Localization and Characterization of the Mitochondrial Isoform of the Nucleoside Diphosphate Kinase in the Pancreatic β Cell: Evidence for Its Complexation with Mitochondrial Succinyl-CoA Synthetase

Abstract

Nucleoside diphosphate kinase (NDPK) catalyzes the transfer of terminal phosphates from nucleoside triphosphates to nucleoside diphosphates to yield nucleotide triphosphates. The present study was undertaken to localize and characterize the mitochondrial isoform of NDPK (mNDPK) in the pancreatic β cell since it could contribute to the generation of mitochondrial nucleotide triphosphates and, thereby, to the mitochondrial high-energy phosphate metabolism of the pancreatic β cell. Mitochondrial fractions from the insulin-secreting β cells were isolated by differential centrifugation. mNDPK activity was assayed as the amount of [3H]GTPγS formed from ATPγS and [3H]GDP. Incubation of isolated mitochondrial extracts with either [γ-32P]ATP or GTP resulted in the formation [32P]NDPK, which could be immunoprecipitated by an anti-NDPK serum. mNDPK exhibited saturation kinetics with respect to its nucleoside diphosphate acceptors and nucleoside triphosphate donors and sensitivity to known inhibitors of NDPK (e.g., uridine diphosphate and cromoglycate). By Western blot analyses, at least three isoforms of NDPK were identified in various subcellular fractions of the β cell. The nm23-H1 (NDPK-A) was predominantly soluble whereas nm23-H2 (NDPK-B) was associated with the soluble as well as membranous fractions. The mitochondrial isoform of NDPK, nm23-H4, was uniformly distributed in the β cell mitochondrial subfractions. A significant amount of NDPK (as determined by the catalytic activity and immunological methods) was recovered in the immunoprecipitates of mitochondrial fraction precipitated with an antiserum directed against succinyl-CoA synthetase (SCS), suggesting that NDPK might remain complexed with SCS. We provide the first evidence for the localization of a mitochondrial isoform of the NDPK in the islet β cell and thus offer a potential mechanism for the generation of intramitochondrial GTP which, unlike ATP, is not transported into mitochondria via the classical nucleotide translocase. Further work will be required to determine the importance of the NDPK/SCS complex to normal β cell function in the secretion of insulin.