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PubMed1992

Hexose metabolism in pancreatic islets. Ca(2+)-dependent activation of the glycerol phosphate shuttle by nutrient secretagogues.

A method is proposed for the measurement of the flux through the glycerol phosphate shuttle in pancreatic islets. Such a flux is taken as the ratio between the production of 3HOH and the specific radioactivity of L-[2-3H]glycerophosphate in islets exposed to [2-3H]glycerol. D-Glucose and non-glucidic nutrient secretagogues, such as 2-ketoisocaproate and 2-aminobicyclo[2,2,1]heptane-2-carboxylate, stimulate, in a Ca(2+)-dependent manner, circulation in the glycerol phosphate shuttle. The shuttle flux is commensurate with the fraction of pyruvate generation which is not coupled with L-lactate production. These findings support the view that a rise in D-glucose concentration leads to activation of the FAD-linked mitochondrial glycerophosphate dehydrogenase through an increase in cytosolic Ca2+ concentration.

Animals↗
PubMed1990

Glucose transport in lysosomal membrane vesicles. Kinetic demonstration of a carrier for neutral hexoses.

Lysosomal membrane vesicles isolated from rat liver were exploited to analyze the mechanism of glucose transport across the lysosomal membrane. Uptake kinetics of [14C]D-glucose showed a concentration-dependent saturable process, typical of carrier-mediated facilitated transport, with a Kt of about 75 mM. Uptake was unaffected by Na+ and K+ ions, membrane potentials, and proton gradients but showed an acidic pH optimum. Lowering the pH from 7.4 to 5.5 had no effect on the affinity of the carrier for the substrate but increased the maximum rate of transport about 3-fold. As inferred from the linearity of Scatchard plots, a single transport mechanism could account for the uptake of glucose under all conditions tested. As indicated by the transstimulation properties of the carrier, other neutral monohexoses, including D-galactose, D-mannose, D- and L-fucose were transported by this carrier. The transport rates and affinities of these sugars, measured by the use of their radiolabeled counterparts, were in the same range as those for D-glucose. Pentoses, sialic acid, and other acidic monosaccharides including their lactones, aminosugars, N-acetyl-hexosamines, and most L-stereoisomers, particularly those not present in mammalian tissues, were not transported by this carrier. Glucose uptake and transstimulation were inhibited by cytochalasin B and phloretin. The biochemical properties of this transporter differentiate it from other well-characterized lysosomal sugar carriers, including those for sialic acid and N-acetylhexosamines. The acidic pH optimum of this glucose transporter is a unique feature not shared with any other known glucose carrier and is consistent with its lysosomal origin.

Animals↗