Increase of serum angiotensin-converting enzyme activity after freezing

A fluorometric assay [4] of angiotensin-converting enzyme (ACE) was adapted to a new fluorometric and nephelometric Transcon 102 FN analyser. The intraassay coefficient of variation for the method was 2.6% (n = 48). The corresponding interassay coefficient was 5.3% (n = 16). Freezing with subsequent thawing of serum increased its ACE activity by approximately 15%. This effect remained constant if serum samples were stored at -70 degrees C, but it disappeared within one month if the samples were kept at -20 degrees C.