Cellular binding of 3H-cytochalasin B

The binding of tritium-labelled cytochalasin B (³H-CB) to a variety of mammalian cells was investigated. Binding studies revealed near-equilibrium binding of ³H-CB within 5 to 10 minutes, but the equilibrium level was influenced by ³H-CB concentration. Binding kinetics revealed strong temperature dependence. Rapid release of up to 70% of cell-bound ³H-CB molecules occurred when cells were washed and returned to fresh medium without CB. The remaining 30% of cell-bound ³H-CB molecules dissociated more slowly. Equilibrium binding studies on a variety of diploid, heteroploid and transformed cells treated with 1 μg/ml ³H-CB revealed between 1.7 X 10⁷ to 5.3 X 10^{7 3}H-CB binding sites per cell. Cellular binding of ³H-CB was not affected by inhibition of cellular energy metabolism, RNA or protein synthesis. Modification of the cell surface by proteases, neuraminidase, hyaluronidase, ribonuclease, or occupation of cell surface saccharide residues by a variety of plant lectins did not significantly alter the pattern of ³H-CB binding. Surface pressure measurements on CB-treated lipid monolayers indicated that CB can interact with lipid molecules. The partition of CB in hydrophobic lipid regions of cell membrane systems as a possible mechanism of cellular binding of CB is discussed. Fractionation of ³H-CB-treated cells revealed binding of ³H-CB to both the plasma membrane and by intracellular membranes.