Search

Two galactosyltransferases involved in the galactolipid metabolism of spinach chloroplast envelopes were studied by specific assays: UDPgalactose: 1,2-diacylglycerol galactosyltransferase, which synthetizes monogalactosyldiacylglycerol from UDPgalactose plus diacylglycerol; and galactolipid: galactolipid galactosyltransferase (GGGT), which forms di- , tri- and tetragalactosyldiacylglycerol by dismutation of galactosyl groups between two galactosyldiacylglycerols. In the assay developed for UDPgalactose: 1,2-diacylglycerol galactosyltransferase, chloroplast envelope membranes were mixed with liposomes of phosphatidylcholine, and the mixture was treated with phospholipase C (from Bacillus cereus). The diacylglycerol produced from phosphatidylcholine is used by UDPgalactose: 1,2-diacylglycerol galactosyltransferase for synthesis of monogalactosyldiacylglycerol in the presence of UDPgalactose. Several characteristics of this enzyme were studied, including the effect of substrate concentrations, pH and temperature. UDPgalactose: 1,2-di-acylglycerol galactosyltransferase did not require cations for activity, but was stimulated by Mg2+ and Mn2+. Of the mono- and divalent cations tested only Zn2+, Cd2+ and Fe2+ were inhibitory. Specific inhibitors for UDPgalactose: 1,2-diacylglycerol galactosyltransferase were UDP and N-ethylmaleimide. In contrast to UDPgalactose: 1,2-diacylglycerol galactosyltransferase, galactolipid: galactolipid galactosyltransferase was strongly stimulated by a series of mono- and divalent cations, most stimulatory being Mn2+ Ba2+, Ca2+ and Mg2+. Galactolipid: galactolipid galactosyltransferase was specifically inhibited by low concentrations of Zn2+ (1 mM) and by chelating anions. UDPgalactose: 1,2-diacylglycerol galactosyltransferase synthetized monogalactosyldiacylglycerol from various molecular species of diacylglycerol; the highest activity was measured with distearoylglycerol and dioleoylglycerol. On the other hand, digalactosyldiacylglycerol synthesis by galactolipid: galactolipid galactosyltransferase proceeded most rapidly by galactosyl transfer to hexaene species of monogalactosyldiacylglycerol.

Two enzymes involved in the galactolipid metabolism of spinach chloroplast envelope membranes were studied. UDPGal: diacylglycerol galactosyltransferase (UDGT) synthetizes monogalactosyldiacylglycerol (MGDG) from UDPGal plus diacylglycerol (1, 2): $$ {\text{UDPGal + diacylglycerol}} \to {\text{UDP + MGDG}} $$ .

Digalactosyldiacylglycerol (DGDG) is a major lipid of the chloroplast membranes (1). in our studies to the galactolipid synthesis in isolated chloroplast envelopes membrane, we could measure only one enzymatic activity involved in DGDG synthesis: galactolipid: galactolipid galactosyltransferase (GGGT). We were unable to measure an alternative reaction involving an UDPGal: monogalactosyldiacylglycerol galactosyl-transferase (2–4). However, apart from DGDG, GGGT produces diacylglycerol and the higher homologues tri- and tetragalactocyldiacylglycerol (TGDG and TeGDG): $$ \begin{array}{*{20}{c}} {MGDG + MGDG \to DGDG + diacy\lg lycerol}\\ {DGDG + MGDG \to TGDG + diacy\lg lycerol.} \end{array} $$

Various authors have noted that isolated chloroplasts, incubated under appropriate conditions, can synthesize small amounts of monogalactosyldiacylglycerol (MGDG) in the absence of UDPgalactose, either from labeled glycerol 3-phosphate [1] or from labeled acetate [2]. Van Besouw and Wintermans [3] tried to explain this phenomenon by a mechanism involving UDPgalactose:diacylglycerol galactosyltransferase (UDGT). In this paper we present evidence that UDPgalactose-independent MGDG synthesis is an enzymatic activity of the chloroplast envelope and that galactolipid:galactolipid galactosyltransferase (GGGT) is involved in the synthesis. Thus, GGGT catalyzes the reversible reaction of Eq.(1), permitted by an expected, low reaction enthalphy: $$\text {DGDG}\;\;+\;\; \text {diacylglycerol}\;\; \;\; \text {MGDG}\;\;+\;\; \text {MGDG}$$ (1) .

UDPGal: diacylglycerol galactosyltransferase (UDGT) and galactolipid: galactolipid galactosyltransferase (GGGT) are two enzymes synthetizing galactolipids in chloroplast envelope membranes. UDGT catalyzes the transfer of a galactosyl group from UDPGal to diacylglycerol (1–3) and produces monogalactosyldiacylglycerol (MGDG). The enzyme has been localized on the inner envelope membrane of spinach chloroplasts (4–5). GGGT produces digalactosylglycerol (DGDG), but also the higher homologues tri- and tetragalactosyldiacylglycerol (TGDG and TeGDG) by transgalacto-sylation of galactosyldiacylglycerols (6,7). It is an enzyme of the outer membrane of spinach chloroplasts (8). in the intact chloroplast it is accessible to the non-penetrating proteinase thermolysin (9). UDGT and GGGT cooperate in isolated envelope membranes to produce a range of galactolipids, when the membranes are supplied with UDP[14C]Gal and MgC2: MGDG, DGDG, TGDG and TeGDG are formed (6,7).

Two galactosyltransferases producing galactolipids have been studied extensively in chloroplast envelope membranes (1). UDPGal: diacylglycerol galactosyltransferase (UDGT) produces monogalactosyldiacylglycerol (MGDG) (2); and galactolipid: galactolipid galactosyltransferase (GGGT) producess di-, tri- and tetragalactosyldiacylglycerol (DGDG, TGDG and TeGDG) (3-6). In the isolated envelope membranes, UDGT and GGGT co-operate in the production of MGDG, DGDG, TGDG and TeGDG (4, 5).