High-yield purification of plasma membranes from transformed human keratinocytes in culture

The density pertubation technique with cationic silica microbeads was applied to prepare highly purified plasma membranes from cultured human keratinocytes. Trypsinized cells were coated successively with the beads (diameter approximately 50 nm, gravity greater than 2 g/cm3) and polyacrylic acid before they were lysed by osmotic shock and mechanical shear. The plasma membranes remained in the form of large open sheets which could easily be separated from other cell organelles and the cytosol by low-speed centrifugation. The membrane preparation was characterized by scanning and transmission electron microscopy, marker enzyme activities, one-dimensional sodium dodecyl sulfate polyacrylamide electrophoresis, and the specific beta-adrenergic receptor count. A yield of 79 +/- 9% was calculated by comparing the amount of beta-adrenoceptors in the purified membrane preparation with that of a crude cellular particulate fraction. The specific beta-adrenoceptor count of these two preparations was 1.2 +/- 0.02 and 0.2 +/- 0.05 pmol/mg protein, respectively, indicating a 6-fold improved purification with this microbead technique. The purified membranes were essentially free from contamination of other cell organelles.