Starch gel immobilized cholinesterase supported on a pad of open cell urethan foam has proved useful in the polarographic detection of cholinesterase inhibitors in air and water (1). The stability of this immobilized enzyme under various storage and use conditions, including those in which no refrigeration is available, is a matter of concern in perfecting the analytical procedures and establishing the reliability of the method under various climatic conditions.
Previous enzyme stability studies were primarily concerned with maintenance of enzyme activity during isolation procedures or in reactions with various substrates. In these studies it was shown that most of the activity of aqueous solutions of cholinesterase and similar enzymes was retained if the temperature was kept low, if the pH was kept near the isoelectric point of the enzyme, and if additives such as the following were employed: serum albumin, gum arabic, gum acacia, insulin, glycogen, boiled starch, castor oil, amino acids, and gum mastic (2-3). It is generally recognized, however, that enzymes are more stable in the dry state than when in solution even though there are little or no published data regarding the effect of elevated temperatures on the stability of dry choline&erase with or without additives.
The original starch gel immobilized enzyme pads for use in the polarographic detection apparatus contained glycerin to promote rapid rewetting of the dried enzyme pads (1). Although these pads kept well at refrigerator temperatures, their use after storage at room temperature and above gave erratic analytical results, indicating the destruction of the enzyme. It was suspected that the glycerin was either reacting with the enzyme or was attracting water that was interfering with the stability of the enzyme. The present study was undertaken to determine whether glycerin or water was responsible for the destruction of the enzyme at temperatures of 3740ยฐC and to find substances which would provide the 587