Structures of diphospho-myo-inositol pentakisphosphate and bisdiphospho-myo-inositol tetrakisphosphate from Dictyostelium resolved by NMR analysis.

Diphospho-myo-inositol phosphates (PP-InsP5 and bis-PP-InsP4) were isolated from Dictyostelium in order to clarify the precise positional isomerism by two-dimensional 1H/31P-NMR analysis. The diphosphorylated inositol phosphates are 4-PP-Ins(1,2,3,5,6)P5 and 4,5-bis-PP-Ins(1,2,3,6)P4 or their corresponding enantiomers. The vicinal arrangement of the diphospho groups with its steric and electrostatic constraints possibly qualifies bis-PP-InsP4 as a metabolite with high phosphate-group-transfer potential in phosphotransferase reactions.

The role of old age in the effects of glucose on insulin secretion, pentosephosphate shunt activity, pyridine nucleotides and glutathione of rat pancreatic islets.

In pancreatic islets of adult (three month) and old (24 month) rats the effect of glucose on glucose oxidation, pyridine nucleotides, glutathione and insulin secretion was studied. DNA content was similar in both groups of animals; however, islets of old rats exhibited 30% less insulin content. While glucose-induced (16.7 mM) insulin secretion in islets of old rats was approximately 50% less than in islets of adults, no significant difference was observed in the insulin releasing effect of theophylline (1 mM). Although islet production of 14CO2 in the presence of 16.7 mM glucose increased equally in both groups, elevation of glucose failed to increase the percentage of total glucose oxidation via the pentose phosphate shunt in islets of old rats. Elevation of glucose increased the NADPH/NADP and the NADH/NAD ratio in both groups of islets in a similar manner. The effect of glucose on the GSH/GSSG ratio revealed a dose-related increase in the islets of adult rats, whereas islets of old rats did not respond to elevation of glucose. Our data seem to indicate that the lower secretory response of islets of old rats is related to the failure of glucose to increase the GSH/GSSG ratio. In contrast the insulin release induced by theophylline does not appear to depend on islet thiols.

A possible role of intracellular and membrane thiols of rat pancreatic islets in calcium uptake and insulin release.

To distinguish between the significance of membrane thiols and intracellular thiols of rat pancreatic islets in calcium uptake and insulin secretion, lanthanum-nondisplaceable uptake of 45Ca and secretion of insulin were studied in the presence of the membrane-penetrating thiol oxidant diazene dicarboxylic acid bis-(N'-methylpiperazide) (DIP), on the one hand, and the corresponding nonpenetrating thiol oxidant salt of DIP (bis-N'-methyl iodide, i.e. DIP + 2), on the other hand. In contrast to DIP + 2, DIP inhibited glucose-induced (20 mM) uptake of 45Ca and insulin secretion. Furthermore, DIP + 2 stimulated 45Ca uptake and insulin secretion in the presence of low glucose (3 mM). DIP inhibited these effects of DIP + 2. 45Ca uptake and insulin release evoked by the ionophore A-23187 were not inhibited by DIP, however. The present data collected from isolated pancreatic islets have several implications. Firstly, the reduced state of the intracellular glutathione to GSSG ratio, which causes the superficial thiol to remain in the reduced state, constitutes a prerequisite for glucose-stimulated 45Ca uptake and insulin release. Secondly, direct oxidation of superficial thiols leads to 45Ca uptake and subsequent secretion of insulin. Finally, the redox state of the intracellular glutathione system does not appear to be of importance in the stimulus-secretion coupling steps following the uptake of calcium.