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Hexose metabolism in pancreatic islets: succinate dehydrogenase activity in islet homogenates.

Succinate dehydrogenase activity was measured in rat pancreatic islet homogenates incubated in the presence of [1,4-14C]succinate, the reaction velocity being judged through the generation of 14CO2 in the auxiliary reactions catalysed by pig heart fumarase and chicken liver NADP-malate dehydrogenase. In the presence of 1.0 mM succinate, the reaction velocity averaged 5.53 +/- 0.44 pmol min-1 microgram-1 islet protein. The Km for succinate was close to 0.4 mM and the enzymic activity was restricted to mitochondria. These kinetic results indicate that, under the present experimental conditions, the activity of succinate dehydrogenase does not vastly exceed that of either NAD-isocitrate dehydrogenase or the 2-ketoglutarate dehydrogenase complex, at least when the latter enzymes are activated by ADP and/or Ca2+. Nevertheless, the activity of succinate dehydrogenase is sufficient to account for the increase in O2 uptake evoked in intact islets by the monomethyl ester of succinic acid. It could become a rate-limiting step of the Krebs cycle in models of B-cell dysfunction.

Animals↗

Monosaccharides as silicon chelators: pentacoordinate bis(diolato)(phenyl)silicates with the cis-furanose isomers of common pentoses and hexoses.

Five-coordinate phenylsilicates are formed from the reaction of trimethoxy(phenyl)silane with monosaccharides in methanol in the presence of a stoichiometric amount of base. Five complexes have been isolated and characterized with two ketoses and three aldopentoses. The silicon central atom in [K([18]crown-6)][PhSi(beta-D-Fruf 2,3H-2)2].MeOH (1, Fru=fructose) is part of two chelate rings, with the ligands being beta-D-fructofuranose-O2,O3 dianions. The beta-furanose isomer is best suited for silicon ligation because it exhibits a torsion angle close to 0 degrees for the most acidic diol function, thus assuring a flat chelate ring. The same structural principles are also found in [K([18]crown-6)][PhSi(beta-D-Araf1,2H-2)2].2 MeOH (2, Ara=arabinose), [K([18]crown-6)][PhSi(alpha-D-Ribf1,2H-2)2] (3, Rib=ribose), [K([18]crown-6)][PhSi(alpha-D-Xylf1,2H-2)2]. acetone (4, Xyl=xylose), and [K([18]crown-6)][PhSi(alpha-D-Rulf2,3H-2)2] (5, Rul=ribulose).

Chelating Agents↗