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Summary:When the horseradish peroxidase reaction is stopped with acid, the decay of unconverted hydrogen peroxide is responsible for the further oxidation of o-phenylenediamine. This leads to a time-dependent ilattening of the Standard curve in the enzyme immunoassay, after the reaction has been stopped. Addition of reducing agents, such äs sulphite ions, to the stopping solution, prevents the further oxidation of 0-phenylenediamine by completely reducing the remaining hydrogen peroxide. The developed colour is then stabilized. Stabilisierung der Substratreaktion mit o-Phenylendiamin und Wasserstoffperoxid für Meerrettich-Peroxidase im EnzymimmunoassayZusammenfassung: Nach Abbruch der Peroxidasereaktion durch Säurezugabe führt der Zerfall von nicht umgesetztem Wasserstoffperoxid zur weiteren Oxidation von o-Phenylendiamin, was zu einer Abflachung der Standardkurven im Enzymimmunoassay mit zunehmender Zeit nach Reaktionsabbruch führt. Der Zusatz von Reduktionsmitteln wie beispielsweise Sulfitionen zur Stopplösung verhindert durch Reduktion des verbliebenen Wasserstoffperoxids die weitere Oxidation von 0-Phenylendiamin und stabilisiert die entwickelte Farbe. Introduction immunoassays (EIA).A disadvantage of o-phe-0-Phenylenediamine is one of the most frequently nylenediamine, however, is its instability. It is thereused chromogens for the determination of horseradish fore advisable to perform the reaction in the dark peroxidase. Under optimal reaction conditions a de-and to measure the colour intensity of the developed tection limit of about 2 ng/1 horseradish peroxidase 2.2 diamino azobenzene immediately after stopping is observed (1), which enables highly sensitive enzyme the reaction by addition of acid (2).

Cu/Zn superoxide dismutase (SOD) was quantified by enzyme immunoassay for prenatal diagnosis of Down's syndrome. Overall, 154 samples of amniotic fluid, 72 samples of amniotic cells and 31 samples of chorionic tissue were investigated. Due to the large biological variance of the SOD concentrations in normal pregnancies (range for amniotic fluid 10.5-154.9, for amniotic cells 40.0-338.8, and for chorionic tissue 132.2-649.5 g SOD/g protein) the cases of Down's syndrome detected by karyotype analysis were not reliably identified by Cu/Zn SOD quantification. As in erythrocytes obtained from patients with Down's syndrome, a trisomy 21 was easily and accurately detected in the erythrocytes from very small quantities (about 50 microliters) of umbilical blood. The SOD concentrations in normal cases (n = 40) varied between 11.4 and 17.3 and in the cases of trisomy 21, as confirmed by karyotyping (n = 4), between 22.5 and 23.2 ng/one million cells. SOD quantification in fetal erythrocyte is a helpful additional method in prenatal Down syndrome diagnosis under certain conditions, which are discussed.

An enzyme immunoassay especially designed for the quantification of Cu/Zn superoxide dismutase (SOD) in erythrocytes has been applied to measure the SOD of outcomes with high risk for Down's syndrome. From 148 fetuses SOD was quantified from erythrocytes of umbilical vein blood and related to the number of cells, the content of haemoglobin (Hb), and to the haematocrit (Hc). Comparative studies between the SOD content of erythrocytes from the fetuses and their mothers resulted in similar SOD levels (14.09 +/- 1.20 for fetal and 14.48 +/- 1.63 for maternal cells) with a 1.84-fold smaller variance for fetal cells. The best differentiation between normal fetuses and fetuses with Down's syndrome resulted from the SOD/cell ratio followed by the SOD/Hb ratio. Fixing a cut-off value from the probability density functions that the method results in a specificity of 99.99 per cent, the sensitivity to detect cases of Down's syndrome was 99.71 per cent for the SOD/cell ratio, 70.92 per cent for the SOD/Hb ratio, and 60.21 per cent for the SOD/He ratio. Nine cases with Down's syndrome were correctly diagnosed by the SOD/cell ratio determination. Eight of these were confirmed as free trisomy 21 by karyotype analysis and one was found to be a triploidy. The latter was not detected by the SOD/Hb and SOD/Hc ratios because of the one-third higher content of haemoglobin and the larger volume of the erythrocytes which resulted in ratios within the normal range.

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