Effects of interaction with gene carrier polyethyleneimines on conformation and enzymatic activity of pig heart lactate dehydrogenase.

Polyethyleneimine (PEI) has long been considered as "golden standard" for polymeric gene delivery carrier, however also induces cytotoxicity. To make a further insight into the molecular basis of PEI cytotoxicity, fluorescence, absorption and circular dichroism spectroscopy were conducted to investigate the influence of PEI (average molecular weight 25,000 and 1800 Da) on the conformation of pig heart lactate dehydrogenase (LDH) and its catalytic efficiency. Zeta-potential measurement and isothermal titration calorimetry were used to reveal the interaction between PEI and LDH. PEI was found to bind onto the surface of LDH predominantly via hydrophobic interaction, inducing a more compact conformation and an increased surface hydrophobicity of the enzyme. The conformational change of LDH induced by PEI binding had little influence on the complex formation between LDH and reduced nicotinamide adenine dinucleotide (NADH, the co-enzyme). However, the nonspecific binding of PEI on the surface of LDH retarded the turnover of the enzyme. Meanwhile, the large quantity of amine groups on the polymer chain made PEI subject to form complexes with NADH and pyruvate (the substrate) via hydrogen bond and electrostatic interaction, which greatly reduced the binding efficient of LDH. The polymer size played an important role in PEI-LDH interaction. The smaller size of lower molecular weight PEI facilitated the close contact with LDH and consequential reduction of the turnover number of the enzyme. However, higher molecular weight PEI was more favorable for competitive binding with NADH and pyruvate and generally decreased the catalytic efficient of LDH.