Intracellular distribution of ribonuclease activity during erythroid cell development

(2'-5') Oligoadenylate synthetase in the maturation of rabbit reticulocytes

The (2'-5') oligoadenylate synthetase normally found in interferon-treated cells has also been detected in considerable amounts in normal rabbit reticulocytes not exposed to interferon. The activity of this enzyme has been followed during the development of the reticulocytes to erythrocytes. A high level was found just after the formation of reticulocytes and this activity decayed with a half-life of about 3 days. In lymphocytes the (2'-5') oligoadenylate synthetase was found to stay at a constant level, which indicates the absence of interferon in the plasma.

Transfer RNA in reticulocyte maturation

The disappearance of tRNA during the maturation of rabbit reticulocytes under the stress of phenylhydrazine-induced hemolysis was studied. The tRNA content of reticulocytes and of erythrocytes derived from them was compared. The results show that tRNA persists longer after reticulocyte maturation than ribosomes and than the ability to incorporate amino acids into protein. Considerable uniformity of tRNA degradation was noted with about 15% of the tRNA for most amino acids remaining after reticulocyte maturation. The half-life of tRNA in the maturing cells is estimated to be 50--60 h. There is little tRNA lacking the 3'-terminal pCpCpA moiety in cells derived from reticulocytes.

Cyclic AMP phosphodiesterase activity during differentiation of rabbit erythroid bone marrow cells

Changes in the activity of cyclic AMP phosphodiesterase during differentiation of rabbit bone marrow erythroid cells were investigated. The cells were separated by velocity sedimentation at unit gravity into six fractions corresponding to different stages of development: proerythroblasts, basophilic cells, polychromatic cells, early orthochromatic and late orthochromatic cells and reticulocytes. Cyclic AMP phosphodiesterase was found to be very active in the most immature cells, the proerythroblasts, which also have the highest content of cyclic AMP. After differentiation into basophilic erythroblasts, a 4-fold decrease in cyclic AMP phosphodiesterase activity was observed. In these cells the amount of cyclic AMP was about 80% lower than that in proerythroblasts. In polychromatic cells a further drop in phosphodiesterase activity occurred. After the final cell division the enzyme activity was very low and the levels of cyclic AMP in the early and late orthochromatic cells remained constant. Kinetic studies demonstrated a heterogeneity of erythroid cell cyclic AMP phosphodiesterase: high affinity, low-Km (5.5 X 10(-6) M) and low affinity, high-Km (0.1 X 10(-3) M) enzymes were found. The phosphodiesterase activity was dependent on the presence of Mg2+ and was activated by Ca2+ at low Mg2+ concentrations (1 mM). The changes in cyclic AMP phosphodiesterase activity during differentiation and maturation of erythroid cells suggest the possible importance of this enzyme in the physiological control of cyclic AMP concentrations in developing erythroblasts. The loss of cyclic AMP phosphodiesterase activity after cessation of cell division supports the concept of the significance of the final cell division in erythroblast differentiation.

Separation of haemopoietic cells for biochemical investigation. Preparation of erythroid and myeloid cells from human and laboratory-animal bone marrow and the separation of erythroblasts according to their state of maturation

The separation of haemopoietic bone-marrow cells by centrifugation through discontinuous density gradients of Percoll is described. This method was used to prepare fractions enriched in erythroblasts, myeloid blast cells or reticulocytes from bone marrow of anaemic and non-anaemic rabbits, from the marrow of other anaemic laboratory animals and from human samples. It is a simple, rapid, reproducible and inexpensive technique that can be readily adapted to suit individual requirements. Secondly, a convenient method is presented for the separation of large quantities of bone-marrow cells into fractions enriched in erythroblasts at different stages of maturation, by velocity sedimentation through a linear gradient of 1-2% sucrose at unit gravity. In vitro, erythroblasts adhere together strongly via a mechanism almost certainly involving a beta-galactoside-specific surface lectin termed erythroid developmental agglutinin. Since the efficiency of cell-separation techniques depends heavily on the maintenance of a single cell suspension in which each unit can move independently, the presence of an adhesive molecule at the cell surface is of considerable significance. The effect of washing the marrow with a lactose-containing medium, which has been shown to remove the agglutinin, was therefore investigated in relation to both methods. The separation on Percoll gradients is considerably enhanced by this treatment. In addition, the unit-gravity sedimentation gradient can be loaded with 5-10 times more cells after lactose extraction in comparison with intact marrow. Although enrichment is less, a useful fractionation according to maturation is still obtained.

Ribonuclease activities in developing experimental granulation tissue with reference to polysomes

The Mg2+-precipitable polysomes in experimental granulation tissue increase up to three weeks and decrease slowly thereafter. Polysomes from young granulation tissue are, on a weight basis, more active in cell-free protein synthesis than polysomes from old granulation tissue. RNase activity is highest in polysomes from 1-week tissue. The polysomes contain both acid and alkaline RNase, but the alkaline RNase is the latent form, manifested by the addition of p-chloromercuribenzoate (pCMB). The high RNase activity of granulation tissue renders the analysis of polysomal patterns and functions of the polysomes difficult. In the post-polysomal supernatant of granulation tissue the RNase activity is highest in 2-4 week old tissue; it is maximal at pH 7-8 and only slightly influenced by pCMB.

Transferrin receptors during rabbit reticulocyte maturation

Experiments were performed to examine the fate of transferrin receptors in reticulocytes as these cells mature in vivo to erythrocytes. Reticulocytosis, synchronized by administration of actinomycin D, was induced in adult rabbits. Simultaneous measurements were made of haematological parameters and the interaction between transferrin and reticulocytes while the cells matured in vivo to erythrocytes. As the reticulocytes matured there was a parallel decline in their ability to take up transferrin and transferrin iron. At the same time, there was a proportionate decrease in the density of receptors for transferrin on the reticulocyte surface. The affinity of the receptors for transferrin remained unaltered during the maturation process. It was concluded that the inability of erythrocytes to take up transferrin or its iron is due primarily to the loss of transferrin receptors from the maturing reticulocyte surface.

Interaction between membrane functions and protein synthesis in reticulocytes. Isolation of RNase M, a membrane component inhibiting protein synthesis through specific endonucleolytic activity

An inhibitor of protein synthesis has been isolated from reticulocyte membranes by solubilization with Triton X-100; it has been purified using heat treatment, filtration on Amicon filters, DEAE-cellulose ion-exchange chromatography and Sephadex G-75 gel chromatography. A final purification of 120-fold was achieved. The purified inhibitor was found to be 95% homogenous when run on a dodecylsulfate/polyacrylamide gel system. Three independent methods were used to estimate the molecular weight of the purified inhibitor: Sephadex G-75 gel chromatography, dodecylsulfate/polyacrylamide gel electrophoresis and sucrose gradient all confirmed that the purified inhibitor was a small molecule with a sedimentation coefficient of 0.7 S and a molecular weight ranging between 5000 and 8000. The purified inhibitor was shown to possess a specific endonucleolytic activity, degrading the 28-S species of ribosomal RNA to species sedimenting between 10 and 14 S. Due to its membrane localisation the name RNase M is proposed. The purified inhibitor's endonucleolytic activity was characterized with regard to its kinetics, concentration dependence, pH optimum and its requirements for divalent cations. Kinetics showed that RNase M retained its specificity after 60 min of incubation with the RNA substrate. Specificity was also demonstrated by incubating the polysomal RNA with high concentrations of purified enzyme. The pH optimum was found to be between pH 6 and pH 7, and the enzyme did not require divalent cations for its activity. Pancreatic RNase B used at a similar protein synthesis inhibitory concentration as the RNase M caused a complete breakdown of ribosomal RNA to oligonucleotides and mononucleotides. The possible biological significance of the purified inhibitor in regulating protein synthesis in the maturing reticulocyte is discussed.