Thyroxine Stimulation of Amino Acid Incorporation into Protein Independent of Any Action on Messenger Rna Synthesis.

The consistent association of the sodiumand potassium-activated ATPase with the immunologically detectable component suggests either that all fragments of plasma membrane maintain their natural "inside-outside" orientation, or that this enzyme system is active only when the original orientation is maintained. The very fact that this ATPase is detectable at all implies that the vesicle walls are either permeable to ATP, or that some of the ATP combining sites of this enzyme system are on the outside surfaces of the surface vesicles and, by inference, on the outside surfaces of intact cells. Summary.-(1) Procedures are described for the analysis of the buoyant density distributions of the membrane components of Ehrlich ascites carcinoma microsomes in gradients of NaBr, sucrose, and the sucrose polymer Ficoll. (2) The microsomal membranes behave as selectively permeable sacs, which expand and contract under electrostatic and osmotic influences. (3) Plasma membrane fragments, localized by their unique enzymatic and immunologic properties, can be separated from the other membrane components by equilibration in the sucrose-polymer gradients.