Protein stabilising effect of polyethyleneimine

The effect of polyethyleneimine (PEI), a cationic polymer, on protein stabi lity was investigated. Shelf stability of lactate dehydrogenase, stored as an aqueous solution, at 36 degrees C was distinctly improved in the presenc e of 0.01-1% (w/v) polymer (except at pH 5.0). PEI also prevented aggregati on of the protein during long-term storage. Both high and low molecular wei ght (ca. 600 000 and 2000, respectively) PEI were equally effective. Stabil ising effect was also observed for other proteins including alcohol dehydro genase, P-galactosidase and trypsin. Circular dichroism scan in the far UV range confirmed the protection offered by PEI against complete loss of prot ein secondary structure during storage. Studies with LDH revealed that the presence of polymer did not increase its denaturation temperature, but on t he other hand, hindered the oxidation of free sulfhydryl groups catalysed b y metal ions and resultant inactivation of the enzyme. Protection by the po lymer was also seen during drying of LDH but not during freeze-thawing. (C) 1999 Elsevier Science B.V. All rights reserved.