Maturation of rabbit reticulocytes: strong decline of the turnover of polyphosphoinositides.

The [32P]phosphate labelling of ATP and polyphosphoinositides (PI) of intact rabbit reticulocytes and mature erythrocytes was compared. Despite the fact that reticulocytes have a 30-fold higher turnover of ATP than mature erythrocytes, the 32P-labelling of the cellular ATP pool was identical. The 32P-Pi entry into the cells is the rate-limiting step for 32P-phosphate isotopic equilibration of the ATP-pool and is independent of maturation. The rates of 32P-phosphate incorporation into PI-4,5-P2 and into PI-4-P, respectively, were 17-fold and 8-fold higher in reticulocyte-rich cell suspensions than in erythrocytes. The specific radioactivity of PI-4,5-P2 labelling reaches 55% and that of PI-4-P 40% of the ATP specific activity. However, a rapid isotopic equilibration of the polyphosphoinositides, was found. These findings indicate the existence of a heterogeneity of polyphosphoinositide pools. Reticulocyte membranes lose about 30% of their polyphosphoinositides during maturation proportional to the total loss of phospholipids.