Studies on plasma membranes XVII. On the chemical composition of plasma membranes prepared from rat and mouse liver and hepatomas

Abstract

Plasma membranes were isolated under hypotonic conditions from rat and mouse livers and five hepatomas, i.e. one rather anaplastic rat hepatoma (and its subline) and three well-differentiated mouse hepatomas. All these membranes contained some 25% protein soluble in 0.15m NaCl. Evidence is presented that this protein is mainly, if not exclusively of nonmembranous origin. Protein/phospholipid P (P=phosphorus) ratios did not differ significantly for the various plasma membrane species except the rat-hepatoma subline, which showed a markedly lower ratio and was thus identified. Hepatoma membranes contained more P of a nonphospholipid nature than did liver membranes and to this increase contributed in all instances an increased RNA content and in some cases also an increased DNA content. The presence of DNA in these plasma membranes is artefactual, but that of RNA is more complicated. Artefactually, Ca²⁺-associated RNA of low mol wt and soluble in 0.15m NaCl, and residual RNA (genuine?, in liver membranes less than 1% in respect of protein) have been demonstrated. The increase in hepatoma-membrane RNA is attributed to the ribosomal RNA of the few microsomal vesicles which are structurally connected with these plasma membranes. The sialic acid content and the percentage of neuraminidase-resistant sialic acid of hepatoma as compared with liver membranes was either similar or changed, depending on the hepatoma strain. Gelfiltration of trypsin-released peptides of liver plasma membranes showed hexosamine and hexose to be confined to the sialic acidcontaining fractions. In spite of quantitative differences among fractions, the relative contents of the three carbohydrates in the combined fractions were (about) similar to those in intact liver membranes. Similar experiments with the rat-hepatoma membranes showed a changed carbohydrate expression.