Decreased inhibitory activity of salt wash preparations toward aminoacyl-tRNA binding to liver ribosomes in vitro after administration of DDT

Ribosomes were prepared from the microsome fraction of a liver homogenate of control and DDT-treated adult male rats using Triton X-100, as well as by several other methods. In every case, a 0.5 M salt wash fraction of the ribosomes from control animals contained an inhibitor of peptide synthesis which exerted its action on the ribosomes by reducing the elongation factor 1-stimulated step measured in vitro, i.e., aminoacyl-tRNA binding. The inhibitory activity was removed by heat treatment and trypsin digestion and by Pronase digestion. The inhibitor was different from the thiol and GTP-sensitive inhibitor of liver microsomal membranes and from the hemin-reversible inhibitor of reticulocyte lysates, but it was similar to the ribosomal inhibitor described for certain nonmammalian cells. In contrast to the situation during liver regeneration and during the response to aminonucleoside nephrosis, no inhibitory activity could be detected in the salt wash fraction after the administration of DDT. These results emphasize the necessity for a salt-wash step in the preparation of ribosomes for measurement of their activity after the induction of microsomal enzymes.