Abstract
Homogenates of isolated pancreatic islets contain 40-70 times as much flavin-linked glycerol-3-phosphate dehydrogenase (EC 1.1.99.5) as homogenates of whole pancreas, liver, heart, or skeletal muscle when the activity is assayed with either iodonitrotetrazolium or with dichloroindophenol as an electron acceptor. Intact mitochondria from islets release 3HOH from [2-3H]glycerol phosphate 7 times faster than do skeletal muscle mitochondria. The activity of the cytosolic, NAD-linked, glycerol phosphate dehydrogenase (EC 1.1.1.8) in pancreatic islets is comparable to that of the mitochondrial dehydrogenase so a glycerol phosphate shuttle is possible in pancreatic islets. Diazoxide, an inhibitor of insulin release in vivo and in vitro, inhibits the islet mitochondrial glycerol phosphate dehydrogenase in all three of the assays mentioned above at concentrations that inhibit insulin release and CO2 formation from glucose by isolated pancreatic islets. Diazoxide does not inhibit the dehydrogenase in mitochondria from skeletal muscle, liver, and heart. A slight inhibition in mitochondria from whole pancreas can be accounted for as inhibition of the islet dehydrogenase because no inhibition is observed in mitochondria from pancreas of rats treated with alloxan, an agent that causes diabetes by destroying pancreatic beta cells. The results of this study are compatible with the hypothesis that the mitochondrial glycerol phosphate dehydrogenase has a key role in stimulus-secretion coupling in the pancreatic beta cell during glucose-induced insulin release.
Highlights
Homogenates of isolated pancreatic islets contain 4070 times as much flavin-linked glycerol-3-phosphate dehydrogenase (EC 1.1.99.5) as homogenates of whole pancreas, liver, heart, or skeletal muscle when the activity is assayed with either iodonitrotetrazolium or with dichloroindophenolas an electron acceptor
It has been proposed that the production of reducing equivalents is an important part of the signal for glucose-induced insulin release (11-E), the means by which the p cell metabolizes reducing equivalents has nporteviously been investigated
For everymolecule of glucose metabolized via glycolysis, two molecules of NADH are formed in the cytosol at the step catalyzed by glyceraldehyde phosphate dehydrogenase
Summary
The rest of the activity was in the fraction containing the nuclei and cell debris and no activity was detected in the fraction containingcytosol and microsomes This pattern corresponds to thatof other exclusively mitochondrial enzymes, such as succinate dehydrogenase, in theislet subcellular fractions [33]. The average activity of the mitochondrial glycerol phosphate dehydrogenase in homogenates of pancreatic islets was 40-70 times higher than in liver, whole pancreas, heart, or skeletal muscle (iodonitrotetrazolium assay, Table I). These relationships were still present when the enzyme activity was assayed in the mitochondrial fractions of these tissues, with the exception thatthe specific activity in skeletal muscle mitochondria was one-ninth that in the islet mitochondria. Separate groups of animals or batches of islets were used for preparing homogenates of whole tissue and for mitochondrial fractions and for each of the two types of enzyme assay
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