Biosynthesis of membrane lipids in the retina: Subcellular distribution and propranolol action on phosphatidic acid, phosphatidylserine and phosphatidylethanolamine.
The de novo biosynthesis of phospholipids and glycerides was followed in subcellular fractions from bovine retinas incubated during short periods with radioactive glycerol. The labeling of lipid classes was determined in the following fractions: rod outer segments, P(1), P(2), microsomes and soluble. A highly labeled phosphatidic acid was found in microsomal fractions with a maximum at 5 min relative to other lipids labeling. The labeling of phospholipids and glycerides from microsomes as well as from other fractions was rapid. The soluble contained minor quantities of phospholipids with high radioactivity. Using radioactive glycerol and amphiphilic drugs the rates of phosphatidic acid synthesis in microsomal membranes were assessed. Moreover, in agreement with previous studies from this laboratory showing that phosphatidic acid contains a high proportion of docosahexaenoate, it is demonstrated that this fatty acid is acylated in the biosynthetic route leading to phosphatidic acid formation. Phosphatidylserine is synthesized from radioactive serine and also through a Ca(2+) independent pathway not involving phosphatidylethanolamine. Phosphatidylethanolamine is also formed by serine decarboxylation. Amphiphilic drugs greatly stimulate phosphatidylserine synthesis. Extracellular Ca(2+) stimulates the synthesis of phosphatidylserine and its decarboxylation. Besides, Ca(2+) potentiates the phospholipid effect exerted by propranolol. The synthesis of phosphatidylinositol as well as that of other minor acidic phospholipids seems to operate in a coordinated manner under different experimental conditions. The microsomal system of the retina generates a docosahexaenoyl enriched phosphatidic acid at high rates. In addition to the turnover of different phospholipid moieties there are active pathways for the de novo biosynthesis of membrane lipids in the retina.
Ilincheta de Boschero
M G
MG
Instituto de Investigaciones Bioquímícas, Universidad Nacional del Sur y Consejo Nacional de Investigaciones Científicas y Técnicas, Bahía Blanca, Argentina.
Giusto
N M
NM
Bazán
N G
NG