Interactions of vanadium(V)-citrate complexes with the sarcoplasmic reticulum calcium pump

Among the biotargets interacting with valladium is the calcium pump from the sarcoplasmic reticulum (SR). To this end, initial research efforts were launched with two vanadium(V)-citrate complexes, namely (NH4)(6)[V2O4(C6H4O7)Z] (.) 6H(2)O and (NH4)(6)[V2O2(O-2)(2)(C6H4O7)(2)] (.) 4H(2)O, potentially capable of interacting with the SR calcium pump by combining kinetic studies with V-51 NMR spectroscopy. Upon dissolution in the reaction medium (concentration range: 4-0.5 mM), both vanadium(V):citrate (VC) and peroxovanadium(V):citrate (PVC) complexes are partially converted into vanadate oligomers. A 1 mM solution of the PVC complex, containing 184 mu M of the PVC complex, 94 mu M oxoperoxovanaditiln(V) (PV) species, 222 mu M monomeric (VI), 43 mu M dimeric (V2) and 53 mu M tetrameric (V4) species, inhibits Ca2+ accumulation by 75%, whereas a solution of the VC complex of the same vanadium concentration, containing 98 mu M of the VC complex, 263 mu M monomeric (VI), 64 mu M dimeric (V2) and 92 mu M tetrameric (V4) species inhibits the calcium pump activity by 33%. In contrast, a 1 mM metavanadate solution, containing 460 mu M monomeric (VI), 90.2 mu M dimeric (V2) and 80 mu M tetrameric (V4) species, has no effect oil Ca2+ accumulation. The NMR signals from the VC complex (-548.0 ppm), PVC complex (-551.5 ppm) and PV (-611.1 ppm) are broadened upon SR vesicle addition (2.5 mg/ml total protein). The relative order for the half width line broadening of the NMR signals, which reflect the interaction with the protein, was found to be V4 > PVC > VC > PV > V2 = V1 = 1, with no effect observed for the V1 and V2 signals. Putting it all together the effects of two vanadium(V)-citrate complexes oil the modulation of calcium accumulation and ATP hydrolysis by the SR calcium pump reflected the observed variable reactivity into the nature of key species forming upon dissolution of the title complexes in the reaction media. (c) 2005 Elsevier Inc. All rights reserved.