Stimulation of rat liver glycogen synthesis by the adenosine kinase inhibitor 5-iodotubercidin.

The adenosine kinase inhibitor 5-iodotubercidin (Itu) was found to have the following effects on glycogen metabolism in hepatocytes of fasted rats. (1) Itu strongly stimulated glycogen synthesis from different substrates (glucose, lactate plus pyruvate, dihydroxyacetone, glycerol and fructose). In cells incubated with these substrates, the well-known stimulating effect of amino acids and that of Itu was more than additive. (2) In parallel with the increase in glycogen deposition, there was an increase in synthase a and a decrease in phosphorylase a concentrations after administration of Itu. Synthase a was increased by Itu and amino acids in an additive manner, whereas the observed activation of phosphorylase after addition of amino acids was antagonized by Itu. (3) In contrast with amino acids, Itu increased neither the cell volume nor the aspartate and glutamate concentrations. (4) Itu enhanced the levels of cyclic AMP. The stimulation of glycogen deposition in the presence of Itu persisted when the cyclic AMP concentration was further increased by adenosine or 2-chloroadenosine. (5) Itu decreased the concentration of ATP, but its effects on glycogen synthesis, synthase a and phosphorylase a concentrations persisted when the ATP catabolism was prevented by adenosine. (6) The effect of Itu on glycogen synthesis was not the result of inhibition of adenosine kinase, since 5'-amino-5'-deoxyadenosine, another inhibitor of this enzyme, had no effect on glycogen deposition.

The common occurrence of ATP diphosphohydrolase in mammalian plasma membranes.

In the plasma membranes from several mammalian tissues (including normal and tumor tissues), a Mg2+ (or Ca2+)-dependent ATP phosphohydrolase activity is present in much greater amount than the (Na+ + K+)-ATPase. The ouabain-insensitive activity can be attributed to at least two enzymes, an ATPase (EC 3.6.1.3) and an ATP diphosphohydrolase (EC 3.6.1.5). The ATPase hydrolyzes ATP and other nucleoside triphosphates and is not inhibited by azide. The ATP diphosphohydrolase hydrolyzes both ATP and ADP (and other nucleoside tri- and diphosphates) and the hydrolysis of adenine nucleotides is strongly inhibited by 10 mM azide. The ratios of these two enzymes in the various membranes (as determined by the extent of azide inhibition) vary widely. The ATP diphosphohydrolase accounts for most of the Mg2+ (or Ca2+)-dependent ATP hydrolysis activity of the plasma membranes of liver (mouse), kidney (dog), two mouse sarcomas, and a human astrocytoma (xenograft in athymic mice). The ATPase is more dominant in the plasma membranes from mouse brain and human oat cell carcinoma. The widespread presence of the ATP diphosphohydrolase in plasma membrane from various types of tissues is demonstrated for the first time and is of particular interest in view of its relatively high activity in the plasma membranes of two sarcomas. The membrane-bound ATP diphosphohydrolase is characterized with respect to its metal ion activators, substrates, and inhibitors. These results should facilitate the distinction of this enzyme from other ATP hydrolyzing enzymes of plasma membranes in future investigations.