Biosynthesis of liver membranes. Incorporation of [3H]leucine into proteins and of [14C]glucosamine into proteins and lipids of liver microsomal and plasma-membrane fractions

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Abstract
1. The smooth-and rough-microsomal and the light and heavy plasma-membrane fractions of mouse liver homogenates were prepared and characterized by using biochemical markers. 2. The hexosamine/protein ratio was threefold higher in the plasma membranes than in the smooth-microsomal fraction. Glucosamine was bound only to protein, and galactosamine was attached mainly to lipids. 3. [3H]-Leucine and [14C]glucosamine were injected into animals and the rates of incorporation of radioactivity into the fractions were determined. Both precursors were rapidly incorporated into the microsomal fractions, but plasma membranes showed a slower rate of synthesis which reached a maximum at 2–4h after intravenous administration. 4. The light- and heavy-plasma-membrane fractions showed similar patterns of incorporation, and therefore a precursor–product relationship appears unlikely. 5. Plasma membranes, especially the light subfraction, showed appreciable incorporation of hexosamine into chloroform–methanol-soluble components which were shown to be mainly glycolipids. 6. The results indicate that liver plasma-membrane proteins and glycoproteins are synthesized at similar rates. However, glycolipid synthesis in plasma membranes occurred more rapidly.