Reproducibility of plasma angiotensin-converting enzyme activity in human subjects determined by fluorimetry with Z-phenylalanine-histidyl-leucine assubstrate

Despite the major physiologic role of angiotensin-converting enzyme (ACE), few studies have evaluated the ideal conditions for measuring human plasma ACE activity specifically when using Z-phenylalanine-histidyl-leucine as su bstrate. This study, performed in volunteer patients. assessed the reproduc ibility of human plasma ACE activity measured by fluorimetry with Z-phenyl- histidyl-leucine as the substrate. After blood centrifugation, plasma was s tored under different conditions until processing. The following sources of variability were evaluated: (1) the interval to centrifugation of blood af ter collection, (2) the temperature and (3) safe time for storing the plasm a after cold centrifugation, (4) the effect of tasting. Plasma ACE activity was 20.6 +/- 7.7 U/ml, 20.9 +/- 8 U/mL, and 20.5 +/- 7.9 U/mL (n = 25) whe n samples were centrifuged immediately, after 1 hour of blood sampling, and after 3 hours of blood sampling, respectively (not significant). In plasma kept at -20 degrees C, ACE activity was not different after 1 week (17.4 /- 4.3 U/mL) nor after 1 month (17.9 +/- 4 U/mL), whereas baseline ACE was 16.7 +/- 4.3 U/mL (n = 10). In plasma stored at -80 degrees C, ACE activity was 15.5 +/- 5.7 U/mL after 1 month (baseline 15 +/- 5.3 U/mL; not signifi cant; n = 12). No evidence for hydrolysis of the reaction product of ACE (h is-leu dipeptide) was observed in plasma samples kept for 1 month at -20 de grees C or at -80 degrees C (by high-performance liquid chromatography anal ysis). In plasma obtained before breakfast, ACE activity was 12.8 +/- 7.1 U /mL, and it was 12.3 +/- 7.5 U/mL 2 hours afterwards (not significant; n = 12), Thus, to determine human plasma ACE activity by fluorimetry with relia bility, with Z-phenylalanine-histidyl-leucine used as a substrate, there is a safe interval of at least 3 hours before blood centrifugation at -4 degr ees C. Plasma may be kept at -20 degrees C or at -80 degrees C for at least 4 weeks before final processing. Easting does not influence its enzymatic activity.