UPD-Galactose-Independent Synthesis of Mono-Galactosyldiacylglycerol (MGDG). An Enzymatic Activity of the Spinach Chloroplast Envelope

Abstract

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)

.