Investigation of manganese(II)-insulin complexes using electrospray ionization mass spectrometry

Manganese is a trace element in human nutrition. It is involved in many enzymes, proteins and biological activities. Mn(II) ion has the capable of binding to protein or peptides. Insulin is a blood glucose-lowering peptide hormone and it is secreted by the pancreatic β-cells. In this study, the binding capability of Mn(II) ions to insulin was studied using ESI-MS, nano-ESI-MS and MS/MS methods. The binding of Mn(II) ions to insulin molecule was studied by examining the effect of pH, the molar ratio of Mn(II) ions to insulin, the flow rate with nano-ESI system and MS/MS spectrometry. The ESI-MS measurements showed that the Mn(II)-insulin complexation mostly produces ML and M₂L type complexes. The highest binding ratio was found at pH 7. The complex formation equilibrium constants of Mn(II)-insulin were calculated as K_f1: 1.03 ± 0.12 × 10⁴ and K_f2: 1.93 ± 0.17 × 10³. The nano-ESI-MS and MS/MS measurements exhibited strong and specific binding of Mn(II) ions to insulin molecule. It was concluded from all the ESI-MS measurements that Mn(II) ion has a high affinity to insulin molecule to form stable complexes.