Studies on plasma membranes: IV. the ultrastructural localization and content of sialic acid in plasma membranes isolated from rat liver and hepatoma

Abstract

ABSTRACT Plasma membranes were isolated from rat liver and a transplanted rat hepatoma of the hepato-cellular type. After glutaraldehyde fixation the membranes were treated with colloidal iron hydroxide (CIH) at pH 1·7, which was found to react specifically with the neuraminidase-sensitive sialic acid of the liver membranes. The CIH-reactive, neuraminidase-sensitive sialic acid, comprising 70 % of the membrane-bound sialic acid, was exclusively located in the outer leaflet of the liver membranes as shown by the rather regular distribution of electron-dense CIH granules. This granular, asymmetric type of staining was also observed in the hepatoma membranes, which contained some 50% more sialic acid than did the liver membranes. In addition, the hepatoma membranes showed an intense and uniform staining by CIH of short segments of both membrane leaflets; the latter type of staining was but little impaired by neuraminidase pre-treatment. None of the junctional complexes of the liver membranes was stained by CIH. Tight junctions were very rarely observed in the hepatoma membrane preparations, and the desmosomes and intermediate junctions of these membranes not infrequently exhibited a loosened appearance exposing CIH-reactive neuraminidase-sensitive sialic acid at their opposite plates. This aspect could be induced in the desmosomes and intermediate junctions, but not in the tight junctions, by pre-treatment of the liver membranes with the chelating agent ethylenediaminetetra-acetate.