Some evidence for a pathway of amino acid incorporation, in rat-liver microsomes, which does not require transfer ribonucleic acid

Abstract

Rat-liver microsomes which exhibit a high activity for the incorporation of [ 14C]-leucine into their proteins in the absence of added pH5 precipitate (unwashed microsomes), are stimulated when the pH5 preparation is separated from the microsomes by a dialysis membrane. A requirement for the complete pH5 preparation is obtained when the microsomes are washed by resuspension in medium A (washed microsomes), and in contrast to the unwashed preparation, the pH5 dialysable materials do not increase such incorporation. The low activity of the washed microsomes alone can be significantly increased by the addition of S 4 supernate (obtained by precipitating the pH5 preparation) containing negligible quantities of RNA, and further increases can now be obtained by the action of the dialysable factors from the pH5 preparation. Such results suggest that washing the microsomes removes those areas of activity associated with simulation by the pH5 dialysable materials, and which are replaceable by components of the S 4 supernate. The results have been interpreted to indicate that in the unwashed microsomal preparation there exist two pathways for amino acid incorporation, one of which is independent of s-RNA and which can be stimulated by the pH5 dialysate. In support to this conclusion, it has been shown that supplementation with both pH5 precipitate and S 4 supernate, can result in lower incorporations than would be obtained in the presence of either one of these fractions alone.