In vitro excitation of purified membrane fragments by cholinergic agonists : IV. Ultrastructure, at high resolution, of the AcChE-Rich and ATPase-rich microsacs.

Membrane fragments rich in acetylcholinesterase (AcChE) or in Na(+)-K(+)-ATPase are observed under the electron microscope on thin sections after fixation, after negative staining of unfixed material, and after freeze-etching. Both classes of membrane fragments make closed approximately spherical vesicles, or microsacs. The preparation appears to be free from mitochondria, nuclear envelopes or other cytoplasmic contamination. A subunit structure is seen with both kinds of microsacs by freeze-etching and negative staining, but the size, shape and arrangement of the subunits are different in the two classes of membrane fragments. On thin sections, globular repeating units are seen only with the AcChE-rich microsacs; the membrane of the ATPase-rich microsacs shows a classic triple-layered structure.