Dual character of lipid composition of the envelope membrane of spinach chloroplasts

Abstract

The envelope membrane was isolated from intact spinach chloroplasts by gentle osmotic treatment in a medium containing appropriate amounts of cations to prevent dissociation and fragmentation of the thylakoids. This treatment allowed us to separate effectively the envelope membranes from the thylakoids with one-step (0.6 M/0.9 M) sucrose density gradient centrifugation. The envelope membrane contained both glyceroglycolipids and glycerophospholipids, as does the thylakoid membrane. There were, however, notable differences in the relative amounts of lipid components between these two membranes. The major glyceroglycolipid in the envelope membrane was digalactosyl diglyceride, whereas monogalactosyl diglyceride was the major one in the thylakoid. The envelope membrane was characterized by a high content of glycerophospholipids, as much as three-fold that in the thylakoid membrane. Phosphatidyl choline, which is known to be minor in the thylakoids and abundant in the microsomal and mitochondrial membranes, was a major component, accounting for 50% of the total glycerophospholipids. The dual character of lipid composition of the envelope membrane is discussed in terms of its chemical and structural connection to the other intracellular membrane systems.