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. 1970 Jun;45(6):800–804. doi: 10.1104/pp.45.6.800

Lecithin Biosynthetic Enzymes of Onion Stem and the Distribution of Phosphorylcholine-Cytidyl Transferase among Cell Fractions 1,2

D James Morré a, Sally Nyquist a, E Rivera a,3
PMCID: PMC396514  PMID: 16657394

Abstract

Enzymatic activities of the cytidine 5′-diphosphate choline pathway for lecithin biosynthesis were demonstrated in homogenates of onion stem (Allium cepa). Choline kinase activity was present in the postmicrosomal supernatant, with less than 3% sedimenting with the particulate fractions. Phosphorylcholine-cytidyl transferase was distributed among all fractions, and phosphorylcholine-glyceride transferase was predominantly found in the particulate fraction.

The phosphorylcholine-cytidyl transferase activity of onion stem required a divalent ion (Mg2+ or Mn2+) for activity, was inhibited by Ca2+, and was specific for cytidine triphosphate, with optimal activity in the range pH 6 to 7. To evaluate the distribution among cell fractions, conditions of pH, cofactors, substrate, and assay were optimized for each fraction. One-third of the transferase activity sedimented with the mitochondria-proplastids fraction, and one-third was in the microsomal supernatant. The dictyosome fraction contained about 10% of the total activity but showed a greater specific activity than the other fractions. Similar results were obtained with homogenates from rat liver, in that purified Golgi apparatus fractions contained the highest phosphorylcholine-cytidyl transferase activity on a protein basis when compared with other cell fractions at pH 7.2.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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