Amino acid incorporation in vitro by ribonucleoprotein particles detached from guinea pig liver microsomes.

Since it has been shown that the recombined mitochondrial, microsomal, and supernatant fractions of rat liver are able to incorporate radioactive alanine into trichloroacetic acid-insoluble protein (1)) many investigators have worked on the purification and characterization of the amino acid incorporation system of liver cells. The results of their efforts have been reviewed most recently by Loftfield (2), Chantrenne (3), and Simkin (4). Zamecnik and Keller (5) showed that the microsomes by themselves were sufficient for incorporation if a soluble nondialyzable fraction, adenosine triphosphate, and an adenosine triphosphate-generating system were present. Littlefield et ~2. (6) focused attention on the attached ribonucleoprotein particles of the microsomes by demonstrating that after an injection in vivo of radioactive amino acid the deoxycholate-insoluble particles became labeled more rapidly than either the whole microsomes or the soluble cell protein. This finding pointed to the ribonucleoprotein particles as the most likely primary site of amino acid incorporation. The experiments reported here were carried out to determine if ribonucleoprotein particles detached from the microsomal membranes still retain their ability to incorporate radioactive amino acids in vitro.