Mn- and Cu,Zn-superoxide dismutase activities during acute myocardial infarction

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A spokesperson triiodothyronine, thyrotropin, prolactin, and progesterone. Clin Chem 1986;32:171-4. 5. Hammarstroem S. Chemistry and immunology of CEA, CA 19-9, and CA 50. In: Holmgren J, ed. Tumor marker antigens. Lund: Studentlitteratur, 1985:34-51. says

of thyroxin,

Gian

Carlo Zucchelli Alessandro Pilo Maria Rosa Chiesa

Istituto di Fisiol. Via Savi 8 56100 Pisa, Italy

Clin.,

Charles Centre de Med. Nuci. Serv. de Radiopharm. Radioanalyse Lyon, France

CNR

Richard Cohen Albert Bizollon et

Table 1. Effect of Glutamate Dehydrogenase Activity on Urea Nitrogen Concentrations Determined in the Chem 1 and RA-1 000 Analyzers Glutamate d.hydrogenas. activity,U/L

30000 15 000

Urea nItrogen, mmol/L _____________ 35.6

7.0

7500

242 16.4

66 6.2

3 750

11.3

6.3

1 875

8.6

6.1

7.6 7.0

6.2

6.5

6.1

6.0 6.1

6.0

938 469 235 118 0

6.0

6.0

terference by adding various amounts of glutamate dehydrogenase (Sigma Chemical Co., St. Louis, MO 63178) to Endogenous Enzyme a serum pool. Our results are shown in Table 1. The urea nitrogen method in To the Editor: the RA-1000 analyzer is unaffected by Recently, during routine clinical valglutamate dehydrogenase activities idation of results from the Chem 1#{176}below -15000 U/L. We believe that analyzer (Miles Inc., Tarrytown, NY the greatest possible physiological se10591), we observed an abnormally rum activity would be -7500 UIL, i.e., high serum urea nitrogen concentra1000-fold the upper limit of the refertion (18.9 mmol/L) that fell outside ence range. However, at this concendelta-limit checks. The urea nitrogen tration there is gross interference in result for this patient determined only the Chem 1 urea nitrogen method. 4 h earlier in the Technicon RA-1000 Even at a serum glutamate dehydroanalyzer (Miles) had been within the genase activity of 500 U/L, often obreference range. When we re-assayed served in patients with less severe the aberrant serum in the RA-1000 liver disease (2), there is a small but analyzer, we obtained a urea nitrogen significant increase in serum urea niconcentration of 6.3 mmol/L, indicating trogen concentration. the presence of a positive interferent in The manufacturer is in the process the sample that affected the Chem 1 of re-formulating the reagent to obviassay. Interestingly, in both analyzers, ate the problem. However, we recomthe urea nitrogen method is based on mend that laboratorians check for this that described by Tiffany et al. (1), type of interference in other analytical utilizing coupled reactions with urease systems that use a urease-based (EC 3.5.1.5) and glutamate dehydrogemethod for measuring urea nitrogen, nase (EC 1.4.1.3). The discordant sambecause it can lead to grossly misleading results. ple was unusual in that it was from a patient with severe liver failure (serum alanine aminotransferase >15 000 References U/L) after acetaminophen overdose. 1. Tiffany TO, Jansen JM, Burtis CA, After several investigations, we found Overton JB, Scott CD. Enzymatic kinetic that the discordant urea nitrogen rerate and end-point analyses of substrate, sult was caused by very high endoguby use of GeMSAEC fast analyzer. Clin nous concentrations of glutamate dehyChem 1972;18:829-40. drogenase in the patient’s serum 2. Schmidt E, Schmidt FW. Enzyme diag(>7000 UIL). In the Chem 1 urea nitronosis in diseases of the liver and biliary gen method, the glutamate dehydrogesystem. Adv Cliii Enzymol 1979;1:239-93. nase activity in the reagent does not Stephen P. Harrison appear to be in excess, and therefore a high amount in the patient’s serum Dept. of Biochem. and Immunol. enhances the final reaction, resulting Bradford Royal Infirmary in a falsely high urea nitrogen value. Duckworth Lane We were able to reproduce this inBradford BD9 6RJ, U.K

Interference In Coupled-Enzyme Assay of Urea NItrogen by Excess

comments: To the Editor: Harrison has raised an interesting in the application of urease/glutainate dehydrogenase-coupled reacpoint

tions

Chem

for the manufacturer

for the measurement

of urea

iii-

trogen in serum or plasma. theInthecaseoftheTechnioonChem1, relatively high ratio of sample to final reaction volumes, 1:15, made the possibility of using a linear assay with zero-order kinetics to monitor the analyte concentration extremely difficult unless the range of the assay was severely limited. In the development of this assay, we chose to increase the range of the procedure to 46.3 mmoliL by utilizing it as a first-order reaction, the rate of which is limited by the concentration of glutamate dehydrogenase as well as urea. After reviewing our original development work on this assay, we concluded that an increase of glutamate dehydrogenase to obviate the effect of any endogenous enzyme would severely limit the linearity of the assay. As a result, we decided that the best way to handle this problem would be to modify the Chem 1 method sheet in the section entitled “Limitations of Method” and include a note regarding the effect of greatly increased glutamate dehydrogenase. From the data presented by Harrison, we calculate that at a glutamate dehydrogenase activity of 375 UIL(50fold the upper limit of the reference interval), the change in apparent urea concentration would be + 0.8 mmol/L at a urea concentration of 6.0 mmol/L. This information will be incorporated in the Chem 1 method sheet as a limitation of the method. Jack

Levine

Clin. Evaluations & Product Labeling Miles Inc., Diagnostics Div. 511 Benedict Ave. Tarrytown, NY 1 0591-5097

Mn- and Cu,Zn-Superoxide Dismutase Activities during Myocardlal infarction

Acute

To the Editor: dismutase (SOD) is the conversion of superoxide radical to hydrogen peroxide and molecular oxygen (1). The enzyme has two forms in humans: manganese superoxide dismutase (Mn-SOD), which Superoxide

enzyme

CLINICAL

catalyzing

CHEMISTRY,

Vol. 39, No. 5, 1993

911

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