Studies on incorporation of amino acids into protein in isolated rat-liver ribosomes

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Abstract
Ribosomes with a high ribonucleic acid protein ratio were prepared from rat liver by 2 methods described. They were shown to incorporate radioactive amino acids into protein in vitro. Ribosomes incorporate more amino acid into protein (per mg of protein or per mg of ribonucleic acid) than micro- somes and they continue to incorporate amino acids for 2 or 3 hr. Adenosine triphosphate, guanosine triphospnate and the soluble fraction of the cell (or a pH 5 enzyme preparation) are required for incorporation but an ATP-generating system is not needed. Ribosomes are more stable than microsomes, being able to withstand preincuba-tion at 37[degree] for an hour, dialysis, and storage at 0[degree] for at least a week without loss of activity. Labelled protein is transferred from the ribosome to the supernatant fluid during incubation. The concentrations of Mg ions and ATP that give maximal incorporation are very critical, but large amounts of either, which in themselves are inhibitory, can be tolerated to some extent if an equimolar amount of the other is present.