Incorporation of amino acids into lipoidal material by microsomal and supernatant fractions of rat tissues.

The function of the microsomes as a primary site of protein biosynthesis is well established (1). Both microsomal and soluble ribonucleic acid have been shown to participate in the incorporation of amino acids into ribonucleoproteins in vitro. This phenomenon has been interpreted as a step in the synthesis de nouo of proteins (2). Hendler (3) has suggested that lipids might also be involved in protein synthesis not only because of the preponderance of cytoplasmic lipid in the microsome but also on the basis of his observation that whole cells of hen oviduct tissue incorporate several amino acids into the lipid fraction obtained bysolventextractionof the trichloroacetic acid-insoluble material. He also showed that agents which attack lipids deter the incorporation of amino acids, presumably into protein. More recently he has reported that the time course of incorporation of labeled amino acids into lipids and proteins of chicken oviduct mince and homogenates is consistent with the role of an amino acid-lipid intermediate in protein synthesis (4). While investigating protein synthesis in cell-free preparations of rat liver we found that both microsomes and the particle-free supernatant solution incorporate radioactive amino acids into lipoidal substances (5). In this communication we compare the metabolic aspects of incorporation of amino acids into lipid with that of incorporation into protein in a cell-free system, and consider the possible relevance of incorporation into lipids to “conventional” protein synthesis. We present also some preliminary results on the nature of the amino-acid bearing lipids involved.