A cell-free system from ethionine-treated rat liver active in initiation of protein synthesis.

A cell-free protein-synthesizing system active in initiation of translation of both endogenous mRNA and exogenous mRNA has been obtained from postmitochondrial supernatant (S-12) of the liver of ethionine-treated rats by adding reticulocyte ribosomal extract as a source of initiation factor. Formation of polysomes in the course of protein synthesis in vitro has also been demonstrated. Homogenization of the liver in the presence of 50 microM hemin stabilizes the initiation activity of S-12 fraction, which otherwise decays rapidly even at 0 degrees C. The mechanism of inhibition of protein synthesis by ethionine is discussed in view of these results.