Biosynthesis of lipids in golgi complex and other subcellular fractions from rat liver

Abstract

1. 1. Golgi complex, rough and smooth microsomes, plasma membranes, mitochondria and nuclei from rat liver were isolated and their purity assessed using specific marker enzymes.

2. 2. The various subcellular fractions were assayed for the following processes: biosynthesis of sphingomyelin, CDPdiglycerides, phosphatidylinositol, phosphatidylserine, the conversion of phosphatidylserine into phosphatidylethanolamine, the formation of lecithin via N-methylation, and the activation of palmitic and octanoic acids.

3. 3. None of these processes were found to be present in Golgi complex.

4. 4. The endoplasmic reticulum appears to be the principal site in the cell for the synthesis of sphingomyelin, CDPdiglycerides, phosphatidylinositol, phosphatidylserine and the formation of lecithin. Interestingly, the biosynthesis of phosphatidylserine appears to be four times more active in rough than in smooth microsomes, which might suggest a ribosomal localization of this process.

5. 5. Except for CDPdiglyceride synthesis, mitochondria do not contain any of the synthesizing activities described in 4. Mitochondria are, however, the only site in the cell where phosphatidylserine is decarboxylated. This activity appears to be localized in the inner membrane.

6. 6. The activation of palmitate is localized predominantly in endoplasmic reticulum and mitochondria, though some activity was detected in plasma membranes as well. All other cell organelles, including Golgi and probably nuclei, did not contain significant palmitoyl-CoA synthetase activity. The subcellular distribution of octanoyl-CoA synthetase resembled that of palmitoyl-CoA synthetase except that the former enzyme is more active in mitochondria than in microsomes.

Abbreviations: HEPES; AT-2-hydroxyethylpiperazme-N'-2-ethanesulfonic acid