Background: K+ has important physiological functions. K+ concentrations in serum are generally determined using ion-selective electrodes (ISEs), though measurement using reagents in aqueous medium is also useful. Methods: K+ concentrations were measured using recombinant inosine 5'-monophosphate dehydrogenase (IMPDH), which was activated only by K+ and NH4+. Exogenous NH4+ and endogenous NH4+ were eliminated using glutamine synthase. Results: Regression analysis of the enzymatic assay (y) vs. the ISE method (x) gave the following relation: y=1.03x+0.09 (n=54, Sy,x=0.06mmol/l). The linear range was up to 12mmol/l when 1U/ml IMPDH was used. Conclusion: Advantages of the proposed assay method are: (i) the measured range is wider than that of existing enzymatic methods; (ii) the conditions for K+ determination can be maintained constant, regardless of the amount of NH4+ in the analyte and reagents; and (iii) the elimination system is simpler because the recombinant IMPDH is stimulated by only K+ and NH4+ and is unaffected by biological materials.
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