APPLICATIONS OF ENZYME SYSTEMS AND ELECTRODES

Enzyme electrodes are modified conventional ion-selective electrodes [362] wherein an enzyme- or substrate-soaked matrix is sandwiched in a gel layer between the sample solution and sensing membrane of the electrode [363—370]. They are covered by the deflnition [371 ] that Enzyme—substrate electrodes are 

The functional principles of these electrodes are illustrated by a Beckman univalent electrode (Model 39137) coated with urease. This electrode when brought into contact with urea solution [364] produced NH4 cations  

Possible applications of enzyme electrodes include the determination of urea in blood (Equation (20)) and for which the optimum pH is about 7 at which 99.7 per cent of the ammonia product exists as NH4 cation, the only form among the species generated that elicits a potential response from the Beckman cation-selective glass electrode. Any sodium or potassium ion interference introduced with the enzyme (or substrate) can easily be detected because of the large positive potential reading that it causes. In such instances Dowex 50 cation exchanger may be used to remove the interferences beforehand [372]. Average differences between the urea values in blood and urine as determined by the urea-urease electrode and a spectrophotometric technique were 2.8 and 2.3 per cent m/m, respectively [373]. 

Blood and serum sodium and potassium interferences may also be overcome by using ion-selective electrodes with better cation selectivity [374—377]. Thus, a silicone rubber matrix containing nonactin X 10 and kNH4Na 

In principle, any metabolic process that terminates in the production of ammonia or carbon dioxide can be similarly quantified in a continuous, automatic, incubation-quenching technique. Already some progress has been made with creatinine, another nitrogenous terminal metabolite, using an ammonia gas electrode [376], while glutaminase activity (arising from isoenzymes present in rat tissues and tumours) may be similarly determined [378]. Urea and tyrosine can be assayed [367] by using the appropriate decarboxylase and a carbon dioxide gas electrode. 

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