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The Journal of Cell Biology, Vol 50, 625-633, Copyright © 1971 by Rockefeller University Press

ARTICLE

ASSEMBLY OF LIPIDS INTO MEMBRANES IN ACANTHAMOEBA PALESTINENSIS

: I. Observations on the Specificity and Stability of Choline-14C and Glycerol-3H as Labels for Membrane Phospholipids



Francis J. Chlapowski 1 and R. Neal Band 1

1 From the Department of Zoology, Michigan State University, East Lansing, Michigan 48823.

Dr. Chlapowski's present address is the Department of Anatomy, University of Massachusetts Medical School, Worcester, Massachusetts 01604

In order to determine the feasibility of using radioactive precursors as markers for membrane phospholipids in Acanthamoeba palestinensis, the characteristics of phospholipids labeled with choline-14C and glycerol-3H were examined. Choline-14C was found to be a specific label for phosphatidyl choline. There was a turnover of the radioactive moiety of phosphatidyl choline at a rate that varied with the concentration of nonradioactive choline added to the growth medium. Radioactivity was lost from labeled phosphatidyl choline into the acid-soluble intracellular pool and from the pool into the extracellular medium. This loss of radioactivity from cells leveled off and an equilibrium was reached between the label in the cells and in the medium. Radioactive choline was incorporated into phosphatidyl choline by cell-free microsomal suspensions. This incorporation leveled off with the attainment of an equilibrium between the choline-14C in the reaction mixture and the choline-14C moiety of phosphatidyl choline in the microsomal membranes. Therefore, a choline exchange reaction may occur in cell-free membranes, as well as living A. palestinensis. In contrast to choline-14C, the apparent turnover of glycerol-3H-labeled phospholipids was not affected by large concentrations of nonradioactive choline or glycerol in the medium. The radioactivity in lipids labeled with glycerol-3H consisted of 33% neutral lipids and 67% phospholipids. Phospholipids labeled with glycerol-3H turned over slowly, with a concomitant increase in the percentage of label in neutral lipids, indicating a conversion of phospholipids to neutral lipids. Because most (sim96%) of the glycerol-3H recovered from microsomal membranes was in phospholipids, whereas only a minor component (sim2%) of the glycerol-3H was in the phospholipids isolated from nonmembrane lipids, glycerol-3H was judged to be a specific marker for membrane phospholipids.

Submitted on November 5, 1970
Revised on January 4, 1971


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