The shift in the distribution of polysome-classes in the course of the maturation process of reticulocytes

Intracellular distribution of ribonuclease activity during erythroid cell development

Five ribonuclease activities, separable by polyacrylamide gel electrophoresis, have been detected in erythroid bone marrow cells from anaemic rabbits. Their intracellular distribution has been investigated and compared with that of the ribonucleases in reticulocytes. Both the acid and alkaline ribonuclease activities of reticulocytes are much lower (30--50 fold) than those of bone marrow erythroid cells. The most marked decrease in enzyme activity occurs in the fractions containing ribosomes and mitochondria plus lysosomes. In these subcellular organelles there was also a qualitative change in the ribonuclease electrophoretic pattern, whereas the cytosol enzymes of marrow erythroid cells and reticulocytes remained largely unchanged. Several ribonucleases released from reticulocyte membranes with urea were similar to those present in the lysosomal plus mitochondrial fraction, as shown by detection of enzyme activity after polyacrylamide gel electrophoresis. The decline in ribonuclease activity was found to begin in the orthochromatic cells, which have a highly condensed nucleus and are no longer active in DNA and RNA synthesis, and to coincide with a decrease in acid phosphatase activity and loss of lysosomes.

Polyribosome profiles and ribonuclease activity in spleens of normal and anaemic mice

The extent to which the polyribosome content in splenic extracts is affected by the level of endogenous ribonuclease activity was investigated. Ribonuclease activity, free and inhibited, was determined under ionic conditions optimal for preservation of polyribosomes. The results show that ribonuclease activity in control spleens varies mainly with the amount of ribonuclease inhibitor, indicating that free ribonuclease activity is stable. In rapidly proliferating splenic tissue the enzymatic activity is regulated by changes in both the amount of inhibitor and enzyme available. The ribonuclease versus ribonuclease-inhibitor balance was found to parallel the proportion of polyribosomes in a polysome profile.