ISOLATION OF THE PLASMA-MEMBRANE AND ORGANELLES FROM CHINESE-HAMSTER OVARY CELLS

Two methods are described enabling the plasma membrane from Chinese hamster ovary (CHO) cells to be obtained rapidly. relatively pure and with a good yield. In both cases, cells were disrupted by nitrogen cavitation in an isoosmotic buffer either at pH 5.4 or at pH 7.4. In the first approach, cells were lysed at pH 7.4 and the plasma membrane and cell organelles were isolated on a self-generated gradient of Percoll, at neutral pH. Mitochondria and endoplasmic reticulum were recovered in the denser fractions, plasma membrane fragments were found in the lighter fractions, but always contaminated by lysosomes. Because lysosomes were found to sediment in acidic conditions, cells were lysed at pH 5.4 and presedimentation (1500 X g) of the cell homogenate at the same pH enabled more than 80% of the lysosomes to be removed. Then, ultracentrifugation of the supernatant over a Percoll gradient at neutral pH yielded plasma membrane fractions practically free of lysosomes with an enrichment ratio of 3 and fractions of mitochondria and endoplasmic reticulum with enrichment ratios of 17 and 6, respectively. A major problem was encountered in the final step of elimination of Percoll from the purified plasma membrane fractions. Whatever the technique used for eliminating Percoll, plasma membranes were observed to be contaminated by a Percoll constituent which prevented further purification and biochemical identification of the lipids extracted from these membrane fractions to be carried out. A second method of plasma membrane preparation was tested consisting first in the coating of the cell surface with positive colloidal silica which was stabilized by an anionic polymer. Then, and through differential centrifugations, plasma membrane fractions were easily obtained within less than 1 h, with a yield of 65% and an enrichment ratio of 7. The coating pellicle was quantitatively removed thus enabling any biochemical manipulation of the plasma membrane to be carried out. The lipids present in the plasma membrane of CHO cells were analyzed and are described, both in terms of headgroup and acyl chain composition.