Axonal growth cones (AGC) isolated from fetal rat brain have an important specific activity of N+/K+-ATPase. Kinetic assays of the enzyme in AGC showed that Kmvalues for ATP or K+are similar to those reported for the adult brain enzyme. For Na+the affinity (Km) was lower. Vmaxfor the three substrates was several times lower in AGC as compared to the adult value. We also observed two apparent inhibition constants of Na+/K+-ATPase by ouabain, one of low affinity, possibly corresponding to the α1 isoform and another of high affinity which is different to that described for the α2 isoform of the enzyme. These results support an important role for the sodium pump in the mantainance of volume and cationic balance in neuronal differentiating structures. The functional differences observed also suggest that the enzymatic complex of Na+/K+-ATPase in AGC is in a transitional state towards the adult configuration. © 1994.
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