in both types of envelope, interlipid galactosyl transferase is active. The highest rates of interlipid galactosyl transferase are seen at pH 6.0 in diacylglycerol-poor envelopes and at pH 7.2 in diacylglycerol-rich envelopes. The latter type of envelope shows considerable activity of monogalactolipid acylase at low pH 6.0, which seems to compete with interlipid galactosyl transferase for monogalactolipid. After removal of UDPGal and transfer of the envelopes to higher pH, acylmonogalactolipid can be transformed again into monogalactolipid and this regenerated lipid can again be transformed into digalactolipid and into higher-homologous galactolipids. Intact spinach chloroplast in similar experiments behave essentially as isolated envelopes. Results are compared with patterns of galactolipid synthesis in vivo and considered as an indication that interlipid galactosyl transferase may be regulated not only by pH but also by a factor originating in the cytoplasm. Results are also discussed in the light of recent suggestions that galactolipids are synthesized in a multi-enzyme complex in the envelope and also considered as a contribution towards understanding the regulation of the mono-/digalactolipid ratio. The observations on the behaviour of acyltransferase and the reversibility of its reaction may constitute a first step to an understanding of a physiological role for this enzyme.

Galactolipid formation in chloroplast envelopes. I. Evidence for two mechanisms in galactosylation

Two different enzymes for galactosylation occur in isolated chloroplast envelopes of spinach leaves, UDPgalactose-diglyceride galactosyltransferase and galactolipid-galactolipid galactosyltransferase. The first enzyme is responsible for the biosynthesis of monogalactosyldiglyceride, UDPgalactose being donor of the galactosyl moiety. The second enzyme is responsible for the biosynthesis of digalactosyldiglyceride and higher homologues. The optimum pH for monogalactosyldiglyceride synthesis was found to be 7.5, for digalactosyldiglyceride synthesis 6.5. After incubation at pH 7.4 a Lineweaver-Burk plot indicated two binding sites for UDPgalactose on the UDPgalactose-diglyceride galactosyltransferase with different affinities for UDPgalactose and different activities. Indirectly the galactolipid-galactolipid galactosyltransferase was shown to respond similarly to various UDPgalactose concentrations. However, the second reaction also proceeds in absence of UDPgalactose. It was concluded that the second enzyme does not require the presence of UDPgalactose, but that galactosyl transfer proceeds by direct exchange of galactosyl groups between molecules of galactolipids, or via unknown lipid intermediates, not detected in our system. Results of other investigations will be discussed in the light of the present data.

Biosynthesis of galactosyl diglycerides by non-green fractions from chloroplasts

When either mitochondria, chloroplast stroma lamellae, or osmotically shocked chloroplasts were centrifuged through sucrose gradients, zones were always obtained at the 0.6 M-0.9 M boundary which were highly active in galactosyltransferase. These activities did not coincide with maxima for chlorophyll or cytochrome c oxidase activity. A second chlorophyll-free fraction was obtained at lower density, showing high galactosyltransferase activity when incubated after isolation. The results indicate that the highly active fractions originate from chloroplast envelopes.