Polyionic complexes of butyrylcholinesterase and poly-L-lysine-g-poly(ethylene glycol): Comparative kinetics of catalysis and inhibition and in vitro inactivation by proteases and heat

We previously reported that recombinant human butyrylcholinesterase (rhBChE) complexed with a series of copolymers of poly-L-lysine (PLL) with grafted (polyethylene) glycol (PEG) (i.e., PLL-g-PEG) showed reduced catalytic activity but relatively similar concentration-dependent inactivation of the organophosphorus inhibitor paraoxon. Herein, we compared the kinetics of catalysis (using butyrylthiocholine as the substrate) and inhibition (using four different inhibitors) of free and copolymer-complexed rhBChE. Using scanning electron microscopy, polyionic complexes of rhBChE with three different PLL-g-PEG copolymers (based on PLL size) appeared as spheroid-shaped particles with relatively similar particle sizes (median diameter = 35 nm). Relatively similar particle sizes were also noted using dynamic light scattering (mean = 26–35 nm). The three copolymer-complexed enzymes exhibited reduced k_cat (30–33% reduction), but no significant changes in K_m. Inhibitory potency (as reflected by the bimolecular rate constant, k_i) was similar among the free and copolymer-complexed enzymes when paraoxon was the inhibitor, whereas statistically significant reductions in k_i (16–60%) were noted with the other inhibitors. Sensitivity to inactivation by proteases and heat was also compared. Copolymer-complexed enzymes showed lesser time-dependent inactivation by the proteases trypsin and pronase and by heat compared to the free enzyme. Understanding the unique properties of PLL-g-PEG-BChE complexes may lead to enhanced approaches for use of BChE and other protein bioscavengers.