[Mechanism of post-synthetic changes in glucose-6-phosphate dehydrogenase in human cells cultured in vitro].

The reasons for the decreased stability of glucose-6-phosphate dehydrogenase in transformed human cells were investigated. The enzyme stability was found to be dependent on its subunit composition; the dimeric form possessed a lower stability in comparison with the tetrameric one. An addition of NADP to cell extracts which had partly lost their glucose-6-phosphate dehydrogenase activity, resulted in reactivation and stabilization of the enzyme. The constants for a forward (k1) and back (k2) reactions during stabilization are equal to 2.87 X 10(-3) and 5.77 X 10(-1) s-1, respectively. The inactivation and reactivation kinetics suggest that the enzyme destabilization may also occur inside the cells. The cells contain more than 40% of glucose-6-phosphate dehydrogenase molecules in an inactive form. A mechanism of destabilization and inactivation of glucose-6-phosphate dehydrogenase is proposed, which consists in NADP hydrolysis and enzyme decomposition to inactive monomers which are less stable to proteolysis.