Rabbit red blood cell hexokinase. Mechanism of decay during cell life-span

Rabbit red blood cells contain hexokinase type I whereas in the reticulocyte two distinct molecular forms (HK Ia and Ib) are present. One (HK Ia) corresponds to hexokinase type I from other tissues, while the other differs from any previously reported isozyme. Rabbit bone marrow cells contain hexokinase type I and II. However, when the erythroid precursor cells become predominant over the non-erythroid cells (during phenylhydrazine anemia) a great increase of HK Ia can be observed concomitant with the appearance of HK Ib. Fractionation of the bone marrow cells on density gradients provides evidence that basophil erythroblasts and proerythroblasts contain only HK Ia while HK Ib appears at the reticulocyte stage. Maturation and ageing of circulating reticulocytes are associated with the decrease of hexokinase activity. Since the decay rate of HK Ib is about three times higher than the decay rate of HK Ia, the mature erythrocytes do not contain appreciable amounts of HK Ib. Furthermore, in vitro, HK Ia and Ib possess similar stabilities so that a cellular mechanism must be responsible of their in vivo different decay rates. This mechanism, as reported in this paper, is ATP-dependent, could be found in the soluble fraction, and is active only at the reticulocyte stage. These properties are similar to those of the ATP-dependent proteolytic system. Pure ubiquitin, an essential polypeptide of the ATP-dependent proteolytic system, is also able to catalyze the decay of hexokinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)