NEW ENZYMATIC ASSAY WITH UREA AMIDOLYASE FOR DETERMINING POTASSIUM INSERUM

We developed a new simple assay for potassium ion in serum using urea amidolyase (UAL) from yeast sp. The method is based on activation of t he enzyme by potassium ion, We eliminated endogenous ammonium ion by u se of glutamate dehydrogenase (GLDH), and then Inollitored the product ion of amtnonium ion by UAL, urea, ATP, bicarbonate and magnesium ions . Ammonium ion was produced proportional to the potassium ion concentr ation and was determined by adding GLDH to produce NADP(+) in the pres ence of 2-oxoglularate and NADRH. We monitored the change of absorbanc e at 340 nm. The inhibitory effect of calcium ion to this assay was el iminated by adding glycoietherdiarnine-N, N`, N', N'-tetraacetic acid to the reaction. The within-assay coeffrcients of variation (CV) of th is method were 0.99-1.55% (n = 10) at 3.32-6.18 mmol/L. Day-to-day CVs ranged from 1.49% to 2.46%. The analytical recovery was 96-108%. The correlation coefiicient between the values obtained by our method (y) and those by the ion-selective electrode (ISE) method (x) was 0.994 (y = 1.032x-0.166 mmol/L, S-y,S-x = 0.110, n = 100). The presence of bij iHIbill, haemoglobin or other ious did not affect this assay, confirmi ng the usefulness of this assay for clinical purposes.