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*Corresponding author
Tel: +64-7-8348800-7554; Fax: +61-3-97312000 E-mail: Fernanda.Costa@wintec.ac.nz
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1
Faculty of Veterinary Science, and 2BIO 21 Institute, The University of Melbourne, Victoria 3010, Australia
3
Universidade Federal de São Paulo, São Paulo 04023-900, Brazil
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Qk。}uxjy@ angiotensin I-converting enzyme, assay agreement, biochemistry, equine, methodology
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Angiotensin I-converting enzyme (ACE) is responsible
The population studied comprised 29 horses, of which 26 (90%) were Thoroughbreds, two (7%) were mixed breeds and one (3%) was a Standardbred. The ages of the horses ranged from 3 to 16 years and there were 16 males (13 geldings and 3 colts) and 13 females. Blood samples (10 mL per horse) were collected between 7 AM and 10 AM by simple venipuncture from one of the jugular veins into heparinized vacuum tubes. The blood was immediately placed and then kept on ice and promptly centrifuged at 3,400 × g for 10 min in a refrigerated centrifuge (GS15R; Beckman, Australia) at 4oC. The resulting plasma was harvested and transferred with sterile disposable plastic pipettes in aliquots into labeled microtubes (Eppendorf, Australia). The sample aliquots were snap-frozen in liquid nitrogen and stored in a −70oC freezer until analysis. Each aliquot was used once, after a single cycle of thawing. Sample aliquots were not thawed and refrozen. From collection to freezing, the process took a maximum of 2 h.
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This method utilized kinetic spectrophotometry and a commercially available assay kit, ACE Infinity (Thermo- Fisher, USA), developed for ACE measurement in both plasma and serum. This assay is based on the chemical reaction between ACE and the substrate furylacryloyl- phenylalanyl-glycyl-glycine (FAPGG), which causes a decrease in absorbance at 340 nm. The method employs a ready-to-use calibrator and substrate reagents and uses 30 µL of plasma and 300 µL of substrate for each sample analyzed. The linearity of the assay is between 1 and 120 U/L and the sensitivity is described by the manufacturer as 0.084ΔmA/min per U/L. Calibration methodology for the spectrophotometer is provided by the kit manufacturer.
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This method is also a kinetic spectrophotometric method based on the chemical reaction between ACE and FAPGG (F7131; Sigma-Aldrich, Australia). The method utilizes raw chemicals instead of a pre-assembled assay kit. Samples must be run with both blanks and substrate
Fig. 1. Bland and Altman plot showing the limits of agreement (two outer lines), 95% confidence interval of the mean difference (two lines on either side of the mean) and the mean difference between the furylacryloyl-phenylalanyl-glycyl-glycine (FAPGG) method and the angiotensin I-converting enzyme Infinity (ACEIn) method. n = 29.
Fig. 3. Bland and Altman plot showing the limits of agreement (two outer lines), 95% confidence interval of the mean difference (two lines on either side of the mean) and the mean difference between the o-aminobenzoic acid-FRK(Dnp)P-OH (ABZ) method and angiotensin I-converting enzyme Infinity (ACEIn) method. n = 29.
Fig. 2. Bland and Altman plot showing the limits of agreement (two outer lines), 95% confidence interval of the mean difference (two lines on either side of the mean) and the mean difference between the furylacryloyl-phenylalanyl-glycyl-glycine (FAPGG) method and the o-aminobenzoic acid-FRK(Dnp)P-OH (ABZ) method. n = 29.
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The mean ± SD of 29 samples was 69.0 ± 16.5 U/L for the ACEIn method, 89.8 ± 19.5 U/L for FAPGG method and 85.1 ± 24.8 U/L for the ABZ method. The CV for each method was 4.3% for the ACEIn method, 2.8% for the FAPGG method and 13.6% for the ABZ method. The respective CVw and CVb values were 2.7% and 10.1% for the ACEIn method, 2.5% and 14.5% for the FAPGG method and 9.5% and 14.8% for the ABZ method. Complete ACE activity inhibition was achieved in the ABZ method by adding captopril to the samples before
analysis. Ninety eight percent of the ACE activity was inhibited by EDTA in the ACEIn method, while 97.7% was inhibited in the FAPGG method. The results (Figs. 1-3) showed that measurements of ACE acquired by different methods had poor agreement. For the FAPGG method and ACEIn method, the lower limit of agreement was −8.4 (95%CI −18.2 to 1.4 U/L) and the upper limit of agreement was 49.9 (95%CI 40.2 to 59.7 U/L). For the FAPGG method and ABZ method, the lower limit of agreement was −28.8 (95%CI −40.1 to −17.6 U/L) and the upper limit of agreement was 38.3 (95%CI 27.0 to 49.6 U/L). Finally, for the ABZ method and the ACEIn method, the lower limit of agreement was −26.3 (95%CI −40.6 to −12.1 U/L) and the upper limit of agreement was 58.4 (95%CI 44.2 to 72.6 U/L). The correlation between the difference and average was 0.23 (p = 0.24) for the FAPGG and ACEIn methods, −0.33 (p = 0.08) for the FAPGG and ABZ methods, and 0.44 (p = 0.02) for the ABZ and ACEIn methods. Removal of the observation with the greatest difference between the ABZ method and ACEIn method (difference of 67.6) resulted in the correlation between the difference and average being 0.34 (p = 0.08). Coefficients of determination were 0.45 between the ACEIn method and FAPGG method, 0.26 between the ABZ method and ACEIn method and 0.53 between the ABZ method and FAPGG method. None of the non-linear quadratic coefficients were significant (p values > 0.75).
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plasma; therefore, these methods should not be used interchangeably. Variation in the mean value of ACE observed with different methods is likely to come from the fact that different protocols (spectrophotometric and fluorimetric) and different substrates (ACEIn and FAPGG vs. ABZ) were used. Nevertheless, a degree of comparability between methods was expected. Judicious analysis of literature utilizing ACE results obtained from one method but utilizing ACE reference values obtained from a second method is necessary. Previous studies on how to determine method agreement for clinical measures have been published and we have followed those guidelines [6,18]. We propose that comparison between results for equine ACE activity obtained from different methods should not be attempted, since this may be misleading. In conclusion, the results of this study demonstrate that different assays have poor agreement and should not be used interchangeably. Accordingly, linear regression should not be used to establish a transformation equation to compare results from different methods, even for clinical purposes. In terms of the precision of each method, the FAPGG method and ACEIn method showed the lowest analytical CV and within-run variation, while the ACEIn method had the lowest between-run coefficient of variation. Overall, the results presented here indicate that it is critical to adhere to a single method of analysis to achieve comparability of samples.
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This research was funded by a Joint Collaborative Grant from The University of Melbourne (Australia) and UNIFESP (Brazil), and by The Pathology Section of The Faculty of Veterinary Science at The University of Melbourne.
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