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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.

The subcellular distribution of alkaline phosphatase in normal and transformed cultivated human cells was investigated. The total activity of alkaline phosphatase in the transformed cells exceeds that in normal cells 10- (or more) fold and is sharply increased with a rise in the cell population density. The proliferating cells have the same specific activity of alkaline phosphatase in the mitochondrial-lysosomal and microsomal fractions. The transition of normal cells to the G0-phase of the cell cycle is accompanied by an increase in the total enzyme activity and selective accumulation of alkaline phosphatase in the microsomes. An addition of exogenous alkaline phosphatase to intact cells leads to the stimulation of DNA synthesis and to changes in cell morphology. The data obtained suggest that alkaline phosphatase can participate in the destabilization of the cytoskeleton and stimulate cell proliferation. However, in normal cells the selective accumulation of the enzyme in the membranes may hamper its biosynthesis. the activity of alkaline phosphatase thus does not reach the level sufficient for unrestricted cell growth.

The causes for different stability of glucose-6-phosphate dehydrogenase in two heteroploid cell strains and in the diploid cell strain of human embryo lungs were investigated. The thermostability of glucose-6-phosphate dehydrogenase was shown to be dependent on the coenzyme (NADP) concentration and to be coupled with the activity of alkaline phosphatase. In diploid and heteroploid cell extracts possessing a low alkaline phosphatase activity glucose-6-phosphate dehydrogenase reveals a high stability. In heteroploid cell extracts having a high activity of alkaline phosphatase a fast hydrolysis of NADP and a decrease of glucose-6-phosphate dehydrogenase stability are observed. Inhibition of alkaline phosphatase by levamisole prior to cell disruption does not increase the stability of glucose-6-phosphate dehydrogenase. Presumably destabilization of glucose-6-phosphate dehydrogenase mediated by alkaline phosphatase occurs in intact cells and is an essential mechanism controlling the enzyme activity.