Galactolipids presence

The galactolipids are known to be distributed asymmetrically in the lipid bilayer - about 60% of the galactoUpids are present in the outer leaflets and 40% in the inner leaflets of the thylakoid-membrane bilayer. The strong interaction between the head-groups of the galactolipid molecules determine then-packing properties and enhance the stability of the membrane. Formation of the bilayer structure of the thylakoid membrane also depends on the presence of proteins. Another characteristic of the thylakoid lipids is their high content ofthe trienoic acid, (C18 3) a-linolenic acid, which contributes to the fluidity of the membrane, necessary for the diffusion of lipophilic compounds such as plastoquinone, and the lateral diffusion of protein complexes. 

The presence of galactolipids outside the chloroplasts or plastids in plant cells is still open to debate. The presence of galactolipids in unpurified mitochondrial preparations from avocado fruits (Schwertner and Biale, 1973), cauliflower buds (Douce et al., 1%8), potato tubers (Abdelkader et al., 1969), and Vida faba leaves (Mackender and Leech, 1974) has been reported. According to Koenig (1971) only 62% of the total leaf galactolipids... 

In our studies, the analysis of galactolipid hydrolysis kinetics of the thylakoid membrane in the presence of the lipase from Rhizopus arrhizus is the basis for the determination of galactolipid transmembrane distribution. These kinetics have been studied under a variety of conditions temperature, ionic strength, surface potential, stacked or unstacked thylakoids. 

Fatty acids in many leaves are desaturated in conjunction with phospholipids before incorporation into galactolipid (the cytosolic pathway). In 16 3-plants, the situation is further complicated by the presence of a chloroplastic pathway in addition to the cytosolic pathway of glycerolipid biosynthesis and fatty acid desaturation. 

The galactolipids of D. salina are almost exclusively of the "prokaryotic" type, meaning that they are assembled entirely within the chloroplast (6). Prokaryotic galactolipids can be conveniently identified by the presence in the -1 position of a fatty acid and in the -2 position of a Ci5 fatty acid. "Eukaryotic" galactolipids, which are constructed using a diacylglycerol moiety donated from outside the chloroplast, characteristically nave a C g fatty acid bound at both the -1 and the -2 positions, and in D. salina are present in small amounts and only in DGDG (Table I). 

The amount of recoverable myelin in GLD is extremely low [41]. Its total lipid and cholesterol are slightly lower than in normal myelin, the PE PC ratio is reversed, while total galactolipids and ratio of cerebrosides to phosphatides are normal. The ratio of cerebrosides to sulphatides is increased [42]. Conspicuous is the presence of gangliosides in the myelin[42]. Galactose is the only carbohydrate present in the glycolipids. Fatty acid distribution in myelin is normal with the exception of a reversed ratio of lignoceric to nervonic acid in the cerebrosides [41]. 

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