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Inhibitors of Protein Synthesis in Rat Liver Microsome Fractions

From the ¹ From the Department of Bacteriology and Immunology, Harvard Medical School, Boston, Massachusetts 02115

Rat liver microsome fractions contain two general kinds of inhibitors of amino acid incorporation in vitro. The first is derived from lysosomes contaminating the microsome fractions. Lysosomes may be removed from rough vesicles (ribosomes plus endoplasmic membranes) by a density centrifugation technique.

Lysosomal inhibition of incorporation in vitro is much more prominent in normal than in regenerating liver microsomes even though the lysosome content is the same, suggesting that normal lysosomes release their degradative enzymes more readily during incubation. Rough vesicles from normal and regenerating livers are alike in activity, as are the polyribosomes derived therefrom.

The second inhibitor is a heat-labile factor associated with the endoplasmic reticular membrane, which requires oxidized glutathione under certain conditions and which can be reversed by an excess of guanosine triphosphate or sulfhydryl reagents. The inhibitor-sensitive step is the transfer of amino acids from aminoacyl soluble ribonucleic acid to protein.

Enzymes catalyzing the transfer of amino acids from soluble ribonucleic acid to protein are less than half as active (per mg of supernatant protein) in normal as in regenerating liver when tested at limiting concentration and in the absence of GTP. Their activity is equalized in the presence of an excess of GTP. The enzymes from normal liver also increase in activity upon aging at 0°, to become more nearly equal in activity to the enzymes from regenerating liver.

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