The microsomal particles (1) have been implicated as the major sites of protein synthesis within the cells of a variety of tissues. The studies of Borsook, Zamecnik, Hultin, and others, with intact animals and various types of whole cell systems (see review by Askonas et al. (2)), as well as electron-microscope studies (3), provided the original evidence for this conclusion. Zamecnik and Keller (4) have, in addition, developed cell-free systems which incorporate C14-labeled amino acids into protein. These include microsomes as well as various soluble enzymes (5). Further studies have partially defined the intermediate stages involved (6). However, it has not yet been possible to equate incorporation of labeled amino acid into protein with actual protein synthesis (see review by Campbell (7))
