The bilayer lipid membrane as a generic electrochemical transducer of hydrolytic enzyme reactions

Bilayer lipid membranes (BLMs) can be used as generic transducers to monitor hydrolytic enzyme reactions occurring at the membrane surface. The representative enzymatic reactions presented herein were between membrane associated urease and penicillinase with urea and penicillin, respectively. Transient electrochemical signals from BLMs which contained enzyme were obtained by proper selection of the lipid composition of membranes. Negatively charged lipid membranes composed of egg phosphatidylcholine (PC) and 35% dipalmitoylphosphatidic acid were used for this purpose. The results were consistent with an electrostatic mechanism of perturbation of the surface structure of the BLMs, where changes of local hydronium ion activity associated with the enzymatic reaction altered the extent of ionization of the headgroups of the acidic constituent of the membranes, thereby providing a transient charging current which lasted for a period on the order of seconds. The delay time for observation of the transient was directly and reproducibly related to the concentration of the substrate which could be determined over a range of microM to mM levels. The results indicate that BLMs can be used as generic selective electrochemical transducers and as switchable biosensors to monitor rapid enzymatic reactions which alter pH.