1. The metabolism of protein and phospholipid in rat liver plasma membranes isolated by the method of Neville [(1960) J. Biophys. Biochem. Cytol. 8, 413-422] was investigated 3 and 6 h after the injection of D-galactosamine in vivo. During this time, all the biochemical and morphological alterations associated with hepatitis developed. 2. After the injection of D-galactosamine the concentration of sphingomyelin in the plasma membrane decreased to below 60% of the control values. 3. The activity of 5′-nucleotidase (EC 3.1.3.5), which has been purified as a sphingomyelin-protein complex, decreased in the total homogenate as well as in the plasma-membrane fraction of livers of rats treated with galactosamine, to about 60% of the control values. 4. Protein synthesis, as measured by the incorporation of [14C]leucine into plasma membranes, was decreased to 45% of that of the controls. However, only small differences were observed in the amino acid composition of the plasma membrane after D-galactosamine treatment. 5. The protein composition of the plasma membranes was determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The results showed a change from low- to high-molecular-weight proteins after the injection of galactosamine. 6. These results demonstrate different metabolic processes of the plasma membrane altered during the induction of galactosamine hepatitis.

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