Optimized determination of angiotensin I-converting enzyme activity with hippuryl-L-histidyl-L-leucine as substrate

The determination of the angiotensin I-converting enzyme activity (ACE, kininase II, peptidyldipeptide hydrolase, EC 3.4.15.1) is necessary to control the course and the treatment of sarcoidosis, as well as to monitor the therapeutic use of enzyme inhibitors such as captopril in hypertension or congestive heart failure. Numerous synthetic substrates are known with which to measure the enzyme activity. A discontinuous method using hippuryl-L-histidyl-L-leucine was tested and improved. The cleavage product, hippurate, reacts with cyanuric chloride to give a yellow complex which can be measured at 405 nm using a spectral line photometer. Enzyme activity, kinetic constants and activation energy are dependent on the chloride ion concentration. Optimal test concentrations are 1.1 mol/l potassium chloride and 3.0 mmol/l hippuryl-L-histidyl-L-leucine at pH 8.3. Higher substrate concentrations effect an inhibition of the enzyme reaction. A Michaelis constant of 0.9 mmol/l was found with serum as enzyme source. An activation energy of 57 kJ/mol was obtained from the relation between the logarithm of velocity of enzyme reaction and reciprocal value of absolute temperature. Furthermore, a linear dependence on chloride ion concentration was observed. The histogram of the enzyme activities in sera from 146 healthy volunteers shows a non-gaussian distribution. The reference interval at 25 degrees C is characterized by a median of 24 units/l with the 2.5th and the 97.5th percentiles at 13 units/l and 42 units/l, respectively. The corresponding values at 37 degrees C are 27 units/l and 86 units/l with a median of 48 units/l. No significant sex and age dependence could be found. A potent ACE inhibitor such as captopril leads to a rapid decrease of the enzyme activity within 60 min after oral administration. In the following hours, the enzyme activity slowly increases.

Optimisation of a new continuous UV assay for the determination of blood coagulation factor XIII activity in human plasma

The new photometric assay described by Fickenscher et al. (Thromb. Haemostas. 65 (1991) 535-540) for the determination of factor XIII facilitates the diagnosis of factor XIII deficiency. In spite of easy handling, this test should be used critically. Patients with hyperfibrinogenaemia showed factor XIII activities of less than 20%, whereas with an optimized assay we found normal factor XIII values. Also, the use of a fixed period of incubation for the analysis is questionable, because the period of constant reaction rate occurs earlier and is shorter with high factor XIII activities and later and longer with low factor XIII activities. A linear relation between factor XIII activity and signal only exists up to 80% of activity. In some plasma samples from patients with hyperfibrinogenaemia the factor XIII determination actually shows decreased values for factor XIII. During the reaction, a fibrin clot is formed. The resulting turbidity simulates an increase in absorbance so that NADH consumption is apparently decreased. In six patients with hyperfibrinogenaemia (8.1-9.4 g/l), a factor XIII activity of 26 U/l or less was determined. Using 50 microliters instead of 100 microliters sample volume, 50% (3/6) of the patients showed a normal factor XIII activities (80-96 U/l), whereas 50% (3/6) values of 6-15 U/l were found. In our modified assay we measured normal factor XIII activities (72-151 U/l) in all 6 patients. The procedure is optimized by reducing the sample volume from 100 microliters to 50 microliters.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of an immunoradiometric assay for bone alkaline phosphatase mass concentration in human sera

The performance characteristics of an immunoradiometric assay for bone alkaline phosphatase mass concentration in human sera are reported. Within-run imprecision (n = 20) was 12.1% (mean = 7.8 micrograms/l) and 3.6% (mean = 22.8 micrograms/l), between-day imprecision (n = 8) was 10.1% (mean = 20.3 micrograms/l) and 2.8% (mean = 84.3 micrograms/l). There was a linear relationship between the concentrations of the standards employed and the counts per minute up to 120 micrograms/l. The detection limit was 0.3 micrograms/l. In 102 apparently healthy persons (51 males and 51 females; range of age: 18-56 years) the following reference intervals were established: 3.8-21.3 micrograms/l (males) and 3.4-15.0 micrograms/l (females). We compared the values obtained using the immunoassay with those obtained by precipitating of bone alkaline phosphatase with wheat-germ lectin (alkaline phosphatase activity concentration was determined at + 25 degrees C by the optimized standard method according to the Recommendations of the German Society for Clinical Chemistry). For the reference individuals the relationship between the results of the two methods is given by the following regression equation: Bone alkaline phosphatase activity concentration [U/l] = 14.81 + 3.28 x bone alkaline phosphatase mass concentration [micrograms/l] (r = + 0.783). In 89 sera from 32 patients before and after renal transplantation (range of bone alkaline phosphatase mass concentration: 2-39 micrograms/l) comparison between the two methods yielded a linear correlation coefficient of r = + 0.886.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of transferrin determination in human sera in the diagnosis of iron deficiency

Apparently healthy persons (n = 425) as well as 264 patients characterized by an iron concentration in serum < 7.2 mumol/l were examined. A latent iron deficiency was defined as a concentration of ferritin < 20 micrograms/l (males) and < 15 micrograms/l (females), without anaemia; manifest iron deficiency defined by an additional presence of hypochromic microcytic anaemia. Fifty-nine of 425 (= 14%) apparently healthy persons showed a latent iron deficiency. In the remaining 366 we established the following reference intervals for the concentration of transferrin in serum [mumol/l]: 25.2-45.3 (males), 29.1-54.5 (females, < or = 25 years of age) and 25.3-48.6 (females, > 25 years of age). Eight of 59 (= 14%) apparently healthy persons with latent iron deficiency had a transferrin concentration above the reference interval. Sixty-one of 264 (= 23%) patients with an iron concentration < 7.2 mumol/l showed a ferritin concentration < 20 micrograms/l (males) and < 15 micrograms/l (females). Thirty-eight of these 61 patients (= 62%) had a manifest iron deficiency. In 18 of these 38 patients (= 47%) the transferrin concentration was increased. For our 264 patients we determined the diagnostic validity of an increased transferrin concentration for diagnosis of iron deficiency, assuming an iron deficiency if the concentration of ferritin remained below the discrimination values mentioned above: The diagnostic sensitivity was 36%, the diagnostic specificity 97%, the predictive value of the positive test result 79% and the predictive value of the negative test result 83%.(ABSTRACT TRUNCATED AT 250 WORDS)

An immunoblotting procedure following agarose gel electrophoresis for subclass typing of IgG paraproteins in human sera

A method for subclass typing of IgG paraproteins in human sera following agarose gel electrophoresis is presented. After electrophoretic separation, serum proteins were blotted by capillary diffusion onto nitrocellulose. Mouse anti-human IgG1-IgG4 monoclonal antibodies were exposed to bound IgG. Goat anti-mouse IgG alkaline phosphatase conjugate was employed as second antibody. Thirty six sera were examined, in which the presence of an IgG paraprotein had previously been proved by agarose gel electrophoresis and subsequent immunofixation with anti-IgG antiserum. The subclass frequency distribution was 27 IgG1, 6 IgG2, 2 IgG3 and 1 IgG4. By means of IgG subclass typing in 8 out of these 36 sera, a total of 12 additional monoclonal bands (1 to 2 bands per serum) were detected which were not seen after immunofixation electrophoresis with anti-IgG antiserum. Eleven of 12 additional bands belonged to an IgG subclass different from that of the bands already detected by immunofixation electrophoresis. Light chain typing was performed in 9 of 12 bands found additionally. Two of these 9 bands belonged to a light chain class different from that of the bands already detected by immunofixation electrophoresis. The method described can be employed to further elucidate the possible diagnostic and prognostic significance of the subclass type of an IgG paraprotein.

[A kinetic method for the direct determination of creatinine in serum with 3,5-dinitrobenzoic acid without deproteinization]

A kinetic method is reported for the determination of creatinine in serum without deproteinization, using alkaline 3,5-dinitrobenzoate solution. The increase in absorbance at 546 nm due to the formation of an orange-red complex is measured. The relationship between the creatinine concentration and absorbance was linear up to a tested concentration of 8840 mumol/l in the aqueous standard solution. Recovery of added creatinine was 98-102%. Compared with the procedure of Jaffé, as modified by Helger et al. (Z. Klin. Chem. Klin. Biochem. 12, 344-349 (1974], the present method shows less interference by pseudo-creatinine chromogens, and no interference by high serum concentrations of bilirubin or triacylglycerols. The measured serum concentration of creatinine was not affected by cephalosporin therapy. Interferences and side reactions have been minimized or eliminated by optimization of the concentrations of all reactants and of the measurement time. The described procedure can, however, only be performed in an instrument, which permits the measurement of absorbance changes in short time intervals after the start of the reaction. In comparative studies on 212 sera, the dinitrobenzoate method and the enzymic UV-test gave similar results, whereas the Jaffé method as modified by Helger et al. gave significantly higher values.

[The effect of storage and temperature on the analysis of steroids in plasma and blood]

In the analysis of steroid hormones careful attention is usually paid to blood collection and plasma storage. However, the appropriate care of samples cannot always be assured in routine work with steroids. Therefore, the stability of cortisol, aldosterone, 17-hydroxyprogesterone, testosterone, androstenedione, dehydroepiandrosteronesulphate, oestrone, oestradiol, sex hormone binding globulin (SHBG) and, the binding of testosterone and cortisol to plasma proteins in blood and plasma were studied before and after various handling procedures. Ten cycles of alternate freezing and thawing of plasma did not significantly affect the levels of the steroids or their plasma binding. The greatest differences, compared with controls, were seen for aldosterone (-6.2%) and oestradiol (-5.3%). Plasma storage at -28 degrees C was hardly superior to a 4 days storage at 4 degrees C (refrigerator) or 22 degrees C (room temperature). Although androstenedione (-10.9%), oestrone (-10.2%) and oestradiol (-12.2%) levels decreased by more than 10%, the means of all analyses were still in the 2 SD range. Even SHBG and the steroid binding were only slightly affected by temperature. When whole blood was stored at 4 degrees C or 22 degrees C, the resulting values differed from those obtained with plasma, but the differences were usually less than 10%. Although the levels were within the 2 SD range, whole blood showed a decrease of 12.3% for aldosterone and 14.5% for androstenedione. In contrast, plasma binding of testosterone (25.9%) and cortisol (15.1%) were substantially affected by storage at 22 degrees C in whole blood. It is concluded that repeated freezing and thawing of plasma, or storage at various temperatures have only a small effect on the measured levels of steroids and their plasma binding. Although it is not advisable, even whole blood may be used for the analysis of steroid concentrations.

[Catalytic activity of serum lipase during continuous temperature- dependent titrimetry]

The catalytic activity of the pancreatic lipase in serum was measured between 20 degrees C and 37 degrees C by the continuous titrimetric assay. As the temperature was increased, the time course of substrate hydrolysis showed an increasing tendency to become non-linear. The degree of non-linearity and its time of onset depended on the sample, the volume of serum in the assay, and the composition of the triglyceride substrate mixture. Twenty seven series of investigations between 20 degrees C and 32 degrees C (maximal) showed a Q10 value of 1.45 (s = 0.03) and an activation energy of mu = 27.42 +/- 0.565 kJ/mol (6550 +/- 135 cal/mol; mean +/- s). At low assay temperature, the addition of colipase caused a slight deterioration of the reaction kinetics, whereas at higher temperatures it caused a marked improvement in the time course of substrate hydrolysis. We were unable, however, to identify a fixed colipase concentration that would promote a linear reaction irrespective of sample origin and volume. These results show that the assay should be performed at 25 degrees C, and that the test conditions described in 1969 and 1982 are valid without modification. If these conditions are observed, there is no experimental basis for the addition of colipase of the quality commercially available at present.

[Determination of the catalytic activity of lipase in serum by measurement or estimation of the pH decrease in a triolein emulsion]

In the enzymic hydrolysis of triolein by lipase in serum, hydrogen ions are produced which lead to a decrease in the pH of the test system. The extent of this pH change depends on the catalytic activity of the lipase and on the buffering capacity of the test system. In the method described for the determination of the catalytic activity of lipase in serum, the decrease in pH is measured and recorded potentiometrically. Comparative studies showed a satisfactory correlation of results with those from the continuous titrimetric method. Further studies were performed with a modification of the detection method described by Härtel et al. ((1971) Z. Klin. Chem. Klin. Biochem. 9, 396-397), based on the estimation of the pH decrease from the colour change of an indicator. With operator experience, this test also gives reproducible results for the catalytic activity of lipase in serum.

[Determination of the catalytic activity of lipase by the continuous titration method]

Development of a standardized procedure for the titrimetric assay of lipase has been suggested. Comparative studies with triolein und olive oil were therefore undertaken. Optimal conditions, such as pH, substrate and glycocholate concentrations, etc. were determined for the assay with triolein. The results agree with those obtained earlier with olive oil as substrate. Experience with the quality of various reagents used since 1966 is reported. Problems of methodology are then discussed, especially the incomplete characterization and purity of the bile acid preparations, and their effects on the test conditions. A critical analysis shows that two point tests with modifications of the continuous titration cannot give reliable results. Haemolysis and its effect on catalytic activity of lipase are used to illustrate the consequences of the uncritical acceptance of data from the literature. Furthermore, the required sensitivity of the analytical equipment, operational errors, and the uncritical evaluation and interpretation of analytical data are discussed. The first experiments to be performed with the addition of colipase are described, and the question of whether this cofactor should be used routinely for test purposes is considered. The problem of whether lipoproteinlipase can be quantitatively inhibited is discussed. In view of the outlay on apparatus and the time required for the assay, continuous titration is definitely not suitable for routine screening purposes. On the other hand, simpler methods that give reproducible quantitative results have not yet been developed.

[Determination of the catalytic activity of lipase with trilinolein as substrate]

The applicability of the two point test reported by Proelss & Wright ((1977) Clin. Chem. 23, 522-531) for the determination of lipase was investigated. The substrate, trilinolein, is indeed cleaved by pancreatic lipase without interference from liver esterase or cholinesterase; on the other hand, the reported colour reaction gives nonreproducible results. Furthermore, the continuous UV-test showed a lag phase, the duration of which depends on the added catalytic activity. For these reasons the method of Proelss & Wright for the determination of lipase in serum does not give diagnostically useful results.