Glucose-6-phosphate dehydrogenase activity and protein turnover in erythroblasts separated by velocity sedimentation at unit gravity and Percoll gradient centrifugation.

This work was undertaken to improve a separation method for preparation of large amounts of erythroid cells of different age with homogeneous and minimal contamination of myeloid cells. Our method was suitably employed in the study of the decay mechanism of glucose-6-phosphate dehydrogenase (G6PDH) during the erythroid cell maturation. Twenty fractions of erythroid cells at different advancing stages of maturation were prepared by fractionating, at unit gravity, bone marrow cells from anaemic rabbit. The specific activity of the G6PDH was assayed and plotted vs the fraction number and the typical sigmoid curve of the activity decay was drawn. The separated cells were then grouped in three sets of fractions following the three phases of the sigmoid curve and the fractions of each set were combined. From the cytochemical analysis of the three main fractions so obtained, we found a 25-30% myeloid cell contamination in the first fraction, while in the other two fractions the myeloid contamination was 10% or less. For this reason we performed a rapid separation of the first fraction on a discontinuous percoll gradient. By this method, the myeloid cell contamination of the first fraction was levelled down to the other two. The fractions, so obtained, (I, II and III in order of increasing cell maturation) showed a four fold decrease of glucose-6-phosphate dehydrogenase activity expressed both per cell number and on protein base. On the contrary the concentration of the total soluble proteins did not change significantly in the three fractions. The three purified cellular populations were used to provide information on the protein turnover of the erythroid cells during their development. We measured, in intact cells, the rate of synthesis and degradation of total proteins and then, in cell lysates, we determined the rate of degradation of G6PDH, purified from rabbit RBC and radiolabeled by reductive methylation with C14-formaldehyde. The rates of proteolysis obtained with total proteins and methyl-G6PDH clearly indicate that the proteolytic machinery of the erythroblasts reduces its activity during the cell maturation.