The role of poly(ethyleneimine) in stabilization against metal-catalyzed oxidation of proteins: a case study with lactate dehydrogenase

The protection provided by poly(ethyleneimine) (PEI) to muscle lactate dehydrogenase (LDH) in metal-catalyzed oxidation (MCO) systems (CuSO4 or FeCl2 combined with H2O2) was studied, and comparisons were made with the chelators EDTA and desferal, respectively. The analytical chelating capacity of PEI was estimated to be around 1 mol Cu2+/10 mol ethyleneimine for all molecular weights of the polymer. The effect of [PEI monomer]/[metal ion] molar ratio on the oxidatively induced aggregation of LDH exhibited a similar trend as that of the other chelators; aggregation was enhanced at lower ratios and subsequently decreased until it was undetectable with increasing ratio. In contrast, the LDH activity showed a monotonic increase with increasing concentrations of the chelator. Total protection to the enzyme by PEI was provided at concentrations lower than that needed for full chelation of the copper ions, i.e. at [PEI monomer]/[Cu2+] ratio above 9 in case of PEI 2000, and above 7 for PEI 25 000 and 2.6×106, respectively. The polymer also provided protection against oxidation in an iron-based MCO system. Hydroxyl radical formation during the MCO reaction was inhibited in the presence of PEI. The polymer of higher molecular weights also exhibited a stronger free radical scavenging effect.