Free and Membrane-bound Ribosomes in Rat Liver

Ribosomes in rat liver are present either free or attached to endoplasmic reticulum and in both states exist largely as polyribosomal aggregates. The present communication presents a quantitative study of the distribution of ribosomes between these two states and examines a number of properties of these two groups of ribosomes. It is shown that the base composition and the rate of synthesis of the ribosomal ribonucleic acid of free and membrane-bound ribosomes are the same, and these tindings, along with the results of previous studies of other properties, are consistent with the possibility that there are no intrinsic differences between the ribosomes found in these two states. Since certain conditions are known to produce alterations in the distribution of free and membrane-bound ribosomes which are rapid with respect to the turnover time of ribosomal RNA, it is further suggested that a given ribosome can exist in either state according to the needs of the cell. Although the administration of hydrocortisone in vivo induces acute rises in the levels of certain specific liver enzymes within a few hours, it has no marked effect upon the ratio of free to membrane-bound ribosomes during this period. The ribosomal aggregates obtained by the methods used here are shown to be sufficiently pure to permit the use of ultraviolet absorption as a direct measure of their concentration without applying a correction for ferritin absorption. Homogenization and zone centrifugation do not produce significant artifacts in the determination of the proportions of free and membrane-bound ribosomes.