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.