New enzymatic assay for calcium in serum.
We established a simple and rapid kinetic assay for measurement of calcium in serum by using urea amidolyase (EC 3.5.1.45) from yeast species. The method is based on inhibition of the enzyme by calcium. In the assay, we eliminated endogenous ammonium ion by use of glutamate dehydrogenase (GLDH; EC 1.4.1.4); then in the presence of urea amidolyase, urea, ATP, bicarbonate, magnesium, and potassium ions, ammonium ion production was inversely proportional to calcium ion concentration in serum. The concentration of ammonium ion formed was determined by adding GLDH to produce NADP+ in the presence of 2-oxoglutarate and NADPH; we then monitored the change of absorbance at 340 nm. The within-run CVs of this method were 1.7-3.2% (n = 10) at 1.53-3.08 mmol/L, respectively. Day-to-day (total) CVs were 2.8-4.1%. Analytical recovery was 92-112%. The presence of other ions, ascorbic acid, reduced glutathione, bilirubin, hemoglobin, citrate, lipemic material, or human serum albumin did not affect this assay system. The correlation between values obtained with our method (y) and o-cresolphthalein complexone method (CPC) (x) was: y = 1.001x + 0.077 mmol/L (r = 0.949, Sy[symbol: see text]x = 0.079, n = 100); with the other enzymatic method (x) it was: y = 0.952x + 0.021 mmol/L (r = 0.955, Sy[symbol: see text]x = 0.074, n = 100). The SEs for each method were: 0.025 mmol/L, our method; 0.023 mmol/L, CPC method; and 0.025 mmol/L, the other enzymatic method.
Kimura
S
S
Central Laboratory for Clinical Investigation, Osaka University Hospital, Japan.
Iyama
S
S
Yamaguchi
Y
Y
Hayashi
S
S
Fushimi
R
R
Amino
N
N
D000255:Adenosine Triphosphate / Q000378:metabolism; D001639:Bicarbonates / Q000378:metabolism; D002118:Calcium / Q000097:blood* / Q000494:pharmacology; D019731:Carbon-Nitrogen Ligases; D004791:Enzyme Inhibitors; D005969:Glutamate Dehydrogenase / Q000378:metabolism; D006801:Humans; D007656:Ketoglutaric Acids / Q000378:metabolism; D007700:Kinetics; D008025:Ligases / Q000037:antagonists & inhibitors / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D009249:NADP / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D011786:Quality Control; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014508:Urea / Q000378:metabolism
D001639:Bicarbonates; D004791:Enzyme Inhibitors; D007656:Ketoglutaric Acids; D000644:Quaternary Ammonium Compounds; D009249:NADP; D000255:Adenosine Triphosphate; D014508:Urea; D005969:Glutamate Dehydrogenase; D008025:Ligases; D019731:Carbon-Nitrogen Ligases; C023270:urea carboxylase (hydrolyzing); D008274:Magnesium; D011188:Potassium; D002118:Calcium